Cardiac Mapping

Cardiac Mapping Second Edition Edited by Mohammad Shenasa, MD Attending Physician Department of Cardiovascular Services O'Connor Hospital; Heart and Rhythm Medical Group San Jose, California

Martin Borggrefe, MD Professor of Medicine (Cardiology) Head, Department of Cardiology, Angiology and Pneumology Klinikum Mannheim GmbH Universitatsklinikum Fakultat fur Klinische Medizin Mannheim der Universitat Heidelberg Mannheim, Germany Gunter Breithardt, MD Professor of Medicine (Cardiology) Head, Department of Cardiology and Angiology and Institute of Arteriosclerosis Research Hospital of the Westfalische Wilhelms-Universitat Munster Munster, Germany

Futura, an imprint of Blackwell Publishing

© 2003 by Futura, an imprint of Blackwell Publishing Blackwell Publishing, Inc./Futura Division, 3 West Main Street, Elmsford, New York 10523, USA Blackwell Publishing, Inc., 350 Main Street, Maiden, Massachusetts 02148-5018, USA Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ Blackwell Science Asia Pty Ltd, 550 Swanston Street, Carlton South, Victoria 3053, Australia Blackwell Verlag GmbH, Kurfurstendamm 57, 10707 Berlin, Germany All rights reserved. No part of this publication may be reproduced in any form or by any electronic or mechanical means, including information storage and retrieval systems, without permission in writing from the publisher, except by a reviewer who may quote brief passages in a review. 03 04 05 06 5 4 3 2 1 ISBN: 0-87993-404-2 Library of Congress Cataloging-in-Publication Data Cardiac mapping / edited by Mohammad Shenasa, Martin Borggrefe, Gunter Breithardt.—2nd ed. p. ; cm. Includes bibliographical references and index. ISBN 0-87993-404-2 (alk. paper) 1. Arrhythmia. 2. Electrocardiography. I. Shenasa, Mohammad. II. Borggrefe, Martin. III. Breithardt, Gunter. [DNLM: 1. Electrocardiography. 2. Electrophysiology. 3. Heart Diseases—physiopathology. WG 140 C267 2003] RC685.A65 C287 2003 616.1'28—dc21 2002014632

A catalogue record for this title is available from the British Library Acquisitions: Steven Korn Production: Joanna Levine Typesetter: International Typesetting and Composition, in New Delhi, India Printed and bound by Walsworth Publishing Company, in Marceline, MO USA For further information on Blackwell Publishing, visit our website: www.futuraco.com

This book is dedicated to those who paved the "roads of cardiac mapping" To all who taught us: our mentors, colleagues, students, and patients We also dedicate this book to our wives, children, and parents for their continuous lifetime support and love

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Acknowledgments

We are grateful to our friends and colleagues for their excellent state-of-theart contribution to the second edition of Cardiac Mapping. We deeply appreciate Ms. Maryam Shenasa for her superb assistance during the preparation of this

work. Our special thanks to Joanna Levine for her tireless efforts in editorial assistance, and to Steve Korn and Jacques Strauss of Blackwell Publishing's Futura Division for their support and advice in completing this project.

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Contributors

Maria Alcaraz, MD Chief, Radiology Service, Hospital Santa Cristina, Madrid, Spain

Jacques Billette, MD, PhD Professor of Physiology, Department de Physiologie, Universite de Montreal, Montreal, Quebec, Canada

Maurits Allessie, MD, PhD Professor of Physiology, Head, Department of Physiology, University of Limburg, Maastricht, The Netherlands

Susan M. Blanchard, PhD Professor of Biological and Agricultural Engineering, North Carolina State University, Raleigh, NC

Thabet Al-Sheikh, MD Consulting Electrophysiologist, Cardiology Consultants, Pensacola, FL

Lucas Boersma, MD Department of Physiology, University of Limburg, Maastricht, The Netherlands

Gregory T. Altemose, MD Consulting Electrophysiologist, Mount Carmel Health System, Columbus, OH

Martin Borggrefe, MD Professor of Medicine (Cardiology), Head, Department of Cardiology, Angiology and Pneumology, Klinikum Mannheim GmbH, Universitatsklinikum, Fakultat fur Klinische Medizin Mannheim der Universitat Heidelberg, Mannheim, Germany

Shlomo A. Ben-Haim, MD, DSc Professor of Medicine, Faculty of Medicine, Technion, Israel Institute of Technology, Haifa, Israel Edward J. Berbari, PhD Professor of Electrical Engineering and Medicine, Director of Biomedical Engineering, Biomedical Engineering Program, Indiana University Purdue University Indianapolis, Indianapolis, IN

Gunter Breithardt, MD, FACC, FESC Professor of Medicine (Cardiology), Head, Department of Cardiology and Angiology and Institute of Arteriosclerosis Research, Hospital of the Westfalische Wilhelms-Universitat Miinster, Munster, Germany

Martin Biermann, MD Josep Brugada, MD, PhD Department of Cardiology and AngiAssociate Professor of Medicine, Arrhythmia Section, Cardiovascular ology, Hospital of the Westfalische Wilhelms-UniversitatTMunster,TER, Institute, Hospital Clinic, University Munster, Germany of Barcelona, Barcelona, Spain vII

viii CARDIAC MAPPING Riccardo Cappato, MD Chief, Center of Clinical Arrhythmia and Electrophysiology, Instituto Policlinico San Donato, San Donato Milanese, Milan, Italy

Andre d'Avila, MD Clinical Arrhythmias and Pacemaker Unit, Heart Institute (InCor)— University of Sao Paulo Medical School, Sao Paulo, Brazil

Corrado Carbucicchio, MD Centro Cardiologico Fondazione Monzino IRCCS, Instituto di Cardiologia dell'Universita, Milan, Italy

Jacques M.T. de Bakker, PhD Professor of Experimental Electrophysiology, Department of Experimental Cardiology, University of Amsterdam, Amsterdam, The Netherlands; Department of Cardiology, University of Utrecht, Utrecht, The Netherlands

Rene Cardinal, PhD Professor of Pharmacology, Department of Pharmacology, Universite de Montreal and Research Center, Hopital du Sacre-Coeur de Montreal, Montreal, Quebec, Canada Edward B. Caref, PhD Research Associate, Cardiology Research Program, NY Harbor VA Health Care Center, Brooklyn Campus, Brooklyn, NY Xu Chen, MD Kardiologisk Laboratorium, Department of Medicine, Rigshospitalet Blegdamsvej, Copenhagen, Denmark Kee-Joon Choi, MD Hopital Cardiologique du HautLeveque, Bordeaux-Pessac, France

Paolo Delia Bella, MD Centro Cardiologico Fondazione Monzino IRCCS, Instituto di Cardiologia deH'Universita, Milan, Italy Igor R. Efimov, PhD Elmer L. Lindseth Associate Professor of Biomedical Engineering, Case Western Reserve University, Cleveland, OH Nabil El-Sherif, MD Professor of Medicine and Physiology and Director of Cardiac Electrophysiology Program, State University of New York, Downstate Medical Center; Chief, Cardiology Division, NY Harbor VA Health Care Center, Brooklyn Campus, Brooklyn, NY

Jacques Clementy, MD Hopital Cardiologique du HautLeveque, Bordeaux-Pessac, France

Sabine Ernst, MD II Medizinische Abteilung, Allg. Krankenhaus St. Georg, Hamburg, Germany

Francisco G. Cosio, MD, FESC, FACC Chief, Cardiology Service, Hospital Universitario de Getafe, Madrid, Spain

Gaetano Fassini, MD Centro Cardiologico Fondazione Monzino IRCCS, Instituto di Cardiologia dell'Universita, Milan, Italy

D. Wyn Davies, MD Consultant Cardiologist, St. Mary's Hospital and Imperial College School of Medicine, London, UK

Vladimir G. Fast, PhD Assistant Professor, Department of Biomedical Engineering, University of Alabama at Birmingham, Birmingham, AL

CONTRIBUTORS ix Peter L. Friedman, MD, PhD Associate Professor of Medicine, Harvard Medical School, Boston, MA; Director, Clinical Cardiac Electrophysiology Laboratory, Cape Cod Hospital, Hyannis, MA; Physician, Brigham and Women's Hospital, Boston, MA Paola Galimberti, MD Centro Cardiologico Fondazione Monzino IRCCS, Instituto di Cardiologia dell'Universita, Milan, Italy Hasan Garan, MD Professor of Medicine, Columbia University College of Physicians and Surgeons; Director of Cardiac Electrophysiology, Columbia Presbyterian Medical Center, New York, NY Stephane Garrigue, MD Hopital Cardiologique du HautLeveque, Bordeaux-Pessac, France Antonio Goicolea, MD, FESC Cardiac Electrophysiology Laboratory, Clinica Nuestra Senora de America, Madrid, Spain Michel Haissaguerre, MD Hopital Cardiologique du HautLeveque, Bordeaux-Pessac, France Helena Hanninen, MD Division of Cardiology, Department of Medicine, and BioMag Laboratory, Helsinki University Central Hospital, Helsinki, Finland

Gerhard Hindricks, MD University of Leipzig Heart Center, Co-director, Department of Electrophysiology, Leipzig, Germany Meleze Hocini, MD Hopital Cardiologique du HautLeveque, Bordeaux-Pessac, France Raymond E. Ideker, MD, PhD Jeanne V. Marks Professor of Medicine, Professor of Physiology, Professor of Biomedical Engineering, University of Alabama at Birmingham, Birmingham, AL Pierre Jais, MD Hopital Cardiologique du HautLeveque, Bordeaux-Pessac, France Michiel J. Janse, MD Professor of Experimental Cardiology, Editor-in-Chief, Cardiovascular Research, University of Amsterdam, Amsterdam, The Netherlands Robert Johna, MD Department of Cardiology and Angiology and Institute of Arteriosclerosis Research, Hospital of the Westfalische Wilhelms-Universitat Miinster, Miinster, Germany Eric E. Johnson, MD Cardiology, Stern Cardiovascular Center, Memphis, TN Mark E. Josephson, MD Chief, Cardiovascular Division, Beth Israel Deaconess Medical Center; Professor of Medicine, Harvard Medical School, Boston, MA

Wilhelm Haverkamp, MD Department of Cardiology and Angiology and Institute of Arteriosclerosis Research, Hospital of the Westfalische Wilhelms-Universitat Minister, Minister, Germany

Alan Kadish, MD Professor and Senior Associate Chief, Division of Cardiology, Northwestern University, Chicago, IL

Francois Helie, MSc Graduate Student, Department of Pharmacology, Universite de Montreal, Montreal, Quebec, Canada

Wilhelm Kaltenbrunner, MD The Ludwig Boltzmann Arrhythmia Research Institute, Wilhelminenspital, Vienna, Austria

x CARDIAC MAPPING Karim Khalife, BSc Department de Physiologie, Universite de Montreal, Montreal, Quebec, Canada Andre G. Kleber, MD Professor of Physiology, Department of Physiology, University of Bern, Bern, Switzerland Petri Korhonen, MD Division of Cardiology, Department of Medicine, and BioMag Laboratory, Helsinki University Central Hospital, Helsinki, Finland Hans Kottkamp, MD University of Leipzig Heart Center, Co-director, Department of Electrophysiology, Leipzig, Germany Karl-Heinz Kuck, MD Chief, II Medizinische Abteilung, Allg. Krankenhaus St. Georg, Hamburg, Germany Kenneth R. Laurita, PhD Assistant Professor of Medicine and Biomedical Engineering, Heart and Vascular Research Center, MetroHealth Campus, Case Western University, Cleveland, OH Michael D. Lesh, MD Section of Cardiac Electrophysiology, Department of Medicine, University of California, San Francisco, San Francisco, CA

Laurent Made, MD Hopital Cardiologique du HautLeveque, Bordeaux-Pessac, France Markku Makijarvi, MD Assistant Professor of Medicine, Division of Cardiology, Department of Medicine, and BioMag Laboratory, Helsinki University Central Hospital, Helsinki, Finland John M. Miller, MD Professor of Medicine, Indiana University School of Medicine, Krannert Institute of Cardiology, Director, Clinical Cardiac Electrophysiology, Indianapolis, IN Maria Antonia Montero, MD Cardiology Service, Complejo Hospitalario de Ciudad Real, Ciudad Real, Spain Juha Montonen, Dr.Tech. BioMag Laboratory, Helsinki University Central Hospital; Laboratory of Biomedical Engineering, Helsinki University of Technology, Helsinki, Finland Reginald Nadeau, MD Centre de Recherche, Hopital du Sacre-Cceur de Montreal; Faculty of Medicine, Universite de Montreal, and Institut de Genie Biomedical, Ecole Polytechnique de Montreal, Montreal, Quebec, Canada

Jukka Nenonen, Dr.Tech. Academy Research Fellow, BioMag Laboratory, Helsinki University Central Hospital; Laboratory of Biomedical Engineering, Helsinki UniPeter Loh, MD versity of Technology, Helsinki, Hospital of the Westfalische WilhelmsFinland Universitat Munster, Department of Cardiology and Angiology and Insti- Ambrosio Nunez, MD Cardiology Service, Hospital Unitute of Arteriosclerosis Research, versitario de Getafe, Madrid, Spain Munster, Germany

Li-Jen Lin, MD Department of Internal Medicine, National Cheng-Kung University Hospital, Tainan, Taiwan

CONTRIBUTORS xi Jeffrey E. Olgin, MD Associate Professor of Medicine, Indiana University School of Medicine, Krannert Institute of Cardiology, Indianapolis, IN

Franz X. Roithinger, MD Electrophysiology Research Group, Department of Cardiology, University Hospital Innsbruck, Innsbruck, Austria

Feifan Ouyang, MD Medizinische Abteilung, Allg. Krankenhaus St. Georg, Hamburg, Germany

David S. Rosenbaum, MD Director, Heart and Vascular Research Center, Associate Professor of Medicine, Biomedical Engineering, and Physiology and Biophysics, MetroHealth Campus, Case Western Reserve University, Cleveland, OH

Pierre L. Page, MD Professor of Surgery, Department of Surgery, Universite de Montreal; Research Center and Staff Surgeon, Division of Cardiac Surgery, Hopital du Sacre-Coeur de Montreal and Institut de Cardiologie de Montreal, Montreal, Quebec, Canada Agustin Pastor, MD Cardiology Service, Hospital Universitario de Getafe, Madrid, Spain Nicholas S. Peters, MD Professor of Cardiac Electrophysiology, Department of Cardiology, St. Mary's Hospital and Imperial College School of Medicine, London, UK Florence Raybaud, MD Hopital Cardiologique du HautLeveque, Bordeaux-Pessac, France

Pierre Savard, PhD Centre de Recherche, Hopital du Sacre-Coeur de Montreal; Faculty of Medicine, Universite de Montreal, and Institut de Genie Biomedical, Ecole Polytechnique de Montreal, Montreal, Quebec, Canada Mauricio Scanavacca, MD Unit of Cardiac Arrhythmia, Clinical Arrhythmias and Pacemaker Unit, Heart Institute (InCor) - University of Sao Paulo Medical School, Sao Paulo, Brazil Christophe Scavee, MD Hopital Cardiologique du HautLeveque, Bordeaux-Pessac, France

Mark Restivo, PhD Richard Schilling, MD Senior Scientist, Cardiology St. Bartholomew's and Queen Mary's Research Program, NY Harbor VA University, London, UK Health Care Center, Brooklyn Campus; Assistant Professor of Med- Wolfgang Schoels, MD Department of Cardiology, Univericine, State University of New York, sity of Heidelberg, Heidelberg, Downstate Medical Center, Brooklyn, NY Germany Stefania Riva, MD Centro Cardiologico Fondazione Monzino IRCCS, Instituto di Cardiologia dell'Universita, Milan, Italy Pierre Rocque, BSc Research Assistant, Research Center, Hopital du Sacre-Coeur de Montreal, Montreal, Quebec, Canada

Dipen C. Shah, MD Hopital Cardiologique du HautLeveque, Bordeaux-Pessac, France Hossein Shenasa MD, MSc, FACC Attending Physician, Department of Cardiovascular Services, O'Connor Hospital; Heart and Rhythm Medical Group, San Jose, CA

xii

CARDIAC MAPPING

Jafar Shenasa, MSc Department of Cardiovascular Services, O'Connor Hospital; Heart and Rhythm Medical Group, San Jose, CA Mohammad Shenasa, MD, FESC, FACC Attending Physician, Department of Cardiovascular Services, O'Connor Hospital; Heart and Rhythm Medical Group, San Jose, CA Haris J. Sih, PhD Research Scientist, Guidant Corporation, Cardiac Rhythm Management, St. Paul, MN Edward Simpson, MS Institute for Global Communications, San Francisco, CA Arne SippensGroenewegen, MD, PhD Arrhythmia Service, Thoracic Cardiovascular Institute, Section of Cardiology, College of Human Medicine, Michigan State University, East Lansing, MI Timothy W. Smith, D.Phil., MD Cardiovascular Division, BarnesJewish Hospital; Assistant Professor of Medicine, Washington University School of Medicine, St. Louis, MO William M. Smith, PhD Professor of Medicine, Professor of Biomedical Engineering, University of Alabama at Birmingham, Birmingham, AL Eduardo A. Sosa, MD Director, Clinical Arrhythmias and Pacemaker Unit, Heart Institute (InCor) - University of Sao Paulo Medical School, Sao Paulo, Brazil Madison Spach, MD James B. Duke Professor of Pediatrics, Emeritus, Department of Pediatrics, Duke University Medical Center, Durham, NC

William Stevenson, MD Associate Professor of Medicine, Harvard Medical School; Director, Clinical Cardiac Electrophysiology Fellowship Program, Cardiovascular Division, Brigham and Women's Hospital, Boston, MA Claudio Tondo, MD Centro Cardiologico Fondazione Monzino IRCCS, Instituto di Cardiologia dell'Universita, Milan, Italy Gioia Turitto, MD Associate Professor of Medicine, Director, Coronary Care Unit and Electrophysiology Laboratory, University Hospital, State University of New York, Downstate Medical Center, Brooklyn, NY Michel Vermeulen, B Pharm, MscA Research Assistant, Research Center, Hopital du Sacre-Coeur de Montreal, Montreal, Quebec, Canada Alain Vinet, PhD Associate Professor, Institute of Biomedical Engineering, Universite de Montreal and Research Center, Hopital du Sacre-Coeur de Montreal, Montreal, Quebec, Canada Jun Wang, MD, PhD Department de Physiologie, Universite de Montreal, Montreal, Quebec, Canada Rukshen Weerasooriya, MD Hopital Cardiologique du HautLeveque, Bordeaux-Pessac, France Stephan Willems, MD Medizinische Klinik und Poliklinik, Abteilung fur Kardiologie, Universitats-Krankenhaus Eppendorf, Hamburg, Germany Andrew L. Wit, PhD Department of Pharmacology, Columbia University College of Physicians and Surgeons, New York, NY

CONTRIBUTORS xiii Patrick Wolf, PhD Associate Professor of Biomedical Engineering, Duke University, Durham, NC Teiichi Yamane, MD Hopital Cardiologique du HautLeveque, Bordeaux-Pessac, France

Douglas Zipes, MD Distinguished Professor of Medicine, Pharmacology, and Toxicology; Director, Division of Cardiology and Krannert Institute of Cardiology, Indiana University School of Medicine, Indianapolis, IN

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Preface

The first edition of Cardiac Mapping stood out as the only textbook in the field with outstanding contributions from world-renowned authors. The book was well received and indeed sold out. Since the release of the first edition, there have been areas of significant progress and even major breakthroughs in the field of cardiac mapping and catheter ablation of arrhythmias. In particular, the technical advancements in noncontact and nonfluoroscopic mapping improved our understanding of the mechanism and thus the appropriate treatment of many arrhythmias, particularly atrial and ventricular fibrillation. The second edition offers a

xv

unique source for the latest developments in cardiac mapping of arrhythmias. This new edition of Cardiac Mapping provides an important resource for the interventional electrophysiologist, rhythmologist, and those who are interested in understanding the mechanism of cardiac arrhythmias. As the field of interventional electrophysiology continues to evolve, cardiac mapping will remain an integral part of the science and practice of electrophysiology. The Editors San Jose, CA, Mannheim and Munster, Germany

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Preface to the First Edition

Cardiac mapping has always been an integral part of both experimental and clinical electrophysiology. Indeed, Sir Thomas Lewis systematically investigated the activation sequence of the dog ventricle as early as 1915. The detailed activation map from that experiment is shown in Figure 1. Since then, cardiac mapping has evolved from single sequential probe mapping to very sophisticated computerized three-dimensional mapping. By the time cardiac mapping began being used in the surgical management of ventricular as well as supraventricular

tachycardias, a large body of literature had already been collected. Despite this significant progress, a collective textbook that attempted to discuss all aspects of cardiac mapping did not exist. When we first considered working on such a project, we were not sure if our friends and colleagues who had paved the road to this point would think it necessary to join us in this effort, especially in this era of implantable devices. We were surprised and encouraged by their unanimous positive support to go ahead with this text. (Many of the contributors have

Figure 1

XVII

xviii CArDIAC MAPPING already asked about the second revised edition!) The contributors unanimously agreed to prepare manuscripts that discussed their latest work and that would subsequently be published in this, the only comprehensive book to present the state of the art on all aspects of cardiac mapping from computer simulation to online clinical application. Thus, we would like to thank all the contributors for presenting

their best work here. Without them, this book would not have been possible. A unique feature of this book is that chapters are followed by critical editorial comments by the pioneer of that specific area, so that the state of the art is discussed. We hope this book will serve as impetus to stimulate new ideas for cardiac mapping in the future. The Editors

Contents

Dedication Acknowledgments Contributors Preface Preface to the First Edition

iii v vii xv xvii

Part 1. Historical Perspectives Chapter 1. Historical Notes on the Mapping of Arrhythmias: The Contributions of George Ralph Mines Michiel J. Janse, MD 3 Part 2. Methodological and Technical Considerations Chapter 2. The Interpretation of Cardiac Electrograms Martin Biermann, MD, Mohammad Shenasa, MD, Martin Borggrefe, MD, Gerhard Hindricks, MD, Wilhelm Haverkamp, MD, and Gunter Breithardt, MD

15

Chapter 3. Methodology of Cardiac Mapping Haris J. Sih, PhD and Edward J. Berbari, PhD

41

Chapter 4. Noncontact Endocardial Mapping Richard Schilling, MD, Nicholas S. Peters, MD, Alan Kadish, MD, and D. Wyn Davies, MD

59

Chapter 5. Principles of Nonfluoroscopic Mapping: Nonfluoroscopic Electroanatomical and Electromechanical Cardiac Mapping Shlomo A. Ben-Haim, MD, DSc

103

Chapter 6. Principles of Magnetocardiographic Mapping Jukka Nenonen, Dr. Tech., Juha Montonen, Dr. Tech., and Markku Makijarvi, MD

119

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xx CARDIAC MAPPING Chapter 7. Fast Fluorescent Mapping of Electrical Activity in the Heart: Practical Guide to Experimental Design and Applications Igor R. Efimov, PhD, Martin Biermann, MD, and Douglas Zipes, MD 131 Chapter 8. Precision and Reproducibility of Cardiac Mapping Martin Biermann, MD, Martin Borggrefe, MD, Robert Johna, MD, Wilhelm Haverkamp, MD, Mohammad Shenasa, MD, and Gunter Breithardt, MD

157

Chapter 9. The Ideal Cardiac Mapping System Raymond E. Ideker, MD, PhD, Patrick D. Wolf, PhD, Edward Simpson, MS, Eric E. Johnson, MD, Susan M. Blanchard, PhD, and William M. Smith, PhD

187

Part 3. Mapping in Experimental Models of Cardiac Arrhythmias Chapter 10. The Role of Myocardial Architecture and Anisotropy as a Cause of Ventricular Arrhythmias in Pathological States Nicholas S. Peters, MD and Andrew L. Wit, PhD

197

Chapter 11. The Figure-of-Eight Model of Reentrant Ventricular Arrhythmias Nabil El-Sherif, MD, Edward B. Caref, PhD, and Mark Restivo, PhD 237 Chapter 12. Demonstration of Microreentry Hasan Garan, MD

275

Chapter 13. Optical Mapping of the Effects of Defibrillation Shocks in Cell Monolayers Vladimir G. Fast, PhD and Andre G. Kleber, MD

291

Chapter 14. Effects of Pharmacological Interventions on Reentry Around a Ring of Anisotropic Myocardium: A Study with High-Resolution Epicardial Mapping Josep Brugada, MD, PhD, Lucas Boersma, MD, and Maurits Allessie, MD, PhD 311 Chapter 15. Microscopic Discontinuities as a Basis for Reentrant Arrhythmias Madison S. Spach, MD 323 Chapter 16. Mapping in Explanted Hearts Jacques M.T. de Bakker, PhD and Michiel J. Janse, MD

341

Chapter 17. Efferent Autonomic Innervation of the Atrium: Assessment by Isointegral Mapping Pierre L. Page, MD and Rene Cardinal, PhD

363

Chapter 18. Mapping of Atrial Flutter Wolfgang Schoels, MD and Nabil El-Sherif, MD

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CONTENTS xxi Chapter 19. Mapping of Normal and Arrhythmogenic Activation of the Rabbit Atrioventricular Node Jacques Billette, MD, PhD, Jun Wang, MD, PhD, Karim Khalife, BSc, and Li-Jen Lin, MD 383 Chapter 20. Mapping of the AV Node in the Experimental Setting Peter Loh, MD, Jacques M.T. de Bakker, PhD, Meleze Hocini, MD, and Michiel J. Janse, MD

403

Part 4. Noninvasive Methods of Cardiac Mapping Chapter 21. Mapping of Atrial Arrhythmias: Role of P Wave Morphology Arne SippensGroenewegen, MD, PhD, Franz X. Roithinger, MD, and Michael D. Lesh, MD

429

Chapter 22. Surface Electrocardiographic Mapping of Ventricular Tachycardia: Correlation with Electrophysiological Mapping John M. Miller, MD, Jeffrey E. Olgin, MD, Thabet Al-Sheikh, MD, and Gregory T. Altemose, MD 455 Chapter 23. Body Surface Potential Mapping for the Localization of Ventricular Preexcitation Sites and Ventricular Tachycardia Breakthroughs Reginald Nadeau, MD and Pierre Savard, PhD 467 Chapter 24. Clinical Application of Magnetocardiographic Mapping Markku Makijarvi, MD, Helena Hanninen, MD, Petri Korhonen, MD, Juha Montonen, Dr.Tech., and Jukka Nenonen, Dr.Tech

483

Part 5. Mapping of Supraventricular Tachyarrhythmias Chapter 25. Endocardial Catheter Mapping in Patients with Wolff-Parkinson-White Syndrome: Implications for Radiofrequency Ablation Karl-Heinz Kuck, MD and Riccardo Cappato, MD

497

Chapter 26. Endocardial Catheter Mapping in Patients with Mahaim and Other Variants of Preexcitation Hans Kottkamp, MD and Gerhard Hindricks, MD

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Chapter 27. Endocardial Catheter Mapping of Atrial Flutter Francisco G. Cosio, MD, Antonio Goicolea, MD, Agustin Pastor, MD, Ambrosio Nunez, MD, Maria Antonia Montero, MD, and Maria Alcaraz, MD

537

xxii CARDIAC MAPPING Chapter 28. Catheter Ablation of Atrial Fibrillation in Humans: Initiation and Maintenance Michel Haissaguerre, MD, Pierre Jais, MD, Dipen C. Shah, MD, Meleze Hocini, MD, Laurent Made, MD, Rukshen Weerasooriya, MD, Teiichi Yamane, MD, Kee-Joon Choi, MD, Christophe Scavee, MD, Florence Raybaud, MD, Stephane Garrigue, MD, and Jacques Clementy, MD 561 Chapter 29. Mapping of Atrial Fibrillation: Clinical Observations Riccardo Cappato, MD, Sabine Ernst, MD, Feifan Ouyang, MD, and Karl-Heinz Kuck, MD

577

Part 6. Mapping of Ventricular Tachyarrhythmias Chapter 30. Substrate Mapping for Ablation of Ventricular Tachycardia in Coronary Artery Disease Timothy W. Smith, D.Phil., MD and Mark E. Josephson, MD

595

Chapter 31. Intraoperative Mapping of Ventricular Tachycardia in Patients with Myocardial Infarction: New Insights into Mechanisms and Electroanatomical Correlations in Septal Tachycardias Pierre L. Page, MD, Wilhelm Kaltenbrunner, MD, and Rene Cardinal, PhD . . . .605 Chapter 32. Dynamic Analysis of Postinfarction Monomorphic and Polymorphic Ventricular Tachycardias Rene Cardinal, PhD, Alain Vinet, PhD, Francois Helie, MSc, Michel Vermeulen, B Pharm, MScA, Pierre Rocque, BSc, and Pierre L. Page, MD 619 Chapter 33. Mapping of Unstable Ventricular Tachycardia William Stevenson, MD and Peter L. Friedman, MD, PhD

635

Chapter 34. Subthreshold Electrical Stimulation: A Novel Technique in Localization and Identification of Target Sites During Catheter Ablation of Cardiac Arrhythmias Mohammad Shenasa, MD, Stephan Willems, MD, Gerhard Hindricks, MD, Jafar Shenasa, MSc, Xu Chen, MD, Hossein Shenasa, MD, MSc, Martin Borggrefe, MD, and Giinter Breithardt, MD

649

Part 7. New Frontiers Chapter 35. Transcoronary Venous Mapping of Ventricular Tachycardia Paolo Delia Bella, MD, Claudio Tondo, MD, Corrado Carbucicchio, MD, Stefania Riva, MD, Gaetano Fassini, MD, and Paola Galimberti, MD

667

Chapter 36. Transthoracic Epicardial Mapping and Ablation Technique EduardoA. Sosa, MD, Mauricio Scanavacca, MD, and Andre d'Avila, MD . . . .681

CONTENTS xxiii Chapter 37. Nonfluoroscopic Mapping of Supraventricular Tachycardia Gerhard Hindricks, MD and Hans Kottkamp, MD

693

Chapter 38. Optical Mapping of Cellular Repolarization in the Intact Heart Kenneth R. Laurita, PhD and David S. Rosenbaum, MD

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Chapter 39. Techniques for Mapping Ventricular Fibrillation and Defibrillation William M. Smith, PhD and Raymond E. Ideker, MD, PhD 729 Chapter 40. Disorders of Cardiac Repolarization and Arrhythmogenesis in the Long QT Syndrome Nabil El-Sherif, MD and Gioia Turitto, MD

747

Index

775

Color Appendix

Al

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t Historical Perspectives

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Chapter 1

Historical Notes on the Mapping of Arrhythmias: The Contributions of George Ralph Mines Michiel J. Janse, MD

and we started to talk about Mines, who at that time was known to only a few people. We founded the G.R. Mines Club, In the first edition of Cardiac Map- whose other members included Maurits A. ping I provided some historical notes on Allessie, Felix I.M. Bonke, Frans J.L. van the early history of mapping of reentrant Capelle, and Robert H. Anderson, and arrhythmias and on the interpretation of Professor Rytand sent me some very extracellular waveforms.1 Rather than interesting correspondence, parts of which repeating these notes, I would like to I quote in this chapter. He wrote to me on elaborate on the role of George Ralph October 19, 1973: "As Founder and PresMines in our understanding of circus ident of the G.R. Mines Club, you should movement reentry, and to introduce some have copies, at the least, of these letters." Although Professor Rytand has by now, "personal" history as well. In the summer of 1973, David A. in his own words, joined "that Great ReRytand, Bloomfield Professor of Medicine entry in the Sky," I feel that he would at Stanford University, visited our depart- not have objected to my quoting certain ment in the Wilhelmina Gasthuis. Pro- passages. Anyone who ever met Dave fessor Durrer telephoned me, asking to Rytand will understand that he used show Professor Rytand ("you know, the terms such as Founder and President in flutterologist") around. I had read his bril- a very tongue-in-cheek manner. He was a liant review on the early history of delightful person with a great sense of arrhythmia research2 (the title suggests humor and the total opposite of a pompous that it only deals with atrial flutter, professor. In his review he discussed A.G. Mayer's which was the subject of Rytand's own research, but it covers a much wider field) experiments, in which circus movement The Early History of Circus Movement: In Search of a Film

From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; ®2003.

3

4 CARDIAC MAPPING was described in ring-like preparations from the muscular tissue of the subumbrella of the scyphomedusa Cassiopeia. "In one record specimen the pulsation persisted for 11 days during which it traveled 457 miles."2 In April 1964, Rytand wrote to Sir Henry Dale: "During a recent sabbatical leave as visiting Professor of Physiology with Dr. S.M. Tenney at Dartmouth College, we were able to make some motion pictures of circus movement in Cassiopea Xamachana, repeating A.G. Mayer's classic observations. It occurred to me to try and find the similar films made by George Ralph Mines 50 years ago, and I learned that you had been on their trail ahead of me." In his reply, Sir Henry denies "any knowledge, or interest of mine, in these, or any other films made by Mines"; however, he passes the request to "The one of my friends still living, who was a partial contemporary at least of Mines at Cambridge, E.D. Adrian, now of course Lord Adrian and Master of Trinity College, Cambridge." In June 1964, Sir Henry writes that through Lord Adrian he has "come across what may be a trail; and although my own rather deep immersion in other, and what seemed more urgent, demands, has prevented me from following it as actively as I should have wished, I have not yet given up all hope," and he provides Professor Rytand with the address of Mrs. Dorothy Thacker, "who at that time was Miss Dorothy Dale (not related to me as far as I know, except by name)," and who "worked with Mines at the time when he described the circus movement." "Mrs. Thacker informs me that she thinks that such a film was taken when she was working with Mines at the Marine Biological Station at Plymouth, and that the circus phenomenon was demonstrated on a ring, cut from the auricles of a skate or ray." Sir Henry mentions that he will write to Dr. F.S. Russell, CBE, FRS, the Secretary of the Marine Biological Association and

Director of the Plymouth Laboratory, but in a later letter he writes that Dr. Russell could not find any record of a film taken there by Mines. Meanwhile, Professor Rytand had contacted Mrs. Thacker, and a lengthy correspondence (hand-written on her part) develops. She confirms that she had the "pleasure and privilege of working with him at Cambridge from 1911 onwards, and at the Marine Biological Laboratory, Plymouth. He was full of ideas and enthusiasm, a clever and ingenious experimentator. Apart from music—he was an excellent pianist—physiology was his overwhelming interest." With respect to the film of "circus rhythm" she writes that she "knew nothing of what happened to his records when Mines left Cambridge in the summer of 1914 to become Professor of Physiology at McGill University, Montreal." She gives a list of Mines' publications, of which the last entry is, "1913. IXieme Congres International des Physiologistes a Groningue - le 2-6 sept. 1913. A short communication about circus rhythm 'with demonstration, projection and cinematografic projection.'" She never indicates that she ever saw the film or that she was present when it was made. "I can only think it might have been taken to Montreal. If so, its fate is probably unknown." Thus ends the trail of the film. However, she sent Professor Rytand some very interesting documents: copies of pages 372 and 383 of Mines' 1913 paper in the Journal of Physiology.3 On page 372 Mines described circus movement in a ring-like preparation of a tortoise heart. In the paper, he provides no mechanical or electrographic records of what he saw and described. In his handwriting, he made the following note: "Later I took electrograms of this expt." On page 383 he wrote: "Repeated this experiment about 6 times on auricle rings from Acanthias vulgaris at Roscoff in Sept. 1913. The best test for a circulating excitation is to cut through the ring at one point. Cinematographed the

HISTORICAL NOTES: GEORGE RALPH MINES 5 ring ext. At Toronto, March 1914, before seeing Garrey's paper, I obtain circulating excitations in rings from excised heart of dog (rt. ventricle), [over] In these preparations the contraction wave coursed round rapidly about once a second. It was instantly stopped by section of the ring." (See Figure 1.) Professor Rytand published these pages in his review "by permission of the Editorial Board of the Journal of Physiology (London) from a reprint presented by Lord Adrian to the Library at the Cambridge 372

0. R. MINKS.

To test the hypothesis, I devised another experiment, which I carried out on the heart of a tortoise. The heart was excised and the sinus venosus cut away. A longitudinal incision was then made extending through the anterior and posterior walls of the auricle and ventricle, so that the heart was converted into a ring, as shown in Fig. 23. The auricles were connected with the ventricle in two places, and Fig. 23. across each of these junctions it was found that excitation could pass. The experiment was made at a temperature of 21°C. On stimulating any part of the heart there was, after a slight pause, contraction in each of the other parts. After stimulating several times, the following condition appeared. The four portions of the heart marked V1, V2, A1, A2, contracted in the order given, with distinct pauses between the successive portions. This cycle of events was repeated over and over again without any further external stimulation. When V, became excited the excitation spread to V2 but not back to A, which was still refractory. From V2 it spread to A1, from A, to A2, now recovered from its refractory state, and then again from A2 to V1. While the cycle was being regularly repeated, the application of an external stimulus to either of the chambers, if out of phase with the cycle, stopped the contractions, showing that they were not originated by an automatic rhythm in any part of the preparation, but were due to a wave of excitation passing slowly round and round the ring of tissue. * It seems then that the reciprocating rhythm may reasonably be regarded as due to a circulating excitation. The circumstances under which the phenomenon made its appearance were such as to produce the favourable conditions of slow conduction and short refractory period. By its continuance the circulating rhythm would tend to maintain these conditions. The conditions are easily upset by the occurrence of an extra systole and they may be re-established by other extra systoles. I venture to suggest that a circulating excitation of this type may be responsible for some cases of paroxysmal tachycardia as observed clinically. The to-and-fro character of the movement in the cases of reciprocating rhythm which I have described recalled in a curious way the appearances sometimes seen in fibrillation of the mammalian heart, with the difference that in fibrillation, different portions of the muscle in a single chamber, instead of separate chambers of the heart, appear to exhibit reciprocating rhythm.

Physiological Laboratory. Mrs. Dorothy Thacker and Prof. Sir Bryan Matthews most kindly made this figure available."2 The Garrey paper to which Mines referred was the 1914 paper on the nature of fibrillation,4 in which Garrey used the term circus contractions. As was the case with Mines, no mechanical or electrographic tracings were presented, only the description of what the author had observed with his own eyes. Still, on the basis of Mines' addenda, we may consider him to be the first arrhythmia mapper. CONTRACTION

OF HEART.

383

ADDENDUM. On circulating excitations in the musculature of a tingle chamber. Since the above paper was sent to press I have made further observations on circulating rhythm, which may be briefly noted here. The experiments were carried out at the Plymouth Marine Laboratory on ring preparations cut from the auricles of large rays. In such preparations a single stimulus applied to any point in the ring starts a wave in each direction. The waves meet on the opposite side of the ring and die out Bat by the application of several stimuli in succession it is sometimes possible to start a wave in one direction while the tissue on the other side of the point stimulated is still refractory. Such a wave runs round the ring sufficiently slowly for the refractory phase to have passed off in each part of the ring when the wave approaches it Thus the wave circulates and may continue to do so for fifty revolutions or more. Usually an interpolated extra stimulus stops the wave at once. The preparation may then remain quiescent or it may start beating with a slow spontaneous rhythm. In the latter case the totally different characters of the spontaneous rhythm and the ulating excitation are very striking.

Erratum. In L 3, p. 232 of my paper in vol. XLVI. of the Journal "the muscle u longer, has " should read " the muscle, Do longer has."

Figure 1. Pages 372 and 383 and its reverse of the article on dynamic equilibrium in the heart by Mines3 with footnotes in his handwriting (see text). These figures were given to the author by the late Professor D.A. Rytand and are reprinted with permission from the Annals of Internal Medicine.

6 CARDIAC MAPPING Mrs. Thacker also sent Professor Rytand 2 photographs of Mines, one of which a snapshot probably taken by herself in the summer of 1911 at Plymouth (Figure 2). It is with some trepidation that I publish this snapshot here, because Mrs. Thacker was rather upset when she learned that Professor Rytand was making copies of the photographs. The more official photograph of Mines was published2 and permission to do so was obtained by Professor Rytand from the photographic studio where it was taken. Whom to ask for permission? Anyway, the harm is already done, because the picture was published in Cardiovascular Research in 19925 by Michael R. Rosen,

who got it from me, who got it from Rytand, who got it from Mrs. Thacker.

Mines' Contributions Throughout the years, 2 causes of tachycardia have been considered: enhanced impulse formation and reentrant excitation. In 1887, McWilliam6 suggested for the first time that disturbances in impulse propagation could be responsible for tachyarrhythmias: "Apart from the possibility of rapid spontaneous discharges of energy by the muscular fibres, there seems to be another probable cause for continued and rapid movement. The peristaltic contraction

Figure 2. George Ralph Mines: a photograph probably taken by Mrs. Dorothy Thacker (at that time Dorothy Dale) in the Marine Biological Laboratory, Plymouth, in the summer of 1911. Professor D.A. Rytand made this picture available to the author.

HISTORICAL NOTES: GEORGE RALPH MINES travelling along such a structure as that of the ventricular wall must reach adjacent bundles at different points in time, and since these bundles are connected with one another by anastomosing branches the contraction would naturally be propagated from one contracting fibre to another over which the contraction wave had already passed... Hence the movement would tend to go on until the excitability of the muscular tissue had been lowered, so that it failed to respond with a rapid series of contractions."6 It is clear that Mc William envisaged the possibility that myocardial fibers could be reexcited as soon as their refractory period had ended by an irregularly propagating impulse, and he therefore may be considered a founding father of reentrant excitation. Yet it was the work of Mines and of Garrey some 30 years later that firmly established the role of reentry as a mechanism for arrhythmias. Both investigators, working independently, were inspired by Mayer's work. Garrey's contributions include the demonstration that fibrillation does not result from a single, rapidly firing focus and that a minimal tissue mass is required for fibrillation.4 Here, we shall concentrate on the work of Mines. Apart from a brief biographical sketch by Rytand,2 the most complete biography of Mines can be found in a recent paper by De Silva.7 Briefly, Mines was born in Bath, England, on May 13, 1886. He entered Sidney Sussex College, Cambridge University at the age of 19. In 1911 he was appointed Assistant Demonstrator in the Physiological Laboratory at Cambridge. As already mentioned, he did important work in the Marine Biological Laboratory in Plymouth (summer of 1912, together with Dorothy Dale) and in Roscoff, France, in August and September 1913 (see Figure 1). In 1914, at the age of 28, he was offered the position of Professor and Chair of Physiology at McGill University in Montreal, Canada. On the

7

evening of Saturday, November 7, 1914, the night janitor found Mines lying unconscious in his laboratory. Mines died shortly thereafter, presumably as a result of self-experimentation.7 The accomplishments of such a young man are truly astonishing. In his 1913 and 1914 papers,3,8 Mines formulated the essential characteristics of reentry: 1. For the initiation of reentry, an area of unidirectional block must be present (Garrey4 also emphasized this point, as was acknowledged by Mines.8) In the 1914 paper, he describes an experiment on an isolated auricular preparation from a large dog-fish (Acanthian), slit up in such a way as to form a ring. The ring was spread on a glass plate and serum was poured on. The preparation remained quiescent. "Pricking with a needle point provokes a strong contraction. Wave runs round ring in each direction; the waves meet on the opposite side of the ring and die out. Repeated the stimulus at diminishing intervals and after several attempts started a wave in one direction and not in the other. The wave ran all the way round the ring and then continued to circulate going round about twice a second. After this had continued for two minutes extra stimuli were thrown in. After several attempts the wave was stopped."8 Thus, not only was unidirectional block found to be essential, the principle of antitachycardia pacing was described as well. 2. Mines described the relationship between refractory period duration and conduction velocity, as shown in Figure 3, and can thus be considered the first to formulate the "wavelength" concept. In the 1913 paper he wrote: "With increasing

8 CARDIAC MAPPING

Figure 3. Mines' diagram to explain that reentry will occur if conduction is slowed and the refractory period duration is decreased. A stimulated impulse leaves in its wake absolutely refractory tissue (black area) and relatively refractory tissue (stippled area). In both A and B, the impulse conducts in one direction only. In A, because of fast conduction and a long refractory period, the tissue is still absolutely refractory when the impulse has returned to its site of origin. In B, because of slow conduction and a short refractory period, the tissue has recovered excitability by the time the impulse has reached the site of origin, and the impulse continues to circulate.

frequency of stimulation, each wave of excitation in the heart muscle is propagated more slowly but lasts a shorter time at any point in the muscle. The wave of excitation becomes slower and shorter. Similarly the refractory phase (towards strong induction shocks) is shortened."3 3. Mines realized that establishing the activation sequence during a reentrant rhythm is not sufficient to prove reentry. "Ordinary graphic records either mechanical or electrical are of no value in attesting the occurrence of a true circulating excitation in rings of this kind, since the records show merely a rhythmic series of waves and do not discriminate between a spontaneous series of beats and a wave of excitation which continues to circulate because it always finds excitable tissue ahead of it. The only method of recording the phenomenon which I have found of any use is cinematography."8 If only the film could have been

found! "The chief error to be guarded against is that of mistaking a series of automatic beats originating in one point in the ring and travelling round it in one direction only owing to complete block close to the point of origin of the rhythm on one side of this point... Severance of the ring will obviously prevent the possibility of circulating excitations but will not upset the course of a series of rhythmic spontaneous excitations unless by a rare chance the section should pass through the point actually initiating the spontaneous rhythm."8 Thus, Mines set the stage for catheter ablation of reentrant rhythm. 4. Mines discovered the vulnerable period for fibrillation. It is remarkable that Mines and Garrey almost simultaneously described fibrillation in terms of reentry. "Garrey arrives independently at a closely similar conclusion to that which I expressed in a recent paper, namely, that fibrillation is due to waves

HISTORICAL NOTES: GEORGE RALPH MINES 9 travelling in closed circuits in the syncytium."8 Mines then describes his experiment showing that fibrillation can be induced by a single induction shock: "The point of interest is that the stimulus employed would never cause fibrillation unless it was set in at a certain clinical instant." He shows that a stimulus falling in the refractory period has no effect, "a stimulus coming a little later in the cycle sets up fibrillation" and a stimulus applied "later than the critical instant for the production of fibrillation merely induces an extrasystole..." "In the production of fibrillation just described, the stimulus apparently arrives at some part of the ventricular muscle just at the end of the refractory phase and probably before the refractory phase has ended in some other regions of the muscle. If this is so, we have again a difference in conditions of different regions of the muscle as a basis for the inauguration of the state of fibrillation." "... Suppose that at the time when excitation is set up in A, B is in the refractory state. It cannot then be excited by A. But the excited state set up in A will persist for a considerable time, and the refractory state will disappear from B before the excited state has ceased in A. The question is: Is it ever possible that under these circumstances A will excite B?" It would be 74 years before Chen and colleagues9 answered this question. As described in detail by Acierno,10 around 1920 a considerable number of people were accidentally electrocuted because more and more electrical devices were installed in households. This prompted electric companies such as Con Edison to provide grants to investigate the effects of electrical currents on human beings. This

led to the (re) discovery of the vulnerable period by Wiggers and Wegria in 1940.11 In that paper Mines is not mentioned, but in another paper by Wiggers in 1940, a brief allusion to Mines is made.12 As also noted by De Silva,7 the extensive and otherwise admirable book The History of Cardiology by L.J. Acierno10 makes no note of Mines' contributions. Fortunately, this is not the case in Luderitz' book History of the Disorders of Cardiac Rhythm.13 Some of the most astonishing paragraphs in the 1913 and 1914 papers describe in detail the mechanisms of 2 arrhythmias that at that time were clinically unknown: atrioventricular (AV) reentrant tachycardia in the WolffParkinson-White (WPW) syndrome and AV nodal reentry. As described in detail by Rosen,5 none of the relevant papers on the WPW syndrome from 1930 to 1967 mention Mines. "As for Mines, who predicted and described it all, his name is known only to those who are aficionados of electrophysiological history (e.g., Rytand)."5 In 1913, after describing circulating excitation in a ring-like preparation of a tortoise heart, Mines wrote: "I venture to suggest that a circulating excitation of this type may be responsible for some cases of paroxysmal tachycardia as observed clinically."3 One year later he repeated this suggestion "... in the light of the new histological demonstration by Stanley Kent ... that an extensive muscular connection is to be found at the right-hand margin of the heart at the junction of the right auricle and right ventricle: Supposing that for some reason an impulse from the auricle reached the main A-V bundle but failed to reach this 'right lateral' connexion. It is possible then that the ventricle would excite the ventricular end of this right lateral connexion, not finding it as refractory as it normally would at such a time. The wave spreading then to the auricle might be expected

10 CARDIAC MAPPING to circulate around the path indicated."8 This was written 16 years before Wolff, Parkinson, and White14 described the clinical syndrome that now bears their name, 18 years before Holzmann and Scherf15 ascribed the abnormal ECG in these patients to preexcitation of the ventricles via an accessory AV bundle, and 53 years before the first studies in patients employing intraoperative mapping and programmed stimulation proved Mines' predictions to be correct.16–18 As already mentioned, Mines' name cannot be found in the bibliography of any of these papers. It is ironic that what Kent described is not at all the usual accessory AV connection, found in the WPW syndrome, as discussed in detail by Anderson and Becker.19 They stated that "... there are indeed good scientific reasons for discontinuing the use of 'Kent bundle' ... the most important being that Kent did not describe connections in terms of morphology we know today... If an eponym is really necessary, then let us call them nodes of Kent."19 From Mines' point of view, the important point was of course that a human heart had been described with multiple connections between atria and ventricles. For AV nodal reentry, the story is different. In the 1913 paper Mines describes what he called reciprocating rhythm. This was based on observations in 3 experiments on the "auricle-ventricle preparation of the heart of the electric ray, and in one experiment on the ventricle-bulbus preparation from the frog..." "After the application of rhythmic stimuli at some particular rate, the cessation of the stimuli was followed by a quick reciprocating movement of auricle and ventricle or of ventricle and bulbus. The appearance of the heart gave the impression that the beats of the ventricle were caused by those of the auricle or bulbus, while these in turn were caused by the ventricle." His explanation is as follows: "The connexion between the auricle and ventricle is never

a single muscle fibre but always a number of fibres, and although these are ordinarily in physiological continuity, yet it is quite conceivable that exceptionally, as after too rapid stimulation, different parts of the bundle should lose their intimate connexion... A slight difference in the rate of recovery of the two divisions of the A-V connexion might determine that an extrasystole of the ventricle ... should spread up to the auricle by that part of the A-V connexion having the quicker recovery process and not by the other part. In such a case, when the auricle became excited by this impulse, the other portion of the A-V connexion would be ready to take up the transmission again back to the ventricle. Provided the transmission in each direction was slow, the chamber at either end would be ready to respond (its refractory phase being short) and thus the condition once established would tend to continue, unless upset by the interpolation of a premature systole."3 I can remember my excitement when I gave a talk in the Hopital Lariboisiere in 1970, describing experiments in isolated rabbit hearts employing multiple microelectrode recordings where we induced and terminated AV nodal reentrant tachycardia by extrasystoles, and meeting Philippe Coumel, Alexandre Fabiato, and Robert Slama, who had treated patients with AV nodal reentry by a pacemaker that could be turned on when a tachycardia occurred and would terminate the arrhythmia when the stimuli would be timed just right. I am pleased to say that both our publications gave full credit to Mines,20,21 as did an earlier paper on AV nodal reentry by Moe and Mendez.22 I have no idea why Mines' name is not associated with the preexcitation syndrome, and why he is given proper recognition in papers on AV nodal reentry. It is remarkable that in the wonderful book by Pick and Langendorf, Interpretation of Complex Arrhythmias,23

HISTORICAL NOTES: GEORGE RALPH MINES Mines' 1913 and 1914 papers are quoted in the chapter on "Reentrant Arrhythmias (Reciprocal Beating)," but not in the chapter entitled "The Preexcitation Syndrome." It is as Rosen5 wrote: "Mines is known only to those who are aficionados of electrophysiological history." One may add that anyone who reads Mines' original papers is likely to become an aficionado. References 1. Janse MJ. Some historical notes on the mapping of arrhythmias. In: Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping. Mount Kisco, NY: Futura Publishing Co.; 1993:3-10. 2. Rytand DA. The circus movement (entrapped circuit wave) hypothesis and atrial flutter. Ann Intern Med 1966;65: 125-159. 3. Mines GR. On dynamic equilibrium of the heart. J Physiol (Lond) 1913;46:349–382. 4. Garrey WE. The nature of fibrillar contract of the heart. Its relation to tissue mass and form. Am J Physiol 1914;33: 397-414. 5. Rosen MR. Did Wolff, Parkinson and White mind their Ps and Qs? Cardiovasc Res 1992;26:1164–1169. 6. Mc William JA. Fibrillar contraction of the heart. J Physiol 1887;8:296–310. 7. De Silva RA. George Ralph Mines, ventricular fibrillation and the discovery of the vulnerable period. J Am Coll Cardiol 1997;29:1397-1402. 8. Mines GR. On circulating excitations in heart muscles and their possible relation to tachycardia and fibrillation. Trans R Soc Can 1914;IV:43-52. 9. Chen P-S, Wolf DD, Dixon EG, et al. Mechanism of ventricular vulnerability to single premature stimuli in open-chested dogs. CircRes 1988;62:1191-1209. 10. Acierno LJ. The History of Cardiology. London, Casterton, New York: The Parthenon Publishing Group; 1994. 11. Wiggers CJ, Wegria R. Ventricular fibrillation due to single, localized induction and condenser shocks applied

11

during the vulnerable phase of ventricular systole. Am J Physiol 1940; 128: 500-505. 12. Wiggers CJ. The mechanism and nature of ventricular fibrillation. Am Heart J 1940;20:399–412. 13. Liideritz B. History of the Disorders of Cardiac Rhythm. Second Revised and Updated Printing. Armonk, NY: Futura Publishing Company; 1998. 14. Wolff L, Parkinson J, White PD. Bundlebranch block with short P-R interval in healthy young patients prone to paroxysmal tachycardia. Am Heart J 1930;5: 685-704. 15. Holzmann M, Scherf D. Ueber Elektrokardiogramme mit verkurzter VorhofKammer-Distanz und positiven P-Zacken. Z Klin Med 1932;121:404–423. 16. Durrer D, Roos JR. Epicardial excitation of the ventricles in a patient with a Wolff-Parkinson-White syndrome (type B). Circulation 1967;35:15–21. 17. Burchell HB, Frye RB, Anderson MW, McGoon DC. Atrioventricular and ventriculo-atrial excitation in Wolff-ParkinsonWhite syndrome (type B). Circulation 1967;36:663-672. 18. Durrer D, Schoo L, Schuilenburg RM, Wellens HJJ. The role of premature beats in the initiation and termination of supraventricular tachycardia in the WolffParkinson-White syndrome. Circulation 1967;36:644-662. 19. Anderson RH, Becker AE. Stanley Kent and accessory atrioventricular connections. JThorac Cardiovasc Surg 1981;81: 649–658. 20. Coumel P, Cabrol C, Fabiato A, et al. Tachycardie permanente par rythme reciproque. Arch Mal Coeur 1967;60:1830– 1864. 21. Janse MJ, van Capelle FJL, Freud GE, Durrer D. Circus movement within the AV node as a basis for supraventricular tachycardia as shown by multiple microelectrode recording in the isolated rabbit heart. Circ Res 1971;28:403–414. 22. Moe GK, Mendez C. The physiological basis of reciprocal rhythm. Prog Cardiovasc Dis 1966;8:461–482. 23. Pick A, Langendorf R. Interpretation of Complex Arrhythmias. Philadelphia: Lea & Febiger; 1979.

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Part 2 Methodological and Technical Considerations

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Chapter 2

The Interpretation of Cardiac Electrograms Martin Biermann, MD, Mohammad Shenasa, MD, Martin Borggrefe, MD, Gerhard Hindricks, MD, Wilhelm Haverkamp, MD, and Gunter Breithardt, MD

in relation to anatomical landmarks of the heart and represents the local activation As early as 1915, Lewis and Roth- of myocardium at each recording site by schild, who studied the cardiac activation a single figure, the time of activation.6,7 In sequence in the dog by recording poten- an isochronal map, only a single activatials directly from the heart, wrote: "It tion time can be represented at each site must be evident that it is a matter of first and all other information also contained concern of us, to ensure a correct inter- in the electrograms is discarded.6 In isochronal mapping, the interpretation of pretation of our curves."1 The term electrogram, as opposed to the excitation sequence of the heart rests the term electrocardiogram (ECG), denotes entirely on the individual activation times a recording of cardiac potentials from elec- assigned to each electrogram, which is why trodes directly in contact with the heart, a the correct interpretation of individual elecdefinition introduced by Samojloff in trograms is of crucial importance. Alterna1910.2–4 Electrograms form the raw data tive mapping methods, namely isopotential for cardiac mapping, which has been and isoderivative mapping,7 place emphadefined as "a method by which potentials sis on interpreting a sequence of maps, recorded directly from the surface of the rather than a set of individual electroheart are spatially depicted as a function grams,7 the correct interpretation of which of time in an integrated manner,"5 and is often difficult and at times uncertain. The objective of this chapter is to which is important as both a research tool review the interpretation of individual carand a method for guiding therapy. The most common method of cardiac diac electrograms in 2 parts: (1) in respect mapping is isochronal or activation map- to timing local activation, and (2) in respect ping. A cardiac isochronal map outlines to information contained in the morphothe locations of the various recording sites logies of electrograms. Throughout this Introduction

From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; ®2003.

15

16 CARDIAC MAPPING chapter it is assumed that negativity of the exploring electrode results in a downward movement in the unipolar electrogram.8,9 The process of creating maps out of electrogram data6,10 is not discussed, and one word of warning by Durrer et al. will have to suffice:"... the presence of time relationships compatible with an excitatory wave progressing in a certain direction does not necessarily prove the existence of such an excitatory wave."11 Activation Detection in Cardiac Electrograms The correct assignment of activation times for electrograms from each recording site is the cornerstone of isochronal mapping. With the transmembrane potential (TMP) as a gold standard, local activation of myocardium at a recording site can be defined as "the time instant when the upslope of the intracellular action potential... is a maximum."6 General Principles Underlying Electrograms

from the recording site, and the reciprocal value of the square of the distance between the dipole layer and the recording site.14,16 Thus, the unipolar electrogram records a combination of local and distant electrical events with the contribution of distant events decreasing in proportion to the square of the distance from the exploring electrode.17 The bipolar electrogram is recorded as the potential difference between 2 closely spaced electrodes in direct contact with the heart; it can be calculated as the difference between the 2 unipolar electrograms at each of the 2 electrode sites.18 These 2 unipolar electrograms differ only in the detail of the local activity at the moment of local excitation, in which case a spike in the bipolar electrogram results,18 the amplitude of which is inversely proportional to the third power of the distance between recording site and dipole.19–21 If the activation front is perpendicular in relation to the electrode pair, the bipolar spike will be of maximum amplitude, while if it is parallel, both electrodes will record the same waveform at the same time and no spike will result.6,22 To compensate the directional sensitivity of bipolar electrodes, investigators have processed bipolar signals from multipolar electrodes in different ways,23,24 or have advocated using bipolar coaxial electrodes,6,25 introduced by Fattorusso et al.26,27 in 1949 for recording precordial ECGs. However, even coaxial bipolar electrodes cannot detect activation fronts parallel to the place of the electrode (Ideker RE, oral communication).

The unipolar electrogram is recorded as the potential difference between a single electrode in direct contact with the extracellular space of the heart, the socalled "exploring electrode," and an "indifferent electrode,"12 which is an electrode placed at a distance from the heart1 or Wilson's central terminal.12 The electrical field produced at the border between resting and excited myocardium can be described as dipole.13–15 During cardiac excitation, the approach of this dipole The History of Activation Detection toward an exploring electrode gives a posAlthough direct cardiac leads had been itive deflection and its passage gives a rapid deflection in the negative direction, recorded2,28–35 (though not by Rothberger with a final return to baseline.5 Depolar- and Winterberg36) and bipolar electrodes ization, too, causes a dipole, albeit of the had been developed22,37,38 before 1914, reverse polarity.14 The amplitude of the Lewis' fame to have performed the first unipolar electrogram is proportional to cardiac mapping in 191439 and 19151 is the area of the dipole layer as "viewed" justified by the scope of his investigations.

INTERPRETATION OF CARDIAC ELECTROGRAMS

17

When Lewis et al.39 examined the spread ventricular outflow tract (RVOT) and the of excitation in the atria of the canine heart, pulmonary artery. The deflection that they used bipolar electrodes, assuming on appeared in all 6 electrograms at practithe ground of various experiments that cally the same time was thought to repre"the prominent spike" in a bipolar lead sig- sent distance cardiac activity and was nified local activation of the myocardium called the extrinsic deflection.1 A second beneath the electrode. When, in their stud- kind of deflection that occurred only in ies on the ventricles, Lewis and Rothschild.1 the 4 leads overlying myocardium and at switched to unipolar leads, they once more progressively later time instants was conhad to solve the question of activation eluded to signal the time of local activation detection. They took a series of electro- and was called the intrinsic deflection.1 grams from 6 equidistant epicardial sites While, according to Lewis' tracings, this arranged in a straight line on the right was the nadir of the S wave1 (Table 1), Table 1 Activation Detection in Unipolar Electrograms Article 1

Lewis T, 1915 Barker PS, 1930102 Wilson FN, 193440 Harris AS, 194146 Wilson FN, 194441 Wilson FN, 194745 Sodi-Pallares D, 195047 Schaefer H, 195154 Veyrat R, 195352 Durrer D, 195455 Durrer D, 195756 Jouve A, I960103 Durrer D, 1961120 Schaefer H, 196214 Durrer D, 196411 Ideker RE, 197980 Smith WM, 1980170 Parson I, 1982171 Cardinal R, 198484 De Bakker JMT, 198478 Parson I, 1984172 Carson DL, 198686 Blanchard SM, 198787 Bonneau G, 198779 Masse s, 198876 Page PL, 198885 Ideker RE, 19896 Paul T, 199090 Pieper CF, 1991168 Pieper CF, 199181 Pieper CF, 199182

Algorithm

Threshold in mV/ms

Evidence

S* S* R* R* R* S* S* FD* FD* FD* FD* FD* FD* FD* FD* MD MD MD* MD MD MD* MD MD MD MD MD MD MD MD MD MD

— — — — — — — — — — — — — — —

E R T E T T E E E E E R E T R R N N E N N E E N E E R R R R R

-2.5 -2.0-5.0 -0.5

-2.5 — —

-0.3 -1.4 -0.5-0.2 -0.5 — — — — —

-2.0

Articles that specified criteria or algorithms for activation detection in unipolar electrograms: fast downstroke (FD), maximum downslope (MD), peak of the R wave (R) or nadir of the S wave (S). Also listed is the type of evidence presented in favor of the criteria: experiment (E), theory (T), references (R), or none (N). It is assumed that negativity of the exploring electrode produces a downward deflection in the unipolar electrogram. An Asterisk (*) marks analog mapping systems.

18 CARDIAC MAPPING Wilson et al.40,41 assumed on the basis of the dipole theory that the time of local activation beneath an epicardial electrode coincided with the peak of the R wave. Later, influenced by the experiments of Cole, Curtis, and Hodgkin,42–44 they identified local activation with the nadir of the S wave.45 Harris46 used a new approach in 1941. Having demonstrated that the closely paired terminals of a bipolar electrode record only local but not distant activity, Harris compared sequential unipolar and bipolar recordings from the same sites and came to the conclusion that the peak of the R wave was "the unipolar manifestation of an action current in the local surface area."46 The investigation by Sodi-Pallares et al.47 in 1950 was based on the same principles. While all these experiments had been performed with the time-honored string galvanometer,9,48,49 Schaefer,50,51 Veyrat,52 Durrer et al.,20 and Scher et al.53 introduced highly accurate cathode tube equipment into their laboratories. The first to make the discovery that the fast downstroke in a unipolar electrogram signifies the moment of local activation were Schaefer54 in 1951, Veyrat52 in 1953, and Durrer and Van der Tweel55 in 1954. Although Schaefer and Trautwein19 had been able to demonstrate that the time of the maximum upstroke of the monophasic action potential precisely coincides with the peak of a bipolar electrogram recorded simultaneously at the same site, Schaefer's experimental evidence concerning unipolar electrograms54 is incomplete. While Veyrat52 essentially applied Lewis' methods of 1915, Durrer and Van der Tweel's approach did not differ markedly from Harris' in 1941: the absence of a fast bipolar complex in leads from thickened epicardium showed "that the fast part of the differential electrocardiogram [=bipolar electrogram] originates from electrical processes directly under the electrode."55 Simultaneous recordings of unipolar and

bipolar electrograms then yielded the following result: "In all cases where a fast part of the intrinsic deflection could be detected, the top of the differential spike [in the bipolar electrogram] was found to coincide with it."55 Thus, the fast downstroke in a unipolar epicardial electrogram was found to represent local activation. Durrer and Van der Tweel56 were able to extend these findings to intramural electrograms by means of a tripolar intramural electrode. Unlike Durrer, Scher et al.16,53 exclusively used bipolar leads for timing local activity, choosing "the positive or negative maxima of the bipolar records"16 as a criterion (Table 2). Intracellular Leads Since the development of suitable microelectrodes,57,58 many investigators have taken intracellular recordings from myocardial fibers to investigate the temporal correlation between extracellular recordings and the TMP (Table 3). The early experiments by Woodbury et al.,59 Sano et al.,60 Hoffman and Cranefield,61 and Dower and Osbourne62 all had serious shortcomings.63 It is, however, interesting to note that the results of the experiments of Sano et al. in 1956, which, due to a methodological error,63 had found no strict temporal correlation between the steep downslope in the unipolar electrogram and the point of the steepest rise of the TMP,60 were addressed by Durrer as late as 1968.64 The first to prove by modern standards "that the steep negative-going downstroke of the local ECG coincides with the upstroke of the transmembrane potential curve of the underlying cell or cells.. ."65 were Dower and Geddes65 in 1960. In their tracings of the in vivo guinea pig heart, the difference between the 2 events is less than 1 ms.65 In 1972, Myerberg et al.66 took simultaneous intracellular and extracellular bipolar recordings from superfused preparations of the right bundle branch of 30 dogs.

INTERPRETATION OF CARDIAC ELECTROGRAMS

19

Table 2 Activation Detection in Bipolar Electrograms Article 22

Clement E, 1912 Erfmann W, 191337 Garten S, 191338 Lewis T, 191439 Harris, AS 1941 46 Schaefer H, 195119 Scher AM, 195353 Durrer D, 1 95455 Durrer D, 195756 Scher AM, 195716 Durrer D, 1961120 Schaefer H, 196214 Durrer D, 196411 Ostermeyer J, 197923 Abendroth RR, 1980108 Rosenfeldt FL, 1984173 Witkowski FX, 1984174 Kaplan DT, 198791 Blanchard SM, 198889 Simpson EV, 198877 Paul T, 199090 Rosenbaum DS, 1990175 Pieper CF, 199181 Pieper CF, 199182

Algorithm

Evid.

onset* onset* onset* peak* peak* peak* peak* peak* peak* MAA* peak* peak peak* MD* 45°* MAA* MAA MAS, morph peak morph MAS, peak, 45° MAS MAS, peak BSS, MAS, morph, peak

N N N E E E E E E N E T R R R N N E E N E N E E

Articles that specified criteria for activation detection in bipolar electrograms in intraoperative or experimental mapping: the baseline crossing with steepest slope (BSS), the maximum absolute amplitude |V|max (MAA), the maximum absolute slope |dV/dt|max (MAS), the major deflection in the rectified bipolar electrogram (MD), morphological algorithms (morph), the onset of the bipolar EG (onset), the maximum amplitude Vmax (peak), the first deflection from baseline steeper than 45° (45°). The type of the main evidence (Evid.) presented in favor of the criteria is also listed: experiment (E), theory (T), references (R), or none (N). An asterisk (*) denotes analog mapping systems.

Bipolar electrograms occurred less than 1 ms before or after the upstroke of the TMP. In case of increasingly premature stimuli, however, disparities up to 10 ms were possible, which the authors explained by the nonuniform arrival time of an impulse across the transverse axis of the Purkinje fiber. Furthermore, the bipolar electrograms usually became unmeasurable some time before conduction finally failed.66 Experiments by Spach et al.67 on dog Purkinje fibers in the same year showed that the maximum downslope of the local unipolar electrogram occurred within less than 0.2 ms from the maximum upslope of the TMP.

Downar et al.68 noted a similar correlation in the acutely ischemic porcine heart. As their and Akiyama's tracings69 from fibrillating hearts show, activity during ventricular fibrillation that is recorded by intracellular electrodes can go undetected in local unipolar electrograms. In 1985, Spach and Kootsey70 published the results of a theoretical model that predicted that "the negative peak of the derivative of the extracellular [unipolar] potential always occurred at the time of dV/dtmax of the transmembrane potential." In experiments on human atrial muscle, Spach and Dolber71 found time differences

20 CAEDIAC MAPPING Table 3 Correlation of Transmembrane Potentials and Local Electrograms Transmembrane Potentials from

Evidence

Article 59

Woodbury LA, 1950 Sano T, 195660 Sano T, 1958176 Dower GE, 195862 Dower GE, 196065 Hoffman BF, 196061 Dower GE, 196263 Myerburg RJ, 197266 Spach MS, 197267 Downar E, 197768 Kleber AG, 1978128 Cinca J, 1980129 Akiyama T, 198169 Spach MS, 1981177 Gardner PI, 1985164 Spach MS, 198570 Spach MS, 198671 Steinhaus BM, 198873 Steinhaus BM, 198974 Haws CW, 199072 Rudy Y, 199175

D, T D, T D, T D, T D, T D, T D, T D, T D, T D, T D D D D D M,T M, D, T M, T M, T M, D,T M, T

Frog Turtle Dog Guinea pig Guinea pig Dog, PM Guinea pig Dog, CS Dog, CS Pig, Al Pig, Al Pig, Al Dog, Al Dog Dog, CMI — Human atrium — — Dog —

Articles that examined the problem of activation detection in extracellular signals by correlating transmembrane potentials and local electrograms, presenting evidence in the form of diagrams with simultaneous recordings (D), theoretical models (M), or remarks in the text (T). Al = acute ischemia; CMI = chronic myocardial infarction; CS = conduction system; PM = papillary muscle.

of less than 50 us between dV/dtmax of the TMP and the maximum downslope of the local unipolar electrogram. The model of Haws and Lux72 made a similar prediction. Steinhaus' computer simulations73,74 in 1988 showed that under conditions of nonuniform coupling resistances and membrane properties, differences in excess of 1.8 ms can occur between the times of maximum negative slope in the unipolar electrogram and the maximum positive slope in the TMP. The criterion of maximum absolute amplitude of electrograms from bipolar electrodes with 0.1-mm interelectrode distance yielded comparable results and performed better during conditions with marked contribution from distant events.73,74 In 1991, Rudy and Quan75 proposed a model that incorporated gap junctions with varying degrees of resistances between cells. They found that in the middle and in the prejunctional area of the cells, the time of the

maximum downslope in the unipolar electrogram and the time of the dV/dtmax in the TMP coincided with a deviation of less than 100 (us. In the postjunctional area of poorly coupled cells, however, deviations of up to 370 (uswere possible.75 Based on this evidence, we may conelude that the time of the maximum downslope in a unipolar electrogram is a valid fiducial point for identifying times of local activation in unipolar electrograms even though error in excess of 1 ms may occur, Computer Algorithms With the advent of computerized multichannel mapping systems, it became necessary to develop computer algorithms for automatic activation detection, manual marking of electrograms being impractical under the severe time constraints imposed on intraoperative mapping studies.76,77

INTERPRETATION OF CARDIAC ELECTROGRAMS 21 Unipolar electrograms

Practically all algorithms for unipolar electrograms are based on the criterion of the "maximum downslope" or "largest negative slope" (Table 1). The typical unipolar activation algorithm is based on the following 2 principles: 1. The algorithm searches each electrogram for time instants in which the negative slope of the electrogram is larger than a threshold slope; these time instants are assumed to represent possible times of local activation. 2. If 2 or more time instants meet criterion 1 within a defined time window, the time instant with the largest negative slope is chosen as the time of local activation. Individual algorithms basically differ in their choice of the threshold value for criterion 1, the width of the time window for criterion 2, for which variable values78 or values of 40 ms76,79 and 50 ms80 have been used, and the way the slope of the electrogram is calculated (e.g., 2-point, 3-point, or 5-point algorithms).81,82 Some unipolar algorithms also provide a refractory period after each time instant for which an activation has been detected using criteria 1 and 2.76,79 A few algorithms use a relative slope threshold in relation to slopes measured in each electrogram analyzed.76 The main controversy concerning unipolar algorithms is the optimum value of the slope threshold, the problem being to reliably distinguish local activity (associated with a steep downslope) from distant activity without local activity (associated with a shallow downslope). Ideker et al.80 used a slope threshold of 5 mV/2 ms, referring to Durrer and Van der Tweel's 1957 mapping study in healthy dogs.56 Roberts et al.83 determined downslopes in unipolar epicardial

electrograms between 11 and 64 mV/2 ms in noninfarcted canine myocardium. In canine chronic myocardial infarction (MI), Cardinal et al.84 were able to observe "organized propagation of wavefronts" at slopes of -0.5 mV/ms. Regardless of slope, a QS complex recorded in epicardial sites overlying transmural infarcts was interpreted as a cavity complex representing no local activity.84,85 Carson et al.86 observed "organized propagation" at slopes of -0.3 mV/ms in acutely ischemic porcine hearts. In a canine model with right ventricular (RV) isolation procedure, Blanchard et al.87,88 were able to simulate various combinations of local and distant activity by sequential or independent pacing of the RV and the left ventricle (LV). A slope threshold of 1.4 mV could reliably distinguish between local plus distant activity and distant without local activity; however, considerable overlap is likely under conditions of ischemia, chronic MI, and ventricular fibrillation.87,88 Checking computer markings against manual markings of electrograms from patients with Wolff-Parkinson-White (WPW) syndrome and ventricular tachycardia (VT), Masse et al.76 found a slope threshold of –0.2 V/s to perform optimally. Thus, recommended slope thresholds for unipolar activation detection algorithms range from -0.2 to –2.5 mV/ms. Bipolar electrograms

There is considerably less agreement on the best computer algorithm for bipolar electrograms. Basically, the following types of algorithms exist for activation detection in bipolar electrograms (Table 2), the respective merits of which are discussed in the succeeding paragraphs: l. the maximum amplitude Vmax of the bipolar electrogram (peak) and the maximum absolute amplitude | V |

max;

22

CARDIAC MAPPING 2. the maximum absolute slope |dV/dt| max of the bipolar electrogram (MAS); 3. the first elevation of more than 45° from the baseline of the bipolar electrogram (45°); 4. the baseline crossing with the steepest slope (BSS); 5. "morphological"82 algorithms.

The peak algorithm: On the conditions that an excitation wave has a constant shape and propagates with a constant velocity—conditions not met on a microscopic level71 and in diseased tissue10—the bipolar electrogram from closely spaced terminals can be considered to approximate the first temporal derivative of the local unipolar electrogram.20,70 In this case, the peak of the bipolar signal would precisely coincide with the maximum downslope of the local unipolar electrogram, which gives the peak criterion some theoretical justification. The maximum absolute amplitude criterion as opposed to the peak criterion has the advantage that it is independent of the polarity of the bipolar electrodes. Recent experimental evidence in favor of the peak criterion came from Blanchard et al.,89 in whose model of the canine RV isolation procedure the peaks of the bipolar electrograms occurred within 5 ms of the maximum downslope of simultaneous unipolar electrograms in 94% of cases regardless of wavefront-to-fiber orientation. Paul et al.,90 who evaluated 3 bipolar algorithms during sinus mapping in dogs, found that bipolar activation times assigned by the peak criterion were closest to those measured in unipolar electrograms from the same sites with the maximum downslope algorithm. Pieper et al.81 found that the peak criterion gave more stable results than the MAS criterion. In another study by Pieper et al.,82 the peak algorithm showed the closest correspondence with manually determined

activation times among all nonmorphological algorithms. The peak criterion has the added advantage that it does not require calculations like MAS.90 The MAS algorithm: Kaplan et al.91 chose the MAS criterion as the nearest equivalent to the criterion of maximum downslope for unipolar electrograms. This is, however, not correct: as each steep slope of the bipolar electrogram coincides with the maximum downslope of either of the 2 component unipolar electrograms,5 the MAS criterion marks the moment of maximum downslope at either of the 2 electrode terminals but not between the terminals. Paul et al.90 found that the MAS criterion corresponded less well than the peak criterion to unipolar activation times based on the maximum downslope criterion. Pieper et al.81 reported similar findings. The 45° algorithm: Scherlag et al.92 used "the first rapid excursion from the isoelectrical line at an angle of 45° or greater" to mark the onset of the A wave in His bundle electrograms. Paul et al.90 found that the 45° criterion led to significantly earlier activation times compared to the unipolar maximum downslope and even to a different localization of the first epicardial breakthrough. The BSS algorithm: Josephson et al.,93 Cassidy et al.,94 and Vassallo et al.95 used the BSS criterion for endocardial catheter mapping with electrode catheters with 10-mm interelectrode distance; these have a wider field of view than bipolar electrodes for intraoperative mapping. In the context of intraoperative mapping, Pieper et al.82 found that the BSS algorithm showed a slightly poorer correspondence with manually determined activation times than the peak algorithm. Morphological algorithms: Kaplan et al.91 compared a morphological algorithm using lead-specific templates with the MAS algorithm and found that the morphological algorithm gave the more

INTERPRETATION OF CARDIAC ELECTROGRAMS consistent results. Simpson et al.77 developed a complex morphological algorithm that marked the point of symmetry in the bipolar waveform. Pieper et al.82 found that their morphological algorithm, which was based on principles similar to those of Simpson's, performed best in every category: of all algorithms tested, it produced the fewest outliers and showed the fewest differences between computer and manual markings. Based on this evidence, we may conclude that the best among the "simple" bipolar activation detection algorithms is the peak criterion, while morphological algorithms perform somewhat better under practical conditions. Still, much remains to be done in the field of activation detection, as Smith et al. stated in 1990: "Much work is required in this area in order to be able to detect and characterize local activations with high sensitivity, specificity, and temporal accuracy, especially in the fast, chaotic milieu of ventricular fibrillation."96

23

"intrinsic deflection" is defined as any fast downstroke in a unipolar electrogram that is interpreted to indicate local activation. Electrograms in the Normal Human Heart The first to map the activation of the in situ human heart were Barker et al.102 in 1930; these investigators upset contemporary views about the ECG patterns of the bundle branch blocks.64 By 1960, Jouve et al.103 were able to list 21 references of mapping studies in humans. Endocardial electrograms

According to the study on 7 Langendorff-perfused preparations of the human heart by Durrer et al.,104 endocardial activation of the ventricles synchronously starts at 3 sites in the LV 0 to 5 ms after the onset of the LV cavity complex (LV-CC): a central area in the interventricular septum, a paraseptal area Morphological Interpretation near the base of the anterior papillary muscle, and a paraseptal area near the of Cardiac Electrograms base of the posterior papillary muscle. The local activation time is but one The endocardial activation becomes conpiece of information contained in an elec- fluent after 30 ms and the posterobasal 104 94 trogram. In the morphology of a unipolar area is activated last. Cassidy et al. electrogram, the activity of the whole largely confirmed these results. Endoheart is encoded, which allows important cardial activation of the RV starts near conclusions under a number of physio- the insertion of the anterior papillary logical and pathological conditions, while muscle between 5 and 10 ms after the 104 the morphology of bipolar complexes may onset of the LV-CC. Unipolar endocardial electrograms contain valuable information about patfrom canine experiments show a QS morterns of local activation. The following phology with a rapid downstroke in the paragraphs concentrate on ventricular first part of the QRS complex.16,53,56,105 electrograms during sinus rhythm in the human heart. We exclude the subjects of Bipolar endocardial electrograms in the mapping of tachycardias, which has normal human LV from catheters with already been amply reviewed,24,97–100 and 10-mm interelectrode distance have of mapping of the atria,17 of the specialized amplitudes of greater than 3 mV and conduction system,17 and of repolariza- durations of less than 70 ms, and no split, 94 tion.72–74,101 For the purpose of this chapter, fractionated electrograms are found.

24 CARDIAC MAPPING Intramural electrograms

The excitation of the thick LV wall proceeds in an almost strictly endocardial to epicardial direction, whereas the activation of the thin RV wall spreads tangentially from the pretrabecular area until, after 60 to 70 ms, the pulmonary conus and posterobasal area are reached. The activation of the interventricular septum proceeds from left to right and in an apical-basal direction.104 Immediately after introduction of a needle electrode, unipolar intramural electrograms show no rapid deflections and an ST segment elevation due to local injury. After 2 to 3 minutes, the ST segment gradually becomes isoelectric and fast downstrokes in the electrogram appear.56 Unipolar intramural electrograms from the normal human heart show a gradual transition from the endocardial QS complex resembling the LVCC to the epicardial complex with a prominent R wave, the greatest increase of which often occurs in the outer layers of the LV wall.106 Bipolar electrograms in the inner layers of the wall are relatively small, sometimes notched, and generally positive, indicating spread of excitation in an epicardial direction. In the middle and outer layers, they are larger, smooth, and always positive.106

The most accurate descriptions of human unipolar epicardial electrograms are those by Jouve et al.103 and Roos et al.106 (Figure 1), other studies109–111 being methodically inferior.103 Unipolar electrograms over the pretrabecular region of the RV surface have an rS morphology.106 As the excitation spreads over the RV, the r wave increases slightly in duration and amplitude106 and rS or RS103 complexes are recorded, which predominantly reflect distant activity of the LV. Electrograms from the RVOT may have an rS (sometimes with notching or r and/or S), rSr', rsr'S', or, rarely, qRS morphology.17 Unipolar complexes over the interventricular septum may have an rS or (v)rS morphology, (v) standing for vibrated initial segment103 and denoting a broad-tipped positive complex with small amplitude (1 to 2 mV) and long duration (20 ms), which begins with the onset of the LV-CC.106

Epicardial electrograms

Epicardial activation of the ventricles begins in the pretrabecular area of the RV about 20 ms after the onset of the LV-CC5,104,107,108 and over the inferior RV.5,107 LV epicardial breakthrough takes place later over the middle portions of the left anterior and left posterior paraseptal regions5,104,107 and occasionally over the left anterior septum near the base.5,104,107 Latest epicardial activation occurs near the base of the posterior LV104,107,108 or of the RV.5,107

Figure 1. Unipolar epicardial electrograms during sinus rhythm in a 61-year-old male patient with normal 12-lead ECG; figures represent the local activation times in milliseconds following the onset of the left ventricular cavity complex. Reproduced from reference 106, with permission.

INTERPRETATION OF CARDIAC ELECTROGRAMS Unipolar complexes from the LV surface may show q waves, the beginning of which coincide with the LV-CC, with amplitudes of up to 3 mV.106 There are no q waves on the anterior RV and on the first 2 cm of the LV lateral to the left anterior descending artery (LAD).112 Q waves with a duration of up to 27 ms can be present or absent over the RVOT and "in a band of 2-4 cm in width located laterally on the left ventricle but parallel to the anterior descending coronary artery and also posteriorly along the course of the posterior descending branch of the right coronary aftery."112 Over the remaining lateral and posterior LV, Q waves with durations between 4 and 32 ms are regularly found.112 Electrograms over the atrioventricular sulcus show long Q waves as the cavity potential is viewed over the rim of the ventricular cavitites.112 At a distance from the anterior attachment of the interventricular septum, R waves increase in size with amplitudes of up to 20 mV over the lateral wall; they can vary very significantly in closely adjacent regions.106 S waves are deep over the anterior aspect of the LV from apex to base and diminish in

25

size in a lateral direction.106 Thus, typical LV epicardial morphologies would be an rS complex on the anterior wall, a qRS complex on the lateral wall, and a qRs complex on the posterior wall. A discussion of epicardial waveforms in RV hypertrophy, RV diastolic overload, and LV hypertrophy is provided by Kupersmith.17 Electrograms in Preexcitation Syndromes The first intraoperative mapping of a patient with WPW syndrome113 was performed by Durrer and Roos114 in 1967, and the first successful mapping-guided ablation by Cobb et al.115 in 1968. In WPW syndrome, the morphology of the earliest recorded unipolar epicardial electrograms may provide information about the location of the accessory pathway (Figure 2).5,100 While a QS morphology indicates epicardial location of the accessory pathway with spread of activation away from the exploring electrode in an epicardial-to-endocardial direction, an rS morphology signifies endocardial location of the accessory bundle.5,116 In case of

Figure 2. Surface ECGs and epicardial unipolar and bipolar electrograms at the earliest site of preexcitation in a patient with Wolff-Parkinson-White syndrome. Reproduced from reference 5, with permission.

26

CARDIAC MAPPING

free wall pathways, the earliest epicardial intrinsic deflection occurs before or simultaneously with the delta wave in the surface ECG.5 Electrogram criteria indicative of septal pathways are earliest ventricular activation over the anterior or posterior septum, an rS morphology, and an intrinsic deflection after the onset of the surface delta wave.5 Electrograms in Acute Myocardial Ischemia and Infarction Since the classic experiments of Johnston et al.117 in 1935, numerous investigators have studied the acute effects of coronary artery occlusion on electrograms in dogs118–126 and pigs.68,127–129 The following paragraphs focus mainly on the porcine

heart, which, in respect to coronary anatomy, resembles the human heart more closely than does the canine heart.130,131 Prinzmetal et al.121 distinguished 2 patterns of ischemia in unipolar epicardial electrograms: "severe acute ischemia," as in acute ligation of the LAD with "elevation of S-T segments, increase in amplitude of R waves (sometimes 'giant' R waves) and decrease in depth or disappearance of S waves," and "mild ischemia,"121 as in hemorrhagic shock, in which epicardial unipolar electrograms showed "numerous islands of S-T depression, often with loss of amplitude of the R wave and increased depth of the S wave." Downar et al.,68 Kleber et al.,128 and Cinca et al.129 gave a precise account of the electrogram changes after acute ligation of the LAD in the porcine heart (Figure 3).

Figure 3. Local transmembrane potentials (upper tracings) and unipolar electrograms (lower tracings) from the ischemic zone (left panel) and from the border zone (right panel) before (control) and 7.5, 14, 30, 42, and 60 minutes after acute left anterior descending coronary artery occlusion in the Langendorff-perfused porcine heart. Note that the monophasic complex at 7.5 minutes is associated with no local activation, while in the other tracings local activation coincides with a shallow intrinsic deflection after the peak of the R wave. Reproduced from reference 128, with permission.

INTERPRETATION OF CARDIAC ELECTROGRAMS

Unipolar electrograms in the ischemic zone show depression of the initially isoelectric TQ segment, caused by a decreased resting TMP of the ischemic myocardium, ST segment elevation due to a decreased amplitude of the action potential, and inversion of the T waves whenever the repolarization of the ischemic cells occurs later than that of the normal cells.128 The main negative deflection in the QRS complex decreases in magnitude and downslope velocity128 while the R wave becomes tall with a delayed intrinsic deflection after the peak of the R (Figure 3, 14),129 the tallness of the R waves being a consequence of the delayed activation because cancellation effects of earlier activated areas of the heart are absent.129 Cells in the ischemic zone then become unresponsive, and unipolar electrograms show monophasic complexes without intrinsic deflection (Figure 3, 7.5).128 When excitability of the ischemic cells transiently returns between 10 and 20 minutes after occlusion in in situ hearts,128 epicardial electrograms show the reappearance of a large intrinsic deflection with diminished TQ depression and ST elevation (electrogram at 42' in Figure 3).129 Unipolar intramural electrograms show essentially the same changes.128 Bipolar epicardial electrograms in canine acute MI show reduced amplitude and increased duration,120,122,124,125 both of which have also been described in human acute MI,122,123 delay or absence of activation,120,124 and, often, ST segment elevation.120,125 Bipolar intramural124 and endocardial126 electrograms show similar changes. Electrograms in Chronic MI Since the classic experiments by Wilson et al.132,133 in 1935, numerous investigators have examined electrogram morphologies in chronic LAD ligation in

27

dogs,11,84,85,112,120,122,134-139 while there are comparatively few studies on chronic MI in humans.112,122,123,137,138 Epicardial electrograms

Durrer et al. made the following generalizations about unipolar epicardial electrograms in canine chronic MI: "(1) The area with abnormal Q waves was always slightly larger than the infarcted area. (...) (2) The beginning of Q and the beginning of the left ventricular cavity potential were synchronous."11 In small canine subendocardial MIs, the only change in the unipolar epicardial QRS complex is deepening and broadening of the Q waves due to loss of depolarizing myocardium.11 In the case of larger subendocardial MIs in which a thin muscle layer overlies a scar devoid of muscle fibers, epicardial electrograms have a qR or QR morphology with a tall R wave and a delayed intrinsic deflection after the peak of the R (Figure 4).11 The tall R waves result from delayed activation of the muscle layer in a tangential direction at a time when the excitatory forces of the remainder of the ventricles are reduced or absent.11 Daniel et al.112 found good correlation between the canine model of subendocardial chronic MI and clinical findings in humans; the concurrence of abnormal Q waves and delayed epicardial activation times permitted the accurate localization of underlying MIs in human hearts.112,137,138 The characteristic unipolar morphology of transmural chronic MI, both in dogs11,20 and in humans,140 is a QS complex which is synchronous with the LV-CC and which may be fractionated.11,120 A smoothlimbed QS complex indicates the absence of local activity11,84,85 regardless of slope.85 Canine septal chronic MI shows a characteristic picture of abnormal Q waves in unipolar electrograms over the RV without delay in epicardial activation.112

28 CARDIAC MAPPING

Figure 4. Cross-section through chronic canine subendocardial infarction (light area) with unipolar epicardial electrograms before (control) and after left anterior descending coronary artery ligation. In the post myocardial infarction electrograms, note the abnormal Q waves, which occur over an area slightly larger than the infarct, the tall R waves, and the delayed intrinsic deflections. The figures indicate activation times in milliseconds. Reproduced from reference 138, with permission.

Page et al.,85 who compared unipolar and bipolar electrograms in canine chronic MI, proposed an interesting classification of unipolar epicardial electrograms in sinus rhythm: "Class A" electrograms were electrograms of rs morphology with an intrinsic deflection within the QRS wave, "Class B" electrograms were electrograms with a wide QS deflection corresponding to a cavity potential, and "Class C" electrograms were electrograms with a QS complex followed by a late intrinsic deflection. Areas of Class C electrograms extending across regions with Class B electrograms were predicative of the inducibility of VT, the Class C areas becoming the common

pathways of figure-of-8 VTs.85 Whether these findings apply to humans remains to be seen.141 Bipolar epicardial electrograms from infarcted areas in both dogs and in humans show reduced amplitude and increased duration and permit accurate localization of infarcts in both dogs122 and humans.122,123 Intramural electrograms

Unipolar complexes from intrainfarction terminals have a QS form.11 In canine subendocardial MI, unipolar electrograms from terminals between the scar and the epicardium have a Qr or Qrs

INTERPRETATION OF CARDIAC ELECTROGRAMS morphology with small r waves (called embryonic r waves), which gradually increase in size toward the epicardium with progressively later intrinsic deflections.11 Bipolar electrograms have a low voltage, are notched, and, by their polarity, indicate predominant outward spread of activation.11 Again, human clinical data and canine experimental data correlate well.112 Intramural unipolar electrograms from all layers of canine transmural MIs show a QS complex synchronous with the LV-CC,11 which results from the unopposed transmission of cavity potential through the ventricular scar.11,132,133 In a scar completely deprived of muscular tissue, the unipolar QS complexes are smooth and the bipolar complexes broad, smooth, and of small amplitude.11 If the scar contains viable muscle fibers, fractionated unipolar and bipolar electrograms can be recorded.11 A classification of canine heart tissue as normal or infarcted based on peak amplitude and maximum slope of intramural unipolar and bipolar electrograms has been proposed.139 Endocardial electrograms

In canine experiments, Purkinje spikes recorded by endocardial electrode terminals have been reported to fall within normal limits.11,112 In human chronic MI, the activation of the endocardial surface over the MI is delayed.95 Fractionated Electrograms Since Durrer et al.120 reported "small, fast deflections" some 75 ms after the beginning of the QRS complex in electrograms over a canine transmural infarct (Figure 5), similar potentials have been recorded in dogs with acute125,142 and chronic11,143,144 MI as well as in patients with chronic MI during intraoperative

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Figure 5. Durrer's original tracings of fractionated electrograms over a transmural canine infarct. Multiple small amplitude deflections occurring 75 ms after the beginning of the QRS complex are recorded in both the local bipolar (top) and unipolar (bottom) electrogram. Reproduced from reference 120, with permission.

mapping112,140,145-150 and endocardial catheter mapping147,151-154 and in patients with arrhythmogenic RV disease.155—157 Electrograms or potentials have been termed late or delayed if they show ventricular activity after the end of the surface QRS complex21,150,152or if they occur "later than normal"158 or are clearly separated from the normal myocardial activation pattern.140 They have been termed fractionated or fragmented if they show several low-amplitude deflections of typically less than 1 mV.158 Late or fractionated potentials can be recorded during sinus rhythm and VT. The term continuous electrical activity denotes a fractionated electrogram that lasts throughout the cycle during VT159 and which is assumed to represent a composite recording of the electrical activity of a reentry circuit.143,160

30 CARDIAC MAPPING The morphology of late potentials is dependent on the recording technique.21 Endocardial bipolar recordings from mapping catheters with 10-mm interelectrode distance show a QRS synchronous potential followed by multiple smaller potentials of amplitudes of less than 1 mV during the ST segment.21,159 Bipolar recordings based on 1-mm interelectrode distance during intraoperative mapping may show one of the following patterns: after QRS synchronous potential, which may be normal,145 small and broad,21 or absent,145 or a delayed sharp potential of high amplitude follows and is often succeeded by another of similar morphology but with different orientation, or the QRS synchronous signal is followed by multiple fragmented potentials.21,145 Unipolar electrograms may show 1 of 3 patterns: a single, rapid biphasic deflection of rs morphology following a wide QS potential, a double rs deflection, or fragmentation with multiple deflections.140 Although fractionated electrograms can be artifacts resulting from distant activation fronts,158 electrode motion,151,161 filter ringing,162 or other sources,18 most instances of late potentials represent true cardiac potentials.158,159,163 According to the experiments in canine chronic MI by Gardner et al.,164 fractionation of electrograms is caused by asynchronous excitation of different poorly interconnected viable muscle fibers separated by fibrosis; the small amplitude of fractionated electrograms is the consequence of the scarcity of myocardium next to the electrode, TMPs in these regions being normal.164 The clinical significance of late fractionated electrograms lies in the fact that they are markers for the electrical and morphological milieu required for VTs.158,165 Among patients with chronic MI, late fractionated electrograms have been recorded more frequently in those with VT than in those without VT.145–147 In patients with VT, however, neither fractionated nor late

electrograms are specific to the site of origin of VT148,149,152 nor are they present at all sites of origin.148,149,152 Artifacts Artifacts in electrograms can be produced at all the different levels of a modern mapping system: electrodes, amplifiers, filters,148,158,162 analog multiplexors,166 analog-to-digital converters,167,168 and the data storage and display system. As a treatise of the technical aspects of multichannel mapping systems169 is beyond the scope of this chapter, our discussion is restricted to artifacts occurring at the level of the electrodes. Local myocardial injury by electrodes results in ST segment elevation in the local unipolar electrogram.56 Polarization of electrodes can cause slow shifts of the baseline of the signals. Electrodes may record pacing artifacts or 50-Hz or 60-Hz noise. Motion artifacts, which are often rhythmic and repeating,158,161 can be sudden shifts of potential,56 which computer algorithms often misinterpret as activations,80 or may mimic fractionated electrograms.158,161 Poor contact between electrode and myocardium leads to wide complexes in bipolar electrograms with heavier weighing of far-field effects and increased 50-Hz or 60-Hz noise.82 Finally, while all noncoaxial bipolar electrodes ignore activation fronts parallel to the line between the electrode terminals,6,22 widely spaced bipolar electrodes may also record a symmetric complex of 2 deflections if a single small activation front passes by.21,82 Conclusion This chapter presents an overview of major aspects of the interpretation of human ventricular electrograms in sinus rhythm. Though most of what is presented is not new, this information is

INTERPRETATION OF CARDIAC ELECTROGRAMS scattered among many articles. With the exception of Kupersmith's review of intraoperative mapping from 1976,17 there seemed to be no single text that could serve as an introduction for newcomers to the method of cardiac mapping, while the historical basis from much of what is now accepted fact appeared to have been largely forgotten. Indeed, we hope that this chapter may be a due tribute to the pioneers of cardiac mapping without whom mapping would not be what it is now: the gold standard in cardiac electrophysiology. Summary Simultaneous recordings of the TMP and of the local electrogram and theoretical models with computer simulations have shown that the time of myocardial activation, defined as the time of the maximum upstroke in the TMP and the time of the maximum downstroke in the local unipolar electrogram, practically coincide with deviations of less than 1 ms in most, though not all, conditions. For activation detection in unipolar leads, there is universal agreement on computer algorithms using the criterion of "maximum downslope." The main controversy is about the optimum slope threshold for distinguishing local from distant activation, recommended thresholds ranging from -0.2 to -2.5 mV/ms. The advantage of bipolar leads lies in the distinction of local versus distant activity, their disadvantage is their directional sensitivity. Among the various algorithms for activation detection, morphological computer algorithms perform the best under practical conditions. Among the simple algorithms, the peak algorithm corresponds the closest with the times of the maximum downslope in simultaneous unipolar leads. During sinus rhythm in the normal heart, the typical unipolar RV epicardial

31

complex has an rS morphology predominantly reflecting activation of the LV. The typical epicardial complexes of the LV are an rS or qrS complex over an anterior wall, a qRS complex over the lateral wall, and a qRs complex over the posterior wall. Q waves of up to 32 ms are typical of the lateral and posterior LV wall. In WPW syndrome, a QS morphology of unipolar epicardial electrograms in the preexcited area indicates epicardial location of the accessory pathway and an rS morphology endocardial location. The characteristic changes of unipolar epicardial complexes during acute ischemia are a reduced amplitude of the initial negative deflection, a tall R wave with or without a shallow intrinsic deflection after the peak of the R, massive ST segment elevation, and TQ segment depression. Unipolar epicardial electrograms overlying subendocardial MI typically show an abnormally deep and long Q wave and a tall R wave with a delayed intrinsic deflection. The typical unipolar complex over a transmural MI is a QS complex, which is synchronous with the LV-CC and which may be smooth or fractionated. Fractionated electrograms consist of several low-amplitude deflections after the QRS complex. They are caused by asynchronous excitation of poorly interconnected muscle fibers and indicate that the electrophysiological milieu for ventricular arrhythmias to occur is present. References 1. Lewis T, Rothschild MA. The excitatory process in the dog's heart. Part II. The ventricles. Philos Trans R Soc Lond [Biol] 1915;206B: 181-226. 2. Samojloff A. Weitere Beiträge zur Elektrophysiologie des Herzens. Pflugers Arch 1910;135:417-468. 3. Lewis T. The Mechanism and Graphic Representation of the Heart Beat. 3rd ed. London: Shaw and Sons; 1925:55.

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4. Wenckebach KF, Winterberg H. Die unregelmäßige Herztätigkeit. Leipzig: Verlag von Wilhelm Engelmann; 1927: 44. 5. Gallagher JJ, Kasell J, Sealy WC, et al. Epicardial mapping in the Wolff-ParkinsonWhite syndrome. Circulation 1978;57: 854-866. 6. Ideker RE, Smith WM, Blanchard SM, et al. The assumptions of isochronal cardiac mapping. Pacing Clin Electrophysiol 1989;12:456–478. 7. Ershler PR, Lux RL. Derivative mapping in the study of activation sequence during ventricular tachyarrhythmias. IEEE Proc Comput Cardiol 1987;623624. 8. Barnes AR, Pardee HEB, White PD, et al. Standardization of precordial leads: Joint recommendations of the American Heart Association and the Cardiac Society of Great Britain and Ireland. Am Heart J 1938;15:107-108. 9. Burch GE, DePasquale NP. A History of Electrocardiography. Chicago: Year Book Medical Publishers; 1964. 10. Berbari EJ, Lander P, Sherlag BJ, et al. Ambiguities of epicardial mapping. J Electrocardiol 1991;24(Suppl): 16-20. 11. Durrer D, Van Lier AAW, Buller J. Epicardial and intramural excitation in chronic myocardial infarction. Am Heart J1964;68:765-776. 12. Wilson FN, Johnston FD, Macleod AG, et al. Electrocardiograms that represent the potential variations of a single electrode. Am Heart J 1934;9:447-458. 13. Wilson FN, Macleod AG, Barker PS. The Distribution of the Currents of Action and of Injury Displayed by Heart Muscle and Other Excitable Tissues. (University of Michigan Studies, Scientific Series. Vol. X.) Ann Arbor: University of Michigan Press; 1933. 14. Schaefer H, Haas HG. Electrocardiography. In: Hamilton WF, Dow P (eds): Handbook of Physiology. Section 2: Circulation. Vol. I. Washington, DC: American Physiological Society; 1962:323–415. 15. Scher AM. The sequence of ventricular excitation. Am J Cardiol 1964;14:287-293. 16. Scher AM, Young AC. Ventricular depolarization and the genesis of the QRS. Ann N YAcad Sci 1957;65:768-778. 17. Kupersmith J. Electrophysiologic mapping during open heart surgery. Prog Cardiovasc Dis 1976;19:167-202.

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of its uses and limitations. Circulation 1972;46:601-613. 93. Josephson ME, Horowitz LN, Spielman SR, et al. Role of catheter mapping in the preoperative evaluation of ventricular tachycardia. Am J Cardiol 1982;49: 207-220. 94. Cassidy DM, Vassallo JA, Marchlinski FE, et al. Endocardial mapping in humans in sinus rhythm with normal left ventricles: Activation patterns and characteristics of electrograms. Circulation 1984;70:37-42. 95. Vassallo JS, Cassidy DM, Marchlinksi FE, et al. Abnormalities of endocardial activation pattern in patients with previous healed myocardial infarction and ventricular tachycardia. Am J Cardiol 1986;58:479-484. 96. Smith WM, Wharton JM, Blanchard SM, et al. Direct cardiac mapping. In: Zipes DP, Jalife J (eds): Cardiac Electrophysiology: From Cell to Bedside. Philadelphia: W.B. Saunders Co.; 1990:849-858. 97. Downar E, Harris L, Mickleborough LL. Direct cardiac mapping of ventricular arrhythmias. Prog Cardiol 1987;1:273288. 98. Josephson ME, Miller JM, Hargrove WC III, et al. Intraoperative mapping of ventricular tachycardia associated with coronary artery disease. In: Aliot E, Lazzara R (eds): Ventricular Tachycardia: From Mechanism to Therapy. Boston: Martinus Nijhoff Publishers; 1987:411-436. 99. Tyagii S, Sharma AD, Guiraudon G, et al. Intraoperative cardiac mapping of preexcitation syndromes and ventricular tachycardia. J Electrophysiol 1989;3: 47-64. 100. Shenasa M, Cardinal R, Savard P, et al. Cardiac mapping. Part I. Wolff-ParkinsonWhite syndrome. Pacing Clin Electrophysiol 1990; 12:223-230. 101. Spach MS, Barr RC. Ventricular intramural and potential distributions during ventricular activation and repolarisation in the intact dog. Circ Res 1975;37:243257. 102. Barker PS, Macleod AG, Alexander J. The excitatory process observed in the exposed human heart. Am Heart J 1930; 5:720-742. 103. Jouve A, Corriol J, Torresani J, et al. Epicardial leads in man. Am Heart J 1960;59:856-868.

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137. Daniel TM, Cox JL, Sabiston DC Jr, et al. Epicardial and intramural mapping of activation of the human heart—a technique for localizing infarction and ischemia of the myocardium. Circulation 1969;39(Suppl III):III-66. Abstract. 138. Daniel TM, Boineau JP, Cox JL, et al. Mapping of epicardial and intramural activation of the heart: A technique for localization of chronic infarction during myocardial revascularisation. J Thorac Cardiovasc Surg 1970;60:704-709. 139. Claydon FJ, Pilkington TC, Ideker RE. Classification of heart tissue from bipolar and unipolar intramural potentials. IEEE Trans Biomed Eng 1985;32:513520. 140. Lacroix D, Savard P, Shenasa M, et al. Spatial domain analysis of late ventricular potentials: Intraoperative and thoracic correlations. Circ Res 1990;66:55-68. 141. Ideker RE, Tang ASL, Daubert JP. On the trail of ventricular tachycardia or the adventure of the unspeckled band. Pacing Clin Electrophysiol 1988;11:650-655. Editorial. 142. Boineau JP, Cox JL. Slow ventricular activation in acute myocardial infarction. A source of re-entrant premature ventricular contractions. Circulation 1973;48: 702-713. 143. El-Sherif N, Scherlag BJ, Lazzara R, et al. Re-entrant ventricular arrhythmias in the late myocardial infarction period. 1. Conduction characteristics in the infarction zone. Circulation 1977;55:686702. 144. Berbari EJ, Scherlag BJ, Hope RR, et al. Recording from the body surface of arrhythmogenic ventricular activity during the S-T segment. Am J Cardiol 1978;41:697-702. 145. Klein H, Karp RB, Kouchoukos NT, et al. Intraoperative electrophysiologic mapping of the ventricles during sinus rhythm in patients with a previous myocardial infarction: Identification of the electrophysiologic substrate of ventricular arrhythmias. Circulation 1982;66: 847-853. 146. Wiener I, Mindich B, Pitchon R. Determinants of ventricular tachycardia in patients with ventricular aneurysms: Results of intraoperative epicardial and endocardial mapping. Circulation 1982;65: 856-861.

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147. Simson MB, Untereker WJ, Spielman SR, et al. Relation between late potentials on the body surface and directly recorded fragmented electrograms in patients with ventricular tachycardia. AmJCardiol 1983;51:105-112. 148. Kienzle MG, Miller J, Falcone RA, et al. Intraoperative endocardial mapping during sinus rhythm: Relationship to the site of origin of ventricular tachycardia. Circulation 1984;70:957-965. 149. Vassallo JA, Cassidy DM, Simson MB, et al. Relation of late potentials to site of origin of ventricular tachycardia associated with coronary heart disease. Am J Cardiol 1985;55:985-989. 150. Schwarzmaier H-J, Karbenn U, Borggrefe M, et al. Relation between ventricular late endocardial activity during intraoperative endocardial mapping and lowamplitude signals within the terminal QRS complex on the signal-averaged surface electrocardiogram. Am J Cardiol 1990;66:308-314. 151. Waxman HL, Sung RJ. Significance of fragmented ventricular electrograms observed using intracardiac recording techniques in man. Circulation 1980;6: 1349-1356. 152. Cassidy DM, Vassallo JA, Buxton AE, et al. The value of catheter mapping during sinus rhythm to localize the site of origin of ventricular tachycardia. Circulation 1984;69:1103-1110. 153. Cassidy DM, Vassallo JA, Buxton AE, et al. Catheter mapping during sinus rhythm: Relation of local electrogram duration to ventricular cycle length. Am J Cardiol 1985;55:713-716. 154. Stevenson WG, Weiss JN, Wiener IW, et al. Fractionated endocardial electrograms are associated with slow conduction in humans: Evidence from pace-mapping. J Am Coll Cardiol 1989; 13:369-376. 155. Fontaine G, Frank R, Gallais-Hamonno F, et al. Electrocardiographie des potentiels tardifs du syndrome de post-excitation. Arch Mal Coeur 1978;71:854-864. 156. Marcus FI, Fontaine GH, Guiraudon GM, et al. Right ventricular dysplasia: A report of 24 adult cases. Circulation 1982;65:384-398. 157. Fontaine G, Frank R, Tonet JL, et al. The Mikamo lecture. Arrhythmogenic right ventricular dysplasia: A clinical model for the study of chronic ventricular tachycardia. Jpn Circ J 1984;48: 515-538.

158. Ideker RE, Mirvis DM, Smith WM. Late, fractionated potentials. Am J Cardiol 1985;55:1614-1621. Editorial. 159. Josephson ME, Wit AL. Fractionated electrical activity and continuous electrical activity: Fact or artifact? Circulation 1984;70:529-532. Editorial. 160. Josephson ME, Horowitz LN, Farshidi A. Continuous local electrical activity: A mechanism of recurrent ventricular tachycardia. Circulation 1978;57:659-665. 161. Ideker RE, Lofland GK, Bardy GH, et al. Late fractionated potentials and continuous electrical activity caused by electrode motion. Pacing Clin Electrophysiol 1983;6:908–914. 162. Simson MB. Use of signals in the terminal QRS complex to identify patients with ventricular tachycardia after myocardial infarction. Circulation 1981;64:235— 242. 163. Wit AL, Josephson ME. Fractionated electrograms and continuous activity: Fact or artifact. In: Zipes DP, Jalife J (eds): Cardiac Electrophysiology and Arrhythmias. Orlando: Grune & Stratton, Inc.; 1985:343-352. 164. Gardner PI, Ursell P, Fenoglio JJ Jr, et al. Electrophysiologic and anatomic basis for fractionated electrograms recorded from healed myocardial infarcts. Circulation 1985;72:596–611. 165. Breithardt G, Borggrefe M, MartinezRubio A, et al. Pathophysiological mechanisms of ventricular tachyarrhythmias. Eur Heart J 1989;10(Suppl E):9-18. 166. Hoeks APG, Schmitz GML, Allessie MA, et al. Multichannel storage and display system to record the electrical activity of the heart. Med Biol Eng Comput 1988; 26:434–438. 167. Barr RC, Spach MS. Sampling rates required for digital recording of intracellular and extracellular cardiac potentials. Circulation 1977;55:40–48. 168. Pieper CF, Lawrie G, Roberts R, et al. Bandwidth-induced errors in parameters used for automated activation time determination during computerized intraoperative cardiac mapping: Theoretical limits. Pacing Clin Electrophysiol 1991;14:214-226. 169. Ideker RE, Smith WM, Wolff P, et al. Simultaneous multichannel cardiac mapping system. Pacing Clin Electrophysiol 1987;10:281-292. 170. Smith WM, Ideker RE, Kinicki RE, et al. A computer system for the intraoperative

INTERPRETATION OF CARDIAC ELECTROGRAMS mapping of ventricular arrhythmias. Comput Biomed Res 1980;13:61-72. 171. Parson I, Mendler P, Downar E. On-line cardiac mapping: An analog approach using video and multiplexing techniques. Am J Physiol 1982;242:H526–H535. 172. Parson I, Downar E. Clinical instrumentation for the intra-operative mapping of ventricular arrhythmias. Pacing Clin Electrophysiol 1984;7:683-692. 173. Rosenfeldt FL, Harper RW, Wall RE, et al. A digital timing and display unit for intra operative mapping of cardiac arrhythmias.

Pacing Clin Electrophysiol 1984;7:985–992. 174. Witkowski FX, Corr PB. An automated simultaneous transmural cardiac mapping system. Am J Physiol 1984;247: H661-H668.

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175. Rosenbaum DS, Kaplan DT, Wilbur DJ, et al. The precision of electrophysiologi

cal mapping: Localizing depolarization

wave front from digital extracellular electrograms and the role of data sampling rate. J Cardiovasc Electrophysiol 1990;1:2-14. 176. Sano T, Tsuchiahashi H, Shimamoto T. Ventricular fibrillation studied by the microelectrode method. Circ Res 1958: 41-46. 177. Spach MS, Miller WTII, Geselowitz DB, et al. The discontinuous nature of propagation in normal canine cardiac muscle. Evidence for recurrent discontinuities of intracellular resistance that affect the membrane currents. Circ Res 1981;48: 39-54.

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Chapter 3 Methodology of Cardiac Mapping Haris J. Sih, PhD and Edward ]. Berbari, PhD

Cardiac mapping involves making some measurement of the heart in 3-dimensional space and then displaying that measurement on a similar 3-dimensional representation. That measurement could be of its mechanical function, electrical function, structure, or some combination of these. To add further complexity, the measurement itself could be a value (such as activation time) or an array (such as conduction velocity, with a magnitude and direction). In the current literature, cardiac mapping usually refers to measuring the electrical activity of the heart in 2 or 3 dimensions and displaying that activity on 2-dimensional representations. The electrical activity is often activation times or isopotentials that are measured directly from electrodes in contact with the tissue or are calculated from body surface electrodes or other electrodes that are not in contact with the tissue. This chapter provides a brief introduction into the techniques of this category of cardiac mapping, and then illustrates how we have dealt with some typical complications to this type of mapping in infarct regions and during atrial fibrillation.

Overview of Current Techniques Most electrical mapping of the heart is done with either unipolar or bipolar electrodes and with either simultaneous multielectrode arrays or sequential recordings from several electrodes on the distal end of a catheter. A unipolar (or single-ended) measurement is usually made relative to some distant, stable reference on the body, the exact position of which is generally not considered critical to the data acquisition. A bipolar (or differential) measurement is between 2 electrodes that are often closely (<1 cm) spaced. Often, the activation time of the cells beneath the electrode must be determined. Criteria used to derive activation times from the recorded signals are discussed in detail elsewhere in this book. Briefly, the maximum negative derivative is the most widely used criterion for defining activation time in a unipolar electrogram. The most widely used criterion for bipolar electrograms is the peak of the main deflection.1

Supported in part by The Herman C. Krannert Fund and a Specialized Center of Research (SCOR) grant (HL52323) from the National Heart, Lung, and Blood Institute of the National Institutes of Health. From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division;©2003. 41

42 CArDIAC MAPPING Using either unipolar or bipolar electrodes, one can map sequentially from a single electrode or simultaneously from multiple electrodes. Aside from the ability to map endocardially without an openchest procedure, mapping using sequential recordings has the advantage of requiring a minimal hardware set-up. Essentially, one only requires a recording catheter, one amplifier, and a display. With such a limited set-up, however, visualization of cardiac activation requires knowledge of the anatomy and a good imagination to mentally reconstruct the sequence of activation. The use of multi-

An epicardial approach avoids these issues, at least to some degree, at the expense of requiring an open-chest procedure. Often, but not always, epicardial electrode arrays are arranged in a regular grid pattern and are of varying construction and dimension. Since epicardial mapping is seldom used in the clinical setting and since each experiment may require unique arrays, most epicardial arrays are made "in-house" with the materials and methods convenient to each researcher. Regardless of the construction, investigators must be aware of the contact quality between the electrodes and the tissue, the necessary interelectrode spacing, whether bipolar or unipolar electrodes are electrode catheters placed under fluoro scopic guidance and the use of multichannel prudent, and the stability and reproelectrophysiology recording systems facil- ducibility of the array placement. Whether itates sequential mapping, but the mental endocardial or epicardial arrays are used, reconstruction of activation is still an inte- the hardware requirements are identical, gral part of this process. While obviously generally consisting of a bank of amplifiers, a useful clinical tool, sequential mapping filters, multiplexers, and analog-to-digital relies on reproducible activation sequences converters, and then some means to write and low spatial variation (i.e., low spatial the data to digital media. As technology develops, the definitions frequency content) of activation, and thus can be insufficient for some research and advantages/disadvantages of multielectrode or sequential mapping become applications. Multielectrode array mapping entails more complex. Two recently developed simultaneously recording the electrical commercial systems are prime examples activity of the heart from a large number of this. The CARTO™ system (Biosense of sites (typically >100). Endocardial map- Webster, Diamond Bar, CA) is essentially ping can be accomplished with basket a sequential mapping system with a catheters, which have electrodes placed sophisticated display.2,3 This system places on thin wire-like splines. When the catheter a catheter tip in an electromagnetic field is inserted into a vein or artery, the and then uses the field to register the splines are contracted to fit into the diam- 3-dimensional location and orientation of eter of the catheter. Once in the chamber the catheter simultaneously with the of interest, the splines are expanded like recorded electrical activity. This allows for the opening of an umbrella, and the elec- a 3-dimensional reconstruction of the endotrodes are exposed and are hopefully in cardial surface and the activation sequence contact with the tissue. While basket across that surface. While this system aids catheters allow for closed-chest, endocar in the visualization of activation, it is still dial recordings, they are limited by sev- limited by its sequential mapping capabileral factors, including the lack of control ities, making it an ineffective tool for mapover which electrodes are in contact with ping nonrepetitive arrhythmias. The EnSite™ system (Endocardial the tissue, unevenly spaced electrodes between splines, and the difficulty in pre- Solutions, Inc., St. Paul, MN) is a multielectrode recording system that records cise anatomical location of the splines.

METHODOLOGY OF CARDIAC MAPPING from 64 sites on a noncontact balloon catheter then reconstructs the endocardial recordings on an ellipsoid-like projection of the endocardium using inverse solution techniques.4 This system changes the definition of spatial resolution for cardiac maps, since "virtual" electrograms can be reconstructed from nearly anywhere on the endocardial projection. The EnSite system can reconstruct activation sequences on the endocardial projection for either repetitive or nonrepetitive arrhythmias. Whether this system can be used to study mechanisms of arrhythmias with high spatial frequency content remains to be determined. As digital hardware speeds have gone up and their prices have come down, the conceivable number of simultaneous channels that can be acquired has increased. However, data management issues, such as electrogram display/review, activation visualization, etc., are more problematic with ever-increasing numbers of channels. These issues are especially troublesome for studying activation in a complex substrate and for studying rapid and irregular rhythms. To illustrate these technical challenges, this chapter focuses on 2 specific areas of cardiac mapping: mapping late potentials and mapping atrial fibrillation. Introduction For studies of late potentials, our original interest in using cardiac mapping was to establish the fundamental bases of the late potentials seen on the body surface ECG.5 The goal was to characterize, in an animal model, the late potential generator, e.g., signal strength, position, and orientation, and to use this information to model and compare with the actual body surface recordings. This prompted a more accurate assessment of the implicit assumptions of cardiac mapping in order to improve the late potential model. Questions concerning the determination of activation time and contour generation became paramount, as

43

did determining which conditions were implicitly and explicitly assumed. While several studies have been performed to correlate activation times to extracellular electrogram features, little has been studied about the mechanics of contour generation for activation maps. Some investigators use a visual approach and manually draw the maps. Others use "canned" contour programs available in scientific subroutine software libraries, and still others use custom-written software. Only a few references exist on the mechanics of contour generation as applied to cardiac activation.6–9 The field of cartography has evolved around, and is usually applied to, geophysical problems,10,11 but a number of newer methods have been used in recent years. It is difficult to determine the extent to which these newer methods have been used in either the published reports or from commercial vendors in cardiac applications. It is our belief that many conclusions about activation sequences have been deduced from poorly constructed contour maps. Some general issues have been discussed by Ideker et al.12 In essence, a fundamental assumption about the underlying structure of activation is implicitly or explicitly made without regard to problems concerning spatial sampling or the assumption of spatial continuity. However, there have been no formal attempts to define the spatial sampling necessary for cardiac activation maps. Spatial continuity is the 2-dimensional property similar to time domain continuity. Most linear mathematical approaches to signal processing require that there be no abrupt changes in the values of the measured quantity; that is, the time derivatives are not infinite. This is also true for the 2dimensional problem where the spatial derivatives are not infinite. In other words, no point in the spatial representation can be multivalued. Unfortunately, mapping in infarct regions almost assures

44 CARDIAC MAPPING discontinuous regions. The inhomogeneity in conduction properties is well known and the presence of dead tissue, i.e., nonconducting regions, must be accounted for in the contour generation process. In geophysical terms, such discontinuities are called faults, and generating contours around a fault region should be considered in cardiac map generation. For atrial fibrillation mapping, the goal has been to use cardiac maps to probe the mechanisms of atrial fibrillation. Some early examples of the successful application of multielectrode mapping to study atrial fibrillation were performed by Allessie et al.13,14 In these studies, the authors were able to verify a long-standing hypothesis on the multiple circulating wavelet behavior of atrial fibrillation.15 One of the greatest complications associated with activation mapping during atrial fibrillation is that the very nature of atrial fibrillation may preclude a succinct presentation of activation. Since a hallmark of atrial fibrillation is its nonrepetitive activation of the atria, categorizing activation can be difficult. Cox et al.16 provided one framework with which to conceptualize possible reentrant patterns during atrial fibrillation by describing the locations of reentrant circuits and how they might activate the atria. More recently, Konings et al.17 categorized activation patterns according to the number of wavelets and the degree of conduction block observed in activation maps. Many other studies resort to a subjective description of the various patterns or the relative complexity of activation with the only quantitative data being cycle length or conduction velocity data. While these observations can be insightful and important, more quantitative measures of activation are needed in order to compare and contrast atrial fibrillation maps. One of our hypotheses is that atrial activation during atrial fibrillation has transient episodes of organization and

that the organization can be quantified. We propose that an organization map may provide new insights into atrial fibrillation mechanisms that could not otherwise be easily discerned with traditional epicardial mapping techniques. Methods

Our mapping system technology has been described previously18,19 and is briefly summarized. The front end consisted of 128 differential amplifiers with programmable gain and bandwidth. Typical settings were a gain of 100 and a bandwidth of 0.1 to 300 Hz. Each signal was sampled at 1000 Hz with a high-speed analog-todigital converter and an interchannel dwell time of 2.0 µs resulting in a maximum time skew of 0.26 ms between channels \ and 128. For late potential mapping, the electrode array was a unipolar 10x10 square grid with a 4-mm spacing between electrode centers. Each side had 6 electrodes, which "rounded out" the edges for a total of 124 epicardial sites within a 6-cm diameter. The remaining 4 channels were for bipolar surface ECG leads. The signal reference was the right leg. The data acquisition computer was an SLS-5450 (Concurrent Computer Corporation, Westford, MA) and was networked to an IBM RS6000 (IBM, Armonk, NY) for data analysis. Experimental data were obtained from the 4-dayold canine infarct model.20 The left anterior descending coronary artery was ligated using the Harris 2-stage tie21 just inferior to the first diagonal branch. All recordings were made during sinus rhythm. For our initial atrial fibrillation studies, we used a commercially available array to epicardially map canine atria during different rhythms. This array had 112 unipolar electrodes in an 8 x 14 grid spanning 2x4 cm. The elements had approximately 1-mm-diameter tips and were constructed of stainless steel. Because of the size of the array, only sections of the

METHODOLOGY OF CARDIAC MAPPING 45 atria could be mapped at any one time. These sequential maps were obtained from the right or left atrial free walls or from the right or left atrial appendages. Two atrial fibrillation models were used: vagal atrial fibrillation induced by atrial burst pacing with superimposed vagal stimulation,22 or self-sustained atrial fibrillation induced by chronic (>4 weeks), continuous rapid atrial pacing.23–25 In our initial experiments on atrial fibrillation organization, we devised a simple algorithm that quantifies the degree of nonlinearity between 2 electrograms on a relatively short time scale (<500 ms). We have shown that this algorithm is more sensitive to changes in atrial fibrillation

organization than 2 previously published algorithms.26 Our initial data are used to compare the relative organization of the vagal model of atrial fibrillation and the chronic pacing model of atrial fibrillation as potential human correlates for nonsustained and chronic atrial fibrillation, respectively. These data demonstrate an alternative method for summarizing mapping data that traditional activation mapping might not as readily quantify. Activation Times in Infarct Regions Figure 1 is an example of a set of recordings overlying the infarct region. Note

Figure 1. A set of 128 electrograms obtained over the infarct in the canine model. The electrode diameter is 6 cm. All recordings are unipolar, with a right leg reference, and were obtained during sinus rhythm.

46 CARDIAC MAPPING the normal biphasic appearance of recordings on the left side. As activation proceeds across the epicardium, the emergence of deep "Q waves" appears and the local late potential appears to migrate after the QRS complex in electrograms on the right side. The top trace in Figure 2 shows the electrograms from channel #71 from the example in Figure 1. The lower traces are the time derivative (dV/dt) and 2 spatial derivatives, dV/dx and dV/dy. The latter are in effect 4-mm spaced bipolar recordings in which the laterally adjacent recording is subtracted to form the horizontally oriented bipole (dV/dx). Similarly, the bottom trace (dV/dy) is formed by subtracting the inferiorly adjacent recording to form the vertically oriented bipole. Note that the x

and y directions only relate to the orientation of the signal plots in Figure 1 and not to any anatomical orientation. In the original electrogram (top trace), the QRS is one of a deep "Q wave" overlying the infarct with a well-defined biphasic late potential well after the QRS complex. The time of local activation of this late potential varies from 109 ms to 104 ms, and 106 ms in dV/dt, dV/dx, and dV/dy, respectively. This slight variation of 5 ms was not considered to be of great significance. Figure 3 is an example of an adjacent electrogram (#84) taken from the same array as the electrogram in Figure 2. The recordings are in the same format. Note now that the activation times differ considerably (=20 ms) between the dV/dt

Figure 2. Electrogram #71, from the same array as in Figure 1, is in the top trace. The second trace is the time derivative, dV/dt. Spatial derivatives, dV/dx and dV/dy were obtained subtracting horizontally and vertically adjacent recordings, respectively.

METHODOLOGY OF CARDIAC MAPPING 47

Figure 3. Electrogram #84 in the same format as in Figure 2.

and the bipolar recordings. The reason for the difference is because the morphology of the late potential has 2 distinct deflections and results in an ambiguous situation. The maximum negative time derivative criterion selected the second deflection, while the maximum of the bipolar recording selected the initial late potential deflection. In our own application for comparing body surface potentials, we selected the end of activation as the parameter of choice and based the selection on different criteria. Another approach is to examine the electrograms in the context of their neighbors. Figure 4 is a column of recordings from Figure 1, showing the time and spatial derivatives adjacent to each. Electrogram #48 shows an ambiguous late potential, i.e., 2 deflections. An arrow

points up from the first late potential deflection to electrogram #37. Note the simultaneous occurrence with the unambiguous late potential. An arrow pointing down from the second deflection of electrogram #48 to the lower electrogram (#59) shows a similar occurrence with a later deflection. The unanswered question is, of course, "Is electrogram #48 recording a local event or just the events occurring at some distance?" In vitro electrograms at an even finer spatial resolution (2 mm) demonstrate that this question may not be easily answered with standard unipolar electrogram analysis. Figure 5 shows an example of 30 unipolar electrograms with a 2-mm interelectrode spacing recording activity over an infarct region. In the top left corner of the map, the unipolar

48 CARDIAC MAPPING

Figure 4. A column of recordings from Figure 1 showing late potentials occurring after the QRS. The arrows imply the possibility that ambiguous recordings (#48 and #84) are recorded from nearby tissue and not directly underneath the electrode.

recordings show fairly typical biphasic electrograms. Recordings in the bottom section of the map, however, are fractionated. Over a distance as small as 2 mm, the character of an electrogram changes dramatically, from unambiguous to highly fractionated. From this example, it is clear that a 2-mm interelectrode spacing is not sufficient to rep-

resent activation through the infarct region. What electrode density is necessary to reveal the nature of the sources of the fractionated electrogram and to identify truly local events has not yet been determined. In vitro correlations with extracellular and intracellular recordings from infarcted regions may be the only way to fully answer this question.

METHODOLOGY OF CARDIAC MAPPING 49 example of how double potentials can occur. In this example, several beats during atrial fibrillation had a repetitive activation sequence. Electrograms in locations A and B are dominated by one repeating wavefront while electrograms in locations D and E are dominated by a different wavefront. Electrograms in location C apparently reflect both wavefronts for approximately 1 second. There is no clear choice between the electrograms to delineate activation times for location C. The choice to delineate activation times for both electrograms, either electrogram, or neither electrogram is arbitrary. One approach to the difficulties associated with activation time delineation Figure 5. A set of 30 electrograms obtained over and the additional problems of concisely the infarct in the canine model. The interelectrode distance is 2 mm. The character of the summarizing multiple activation maps electrograms can change from a simple bipha- is to circumvent the issue by choosing an sic deflection to a multiphasic, fractionated elec- alternative representation of the map. trogram within 2 mm. Such representations could include almost any imaginable signal processing function that might extract some useful Organization Mapping in parameter about activation. Several recently proposed signal processing algoAtrial Fibrillation rithms claim to measure the "organizaActivation maps of atrial fibrillation tion" of atrial fibrillation activation. We can be difficult to interpret because of are currently exploring such a technique the varying patterns and the large that quantifies "disorganization" as the number of activations per second. degree of nonlinearity between 2 elecBecause of these difficulties, alternative trograms. Preliminary results indicate representations of fibrillation mapping that this measure has good sensitivity data have been explored, including maps in discriminating different levels of fibof average cycle length,25 consistency of rillation organization.26 This and simiactivation direction,27 and organization lar methods replace the subjective mapping. 28 Some of these methods delineation of activation times by objecrequire that activation times be detected tively and automatically calculating the from the atrial electrograms. However, relative organization. In one of our first similar to how fractionated electrograms applications of this measure to mapcomplicate activation detection in the ping data, we compared organization betinfarct zone, multiphasic electrograms ween vagal and chronic models of atrial and double potentials complicate atrial fibrillation. Organization as we have defined it fibrillation. During atrial fibrillation, multiphasic electrograms and double can be measured between any pair of elecpotentials often appear, usually because trograms. So, to derive some measure of of the electrode's sensing multiple, the regional organization, we used our nearby wavefronts. Figure 6 shows one technique of quantifying nonlinearity on

50 CARDIAC MAPPING

Figure 6. Double potentials during atrial fibrillation. The activation map with hand-drawn contours is shown in the top and taken during the interval shown by the boxed region on the signals. The double potentials in channel C are a result of recording activity over an arc of block, where 2 wavelets pass on either side of the arc at different times.

multiple pairs. Our 8 x 14 electrode array mapped the left and right atrial free walls and the left and right atrial appendages sequentially. By calculating the nonlinearity between multiple pairs

of electrodes in the 8 x 14 array and then plotting the nonlinearity versus the interelectrode distance, one could derive an exponential-like plot of the regional nonlinearity (Figure 7). Thus in, for

METHODOLOGY OF CARDIAC MAPPING

51

Figure 7. Example of nonlinearity between sites versus interelectrode distance. To summarize regional organization, the nonlinearity is fit to an exponential function, shown in the black line. The exponential decay constant, which has units of distance, can be interpreted as a measure of the spatial extent to which atrial fibrillation is organized. Smaller values of the decay constant indicate less organization, and larger values of the decay constant indicate more organization.

example, the right atrial free wall, one can plot how rapidly over distance the organization decreases between sites during fibrillation. By then determining the exponential decay constant, a measure of the spatial extent of organization (in mm) can be derived, with larger values of the decay constant implying a greater spatial extent of organization and smaller values of the decay constant implying a smaller extent of organization. Over a series of 13 vagal atrial fibrillation dogs and 18 chronically paced atrial fibrillation dogs, we found differences in organization, primarily in the right atrium. The right atria in the vagal atrial fibrillation model were less organized than right atria in the chronic atrial fibrillation model, while left atrial organization was unchanged (Figure 8).

After bilaterally stimulating the vagus in the chronic atrial fibrillation model and remeasuring organization, we found that bilateral vagal stimulation reduced the organization, but only significantly in the right atrium (P < 0.0001). These differences were attributed to the differences in left and right vagal innervation of the atria and to the electrical remodeling that occurs during the longterm pacing in the chronic model. This representation of organization can provide additional insight into atrial fibrillation mechanisms that might not be apparent with traditional activation maps. We are currently pursuing means to quantify the spatial extent of organization over smaller regions to achieve a true map of atrial fibrillation organization.

52 CARDIAC MAPPING

Figure 8. Comparison of organization between vagal atrial fibrillation and chronic atrial fibrillation. Organization was not statistically different between the left atrium (LA) and right atrium (RA) in the vagal atrial fibrillation group. Left atrial activation in the chronic atrial fibrillation group was more disorganized than right atrial. Superimposed vagal stimulation in the chronic group caused organization to decrease, but only in the right atria.

Generating Contours Contour generation can be done manually. This obviously interjects an element of bias, but more importantly it does not allow for a mathematical description of the data. Such descriptions allow the use of transformations such as directional derivatives, smoothing with 2-dimensional filters, and the measurement of error in the contours when compared to the actual underlying data. At this stage, the values chosen from the individual raw data waveforms would be considered

unambiguous. The next assumption in contour generation is that the spatial sampling is adequate. The previous discussion implied that at present there is no known way to assure this since the minimum wavefront length is not known. The next assumption in contour generation is that the data fit some underlying mathematical structure. The simplest structure is the linear model assumed by simple triangulation methods. In essence, this assumes that, if a straight line connects the sample data points, the values under the line vary linearly between the

METHODOLOGY OF CARDIAC MAPPING 2 points. Triangulation has several drawbacks in that there is no physiological basis for the linear assumption, and that depending on how the data points are linked, there is no unique solution. For unevenly spaced recording sites, there are many ways in which the data points can be linked. There are no a priori restrictions on the formation of triangles as the data in the entire region are linked together. Older software algorithms were even susceptible to the order of data entry. Hence, in the early 1970s, triangulation fell into disuse by cartographers because the method is not well defined. More recently, some efforts have been made to regularize the triangulation methods, but other more mathematically based methods are favored. Gridding is a method whereby the data points (usually unevenly spaced) are converted to an underlying, evenly spaced set of points. The grid can be defined to have many more points than the sample points. The value assigned to each grid can be a linear combination, e.g., an average, of nearest data points. The term nearest can be defined in a radial sense, e.g., all data points within 5 mm. Alternatively, using the radial search criterion, the grid value can be weighted with a distance measure. Thus, data points closer to the grid point will have a larger influence in calculating the grid value than data points further away. Much of what has been said can be more succinctly stated in mathematical terms (equation 1).

Here, VG is the value computed at the gridded data point, Vi are the values at the 4 original data points, and DiG are the distances from those original data points to the new grid point. For a more intuitive approach, however, these concepts can be described graphically with a set of simu-

53

lated data points. Figure 9 has 6 panels. Panel A shows a set of irregularly spaced data points with values ranging from 5 to 8. Panel B is a regularly spaced grid, with the open circles at the line intersections representing the new underlying grid points. Panel C demonstrates the variable spacing between the data and grid points by overlapping panels A and B. Panel D is an example of one grid point and its 4 surrounding nearest neighbors. The simplest way to evaluate the value of the grid point is to just average the surrounding data points. This value is 6.5. However, one can also use the weighting approach described by equation 1. Table 1 shows the actual coordinates and data points for this problem. Substituting values into equation 1, VG = (7.69 + 6.0 + 11.67 + 7.0)/4.95 = 6.54. In this example, this is very close to the simple average of 6.5. The rest of the grid points, to one decimal point, are shown in Figure 9F. Now that the data have been converted to a regularized grid, many types of operations can be performed. The method of deriving the contours can be based on one of many different schemes, such as cubic spline fitting or even linear interpolations. Many schemes can be used to form the grid. For example, one could require that there be data points in all 4 quadrants surrounding the grid point, except in the case of boundary grid points. Krige29 proposed a statistically based method that minimizes the variance of data points that coincide to or are very close to the grid points. This minimum Table 1 Coordinates and Data Points Grid Data A Data B Data C Data D

X

Y

2 1.5 3.0 2.0 1.0

3 3.6 3.0 2.4 2.9

D

V

V/D

0.78

6 6 7 7

7.69 6.00 11.67

1.0 0.6 1.0

7.0

54 CARDIAC MAPPING

Figure 9. A simple example of how a grid is formed from a set of unevenly spaced observation points (A). B. The evenly spaced grid points are represented as open circles. C. Superimposition of A and B. D. One grid point and its 4 nearest neighbors. E. Superimposition of C and D. The final grid values (from equation 1 and summarized in Table 1) are shown in F.

variance method, often referred to as kriging, now allows for estimates of error in the map. Detailed discussion of this is beyond the scope of this chapter, but in essence such a statistical approach would allow for the generation of an optimal map and clear delineation of regions with the highest uncertainty. Some examples of gridding to generate activation maps over the infarct are shown in Figure 10. These maps are derived from the electrograms shown in Figure 1. In these maps, we have assumed appropriate spatial sampling, the data are spatially continuous (no dead inac-

tive regions), and each electrogram has a unique activation time. Each isochrone represents 3 ms, with early activation on the left and late activation on the right. The specifics of timing are not considered, and the emphasis is on the actual generation of the contour lines. The dots on each map represent the electrode recording sites. Figure 10A shows a low-resolution map with grid nodes arranged in a 15 x 15 matrix. In generating this grid matrix there is little interpolation since the matrix nodes roughly correspond to the original measurement points. An infarct border zone is encountered approximately

METHODOLOGY OF CARDIAC MAPPING

55

Figure 10. Activation maps from Figure 1, assuming continuous activation. A. A 15 x 15 grid. B and C use a 25 x 25 and 100 x 100 grid, respectively. D. A smoothed version of C.

halfway across the map, and conduction in this region is delayed and nonuniform. Figure 10B is the same map with a 25 x 25 grid matrix. This has twice as many columns and rows as the electrode layout. Artifacts now begin to appear as extreme regions. This is fully appreciated in Figure 10C as the grid matrix is increased to 100 x 100. Figure 10D shows the smoothed version of Figure 9C and appears more like the map in Figure 9A.

However, the high-resolution grid after smoothing contains more spatial temporal information. This is demonstrated by creating a trend map. Figure 11A is the trend map of Figure 10D and is the locus of normal activation extrapolated from the left side of the activation map (the normal region) to the entire epicardial surface under the electrode. It is a first-order polynomial approximation and assumes uniform, constant velocity, conduction. It was derived

56 CARDIAC MAPPING

Figure 11. A. Trend map obtained from the region of Figure 10D showing uniform conduction; roughly the left half of Figure 10D. B. Residual map obtained by subtracting A from Figure 10D. This is a form of high-pass filtering.

from the grid. Figure 11B is a residual map derived by removing the trend map (Figure 11A) from the activation map (Figure 10D). Such mathematical operations are easily performed once the underlying grids are formed. The contours in Figure 11B represent "lateness" or departure from uniform conduction, and it appears that there are 2 regions of localized late activity in the lower right quadrant of the map. The residual map was applied to the late potential problem, but is offered here as an example of map manipulation. The creation of a contour map that takes into consideration a faulted region, e.g., dead nonconducting tissue, is another example of how a gridded structure can be used. It is not enough to just declare that an electrogram site generates no activation time. Without a clear definition of a fault zone, most algorithms will simply interpolate across the dead tissue. The drawing of an isochrone or the inclusion of a data point in the interpolation that is "across" the fault should be considered an invalid approach. It is not known how this has been dealt with in prior studies.

Conclusions The question may be asked, "Why are these details about map generation important?" Most of the theories regarding cardiac activation and arrhythmogenesis are derived from the visual examination of contour maps. However, very little attention has been given to the creation of these maps. Even if the problems of selecting activation times from the electrograms are solved, the validity of the mapping assumptions has not been critically examined. It is quite possible to generate visually different maps, all correct according to their mathematical basis, from the same data sets. Hence, differences in interpretation of a particular activation sequence may in fact be due to different algorithms used in contour generation rather than the underlying pathophysiology. To some degree, organization mapping or similar "parameter extractions" from epicardial mapping data attempt to circumvent these problems by avoiding activation sequence reconstruction entirely.

METHODOLOGY OF CARDIAC MAPPING The methods used to generate a contour map should be examined in greater detail as it applies to cardiac mapping. Investigators must become cognizant of various methods and the strengths and weaknesses of these approaches. Until that time, the conclusions drawn from contour maps should be tempered so that investigators can develop the technical skills to deal with the problems associated with the data presentation. Various mapping approaches will change the interpretation of the data and greater care must be taken in presenting the methods used and the impact these methods have on interpretation. References 1. Sodi-Pallares D, Rodriguez MI, Chair LO, Zuchermann R. Activation of the interventricular septum. Am Heart J 1951;41: 569. 2. Nakagawa H, Jackman WM. Use of a three-dimensional, nonfluoroscopic mapping system for catheter ablation of typical atrial flutter. Pacing Clin Electrophysiol 1998;21:1279-1286. 3. Gepstein L, Evans SJ. Electroanatomical mapping of the heart: Basic concepts and implications for the treatment of cardiac arrhythmias. Pacing Clin Electrophysiol 1998;21:1268-1278. 4. Peters NS, Jackman WM, Schilling RJ, et al. Images in cardiovascular medicine: Human left ventricular endocardial activation mapping using a novel noncontact catheter. Circulation 1997;95:1658-1660. 5. Berbari EJ, Scherlag BJ, Hope RR, Lazzara R. Recording from the body surface of arrhythmogenic ventricular activity during the S-T segment. Am J Cardiol 1978;41:697-702. 6. Barr RC, Gallie TM, Spach MS. Automated production of contour maps for electrophysiology: I. Problem definition, solution strategy, and specification of geometric model. Comput Biomed Res 1980; 13:142-153. 7. Barr RC, Gallie TM, Spach MS. Automated production of contour maps for electrophysiology: III. Construction of contour maps. Comput Biomed Res 1980; 13: 171-191.

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8. Barr RC, Gallie TM, Spach MS. Automated production of contour maps for electrophysiology: II. Triangulation, verification, and organization of the geometric model. Comput Biomed Res 1980;13:154-170. 9. Monro DM. Interpolation methods for surface mapping. Comput Programs Biomed 1980;11:145-157. 10. Robinson JE. Computer Applications in Petroleum Geology. New York: Van Nostrand Reinhold; 1982. 11. Davis JC. Statistics and Data Analysis. New York: John Wiley; 1986. 12. Ideker RE, Smith WM, Blanchard SM, et al. The assumptions of isochronal cardiac mapping. Pacing Clin Electrophysiol 1989;12:456-478. 13. Allessie MA, Bonke FIM, Schopman FJG. Circus movement in rabbit atrial muscle as a mechanism of tachycardia, III: The 'Leading Circle' concept: A new model of circus movement in cardiac tissue without the involvement of an anatomical obstacle. Circ Res 1977;41:9-18. 14. Allessie MA, Lammers WJEP, Bonke FIM, Hollen J. Experimental evaluation of Moe's multiple wavelet hypothesis of atrial fibrillation. In: Zipes DP, Jalife J (eds): Cardiac Electrophysiology and Arrhythmias. Orlando: Grune and Straton; 1985:265-275. 15. Moe GK. On the multiple wavelet hypothesis of atrial fibrillation. Arch Int Pharmacodyn Ther 1962; 140:183-188. 16. Cox JL, Canavan TE, Schuessler RB, et al. The surgical treatment of atrial fibrillation: II. Intraoperative electrophysiologic mapping and description of the electrophysiologic basis of atrial flutter and atrial fibrillation. J Thorac Cardiovasc Surg 1991;101:406-426. 17. Konings KTS, Kirchhof CJHJ, Smeets JRLM, et al. High-density mapping of electrically induced atrial fibrillation in humans. Circulation 1994;89:1665-1680. 18. Berbari EJ, Lander P, Geselowitz DB. A cardiac mapping system for identifying late potentials: Correlation with signal averaged surface recordings. Proceedings of the Computers In Cardiology Conference. Washington, DC: IEEE Computer Society Press; 1988:369-372. 19. Berbari EJ, Lander P, Scherlag BJ, et al. Ambiguities of epicardial mapping. J Electrocardiol 1992;24(Suppl): 16-20. 20. Scherlag BJ, El-Sherif N, Hope R, Lazzara R. Characterization and localization of

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21. 22.

23.

24.

25.

CARDIAC MAPPING ventricular arrhythmias resulting from myocardial ischemia and infarction. Circ Res 1974;35:372-383. Harris AS, Rojas AG. The initiation of ventricular fibrillation due to coronary occlusion. Exp Med Surg 1943; 1:105-122. Lewis T, Drury AN, Bulger HA. Observations upon flutter and fibrillation: VII. The effects of vagal stimulation. Heart 1921;8:141-169. Morillo CA, Klein GJ, Jones DL, Guiraudon CM. Chronic rapid atrial pacing: Structural, functional, and electrophysiological characteristics of a new model of sustained atrial fibrillation. Circulation 1995;91:1588-1595. Elvan A, Wylie K, Zipes DP. Pacinginduced chronic atrial fibrillation impairs sinus node function in dogs: Electrophysiological remodeling. Circulation 1996; 94: 2953-2960. Wijffels MCEF, Kirchhof CJHJ, Borland R, Allessie MA. Atrial fibrillation begets

atrial fibrillation: A study in awake chronically instrumented goats. Circulation 1995;92:1954-1968. 26. Sih HJ, Zipes DP, Berbari EJ, Olgin JE. A high-temporal resolution algorithm for quantifying organization during atrial fibrillation. IEEE Trans Biomed Eng 1999;46: 440-450. 27. Damle RS, Kanaan NM, Robinson NS, et al. Spatial and temporal linking of epicardial activation directions during ventricular fibrillation in dogs: Evidence for underlying organization. Circulation 1992; 86:1547-1558. 28. Sih HJ, Sahakian AV, Arentzen CE, Swiryn S. A frequency domain analysis of spatial organization of epicardial maps. IEEE Trans Biomed Eng 1995;42:718727. 29. Krige DG. Two dimensional weighted moving average trend surfaces for ore evaluation. J S Afr Inst Mining Metallurg 1966:13-38.

Chapter 4 Noncontact Endocardial Mapping Richard Schilling, MD, Nicholas S. Peters, MD, Alan Kadish, MD, and D. Wyn Davies, MD

ablation of complex cardiac arrhythmias such as ventricular tachycardia (VT) in structural heart disease, and atrial fibrillation (AF). The importance of accurate mapping is exemplified in ablation of VT in the setting of ischemic heart disease where identification of relevant diastolic activation is critical. Surgical ablation of the diastolic pathway has achieved primary success rates of 63%12 to 76%13 when guided by preoperative and intraoperative mapping, but the mortality associated with this procedure is 9% to 23%.14 Catheter ablation of VT is an attractive alternative approach because it might avoid the risks associated with surgery. Localization of diastolic activation is achieved by sequential acquisition of contact catheter data points from which the operator develops a mental image of the VT circuit. However, only a minority (10%) of patients are considered able to tolerate VT for long enough to allow such conventional mapping to identify the critical diastolic activity that maintains VT.15 The current complication and failure

Introduction Accurate mapping is the cornerstone to successful ablative therapy of cardiac arrhythmias because it provides insight into the arrhythmia mechanism and identifies the location of a suitable target for ablation. Reentry is the mechanism responsible for the majority of sustained arrhythmias in humans.1–9 Ablation of these arrhythmias is critically dependent on locating the abnormal depolarization that completes the diastolic pathway that maintains the reentrant circuit.10–11 Using conventional mapping techniques it has been possible to treat a wide range of arrhythmias with high success and low complication rates. There remain some arrhythmias that are difficult to treat with catheter ablation because of a number of factors, including the small lesion sizes produced by radiofrequency (RF) energy and difficulties with precise localization of the arrhythmia substrate. For this reason, developments in mapping techniques are fundamental to the success of catheter

From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; ®2003.

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CARDIAC MAPPING

rates therefore largely reflect the lack of rapidity and precision of mapping to guide delivery of sufficient ablative energy to the correct site. Thus, the results of catheter ablation of VT have been disappointing, with immediate success rates of 69% to 90%9,10,15,16 and high recurrence rates. Attempts to improve these results have been directed at increasing the size of ablation lesions and at improving mapping techniques to increase the precision of delivery of lesions by elucidating the full complexity of the substrate underlying VT in ischemic heart disease. A system for 3-dimensional electroanatomical reconstruction of sequentially acquired contact catheter data has recently been described and validated.17 Although this development is a significant advance in mapping technology, like all sequential systems resolution is limited by the time available to acquire data points, and its use remains restricted in cases of nonsustained or hemodynamically unstable arrhythmias. It is therefore apparent that an ideal cardiac mapping system should collect data simultaneously from the entire cardiac chamber, from which high-resolution maps would be produced on which the position of mapping catheters can be shown so that the catheters may be guided to sites of interest on the map. It should be capable of deployment using a percutaneous minimally invasive technique. Simultaneous complete data acquisition will allow mapping of nonsustained and hemodynamically unstable arrhythmias such as VT or arrhythmias that have beat-tobeat changes in activation such as AF. The development of noncontact mapping techniques has been an attempt to produce such a mapping system. Such systems will be commercially available in the near future and this chapter examines their development and describes the validation and initial clinical experience of one such system.

From the Body Surface to the Endocardium: The Development of Noncontact Mapping Body Surface Mapping Body surface mapping is a method for improving the resolution and sensitivity of recordings of surface electrograms. It involves the recording of 2 or more electrograms from the subject's body surface and using these data to interpret the underlying cardiac activation patterns. Recordings made by surface electrodes reflect data from points around the entire cardiac chamber and are a summation of these data which thus varies according to the positions of the electrodes relative to each of these infinite number of cardiac points. The data acquired from each electrode are different from the data of their neighbor data because the degree of influence of each myocardial point on an individual electrode is related to the distance of each point from the electrode and the nature of the tissue interposed between the two.18 If this geometry and the nature of the thoracic tissues are known, it is theoretically possible to determine myocardial potentials from the body surface potentials. This process is known in electrocardiography as solution of the inverse problem. The clinical application of body surface mapping has been limited by the resolution of the data provided. From as early as 1955,19 efforts were made to describe the complex relationships of the cardiothoracic geometry, tissues, and potential fields as mathematical equations so that the nature of unknown epicardial potentials could be inferred from body surface recordings. These solutions were developed by mathematically describing the "forward" relationship between a defined experimental model of cardiac activation and the

NONCONTACT ENDOCARDIAL MAPPING

resulting body surface potentials. Under these in vitro conditions the activation patterns of the source, the resulting potential distribution on the body surface, and the geometric relationship between surface electrodes and each finite point on the source are all known. The forward solution describing the body surface potentials resulting from a known cardiac source can then be "inverted" to calculate cardiac source potentials from the body surface potentials. The models used to simulate cardiac activation were constructed so that they produced body surface maps similar to those seen in clinical practice. However, it must be remembered that body surface potentials are an averaged and simplified version of true activation patterns on the cardiac surface and, as a result, the models used to describe this relationship, although they were of increasing complexity, evolving from single dipoles,19,20 multiple dipoles,21,22 and higher order multipoles,23,24 did not truly describe the cardiac source. This meant that the resulting solutions were not unique, that is, a variety of electrical generators (i.e., myocardial potential fields) could have produced identical body surface maps. For this reason the noninvasive nature of body surface mapping, which is its attraction, is also a limitation, because the development and validation of an accurate inverse solution requires direct measurement of the epicardial potentials while leaving the geometry of the thorax and heart undisturbed, as it would be in the closed-chested living patient. This has been overcome to some extent by directly recording the epicardial potentials of animal hearts suspended in electrolytic tanks.25 With use of this preparation, the complex potential field generated by the heart is equivalent to that in vivo and the geometry between the heart and the body surface (the surface of the electrolytic tank) can be measured

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accurately. However, this model does not account for the nature of structures between the cardiac surface and the body surface. The boundaries between these structures (lung, muscle, bone, and fat) exert complex influences to distort the potential field as it appears on the body surface and must be accounted for in any inverse solution. This may be achieved by using a finite element method (FEM)26 that divides the thorax into a series of small finite elements whose influence on the potential field may be calculated and accounted for by performing a series of highly complex integrations (described in more detail later). This technique is highly complex and thus increases the time taken to construct body surface maps. Endocardial Noncontact Mapping An alternative "noncontact" concept was introduced by Taccardi et al.27 in which intracavity potentials were measured from electrodes on an olive-shaped probe introduced through the left ventricular (LV) apex of animal hearts. Taccardi and colleagues noted that the pattern of endocardial activation was not precisely reflected in the raw cavity potentials recorded by the probe, which exhibit spatially averaged, lower amplitude distributions. This phenomenon closely resembled the noncontact recordings made during body surface maps. It was therefore a logical progression to conceive a mathematical solution to these noncontact endocardial potentials in a manner similar to that applied to body surface maps. Application of inverse-solution methods to raw noncontact endocardial recordings to reconstruct endocardial potentials has been investigated by several groups.28–32 This approach has several advantages over inverse solution of body surface potentials. First, the noncontact

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Figure 1. Multielectrode array (MEA) catheter with 9F shaft (left), deployed 7.5-mL balloon with braided MEA (center), and micrograph of 0.025" long electrode created by removing a spot of insulation with a laser (right).

probe is in closer proximity to the source than is achieved by body surface electrodes, thus reducing distance-related blurring of the endocardial potentials. Second, the space between the intracavitary probe and the endocardium is filled with a medium that may be treated as uniform and electrically inert, i.e., blood. This also means that development and validation of solutions to the inverse problem is easier because in vitro studies may be performed in a tank without having to account for the complexities of extracardiac thoracic tissues. Furthermore, validation of such techniques is feasible both in vitro and in vivo because it is possible to percutaneously access the endocardium to make recordings of contact endocardial electrograms with which to compare the reconstructed electrograms. The first commercially available percutaneous noncontact mapping system uses advanced mathematical methods that provide detailed mapping of the entire endocardial surface of either atrial or ventricular chambers by simultaneously computing electrograms and rapidly displaying

high-resolution color maps of activation33 in the intact beating heart. The system uses a catheter-based noncontact multielectrode array (MEA) to detect far-field endocardial potentials from within a cardiac chamber. From these potentials, the system reconstructs instantaneous endocardial electrograms and isopotential maps on a computer-generated "virtual" endocardium. This noncontact mapping system is also capable of locating and guiding a conventional contact catheter to an area of interest on the isopotential map. Materials The noncontact mapping system consists of a catheter with its MEA (Figure 1) and a custom-designed amplifier system connected to a Silicon Graphics (Mountain View, CA) workstation that is used to run specially designed system software (Figure 2). The MEA catheter consists of a 7.5-mL balloon mounted on a 9F catheter around which is woven a braid of 64 insulated

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Figure 2. Noncontact endocardial mapping system, consisting of the multielectrode array catheter, patient interface unit (rear left), and Silicon Graphics display workstation.

0.003"-diameter wires (Figure 1). During assembly, a laser is used to remove a single spot of insulation on each wire, 0.025" in length, producing 64 noncontact unipolar electrodes. Each electrode is etched at a specified location on each wire in a pattern of 8 rows by 8 columns around the braid, thereby defining the MEA. The raw far-field electrographic data are acquired and applied to a computerized multichannel amplifier (patient interface unit) and digital recording system (display workstation), as shown in Figure 2. The unipolar MEA signals are recorded using a ring electrode, which is located on the proximal shaft of the MEA catheter, as a reference. This reference is at the level of the descending aorta when the MEA is deployed in the LV and in the inferior vena cava (IVC) when the MEA is deployed in the right atrium (RA). Data are sampled at 1.2 kHz and filtered with

a programmable bandwidth between 0.1 and 300 Hz. An electrically based locator signal is also generated by the system and simultaneously sensed by the MEA to track the position of any contact catheter in the same chamber. In addition to electrographic and location data acquired from the MEA, the recording system has 16 channels for contact catheters and 12 channels for the surface ECG. The system is applied in 3 steps: 1. Establish cardiac chamber geometry 2. Identify site(s) critical for maintenance of reentry circuit(s) 3. Navigate ablation catheter to critical site(s) The locator signal is central to steps 1 and 3 while the inverse solution for reconstructing endocardial electrograms is central to step 2.

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Figure 3. The contoured model of the chamber geometry is created by tracing the endocardial surface with a conventional roving catheter while the system tracks its position. The geometry is partially defined midway through the process in the left panel, completed and smoothed as a bicubic spline in the center panel, and rendered in the right panel as an anatomically contoured, wire-frame 3-dimensional model. See color appendix.

Catheter Locator System The system locates any conventional catheter with respect to the MEA by sourcing a 5.68-kHz, low-current "locator" signal from the roving catheter and alternately sinking the current in ring electrodes approximately 1 cm proximal and distal to the MEA on the noncontact catheter shaft. Given the known positions of the array electrodes and the currentsink electrodes, a custom algorithm can determine the position of the roving source by demodulating the 5.68-kHz potentials on the MEA. This locator system serves 2 purposes. First, it can be used to construct a 3-dimensional computer model of the endocardium (virtual endocardium), which is required for the reconstruction of endocardial electrograms and isopotential maps. This model is acquired by moving a conventional catheter around the cardiac chamber, building up a series of coordinates for the endocardium, and gen-

erating a patient-specific, anatomically contoured model of its geometry. To accomplish this, the system automatically stores only the most extreme points visited by the roving catheter in order to ignore those detected when the catheter is not in contact with the endocardial wall. This method is demonstrated midway through the process in the left panel of Figure 3. The green vector provides a graphic representation of the locator signal, which originates at MEA center and indicates the position of roving electrode catheter. As the roving catheter is moved, points accumulate and serve as the vertices of translucent red polygons that are drawn between the points. The set of vertices shown represents the most distant points along each vector, visited by the roving catheter midway through the process, with the chamber geometry partially defined. As the process continues, the number of facets grows and the model evolves to a polygonal representation of the chamber surface at its completion.

NONCONTACT ENDOCARDIAL MAPPING

The sampled coordinates are then automatically fitted to a bicubic spline surface in order to more optimally estimate the actual chamber surface, as shown in the center panel of Figure 3. The total time required to define the contoured geometry is typically between 5 and 10 minutes. The locator signal can then be used to display and log the position of any catheter on the endocardial model. During catheter ablation procedures, the locator system has been used to guide the catheter to sites of interest identified from the isopotential color maps and to log the position of RF energy applications on the virtual endocardium. In Figure 4 the green vector shows the position of the tip of an ablation catheter. Because the geometry of the cardiac chamber is taken with reference to the MEA and is used to produce the inverse solution, it is critical that once geometry is established the MEA should not move. If the MEA is moved, then cardiac geometry must be reestablished. In practice, this is not a problem. The stiffness of the MEA catheter shaft and weight of the balloon means that once sited its position remains stable. A stable position for the MEA catheter pigtail in the LV was the apex, and for the RA either the superior vena cava (SVC) or across the tricuspid annulus (TA). The stability of the MEA is verified during studies by use of a reference quadripolar catheter positioned in the right ventricular apex. The locator signal is passed through the reference catheter immediately after the geometry is established. This locator signal is then used to mark the reference catheter position on the virtual endocardium. The locator signal is then passed through the reference catheter at regular intervals to confirm that the locator signal bisects the previously marked position of the catheter. If the MEA catheter has moved, the reference catheter will have moved relative

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to the MEA, this displacement being shown by the locator signal. Inverse Solution Reconstruction of Endocardial Potentials The electrical activity detected by the electrodes on the surface of the MEA is generated by the potential field on the endocardial surface. Cavity electrograms detected by noncontact electrodes are of lower amplitude and frequency than the source potentials on the endocardium, which limits their clinical utility in raw form.27,28,34 The technique to enhance and resolve the actual endocardial surface potentials has been devised based on an inverse solution to Laplace's equation using a boundary element method (BEM).35 Specialized algorithms have also been developed to further improve the accuracy and stability of the reconstructions. The potential distribution on the MEA created by endocardial activation is described by Laplace's equation. The potential field at any one electrode is influenced to a degree by the potentials from the entire endocardium, with the degree of influence diminishing with the distance between the electrode and each endocardial point. The potential field created on the MEA surface therefore depends on the geometry of both the MEA and the endocardium, and on their relationship in space. The geometry matrix defines the relationship between the location of the 64 electrodes on the MEA and 3360 points on the endocardium where the reconstruction is computed. The locations of the MEA electrodes relative to the center of the balloon are precisely determined at manufacture (Figure 5). The locations of the 3360 endocardial points are obtained during the geometry acquisition described above. With this information, it is possible to compute endocardial electrograms from the MEA potentials by an inverse solution

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Figure 4. Detail from the screen of the noncontact system workstation. Within the geometric contour of the computer-generated "virtual" left ventricular endocardium (top), the position of the multielectrode array (MEA) is represented as a yellow frame. The "virtual" endocardium has been opened and unfolded along the inferior septum (the edges are therefore in continuity) to reveal the endocardial surface. The small yellow letters a, e, and g are an engineering feature of the system, which marks the positions of the columns of MEA electrodes on the virtual endocardium. Some anatomical locations are labeled in yellow having been identified on fluoroscopy and the locations are marked on the map using the catheter locator system. Inf = inferior; Post lat = posterolateral; Ant lat = anterolateral; Ant = anterior; Sept = septal. Superimposed onto the "virtual" left ventricular endocardium is an isopotential map generated from reconstructed electrograms. The color scale for the isopotential map is shown on the line below the "virtual" endocardium and has been set so that white represents endocardial regions where the potential is -1.5 mV or less and purple represents endocardium where the potential is -1.49 mV or more (thus producing an activation map). The green line emerging from the MEA marks the position of a mapping catheter electrode on the endocardium using the locator signal. The isopotential map is set to show the exit site of a ventricular tachycardia as a white dot of activation. Below the "virtual" endocardium is a waveform window showing reconstructed electrograms from the exit site (B and C), the surface ECG, and the contact electrogram from the mapping catheter tip (which is remote from the exit). The white line on the waveform window represents the position in time shown by the isopotential map. See color appendix.

of Laplace's equation. This inverse solution is based on Green's second formula:

where D is a domain (the blood pool), 3D is the boundary of D (the endocardium and the MEA), 3/3n represents the outward normal on D, V2 is the Laplacian, dA is the surface area differential and

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Figure 5. Diagrammatic representation of the balloon showing the positions of electrodes around the balloon. Their exact positions may vary with each balloon. Therefore, the electrode positions on each multielectrode array are determined at manufacture and recorded on a microchip, which is used to calibrate and orientate the computer. See color appendix.

dD is the volume differential, w is the many smaller regions, small enough so potential field created by a unit charge that the behavior of a potential field within in free space, and v is the potential field each element can be accurately approxiin the domain. This equation is dis- mated by an algebraic expression. In addicretized using the BEM, which is a tion, the interactions between each element numerical approach to solving integral are described by other algebraic expresequations such as those governing the sions that equate the properties of electribehavior of electrical fields in volume cal conduction between each element. conductors. This results in a linear system of many In summary, the inverse solution con- equations with many unknowns (one for siders how a signal detected at a remote each element), which can be solved by point will have appeared at source and standard techniques. The BEM works simthe BEM is a method for applying the ilarly, but it only needs to model the 2inverse solution to resolve a matrix of such dimensional surfaces ("boundary") instead signals from a source at a known bound- of the entire 3-dimensional domain. This ary (the blood-endocardial boundary). results in far fewer elements and a much The BEM is related to the more smaller system of equations, small enough widely known FEM. The FEM chops up to be feasibly solved in real time (1200 Hz) the region or space where complex elec- for each instant of voltage potential meatrical phenomena are to be computed into sured by the MEA electrodes.

68 CARDIAC MAPPING Inverse problems are mathematically "ill posed," meaning that small errors (noise) in the measured cavity potentials will be inherently magnified into large errors in the reconstructed endocardial potentials. In order to overcome this problem, stability is provided by applying a mathematical constraint (with a physiological basis) on the solution, using a technique called regularization.36,37 Noise emerges in the recorded data from several sources including electrical interference coupled to the body from the surrounding environment, electrochemical fluctuations at the surface of electrodes, and electrical fluctuations in the amplifier circuitry. Environmental noise comes primarily from power lines and manifests as a common-mode noise across the body. Common-mode noise is largely rejected by connecting the unipolar reference to the biopotential amplifiers in the standard differential configuration. Noise due to electrochemical fluctuations at the surface of electrodes is primarily low frequency in nature and is reduced by coupling the electrodes to the amplifiers through a high-pass filter that attenuates frequencies below 0.1 Hz. The small noise fluctuations in amplifier circuitry are generally broad-band in frequency and are not correlated between channels. Regularization is required to attenuate these random fluctuations plus any residual noise that remains after reduction of the other types of noise described above. Note that the random fluctuations among amplifier channels translate back to an error in the measurement at the recording sites which is also randomly associated between the sites. These errors can be thought of as small and rapid "steps" of potential across the surface of the MEA. Accordingly, regularization attenuates these rapid fluctuations across space at any given instant in time, much as a standard time-domain low-pass filter attenuates

rapid fluctuations across time at any given point in space. In general, accuracy of the system depends on the regularization technique applied, the underlying solution method for Laplace's equation, and the accuracy of the geometry information. In order to improve this accuracy, several modifications have been applied to previously described techniques for inverse solution of endocardial electrograms.34 The regularization scheme used in the noncontact mapping system includes a proprietary enhancement of methods described by Tikhonov and Arsenin.36 The geometry and voltage are modeled using bicubic spline surfaces instead of the standard linear elements. This allows for a physiologically more realistic model of the endocardium in addition to a more accurate application of the boundary element solution method, because it fits the sampled endocardial points with curves rather than lines. Small errors in modeling the geometry may still result from undersampling of endocardial points during geometry acquisition or because of the complexity of the chamber geometry itself. The total system accuracy, including these errors, will be reflected in the results described later. Using these techniques, the system is able to reconstruct and interpolate thousands of unipolar electrograms simultaneously over the entire virtual endocardium (Figure 4). These electrograms may be superimposed on the virtual endocardium to produce isopotential or isochronal maps with a color range representing voltage or timing of onset. In addition, the electrograms may be interactively selected with the mouse pointer as individual points, points along a line, or points within a rectangle from anywhere on the virtual endocardium, and displayed as waveforms. By allowing digital subtraction of adjacent unipolar reconstructed electrograms, bipolar electrograms may

NONCONTACT ENDOCARDIAL MAPPING

69

also be displayed as waveforms by the muscle and endocardium, then theoretisystem. cally the influence of endocardial and Because a BEM is used for the inverse papillary electrical activity on the consolution, the 3-dimensional myocardium, tact electrogram at this point may be with its finite muscle thickness, is treated significantly different from a reconas a 2-dimensional endocardial surface. The structed electrogram which assumes a reconstructed electrograms are analogous 2-dimensional surface. A further theoto those measured by a standard recording retical example may be if the contact catheter in contact with this surface. catheter penetrates through the endoThus, the reconstructed electrograms are cardium into myocardium, which again subject to the same electrical principles as will be indicated to the operator by the contact catheter electrograms insofar as locator signal cross-hair showing the both contain far-field electrical informa- catheter beyond the boundary of the virtion from the surrounding endocardium tual endocardium. as well as the underlying myocardium. The contribution of each such component Mapping Protocols to the electrogram is weighted by the signal vector, amplitude, and distance from the site of measurement. Therefore, Preoperative Investigations anatomical details such as papillary muscles or trabeculae will contribute to The first human clinical studies of both a contact and reconstructed elec- the noncontact system were carried out at trogram recorded at an adjacent site, but St. Mary's Hospital, London, and the meththis contribution is small when compared ods used for these studies are described to that from the larger area of endocar- briefly here. All patients had a full physdial tissue surrounding these structures. ical examination before and after the proThere are several situations in which cedures, with particular attention paid contact and reconstructed elements may to the cardiac and neurological examinatheoretically vary, the first being if a con- tion in order to identify any complications tact catheter rests on a papillary muscle resulting from the procedure. Before and that has extended a significant distance after the studies all patients had a full into the chamber away from the endo- blood count, urea and electrolytes, carcardium. Because the geometry is based diac enzyme assay, and clotting screen. on extreme points collected by moving a Echocardiography was used to determine contact catheter, the papillary muscle LV dimensions and function and to elimanatomy may be ignored in favor of the inate pericardial effusion before and after more distant outlying endocardium. This the study. Chest x-rays were performed may result in some rejection of the pap- before and after the procedure to deterillary muscle electrogram during recon- mine the heart size and identify any pulstruction by the regularization process monary congestion. Oral anticoagulants because of the significant deviation from were stopped, allowing the international established geometry. The locator signal normalized ratio to reach normal values will also alert the operator to the fact that prior to the mapping and ablation procethe catheter is not on the virtual endo- dures, during which time the patients cardium with a cross-hair showing dis- were anticoagulated with heparin. Antitance of the catheter from the MEA and coagulants were recommenced if the risks virtual endocardium. Second, if a contact of thrombosis or embolization were not catheter is wedged between papillary eliminated by the ablation procedure.

70 CARDIAC MAPPING Antiarrhythmics were discontinued after the procedure in patients undergoing successful ablation of atrial flutter (AFL). Antiarrhythmics were also discontinued in those patients with no structural heart disease who had successful ablation of VT. Otherwise, antiarrhythmic therapy was continued as before the mapping or ablation procedure. Mapping Procedure For all studies a conventional mapping system was used to record data in parallel to the noncontact system in order to assist with validation. Before deployment of the MEA, patients were given 10,000 international units heparin and the activated clotting time (ACT) was measured. In order to prevent thrombus formation on the MEA, the MEA was only introduced once the ACT was above 300 seconds. After introduction of the MEA, the ACT was monitored every 30 minutes and further boluses of heparin were given to maintain the ACT between 300 and 400 seconds. At the end of the first 4 studies (all of which were LV), the femoral sheaths were left in situ until the heparin had been eliminated and the sheaths were then removed. Two patients suffered femoral vascular complications so that for subsequent patients protamine was used to reverse the heparin and the sheaths were removed immediately when the ACT was below 120 seconds. LV Mapping Procedures Transesophageal echocardiography was performed prior to LV mapping procedures in the last 20 patients in order to identify any thrombus in the left atrium and LV or significant atheroma in the ascending aorta as potential hazards to catheterization via the transaortic or trans-septal route.

Local anesthetic was used to introduce sheaths in both femoral veins and arteries. Two 7F sheaths in the left femoral vein were used to introduce a standard quadripolar catheter to the right ventricular apex, and a 7F multipurpose catheter was used to monitor pulmonary artery pressure. The right femoral vein was used to introduce a trans-septal sheath, which allowed introduction of a deflectable mapping/ablation 4-mm-tip catheter to the LV. An 8F sheath in the right femoral artery was used to pass an additional 7F mapping/ ablation 4-mm-tip catheter to the LV using the retrograde transaortic route. The same arterial sheath was used to monitor systemic arterial pressure around the mapping catheter. A 9F sheath in the left femoral artery was used to introduce the MEA catheter. The MEA catheter was deployed in the LV of all patients using the retrograde transaortic route. To obtain a stable position and optimal recordings from within the LV, it was necessary to site the pigtail of the MEA at the LV apex and this was done over an exchange length 0.032" J-tipped guidewire previously positioned in the LV apex using a 6F pigtail catheter. With the pigtail of the MEA in the LV apex, the guidewire was withdrawn and the balloon inflated with a contrast-saline mixture (Figure 6). RA Mapping Procedures Sheaths in the left femoral vein were used to introduce a 7F quadripolar catheter (placed in the right ventricular apex) and the MEA catheter. A 7F deflectable mapping/ ablation 4-mm-tip catheter was placed in the RA via a right femoral venous sheath. Systemic arterial blood pressure was continuously monitored via a 6F sheath in the right femoral artery. A further 7F sheath in the right subclavian vein was used to introduce an electrode catheter into the coronary sinus (CS).

NONCONTACT ENDOCARDIAL MAPPING

71

Figure 6. Posteroanterior radiograph showing the noncontact mapping catheter with contrast medium/saline in the balloon, deployed in the left ventricle (LV) (A). Also seen in the LV is a transseptal mapping catheter (B) and a retrograde transaortic mapping catheter (C) in an equatorial position to the multielectrode array. Other catheters are positioned in the high right atrium, middle cardiac vein, and right ventricular outflow tract. This patient also has an implantable defibrillator lead (D).

The MEA catheter was deployed in the RA of all patients over a 0.035" Jtipped guidewire advanced to the SVC (Figure 7) or the pulmonary outflow tract having been positioned using a 7F multipurpose catheter (Figure 8). This technique was developed to cant the balloon toward an area of interest (the atrioventricular junction or IVC-tricuspid isthmus) in patients with a posteroseptal accessory pathway or AFL. This was done to improve the mapping resolution of these areas, because the accuracy of electrogram reconstruction may theoretically improve with decreasing distance between the MEA and to potential field source.

Initial Validation Contact unipolar recordings from a mapping catheter were made in 13 patients during sinus rhythm at up to 6 separate endocardial locations with as varied an anatomical distribution as possible. All locations were marked using the locator signal and these were used to reconstruct electrograms at the same site for comparison with contact electrograms for the following criteria: 1. Timing of activation, indicated by the point of maximum negative dV/dt.

72 CARDIAC MAPPING

Figure 7. Posteroanterior radiograph showing the multielectrode array deployed around the balloon filled with 7.5 mL of contrast (B) in the right atrium, with the pigtail tip positioned near the superior vena cava. Also seen in this image is a mapping catheter positioned on the tricuspid annulus-inferior vena cava isthmus (Map), a coronary sinus catheter (CS), and a halo catheter (H), which were used to map and ablate this patient's atrial flutter.

2. Electrogram morphology, with reference to the polarity and relative amplitude and frequency of electrogram components using a visual morphology score (1 to 5), was used: 5: exact match 4: features exact with phase shift 3: many features match with or without phase shift 2: few features match with or without phase shift 1: no match.

3. In addition to manual analysis, electrogram morphologies were compared using a template comparison algorithm to calculate the crosscorrelation using the formula:

where C(k) is the correlation coefficient and Xj and Yj are a corresponding series of amplitudes,

NONCONTACT ENDOCARDIAL MAPPING

73

Figure 8. Posteroanterior radiograph showing the multielectrode array (B) deployed in the right atrium and canted over toward the tricuspid annulus by a guidewire passed through the central lumen and out through the right ventricular outflow tract, so that the pigtail of the multielectrode array is straightened and sited on the ventricular side of the tricuspid annulus. Also seen in this image are catheters sited at the high right atrium (HRA), right ventricular apex (RVA), His bundle (His), and coronary sinus (CS). A mapping/ablation catheter has been positioned on the tricuspid annulusinferior vena cava isthmus (arrow).

sampled at 1200 Hz, from the contact and reconstructed electrogram waveforms, respectively. X and Y are the mean amplitudes of these series.38,39 4. The cross-correlation computer algorithm was used to produce a measure of difference in timing between reconstructed and contact catheter electrograms that was termed correlation timing.

Results: Data from a total of 76 LV points were collected. The distance of equatorial points from the center of the MEA was 32.12 ± 12.12 mm (mean ± SD). Timing: Reconstructed electrograms differed from the maximum negative dV/dt for the contact electrogram at the same site by -6.44 ± 14.17 ms (95% CI -3.26 to -9.63) (Figure 9). Perfect matches in timing were obtained as far as 52 mm

74 CARDIAC MAPPING

Figure 9. Graph of the difference in timing between maximum negative dV/dt of unipolar reconstructed and mapping catheter tip contact electrograms (ms) versus the distance from the center of the multielectrode array (mm). A negative timing value indicates that the maximum negative dV/dt of the reconstructed electrogram precedes the contact electrogram.

from the MEA center, but differences in timing of maximum negative dV/dt for reconstructed electrograms with respect to corresponding contact electrograms increased gradually with distance but significantly at data points greater than 34 mm from the center of the MEA (Figure 9). For this reason, data were handled in 2 groups of points within and beyond 34 mm from the center of the MEA. Timing differences for electrograms recorded at points less than 34 mm were -1.94 ± 7.12 ms (95% CI -3.96 to 0.07), and at points greater than 34 mm was -14.16 ± 19.29 ms (95% CI -21.08 to -7.01). The difference at distances greater than 34 mm from the balloon appeared to be predominantly negative indicating that the system appears to reconstruct the electrograms earlier than the contact counterparts. Computer timing analysis did not demonstrate this predominantly negative

shift of reconstructed electrograms with respect to contact electrograms. Mean timing shift was -1.93 ± 11.71 ms. Timing differences for reconstructed electrograms compared with contact electrograms at endocardial points less than 34 mm was -1.30 ± 6.34 ms (95% CI -3.20 to 0.59), and for points greater than 34 mm was -3.02 ± 21.03 (95% CI -10.95 to 4.91) suggesting that timing differences do increase at endocardial points greater than 34 mm from the balloon, but the computer algorithm to determine timing difference demonstrates no trend for reconstructed electrograms to be earlier or later than the contact electrogram. Morphology: The mean subjective morphology score for equatorial points was 3.45 ± 0.99 (Figure 10). A general linear model analysis of variance of morphology scores demonstrated a significant (P< 0.001)

NONCONTACT ENDOCARDIAL MAPPING

75

Figure 10. Histogram of mean and standard deviation of subjective morphology scores comparing reconstructed and mapping catheter tip electrograms at equatorial points plotted versus 10-mm ranges of endocardial distance. *P< 0.05, **P< 0.005.

decrease in accuracy of morphology of reconstructed electrograms when compared against contact electrograms with increasing endocardial distance from the MEA (Figure 10). There was no apparent threshold distance beyond which the morphology of reconstructed electrograms worsened. Cross-correlation of reconstructed and contact electrograms was 0.83 ± 0.16 (mean ± SD). Correlation worsened at greater endocardial distances and correlation for points greater than 34 mm from the balloon was 0.76 + 0.18 (95% CI 0.69 to 0.83) compared with correlation at distances less than 34 mm from the balloon 0.87 ± 0.12 (95% CI 0.84 to 0.91) (P < 0.01), but a threshold distance at which this difference occurred was not clear. The noncontact mapping system described here was first validated using

tank tests with accurate catheter location and good correlation between reconstructed and contact potentials using a lower order inverse solution than that used clinically.31 It has also been demonstrated that, using the presently employed inverse solution,35 reconstruction of electrograms and catheter location can be performed with accuracy decreasing only at distances greater than 40 mm from the MEA center. The validation demonstrated that this system is capable of reconstructing electrograms at distances of up to at least 50 mm from the center of the MEA but that accuracy decreases with distance and significantly when the endocardium is greater than 34 mm from the MEA center. There is no significant difference in timing of maximum negative dV/dt between contact catheter and reconstructed

76

CARDIAC MAPPING

electrogram less than 34 mm from the MEA center, but the timing of electrogram maximum -dV/dt becomes earlier than contact electrograms greater than 34 mm from the MEA center. The trend for the reconstructed electrogram to be displayed earlier than the contact electrogram at distances greater than 34 mm from the MEA center is probably a result of a number of combined errors. These may be errors introduced by distance of the endocardium from the MEA, the signal-to-noise ratio, the complexity of anatomy and motion of different regions of the LV, and the presence of scar. These possible sources of error may be further compounded by the need for regularization in which subtle features of the endocardial potentials may be rejected because of rapidly changing geometry or amplitude at various cardiac regions. This means that the complex relationship between errors in reconstruction and MEA-endocardial distance can be demonstrated but not easily defined. If it is eventually possible to define this complex reproducible error in the reconstruction algorithm, it may be possible to modify the mathematical solution to adjust for this. The computerized correlation timing data also demonstrated the phenomenon of differences in electrogram timing increasing with increasing endocardium to MEA distances, but there was not a consistent shift in timing of reconstructed electrograms earlier than contact electrograms, and differences in timing appeared to be less than those measured by maximum -dV/dt. It is possible that a more global measurement of electrogram timing as provided by computerized correlation timing is a better measure of electro-gram timing than maximum -dV/dt, but it is also possible that the computerized correlation timing technique underestimates errors in timing of reconstructed electrograms and requires further validation.

Initial Experience of Noncontact Mapping of Human VT Validation of electrogram reconstruction during VT

A comparison between reconstructed and contact unipolar electrograms recorded at the same site, as identified by the locator system, was made for all patients in all recordings made during VT. Analysis was performed offline by using the previously described computer cross-correlation technique. Isopotential mapping: Voltages are displayed on the isopotential map as colors on the virtual endocardium (Figure 4). The color scale is adjusted so that it covers a narrow range of voltages thus producing a binary display of colors with voltages above a threshold voltage shown as purple and below this threshold shown as white. Adjusting the offset of the display to be just below 0 mV creates a unipolar activation map, with activation shown as white on the virtual endocardium. The sensitivity of the activation map is altered using the color offset adjustment. Identification of diastolic depolarization during VT

Our initial experience indicated that depolarization of the diastolic reentrant pathway could be confused with ventricular repolarization. In order to avoid this problem, we strictly defined diastolic depolarization as activity on an isopotential map that could be clearly and continuously tracked back in time from the exit site of tachycardia (see results). Thus, relevant diastolic depolarization was that which resulted in activation of the ventricle, avoiding confusion with repolarization and bystander activity. Exit sites

NONCONTACT ENDOCARDIAL MAPPING

were seen on the isopotential map synchronous with the onset of the surface ECG QRS and were defined as rapidly spreading white spots from which continuous activation of the entire ventricle occurred. The exit sites were confirmed by selecting reconstructed electrograms from these sites and comparing their timing with that of adjacent reconstructed electrograms and QRS onset on the surface ECG (Figure 4). The sensitivity of the isopotential map was then increased and an attempt was made to progressively trace activity as a discrete "spot" back through diastole until continuity was lost. Diastolic activity seen prior to this earliest point was ignored. Areas of relevant diastolic depolarization and exit sites were then marked on the virtual endocardium and the catheter location system used to guide the mapping catheter to them. If the suitability of these sites for RF ablation of VT was then confirmed by conventional mapping criteria,40–42 the catheter position was recorded using the locator system and RF energy was delivered. Validation of reentry circuits identified Analysis was performed offline that allowed identification of the ablation site within the isopotential map of the reentry circuit of the ablated VT. For the purposes of this analysis, successful ablation was defined as delivery of RF energy resulting in termination of VT with partial success being a change in morphology of VT, after which, in both cases, the original VT morphology was noninducible.

Results

77

was followed by a period of cardiogenic shock. The other patient developed rightsided facial weakness after introduction of the trans-septal mapping catheter and the procedure was abandoned. Validation of reconstructed electrograms during VT: A total of 7593 (range 40–1731 per patient) unipolar electrogram complexes recorded during VT were analyzed using a cross-correlation algorithm. The cross-correlation between contact and reconstructed unipolar electrograms was 0.86 ± 0.16 (mean ± SD) (Figure 11), and the timing shift required to produce the closest electrogram match between these reconstructed and contact unipolar electrograms was -1.67 ± 10.46 ms. VT morphologies induced: 81 of 97 morphologies of VT were mapped using the noncontact system (3.4 per patient) (Table 2). Sixteen morphologies terminated prior to introduction of the MEA. Twenty-four of the mapped tachycardias were clinical morphologies of VT. Fiftyseven morphologies were either previously undocumented or only induced at previous electrophysiological studies. Identification of VT exit: Noncontact mapping identified exit in 80 (99%) of 81 VT morphologies. In 1 patient who had an LV ectopic tachycardia, an exit was identified approximately 1 cm (as assessed by multiple radiographic views) from the final successful ablation site, both being located in the outflow tract of the LV.

Identification of diastolic activity: Despite demonstration of an exit site, diastolic Data were collected from 24 patients activity could not be identified in 27 mor(Table 1). Ablation procedures were com- phologies of tachycardia, including the pleted in 22 patients. Of the 2 patients in fascicular tachycardia and ectopic VT. whom ablation was not performed, ven- Activity in some or all of the diastolic tricular fibrillation was induced in 1 and interval was thus identified in 54 of 81 VT

Table 1 Patient Demographics Patient

1 2 3 4 5 6 7 8 9 10 11 12 13 14* 15 16 17 18 19 20 21 22 23 24*

Mean SD

Age (yrs)

Ml

Follow-up (days)

CABG

Defib

Defib Post

Shock Pre

Shock Post

Aneurysm

EF (%)

LVEDD (cm)

LVESD (cm)

60 65 39 32 60 69 65 61 60 66 64 55 59 64 75 56 65 55 54 48 76 65 55 19

INF ANT — — ANT M M M M ANT ANT ANT M M M M M M — — M LAT M —

746 745 744 743 498 497 496 428 405 400 372 370 365 326 319 258 235 190 176 144 131 130 47 46

N Y N N N N Y Y N N N N Y N Y N Y Y N N Y N N N

Y Y N Y N N N Y N N N Y Y N N N N N Y N N N Y N

n/a n/a N n/a N Y N n/a Y Y N n/a n/a Y N Y Y N n/a Y N Y n/a N

6 unavailable n/a 11 (15 Pace) n/a n/a n/a unavailable n/a n/a n/a 11 2 n/a n/a n/a n/a n/a 3 n/a n/a n/a 23 n/a

none n/a n/a 1 (5 Pace) n/a n/a none n/a n/a none none none none n/a n/a n/a n/a n/a n/a n/a n/a n/a none n/a

INF ANT/AP — — — — LAT ANT/AP — ANT/AP — — — — — — — — — — — — ANT/AP —

33 38 70 40 30 35 45 35 40 35 40 30 29 30 35 38 32 30 35 60 30 40 35 70

6.32

5.14

6.2

4.7

4.47 5.61 7.07 6.49 5.36 6.99 4.73 6.61 6.46

2.98

349

38.91

218.2

1.5

57.79 12.84

LVL

(cm)

6.9 4.8

5.2 6.3 2.7

6.19 6.57 6.83 5.22 7.05 6.89

5.81 5.65 5.96 4.49 6.61 6.06

5.7 6 6.1 6.3 4.1

4.3 5.4 5.7 5.4

3.58

8.56 6.54 7.05 6.89 8.41 9.07 7.48 7.57 6.96 8.08 8.14 9 7 9.87 8.39 8.67 9.63 8.76 8.62 7.8 8 7.8 9.2 7.19

6.04 0.86

5.08 1.06

8.11 0.91

4.1

5.36 5.79 4.78 5.93 3.69 6.19

ANT - anterior; AP = anteroposterior; CABG = previous coronary artery bypass grafting; EF = ejection fraction; LAT = lateral; LVEDD = left ventricular end-diastolic dimension; LVESD = left ventricular end-systolic dimension; LVL = left ventricular length; INF = inferior; M = mid; Ml = myocardial infarction; SD = standard deviation.

NONCONTACT ENDOCARDIAL MAPPING

79

Figure 11. Histogram showing the cross-correlation between contact and reconstructed electrograms recorded during ventricular tachycardia. The majority of compared electrograms had a perfect correlation (1) and almost all comparisons gave a correlation greater than 0.8, which was considered to be an acceptable value. LV = left ventricular.

morphologies (67%). In 17 of these, the noncontact mapping system demonstrated a complete reentrant circuit (Figure 12). Of these 17 VTs, 7 were terminated by application of RF energy, and the position of the successful ablation site was located within the diastolic pathway identified by the noncontact system in all cases. The remaining 10 VT morphologies were not ablated because they were hemodynamically unstable tachycardias (n = 2) or the tachycardia was induced only once during the procedure (n = 8) and the circuits were subsequently identified during analysis offline. In the remaining 37 circuits, diastolic activity was traced over 36 ± 30% (mean ± SD) of the diastolic interval (range 1-95%). Ablation results: 154 RF applications (range 0-34) were used to ablate a total of 38 tachycardias (4 applications per VT ablated). Fifteen clinical VT morphologies and 23 nonclinical VT morphologies were ablated. Four VT were ablated by 2

RF applications on common shared diastolic pathways (Figure 12). Validation of VT circuits identified

The relationship between ablation catheter and the diastolic pathway location to the outcome of RF application is shown in Table 3. A total of 76 RF energy applications were delivered during VT of which 22 were successful, 9 were partially successful, and 45 were unsuccessful. Application of RF to a diastolic pathway identified by the noncontact mapping system resulted in successful ablation in 77% of cases. The success rate of RF delivery at an exit site was significantly lower (P < 0.005) and resulted in failure to eliminate VT in 79% of cases because of a change in morphology or no change to the VT. Delivery of RF at locations remote from the VT diastolic pathway also resulted in a significantly lower incidence of successful ablation (P< 0.0001) resulting in failure of ablation in 91% of RF applications.

Table 2 Procedure and Follow-up Results Ensite Time (min)

VT #

Cycle Length Mean (Range) ms

Clinical VTs

Ablated Clin/Nonclin

426 (250-520) 426 (348-538)

1 1 1 1 1 0 0 1 1 1

1/1 1 /2

345 363 436 370 145 275

4 3 1 2 2 6 6 2 1 3

250 365 73 188 234

3 1 1 1 6

360(316-424) 382 (322-522)

65.4 44.1

231 360 194 20

5 8 2 1

323(170-405) 415(330-446) 328 (308-347)

300 260 140 430 290

55.1 47.9 19.2 44.8 57.4

150 150 90 245 145

5 1 1 6 1

381 (254-352)

400.83 105.47

77.01 65.16

231.45 114.56

81 VTs

399.35 89.76

Screening Time (min)

PT

Procedure Time (min)

1 2 3 4 5 6 7 8 9 10

520 420 435 440 570 495 585 460 315 395

11 12 13 14 15

435 515 390 310 365

363

16 17 18 19

425 460 395 270

102.7

20 21 22 23 24

Mean SD

43 46

48.6 89.6 86.3 105

74.6 72.1 66.3 89.8 61.9 41.2 43.1 73.1 108

PT = patient; VT = ventricular tachycardia.

280

292 (264-320) 365(310-420) 467 (374-600) 426 (338-470) 449 (278-620) 374

512(474-560)

450 370

483 (370-600)

410

458 332

389 (280-528) 448

1/0

1/1 1/1

0/2 0/4 1/0 1/0 1 /2

0 4 0 1 2

0/2

1 1 2 0

?/0 1/1 2/0 0/0

1 1 1 2 1

0/1 ?/0 ?/0 ?/0 1/0

25 VTs

38 VTs 15/23

1

/2

0/0 ?/0 2/4

Recurrent 1 Self terminating None None New fast VT NewVT None None Died at 3 days None New fast VT detected by ICD None None Not ablated Early recurrence Early recurrence of slow nonsustained VT None None None Not ablated, Died at 4 wks None Died at 12 hours None None None

NONCONTACT ENDOCARDIAL MAPPING

81

Figure 12. A series of activation maps recorded during ventricular tachycardia (VT) in a patient with 2 VT morphologies using the same reentry circuit in contrarotation. The virtual endocardium has beenopenedalongtheanteriorseptumsothatthe2edgesare in continuity. Labels have= been placed on sites identified using fluoroscopy as follows: Basal = left ventricular (LV) base; Apex LV apexlat = lateral. Activation is indiCated by White areas. The successful

radiof requency site is shown with

a green dot. A. VT1: frame 1 showS activation in the diastolic component of the reentry circuit just prior to the exit and systolic activation of the LV. Frames 2 through 4 show systolic activation of the

LV; note the sparing of the region of the diastolic pathway. Frames 5 through 8 show activation during diastole Progressing from the apicoseptal end of the diastolic component of the reentry circuit to the exit at the basolateral end. B. VT4: frame 1 shows activation in the diastolic component of the reentry circuit just prior to reaching the exit site and systolic activation; note that the exit site is at the apicoseptal end of the Same diastolic pathway as in A. Frames 2 through 4 show systolic activaion of the LV; note the sparing of the region of the diastolic pathway. Frames 5 through 7 show activation in diastole progressing from base and lateral

(the exit site for the VT morphology shown in A) toward the septum and apex before returning to the exit. See color appendix

82 CARDIAC MAPPING Table 3 Results of Radiofrequency Application During Ventricular Tachycardia On Exit*

Near Diastolic Pathway

Remote from Diastolic Pathway**

Total

5 (21%) 7 (29%) 12 (50%)

4 (80%) 0 (0%) 1 (20%)

3

22

(77%) 2 (15%) 1 (8%)

(9%) 0 (0%) 31 (91%)

13 (100%)

24 (100%)

5 (100%)

34 (100%)

On Diastolic Pathway Success Partial Failure Total

10

9 45 76

Chi-square analysis for all data = P< 0.0001. With respect to the "On diastolic pathway" column: *P< 0.005 and **P< 0.0001.

documented but not targeted for ablation. Two patients had VTs not previously documented, more than 3 months after the procedure. Five patients in the series died of causes unrelated to the use of the nonProcedural Complications contact mapping system. Further details 43 No patient suffered a cardiac com- are given elsewhere. Of 8 patients with defibrillators preplication as a result of deployment of the noncontact mapping system. Procedural sent before the procedure, 6 had therapy complications were arteriovenous fistu- histories available from the device both lae (n = 2), cerebrovascular event (n = 1) before and after the procedure. Over a prior to deployment of the MEA, cardio- follow-up period of 1.19 ± 0.7 years, the genic shock following ventricular fibril- mean shock frequency was reduced from lation (n = 1) and hemothorax (n = 1) after 9.2 (range 2-23) to 0.16 per year (P < 0.05) a difficult trans-septal puncture that was (1 shock for a fast, previously documented but untargeted VT). treated by percutaneous drainage. RF applications made at sites close to an incompletely identified circuit succeeded in ablating VT in 80% of cases.

Patient Follow-up Of 22 patients who underwent ablation, 14 (64%) had no recurrence of VT over a mean follow-up of 1.5 years (range 0.6-2.5 years). Of 38 target VTs, there was sustained recurrence of only 2 (5.3%) (both within 1 week), which were subsequently ablated at repeat procedures without the noncontact system. Two patients had slower nonsustained VT of the same morphology as that ablated, occurring within 1 week in 1 patient and 1 year after the procedure in the other. One patient had recurrence of a fast VT, 6 months after the procedure, which had been previously

Noncontact Mapping of Atrial Arrhythmias Atrial Flutter

A mapping catheter was positioned in the RA using fluoroscopic and electrogram data and the locator signal used to mark anatomical locations on the virtual endocardium. RF applications were then made in sequence from the ventricular aspect of the TA-IVC isthmus to the IVC in order to complete a line of conduction block between the TA and IVC. The location of each RF application was

NONCONTACT ENDOCARDIAL MAPPING

marked on the virtual endocardium using the catheter locator system. If a line of lesions from TA to IVC failed to either terminate AFL or produce conduction block across the isthmus during pacing at the CS os, then the noncontact system was used to identify the point (or points) at which the activation front passed through the isthmus. The mapping/ablation catheter was then moved to these locations and further RF delivered. If AFL terminated prior to completion of this line or was not initiated during the procedure, then ablation was performed during CS pacing, guided using the noncontact system to create bidirectional block. This was then confirmed using a halo catheter at the end of the procedure.3,44 Distance and Conduction Velocity Measurements The coordinates of points on the virtual endocardium may be displayed as a distance from the MEA and used to calculate the straight line distance between 2 points across the endocardium using the formula:

D = ^(Xl -x 2 ) 2 +( yi -y 2 )

-z 2 ) 2 ,

where x1? y1? z1? and x2, y2, z2 are the coordinates of the 2 points (in mm) and D is the straight line distance between the points. This formula was used to calculate a series of short straight-line distances describing the passage of the AFL wavefront and summed to calculate the isthmus length. Wavefront velocity through the isthmus was then calculated by dividing this distance by the time taken for the wavefront to pass through the isthmus. Definitions Anatomical terminology: For the purposes of anatomical orientation, a format of anatomical nomenclature was used

83

that reflected the true orientation and positioning of the human heart in the thorax. With this format, the SVC and right atrial appendage are superior and the IVC and the TA-IVC isthmus are inferior to the TA. TA-IVC isthmus: was defined by locator signal labeling of anatomical landmarks identified using fluoroscopy and electrogram characteristics and was considered to extend from the CS os posteriorly, the TA superiorly, and the IVC inferiorly. The anterior border of the isthmus was considered a line joining the anteroinferior border of the TA with the anterior border of the IVC. It was further defined by the demonstration of a distinct narrowing and expansion of the AFL wavefront on isopotential maps as the wavefront respectively entered and exited the isthmus. Results Patients Eight patients, all males, mean age 49 years (range 33-68 years), were studied. All patients had begun antiarrhythmic therapy prior to the procedure (either flecainide [n = 2], verapamil [n = 1], amiodarone [n = 5], sotalol [n = 2], or digoxin [n = 1]). Three patients were taking multiple antiarrhythmic drugs. After mapping, 7 patients underwent RF ablation of AFL by creation of a line of block along the TA-IVC isthmus. One patient with AF as the predominant arrhythmia did not have an ablation procedure. Six patients were in AFL at the start of the procedure and no attempt was made to initiate AFL in 1 patient who was in sinus rhythm at the onset of the study and in whom RF application was performed during CS pacing. The rotation of the AFL circuit was counterclockwise in 5 patients and clockwise in 1.

84

CARDIAC MAPPING

Characteristics of Isthmus Activation The passage of the flutter wavefront was seen throughout the entire isthmus in 5 patients, with some signal attenuation seen in 2 patients who had had previous ablation procedures. Passage of the flutter wavefront through the isthmus took 78.3 ± 49.5 ms before RF application and 110 ± 57.9 ms (95% CI 63.3 to 156.0) after delivery of the penultimate RF application (P < 0.05). Isthmus activation occupied a mean 32.16 ± 16.0% of the flutter cycle length before delivery of RF and 39.8 ± 16% (95% CI 27% to 52.6%) after the penultimate RF application before flutter terminated (P< 0.05). The velocity of the wavefront in the isthmus was 0.93 ± 0.32 m/s before ablation and slowed to 0.67 ± 0.21 m/s (95% CI 0.5 to 0.84) after the penultimate RF application prior to AFL termination (P < 0.05). In 2 patients the AFL wavefront split into 2 separate wavefronts as it passed the CS os at the region of the eustachian ridge, one wavefront passing inferiorly and one superiorly to the CS os. They then fused into a single wavefront. In 3 patients a single activation wavefront slowed and turned superiorly toward the annulus in the region of the CS os and eustachian ridge, and in an additional 2 patients the wavefront slowed and emerged from the isthmus inferiorly. The pattern of isthmus activation was changed by delivery of RF energy in 5 of the 6 patients in whom RF was applied during AFL. Lateral Wall Activation Patterns During AFL A line of block was seen on the posterolateral RA wall on all isopotential maps recorded during AFL. The presence of this line of block was confirmed by the presence of double potentials in electrograms reconstructed along the

line (Figure 13). In 4 patients the line of block extended from the IVC to the SVC. In 1 patient the line of block extended superiorly from the IVC to the midlateral RA but did not reach the SVC. In another patient the line of block extended from the SVC to the low RA but slow activation was seen progressing over a 70-ms interval between the IVC and the crista terminalis which was further confirmed by long fractionated reconstructed electrograms in this region (Figure 14). Activation Around the TA The pattern of AFL activation wavefront in relation to the portions of the TA not adjacent to the isthmus (nonisthmus TA) was seen in all patients. AFL activation progressed toward the nonisthmus TA from the surrounding RA in 4 patients, suggesting that endocardium around the line of block in the lateral RA activated before the TA and was the critical region of conduction block around which the activation wavefront rotated (Figure 15). In 2 patients activation progressed away from the nonisthmus TA suggesting that, in these patients, one of whom had a massive RA (6 cm x 8 cm), the TA was the critical region of conduction block around which the activation wavefront rotated. AFL Ablation Radiofrequency energy was used to produce a line of block in the isthmus in 7 patients with 13.3 ± 19.5 RF application, and the continuity of the line of block was confirmed using a halo catheter in 6 of these patients; a halo catheter could not be positioned adequately in the patient with the massive RA. The location of a break in the line of block was successfully detected during AFL by the noncontact system during online analysis in 5 patients

Figure 13. The virtual endocardium has been opened and unfolded from superior to inferior along the anterior tricuspid annulus. Anatomical locations have been identified using the fluoroscopic position and electrograms recorded from catheter electrodes and marked on the virtual endocardium using the catheter location system. Locations marked in this manner are as follows: IVC = inferior vena cava; TA = tricuspid annulus; CS - coronary sinus os. Also marked with a blue line are the possible positions of the crista terminalis (CT) and eustachian ridge (ER). A. A series of activation maps recorded from the right atrium (RA) during atrial flutter (AFL) showing counterclockwise flutter. The positions at which reconstructed bipolar electrograms have been displayed in B are labeled a through f, from superior to inferior, along the blue line indicating a line of conduction block and the possible position of the CT. Activation is displayed on the virtual endocardium as a white and colored area. Activation is seen progressing from the anterolateral RA in near the IVC (frame 1) to the superior anterolateral RA (frames 2 and 3) along a line of block. Activation then turns at the superior vena cava and progresses down the opposite side of the line of block from superior to inferior (frames 3 through 5) before it reaches the isthmus. Activation progresses slowly through the isthmus (frames 6 to 1), before activation of the anterolateral RA occurs once more (frame 1). See color appendix. B. Reconstructed bipolar electrograms from positions a through f along a line of block shown in A. Two AFL cycles are shown, and the numbers 1 through 6 indicate the point at which the activation maps (A) have been displayed. The electrograms show double potentials with the isoelectric intervals between potentials increasing from a to f, reflecting the activation pattern shown on the activation maps. Note the long interval between the second potential on electrogram f and the first potential on the subsequent electrogram indicating the time for activation to pass through the isthmus and activate the anterolateral RA.

86

CARDIAC MAPPING

Figure 14. Virtual electrogram as described for Figure 13. A. A series of activation maps recorded during counterclockwise atrial flutter. The positions at which bipolar electrograms have been reconstructed and displayed in B are shown labeled as a through e, from superior to inferior, along the line of block representing the possible position of the CT. Activation passes through the isthmus (frame 1) and turns to toward the TA around a line of block (possibly the ER) (frame 2). The wavefront then begins to split (frame 3), with one front moving superiorly along the line of block while the other moves slowly along the ER. The first wavefront turns at the superior vena cava (frame 4) and then passes inferiorly along the line of block (frames 5 and 6). The second front passes slowly through a break in the line of block (frames 4 to 6). The first and second front then fuse (frame 7) and pass around the IVC to enter (frame 8) and pass through the isthmus (frame 1). See color appendix. B. Waveform window from the noncontact mapping system showing the surface EGG lead II, an electrogram from a catheter at the CS os, and reconstructed bipolar electrograms from points a through e on the activation maps in A. The numbers 1 through 8 represent the points at which the activation maps have been displayed in A. The electrograms from the line of block show double potentials apart for those recorded at the break in the line of block that are long and fractionated representing the slow activation at this point.

Figure 15. A series of activation maps superimposed onto the virtual endocardium with the lateral wall of the right atrium (RA) cut away so that the view is looking toward the tricuspid annulus (TA). Anatomical locations are marked in green as follows: isthmus, ant annulus = anterior TA; ant sup ann = anterior superior annulus; his = His bundle; post sup TA = posterior superior TA; post mid ann = posterior middle TA; mid septal = middle septal TA; post annulus = posterior TA; cs os = coronary sinus os. Also marked as black circles and labeled in yellow are the boundaries of the tricuspid valve (TV) and inferior vena cava (IVC). The possible position of the eustachian ridge is drawn onto the virtual endocardium as a yellow line. Activation is seen as white and colored regions. The atrial flutter wavefront enters and passes through the isthmus (frame 1) and encounters a line of block at the posterior isthmus. The wavefront turns toward the TA (frame 2) and passes up over this line of block and passes back down the posterior aspect of the line of block (frame 3). Activation exits the isthmus and activates the posterior RA (frames 4 to 6), but activation is earliest in the posterolateral RA and progresses toward the TA (frames 6 and 7) and then activates the anterior RA before progressing toward the TA (frame 8). The wavefront then passes into the anterior isthmus (frame 8) and through the isthmus to repeat the circuit (frame 1). See color appendix.

88 CARDIAC MAPPING (Figure 16). The noncontact system was then used to guide a catheter to this break, where energy was then delivered. CS Pacing During Sinus Rhythm At the end of the ablation procedure, CS pacing confirmed a line of block in the isthmus in 5 cases. In 2 patients the size and shape of the RA prevented adequate electrogram recording using a halo catheter, and offline analysis of noncontact data identified the location of a break in the line of block. A line of block in the lateral RA in the same location as seen during AFL was identified during slow pacing at the CS os in 6 patients. In 1 patient CS pacing was performed at a distal CS position, resulting in an RA activation wavefront emerging from the superior RA septum in the region of Bachmann's bundle. The paced RA wavefront thus progressed simultaneously down the RA, anterior and posterior to the line of block identified during AFL.

The AFL wavefront has been seen to turn from the posteroseptal RA to the anterolateral RA at the level of the midlateral RA in 1 patient and in another patient the main wavefront was seen to split, with 1 daughter wavefront turning at the level of the IVC with the slowness of conduction of the wavefront through this region maintaining the reentry circuit. A line of block is also seen during slow CS pacing which suggests that the crista terminalis is a fixed line of block to transverse depolarization. Although this line of block is fixed (i.e., present during slow and fast activation rates) there are differences in the extent of this line of block between patients. Activation in the Region of the CS Os

The anatomy of the triangle of Koch and posterior isthmus is complex and is composed in part by the eustachian ridge extending into the tendon of Todaro, and the CS os. Slow and irregular activation seen in Koch's triangle may be due to fiber Role of the Crista Terminalis arrangement,49 and it has also been noted that circumferential fibers found around The presence of double potentials45,46 the rest of the TA may be absent in this during AFL has suggested the presence of area.50 Noncontact mapping demonstrated barriers to conduction,45,47,48 and intra- that the wavefront slows and turns in the cardiac echocardiography has confirmed posterior region of the triangle of Koch in that these electrical barriers are related the majority of patients, suggesting that it to the anatomical structures of the crista encountered a line of block compatible with terminalis and the eustachian ridge.45 the site of the eustachian ridge. Splitting of Noncontact mapping during AFL has con- the wavefronts in this region was also seen firmed the presence of a line of conduction and may have been a feature of the waveblock in the lateral RA that is compatible front passing around the CS os or it may with the location of the crista terminalis have been that part of the activation front with both double potentials on recon- passed under the eustachian ridge, becomstructed electrograms in this region and ing intramural and thus not visible to the demonstration of block on isopotential noncontact system. While only 1 patient maps. Uniquely, noncontact mapping has with clockwise AFL was included in this demonstrated that this line of conduction series, a similar activation pattern was block does not necessarily extend for the seen indicating that it is not specific to the entire distance between the SVC and IVC. direction of the wavefront.

NONCONTACT ENDOCARDIAL MAPPING

89

Figure 16. Activation maps recorded during atrial flutter (AFL) demonstrating identification of a break in a line of block in the tricuspid annulus-inferior vena cava (TA-IVC) isthmus and the positioning of a catheter on this break with the aid of the noncontact mapping system. The virtual endocardium has been opened along the lateral wall and tilted so that the view is looking from the lateral wall toward the TA with the image focused on the TA-IVC isthmus. The anatomical labels marked during the procedure are shown in green as follows: HRA = high right atrial catheter position; POST ANNULUS = posterior tricuspid annulus; ANT ANNULUS = anterior tricuspid annulus; HIS = his bundle position. The positions at which radiofrequency (RF) applications have been made are marked with green stars. The TA and IVC are marked as black circles and labeled in yellow. Also marked are the positions of a mapping/ablation catheter (RF catheter) and the break in the line of block. Activation is shown as white and colored regions on the purple virtual endocardium. A. The activation map is shown as the wavefront of counterclockwise flutter enters the isthmus and encounters an incomplete line of block produced by the previous RF applications. B. The activation map is now shown with the AFL wavefront seen emerging as a discrete point of activation emerging through the break in the line of block. The catheter location system has been used to position the ablation catheter at this break in the line of block, and delivery of RF at this point resulted in termination of AFL and completion of the line of block in the isthmus. This was confirmed using a halo catheter deployed around the right atrium. See color appendix.

90 CARDIAC MAPPING Conduction Velocity Within the Isthmus It is not possible to conclude from these data that the TA-IVC isthmus is a region of slow conduction because 4 of the patients studied had had previous attempts at AFL ablation and our data suggest that application of RF within the TA-IVC isthmus may result in a significant slowing of conduction velocity in the absence of a complete line of block or termination of the arrhythmia. Catheter Ablation of AFL Ablation of AFL by delivering RF energy to produce a complete line of conduction block between the TA and the IVC is well described.51 Noncontact mapping successfully localized breaks in the line of block in the isthmus during AFL and guided delivery of RF energy at these sites to terminate AFL. The confirmation of a complete line of block was possible in most patients, but in 2 patients further offline analysis identified breaks in the TA-IVC line of block that were associated with subsequent recurrence. The noncontact system proved particularly useful

in 1 patient with a massive RA in whom it was not possible to deploy a halo catheter to confirm the RA activation pattern during CS pacing. Thus, noncontact mapping in the human RA during AFL has confirmed observations made during previous studies. The AFL reentry circuit rotates around anatomical obstacles and is dependent on a fixed line of block created by the TA or, more often, the crista terminalis. The AFL wavefront conduction velocity within the TA-IVC isthmus has been measured for the first time in humans; while it has been confirmed that conduction may be slow in this region, a cause for this has not been identified. Atrial Fibrillation The RA was mapped during AF in 11 patients (8 male, mean age 55 years, range 34-76 years) (Table 4). Three patients had a history of chronic AF and others developed AF during the procedure. A 12-lead ECG was recorded during AF either immediately prior to the mapping procedure in the patients with chronic AF or during AF in the patients with paroxysmal AF. The AF was categorized as "coarse" or "fine"

Table 4 Patient Demographics Patient

1 2 3 4 5 6 7

8 9 10 11

Age 55 40 66 42 34 76 73 75 36 34 74

Underlying Arrhythmia

AF P Septal WPW A Flutter Paroxysmal AF

AF AF A Flutter A Flutter A Flutter AVNRT A Flutter

Drugs

RA lat/TA (cm)

RA AP (cm)

Digoxin, Warfarin Disopyramide Verapamil, Warfarin None Amiodarone, Warfarin Digoxin, Diltiazem, Warfarin Diltiazem, Metoprolol None Amiodarone None Amiodarone

5.5 4.3 4 4 5.6 5.4 5.8 6.2 9.9 7.4 8.4

4.3 3.5 3.2 3.2 4.7 3.7 3.6 2.8 5.1 4.3 5.3

AF = atrial fibrillation; A Flutter = atrial flutter; AVNRT = atrioventricular nodal reentrant tachycardia; P Septal WPW = posteroseptal Wolff-Parkinson-White; RA lat/TA = right atrium lateral/tricuspid annulus; RA AP = right atrial antero-posterior dimension.

NONCONTACT ENDOCARDIAL MAPPING

according to the appearance of the diastolic interval on the surface EGG.52 Entrainment of AF was attempted in 7 patients at multiple sites by bipolar pacing using the mapping/ablation catheter at cycle lengths shorter than the median RA cycle length and at twice the capture threshold determined by pacing at a cycle length 20 ms less than the median RA cycle length. If AF was chronic no attempt was made to terminate it, but persistent AF induced during the procedure was terminated using up to 2 mg/kg intravenous flecainide (to a maximum of 150 mg) infused over 10 minutes. The infusion was stopped as soon as the patient reverted to sinus rhythm. Endocardial activation was recorded throughout the infusion and noncontact mapping data were analyzed offline. Definitions Focal activation: or breakthrough of electrical activity was defined as a wavefront emerging from a point on the endocardium where the surrounding endocardium was electrically silent so that activation was not propagating from a detectable RA wavefront. Entrainment: was defined as activation of an area of endocardium that propagated from a pacing source for at least 2 pacing cycles so that the cycle length of activation of that area equaled the pacing cycle length. Entrainment was confirmed by examination of both the noncontact activation maps, which provided data of the origin of the wavefront and direction of propagation, and examination of individual reconstructed unipolar electrograms, which provided cycle length data. AF pattern: Activation of the RA during AF was divided into 3 categories

91

using a classification similar to that used by Konings et al.53 and was defined as follows: 1. type I AF: only 1 wavefront present in the RA for greater than 50% of the time; 2. type II AF: 2 independent wavefronts present in the RA for greater than 50% of the time; 3. type III AF: 3 or more wavefronts present in the RA for greater than 50% of the time. The definitions used differed from those of Konings et al. in that data were acquired from the entire RA and it was stipulated that a set number of wavefronts were present for greater than 50% of the time period analyzed rather than for the entire time. This was because with noncontact mapping larger areas of the RA were analyzed and for much longer periods than had been possible in the study by Konings et al., so that there was a variation in the number of wavefronts present in most of the cases analyzed. After AF had been established for at least 30 seconds and prior to any drug or pacing intervention, AF was then categorized as type I, II, or III AF according to the number of wavefronts present on activation maps for the majority of the time period analyzed. Results Initiation of AF Initiation of AF in 4 patients without chronic AF was caused by catheter manipulation, and in 4 AF occurred during attempted entrainment of AFL. This occurred before introduction of the MEA in 1 patient. Initiation of AF was recorded by the noncontact mapping system in 1 patient. Noncontact mapping and catheter location demonstrated that the mapping/

92

CARDIAC MAPPING Table 5 Results

AF on ECG

AF type

Focal Activation

1 2*

Coarse Fine

I III

3*

Fine

III

4* 5 6 7 8 9 10 11

Fine Coarse Fine Coarse Fine Coarse Coarse Fine

III II III I II I I III

No No (pace terminated) Sup and CS sept (blocked in isthmus) No Sup sept Sup mid and CS sept Sup and CS sept Sup septum Sup mid and CS sept Sup septum Sup mid and CS sept

PT

Termination by Flecainide

No To A flutter To A flutter Atrial ectopics No No No No No No No

"Indicates patients in whom AF was initiated by catheter manipulation. AF = atrial fibrillation; CS = coronary sinus; PT = patient; sept = septum; Sup = superior.

ablation catheter had provoked 2 atrial ectopic beats. The inferior aspect of the second resulting wavefront encountered a line of block in the IVC-TV isthmus, possibly established by the first wavefront, while the superior portion of the wavefront continued around the superior and anterior TA to form a single macroreentrant wavefront following a path similar to counterclockwise AFL. After 2 complete circuits, lines of conduction block developed causing the wavefront to divide into 3 in the septum that then continued to propagate in a random manner resulting in changing wavefront numbers and vectors as AF established. Mapping of Established AF Atrial fibrillation that had been established for at least 1 minute was recorded in all patients. The results of AF mapping and termination are shown in Table 5. By the criteria described above, 4 patients had predominantly type I AF, 2 patients had predominantly type II AF, and 5 patients had predominantly type III AF in the RA. The patients with type I AF had coarse atrial activity on the surface EGG. Of

patients with type II AF, 1 had coarse and 1 had fine atrial activity on the EGG. All of the patients with type III AF had fine atrial activity on the surface EGG. Periods of electrical silence lasting greater than 50 ms were observed in the RA of 8 patients (3 type I, 2 type II, 3 type III). In all patients the re-initiation of RA activation was from consistent locations on the RA septum probably representing breakthrough from the left atrium. In 3 patients RA reactivation was in the superior septum in the region of Bachmann's bundle; in 2 patients 2 areas were observed in the superior septum (Bachmann's bundle) and near the CS os. In 3 patients 3 sites were observed in the superior septum/ Bachmann's bundle, mid septum, and near the CS os. Example of type I AF

An example of11/2cycles of type I AF is shown in Figure 17. The reentry wavefront rotates around a line of block situated in the posterior septum and postero- lateral wall near the crista terminalis. This line of block did not vary during AF but slight variations in the path of the wave front were seen in other regions of the RA.

Figure 17. A. Unipolar activation maps, shown at 30-ms intervals, of type I atrial fibrillation showing a macroreentry wavefront rotating around a line of block indicated by the blue line. See color appendix. B. Unipolar reconstructed electrograms taken from points on the activation maps a through g showing sequential activation around the right atrium. The larger numbers at top indicate the points at which the corresponding sequential activation map frame has been displayed. Abbreviations as in previous figures.

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Example of type III AF

An example of type III AF is shown in Figure 18. After a period of electrical silence, activation emerges from an area on the septum near the CS. This rapidly splits into 3 activation wavefronts, the central one being of lowest amplitude and velocity possibly representing slowed conduction in the region of the fossa ovalis. This wavefront makes a 180° turn and passes around the inferior TA through the TA-IVC isthmus to collide with a wavefront passing around the anterior RA.

the left atrium or left atrial septum with the wavefront returning to the RA at Bachmann's bundle. Distal CS pacing did not alter activation patterns within the RA but did shorten the cycle lengths of electrograms recorded on the CS catheter. Termination of AF Termination of AF following flecainide infusion was observed and recorded in 3 patients, all with type III AF. Mean flecainide dose was 140 mg. Termination of AF example 1

Entrainment During AF Entrainment was demonstrated during AF in 2 patients. Entrainment was attempted in the 6 other patients, but either the stimulus amplitudes required to achieve local capture of the RA were large enough to saturate the amplifiers of the noncontact system and therefore prevent meaningful analysis of data (n = 4), or it was not possible to demonstrate local or remote entrainment (n = 2). Entrainment example

Pacing at the high RA (in the region of the crista terminalis), IVC-TV isthmus, and CS os resulted in entrainment of the entire RA during type I AF. Pacing in the lateral RA resulted in a wavefront that fused with the advancing spontaneous wavefront at successively earlier stages until the entire RA was entrained. Pacing from the CS os did not locally capture the RA but was followed by remote RA activation from the superior septum in the region of Bachmann's bundle that advanced the entire macroreentry circuit (Figure 19). This phenomenon stopped on cessation of CS pacing and the previously mapped reentry circuit resumed indicating a possibility of entrainment of

Flecainide infusion in a patient with type III AF resulted in progressively fewer wavefronts with longer periods of RA electrical silence which then evolved to a single reentry wavefront, which encountered a line of block in the superior posteroseptal wall. The RA was then reactivated several times from the septum presumably as breakthrough from the left atrium. Following one period of electrical silence, RA tachycardia emerged as a uniform wavefront from a focus on the lateral RA wall in the region of the crista terminalis. A second wavefront emerged from a similarly located focus that was then followed by a longer pause. This was followed by slow regular activity emerging from the same RA point as recorded during sinus rhythm indicating the resumption of sinus rhythm. Termination of AF example 2

Flecainide infusion resulted in progressively fewer wavefronts until a single wavefront remained rotating in a macroreentry circuit around the RA consistent with typical AFL. This terminated several times and random reentry54,55 was reestablished by local reactivation from the septum. After a further 2 rotations,

NONCONTACT ENDOCARDIAL MAPPING

95

Figure 18. A. A series of activation maps, shown at 20-ms intervals, during type III atrial fibrillation (AF). Activity emerges from the coronary sinus (CS) septum and splits into 3 separate lateral, central, and medial wavefronts (frames 1 and 2). The central of these wavefronts is slower and of lower amplitude and splits into 2 further wavefronts traveling superiorly and inferiorly (frame 3). The more inferior wavefront collides in the isthmus with the medial wavefront that has rotated around the tricuspid annulus (TA) (frames 3 and 4) while the more superior wavefront collides with a line of block established by earlier activation by the same medial wavefront (frame 4). The lateral and medial wavefronts fuse in the anterolateral right atrium (RA) (frames 2 and 3) and the resulting wavefront divides in the superior RA with one front rotating around the TA (frames 3 and 4) and the other rotating in the superior septum before fusing with the central superior wavefront (frames 4 and 5). Subsequent activation maps show the continuing complex patterns of activation seen in type III AF. See color appendix. B. Unipolar reconstructed electrograms from points a through e in A demonstrate the complexity of electrograms compared with type I AF (Figure 17B). The point at which each activation map is displayed is shown by the numbers at top.

96

CARDIAC MAPPING

Figure 19. A. Unipolar activation maps at 30-ms intervals during type I atrial fibrillation. Entrainment from a contact catheter sited in the high right atrium lateral wall (indicated by locator signal green line) pacing at a cycle length of 170 ms is shown. See text for detailed descriptions of activation maps. The approximate timing of pacing signals is shown with white arrows and the label "Pace." See color appendix. B. Unipolar reconstructed electrograms from positions a through g on the virtual endocardium in A. Times at which activation maps are shown are indicated by the corresponding frame numbers at the top of the figure.

NONCONTACT ENDOCARDIAL MAPPING

this wavefront then blocked in the region of the TA-IVC isthmus resulting in termination of tachycardia and resumption of sinus rhythm. Termination of AF example 3

In the remaining patient an infusion of flecainide resulted in organization and slowing of the wavefront so that a consistent AFL reentry circuit was formed. Overdrive pacing was required to terminate tachycardia.

97

majority of patients described in the study by Konings et al.53 had a predominantly type I pattern, whereas most of the patients included in this study had predominantly type III patterns. This may be explained by bias resulting from the small numbers included, or by the global nature of the mapping technique used which revealed the presence of other wavefronts that may not have been apparent to Konings' group when mapping a smaller atrial area. Focal atrial activation

Focal atrial activation during AF has been described by several published map53,61,64,68 Of those that used Detailed mapping studies such as ping studies. these are likely to increase our under- high-density mapping, the majority used standing of the mechanisms of AF. epicardial arrays applied during surgery Many mapping studies of human AF and concluded that the focal activation have been published to date,1,2,56–61 but seen was a result of epicardial breakcontain recorded data from limited through of an AF wavefront propagating 53,64,65 endocardial contact sites62,63 or describe in a free-running atrial trabeculum. epicardial electrode arrays used to One study used simultaneous endocardial record high-resolution data over lim- and epicardial mapping in canines to con69 ited areas53,58,64,65 or low-resolution data firm the existence of this phenomenon. The data presented in this chapter over large areas,66,67 often in patients with normal atria who had AF induced demonstrate activity emerging solely from the septum in the region of Bachmann's during a surgical procedure. These are the first experiences of bundle and the septum near the CS where noncontact mapping of human AF and the orientation of atrial fibers might are the first to provide maps of the entire encourage interatrial conduction. One RA endocardium including the septum can surmise from these data that this during AF. Noncontact mapping of AF in activity is the result of breakthrough of the human RA has confirmed the pres- activation from the left atrium. These ence of a variation in AF patterns in dif- conclusions are supported by previous left ferent patients that correspond with the studies that have used simultaneous RA mapping in canines61 and classification of types I, II, and III described atrial and 68 by Konings et al.53 The data have also humans to identify breakthrough of demonstrated entrainment of large areas activation from one atrium to the other, of the RA and recorded the termination of in some cases resulting in reactivation of unstable reentry circuits. AF by intravenous flecainide. Discussion

Atrial reentry

Termination of AF

The range of activation patterns presented here are compatible with previous studies.53,61 It is of interest that the

Previous investigators have used either low-resolution endocardial catheter mapping in humans70 or high-resolution

epicardial mapping of canines57,59,60 and entrained in this study and that seen in humans71 to demonstrate the mechanism previous studies may be because previof termination of a number of different ous high-resolution mapping studies of models of AF and using a variety of drugs. entrainment in AF have been restricted Flecainide has been shown to increase to canines. In addition, the global data the size and reduce the number of reen- provided by the noncontact mapping try circuits.57 In addition, flecainide may system may facilitate the differentiation convert AF to a regular tachycardia before between an entrained wavefront and an terminating the arrhythmia,57,59,71 and arrhythmia wavefront, so that fusion and termination of AF may follow the estab- modification of the native wavefront and lishment of a macroreentry circuit.57 entrainment can be confirmed. NonconMechanisms of termination of AF, similar tact mapping could therefore potentially to the cases described in this study, have demonstrate entrainment that may have been demonstrated previously, showing been overlooked by other techniques. It should be remembered that none of that failure of reexcitation of the RA from the left atrium was important, and that the studies described here examine actitermination may be preceded by wave- vation within the left atrium. It is well fronts emerging from a focus.59 A consid- known that the conduction properties of erable interrelation between AF and AFL the left atrium and the RA are inhomo72,73 and that the fibrillation rates and spontaneous conversion between the geneous 74 60 2 has also been shown previously and may differ between them. fits with the observations of the development of a reentry circuit identical to RA Summary flutter preceding the termination of AF presented in this study. Noncontact mapping has been validated in both sinus rhythm and VT. The Entrainment of AF VT and AFL reentry circuits identified by the noncontact mapping system have In 2 patients with type I AF, it was also been validated by examining the demonstrated that entrainment of large outcome of application of RF energy on areas of the RA was possible from more portions of the circuits critical to maintethan 1 site. Studies using high-resolution nance of the arrhythmias. Electrogram mapping have demonstrated entrainment reconstruction is accurate but this accuof smaller areas of the RA than the pre- racy decreases as the MEA-endocardial sent study, of between 4 cm58 and 6 cm.56 distance increases and this becomes sigLeft atrial activation was not examined in nificant when this distance is greater than our studies, except from the CS electrode, 34 mm. Delivery of RF energy to portions but pacing at the CS os in 1 patient with of the diastolic pathway VT is signifitype I AF resulted in activation of the right cantly more likely to successfully ablate superior septum remote from the pacing VT than energy applications at other site with a coupling interval identical to places. Noncontact mapping has also pacing cycle length suggesting entrain- been able to provide new insights into ment, presumably as a result of capture the mechanisms of AFL and fibrillation. of the left atrium or left atrial septum. Noncontact mapping appears to have 2 Therefore, it may also be possible to entrain major applications, assisting and guiding significant areas of the left atrium. The ablation of arrhythmias, and providing a difference between the proportion of RA better understanding of arrhythmias

NONCONTACT ENDOCARDIAL MAPPING

such as AF that have so far proved difficult and complex to treat using currently available technology.

References 1. Allessie MA, Lammers WJEP, Smeets JRLM, et al. Total mapping of atrial excitation during acetylcholine-induced atrial flutter and fibrillation in the isolated canine heart. In: Kulbertus HE, Olsson SB, Schlepper M (eds): Atrial Fibrillation. Molndal, Sweden: Lindgren and Soner; 1982:44-62. 2. Allessie MA, Lammers WJEP, Bonke FIM, Hollen J. Experimental evaluation of Moe's multiple wavelet hypothesis of atrial fibrillation. In: Zipes DP, Jalife J (eds): Cardiac Arrhythmias. New York: Grune & Stratton; 1985:265-276. 3. Cauchemez B, Haissaguerre M, Fischer B, et al. Electrophysiological effects of catheter ablation of inferior vena cavatricuspid annulus isthmus in common atrial flutter. Circulation 1996;93:284294. 4. Cosio FG, Lopez-Gil M, Arribas F, et al. Mechanisms of entrainment of human common flutter studied with multiple endocardial recordings. Circulation 1994; 89:2117-2125. 5. Josephson ME, Horowitz LN, Farshidi A, Kastor JA. Recurrent sustained ventricular tachycardia. I. Mechanisms. Circulation 1978;57:431-439. 6. Karagueuzian HS, Fenoglio JJ, Weiss MB, Wit AL. Protracted ventricular tachycardia induced by premature stimulation in the canine heart after coronary artery occlusion and reperfusion. Cire Res 1979;44:833-846. 7. deBakker JMT, Coronel R, Tasseron S, et al. Ventricular tachycardia in the infarcted, Langendorff-perfused human heart: Role of the arrangement of surviving cardiac fibers. J Am Coll Cardiol 1990;15:1594-1607. 8. Wellens HJJ, Schuilenberg RM, Durrer D. Electrical stimulation of the heart in patients with ventricular tachycardia. Circulation 1972;46:216-226. 9. Stevenson WG, Friedman PL, Kocovic D, et al. Radiofrequency catheter ablation of ventricular tachycardia after

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myocardial infarction. Circulation 1998; 98:308-314. 10. Stevenson WG, Khan H, Sager P, et al. Identification of reentry circuit sites during catheter mapping and radiofrequency ablation of ventricular tachycardia late after myocardial infarction. Circulation 1993;88:1647-1670. 11. El-Sherif N, Mehra R, Gough WB, Zeiler RH. Reentrant ventricular arrhythmias in the late myocardial infarction period: Interruption of reentrant circuits by cryothermal techniques. Circulation 1983; 68:644-656. 12. Borggrefe M, Podczeck A, Ostermeyer J, Breithardt G. Long term results of electrophysiologically guided antitachycardia surgery in ventricular tachyarrhythmias. A report on 655 patients. In: Breihardt G, Borggrefe M, Zipes DP (eds): Nonpharmacological Therapy of Tachyarrhythmias. Mount Kisco, NY: Futura Publishing Co.;1987:109. 13. Davis LM, Cooper M, Johnson DC, et al. Simultaneous 60-electrode mapping of ventricular tachycardia using percutaneous catheters. J Am Coll Cardiol 1994;24; 709-719. 14. Bockner D, Breithardt G, Block M, Borggrefe M. Management of patients with ventricular tachycardia: Does an optimal therapy exist? Pacing Clin Electrophysiol 1994;17:559-570. 15. Morady F, Harvey M, Kalbfleisch SJ, et al. Radiofrequency ablation of ventricular tachycardia in patients with coronary artery disease. Circulation 1993;87:363-372. 16. Borgreffe M, Chen X, Hindricks G, et al. Catheter ablation of ventricular tachycardia in patients with heart disease. In: Zipes DP (ed): Catheter Ablation of Arrhythmias. Armonk, NY: Futura Publishing Co.;1994:277-308. 17. Gepstein L, Hayam G, Ben-Haim S. A novel method for nonfluoroscopic catheterbased electroanatomical mapping of the heart. Circulation 1997;95:1611-1622. 18. Gulrajani R, Roberge F, Savard P. The inverse problem of electrocardiography. In: Macfarlane P, Veitch Lawrie T (eds): Comprehensive Electrocardiology. New York: Pergamon Press; 1989:237-288. 19. Frank E. Absolute quantitative comparison of instantaneous QRS equipotentials on a normal subject with dipole potentials on a homogenous torso model. Circ Res 1955;3:243-251.

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20. Savard P, Roberge FA, Perry JB, Nadeau RA. Representation of cardiac electrical activity by a moving dipole for normal and ectopic beats in the intact dog. Circ Res 1980;46:415-425. 21. Barnard ACL, Holt JH, Kramer JO. Models and methods in inverse electrocardiology: The UAB choice. In: Nelson CV, Geselowitz DB (eds): Theoretical Basis ofElectrocardiology. Oxford: Clarendon; 1976:305-322. 22. Ideker RE, Brody DA, Cox JW, Keller FW. Examination of a multiple dipole inverse cardiac generator based on accurately determined model data. J Electrocardiol 1973;6:197-209. 23. Arthur RM, Geselowitz DB, Briller SA, Trost RF. Quadrupole components of the human surface electrocardiogram. Am Heart J 1972;83:663-677. 24. Geselowitz DB. Multipole representation for an equivalent cardiac generator. Proc IRE 1960;48:75-79. 25. Oster HS, Taccardi B, Lux RL et al. Noninvasive electrocardiographic imaging. Reconstruction of epicardial potentials, electrograms and isochrones and localization of single and multiple electrocardiac events. Circulation 1997;96:1012-1024. 26. Colli-Franzone P, Taccardi B, Viganotti C. An approach to inverse calculation of epicardial potentials from body surface maps. Adv Cardiol 1978;21:50-54. 27. Taccardi B, Arisi G, Macchi E, et al. A new intracavitary probe for detecting the site of origin of ectopic ventricular beats during one cardiac cycle. Circulation 1987;75:272-281. 28. Khoury DS, Rudy Y. A model study of volume conductor effects on endocardial and intracavitary potentials. Circ Res 1992;71:511-525. 29. Khoury DS, Rudy Y. Reconstruction of endocardial potentials from intracavitary probe potentials: A model study. Proc Comput Cardiol Durham NC: Oct 11-14, 1992:9-12. 30. Macchi E, Arisi G, Colli-Franzone P, et al. Localization of ventricular ectopic beats from intracavitary potential distributions: An inverse model in terms of sources. Proc llth IEEE/EMBS. 1989:191-192. 31. Beatty GE, Remote SC, Hansen R, et al. Noncontact electrical extrapolation technique to reconstruct endocardial potentials. Pacing Clin Electrophysiol 1994; 17:765. Abstract.

32. Derfus DL, Pilkington TC. Assessing the effect of uncertainty in intracavitary electrode position on endocardial potential estimates. IEEE Trans Biomed Eng 1992; 39:676-681. 33. Peters NS, Jackman W, Schilling RJ, et al. Human left ventricular endocardial activation mapping using a novel non-contact catheter. Circulation 1997;95:1658-1660. 34. Khoury DS, Taccardi B, Lux RL, et al. Reconstruction of endocardial potentials and activation sequences from intracavity probe measurements. Localization of pacing sites and effects of myocardial structure. Circulation 1995;91:845-863. 35. Schilling RJ, Peters NS, Davies DW. A non-contact catheter for simultaneous endocardial mapping in the human left ventricle: Comparison of contact and reconstructed electrograms during sinus rhythm. Circulation 1998;98:887-898. 36. Tikhonov AN, Arsenin VY. Solutions of Ill-Posed Problems. Washington, DC: VH Winston & Sons; 1977:27-94. 37. Messinger-Rapport BJ, Rudy Y. Regular ization of the inverse problem in electrocardiography: A model study. Math Biosci 1988;89:79-118. 38. Chiang CM, Jenkins JM, DiCarlo LA. Digital signal processing chip for detection and analysis of intracardiac electrograms. Pacing Clin Electrophysiol 1994;17:13731379. 39. Greenhut SE. Identification of ventricular tachycardia using intracardiac electrograms: A comparison of unipolar versus bipolar waveform analysis. Pacing Clin Electrophysiol 1991;14:427-433. 40. Morady F, Kadish A, Rosenheck S, et al. Concealed entrainment as a guide for catheter ablation of ventricular tachycardia in patients with prior myocardial infarction. J Am Coll Cardiol 1991;17: 678-689. 41. Khan H, Stevenson WG. Activation times in and adjacent to reentry circuits during entrainment: Implications for mapping ventricular tachycardia. Am Heart J 1994; 127:833-842. 42. Josephson ME, Horowitz LN, Spielman SR, et al. Role of catheter mapping in the preoperative evaluation of ventricular tachycardia. Am J Cardiol 1982;49:207221. 43. Schilling RJ, Peters NS, Davies DW. Feasibility of a noncontact catheter for endocardial mapping of human ventricular

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tachycardia. Circulation 1999;99:25432552. 44. Poty H, Saoudi N, Abdel Aziz A, et al. Radio-frequency catheter ablation of type 1 atrial flutter. Prediction of late success by electrophysiological criteria. Circulation 1995;92:1389-1392. 45. Olgin JE, Kalman JM, Fitzpatrick AP, Lesh MD. Role of right atrial endocardial structures as barriers to conduction during human type I atrial flutter. Activation and entrainment mapping guided by intracardiac echocardiography. Circulation 1995; 92:1839-1848. 46. Kalman J, Olgin J, Saxon L, et al. Electrophysiologic characteristics of atypical atrial flutter Circulation 1995;92:I-406. Abstract. 47. Feld GK, Shahandeh RF. Mechanism of double potentials recorded during sustained atrial flutter in the canine right atrial crush-injury model. Circulation 1992;86:628-641. 48. Cosio F, Arribas F, Barbero JM, et al. Validation of double spike electrograms as markers of conduction delay or block in atrial flutter. Am J Cardiol 1988;61:775780. 49. Racker DK. Atrioventricular node and input pathways: A correlated gross anatomical and histological study of the canine atrioventricular junctional region. Anat Rec 1989;224:336-354. 50. Racker DK, Ursell PC, Hoffman BF. Anatomy of the tricuspid annulus: Circumferential myofibers as the structural basis for atrial flutter in a canine model. Circulation 1991;84:841-851. 51. Cosio FG, Lopez-Gil M, Goicolea A, et al. Radiofrequency ablation of the inferior vena cava-tricuspid valve isthmus in common atrial flutter. Am J Cardiol 1993; 71:705-709. 52. Nelson RM, Jenson CB, Davis RW. Differential atrial arrhythmias in cardiac surgical patients. J Thorac Cardiovasc Surg 1969;58:581-587. 53. Konings KTS, Kirchhof CJHJ, Smeets JRLM, et al. High density mapping of electrically induced atrial fibrillation in humans. Circulation 1994;89:16651680. 54. Hoffman BF, Rosen MR. Cellular mechanisms for cardiac arrhythmias. Circ Res 1981;49:1-15. 55. Allessie MA, Kirchhof CJ, Konings KTS. Unravelling the electrical mysteries of

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atrial fibrillation. Eur Heart J 1996;17 (Suppl C):2-9. 56. Allessie MA, Kirchhof CJ, Bonke FIM. The role of the sinus node in supraventricular arrhythmias. In: Mazgalev T, Dreifus LS, Michelson EL (eds): Electrophysiology of the Sinoatrial and Atrioventricular Nodes. New York: Alan R Liss; 1988:53-66. 57. Wang Z, Page P, Nattel S. Mechanism of flecainide's antiarrhythmic action in experimental atrial fibrillation. Circ Res 1992;71:271-287. 58. Kirchhof C, Chorro F, Scheffer GJ, et al. Regional entrainment of atrial fibrillation studied by high-resolution mapping in open chest dogs. Circulation 1993;88:736-749. 59. Wang J, Bourne GW, Wang Z, et al. Comparative mechanisms of antiarrhythmic drug action in experimental atrial fibrillation importance of use-dependent effects on refractoriness. Circulation 1993;88: 1030-1044. 60. Ortiz J, Niwano S, Abe H, et al. Mapping the conversion of atrial flutter to atrial fibrillation and atrial fibrillation to atrial flutter: Insight into mechanisms. Circ Res 1994;74:882-894. 61. Kumagai K, Khrestian C, Waldo AL. Simultaneous multisite mapping studies during induced atrial fibrillation in the sterile pericarditis model: Insights into the mechanism of its maintenance. Circulation 1997;95:511-521. 62. Daoud EG, Pariseau B, Niebauer M, et al. Response of type I atrial fibrillation to atrial pacing in humans. Circulation 1996;94:1036-1040. 63. Jais P, Haissaguerre M, Shah DC, et al. Regional disparities of endocardial atrial activation in paroxysmal atrial fibrillation. Pacing Clin Electrophysiol 1996; 19: 1998-2003. 64. Holm M, Johansson R, Brandt J, et al. Epicardial right atrial free wall mapping in atrial fibrillation: Documentation of repetitive activation with a focal spread— a hitherto unrecognised phenomenon in man. Eur Heart J 1997;18:290-310. 65. Holm M, Johansson R, Olsson SB, et al. A new method for analysis of atrial activation during chronic atrial fibrillation in man. IEEE Trans Biomedical Eng 1996; 43:198-210. 66. Cox JL, Canavan TE, Schuessler RB, et al. The surgical treatment of atrial fibrillation II. Intraoperative electrophysiologic

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67.

68.

69.

70.

CARDIAC MAPPING mapping and description of the electrophysiologic basis of atrial flutter and atrial fibrillation. J Thorac Cardiovasc Surg 1991;101:406-426. Chang BC, Schuessler RB, Stone CM, et al. Computerized activation sequence mapping of the human atrial septum. Ann Thorac Surg 1990;49:231-241. Harada A, Sasaki K, Fukushima T, et al. Atrial activation during chronic atrial fibrillation in patients with isolated mitral valve disease. Ann Thorac Surg 1996;61: 104-112. Schuessler RB, Kawamoto T, Hand DE, et al. Simultaneous epicardial and endocardial activation sequence mapping in the isolated canine right atrium. Circulation 1993;88:250-263. Sih HJ, Ropella KM, Swiryn S, et al. Observations from intraatrial recordings

on the termination of electrically induced atrial fibrillation in humans. Pacing Clin Electrophysiol 1994;17:1231-1242. 71. Brugada J, Gursov S, Brugada P, et al. Cibenzoline transforms random reentry into ordered reentry in the atria. Eur Heart J 1993; 14:267-272. 72. Alessi R, Nusynwitz M, Abildskov JA, Moe GK. Non-uniform distribution of vagal effects on the atrial refractory period. Am J Physiol 1958; 194:406-410. 73. Han J, Millet D, Chizzonitti B, Moe GK. Temporal dispersion of recovery of excitability on atrium and ventricle as a function of heart rate. Am Heart J 1966;71: 481487. 74. Allessie MA, Kirchhof CJHJ, Scheffer GJ, et al. Regional control of atrial fibrillation by rapid atrial pacing in conscious dogs. Circulation 1991;84:1689-1697.

Chapter 5 Principles of Nonfluoroscopic Mapping: Nonfluoroscopic Electroanatomical and Electromechanical Cardiac Mapping Shlomo A. Ben-Haim, MD, DSc

Introduction Cardiac mapping is commonly associated with cardiac electrophysiology; it is usually performed by cardiac electrophysiologists, and is used in the diagnosis and treatment of cardiac arrhythmias. Nevertheless, more "cardiac maps" (echocardiograms, ventriculograms, radionuclide imaging, etc.) are performed by nonelectrophysiologists than by cardiac electrophysiologists. Usually, the nonelectrophysiological "cardiac maps" lack electrical information while conventional electrophysiological maps lack mechanical or geometrical information. The recent introduction of modern technologies to the field of cardiac mapping widened the boundaries and scope of conventional cardiac mapping and catalyzed the development of an integrated mapping approach. Conventional cardiac mapping entails recording electrograms from a multiplicity of sites within the heart, the results of which are presented as an activation map.

Advanced cardiac mapping portrays cardiac electromechanical, or the spatial distribution of several different parameters (e.g., metabolism, perfusion, electrical activity). A typical map is usually made up of several data points, each having 2 values: (1) the local information (i.e., activation time in electrophysiological maps, local shortening in mechanical maps, and electromechanical coupling in combined maps), and (2) the location coordinate within the heart. Significant research efforts have been directed toward accurately determining local information. Analysis of electrograms was used for determining local activation time (LAT)1; regional wall motion was evaluated for determining local cardiac mechanics.2 However, only a negligible effort was made to better understand the importance of accurate location determination. In the early days of cardiac electrophysiological tracking, the effects of inaccurate mapping electrode location may have been appropriately disregarded, as the recording electrodes were

From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; ®2003.

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104 CAEDIAC MAPPING hard fixed to a recording array (e.g., epicardial sock), and the location error was therefore relatively small. However, in modern practice, where catheter ablation is commonly used to treat arrhythmias, determining the location of the mapping electrode in relation to the heart becomes critical. At present, localization of the mapping catheter is based on 2-dimensional fluoroscopic images. These images clearly identify the location coordinates of the electrode within the patient's chest. However, since fluoroscopic images do not reveal endocardial information, they are insufficient to record the electrode coordinates within the patient's heart. Similarly, the nonelectrophysiological "cardiac maps" initially had a poor spatial accuracy, which in turn limited their ability to perform regional therapy. Later on, more precise regional "cardiac maps" became a critical part of guiding regional therapy in patients (e.g., angioplasty). As new local therapies become available to the interventional cardiologist (e.g., laser-induced angiogenesis, growth factor injection, or other forms of local genetic therapies), the need for precise advanced cardiac mapping systems will increase for obtaining the correct diagnosis and for delivering the proper therapy. The nonfluoroscopic mapping system described in this chapter, first described in 1996,3 simultaneously records the location and electrograms of the mapping electrode. This chapter describes the method for cardiac electroanatomical mapping using an intrabody, real-time, high-resolution, nonfluoroscopic location and navigation system. Several applications of technology in electrophysiology as well as in interventional cardiology are presented.

Method Concept The nonfluoroscopic system used for cardiac electrophysiology (CARTO™, Biosense Webster, Diamond Bar, CA) is

an integrated electroanatomical catheterbased cardiac mapping technique that uses a location sensor incorporated into a mapping catheter to allow automatic and simultaneous acquisition of the catheter tip electrogram and its 3-dimensional location coordinates. The mapping system acquires the location of the catheter tip electrode (single location at a fixed time during the cardiac cycle), together with its local electrogram (throughout the cardiac cycle), and reconstructs the 3-dimensional electroanatomical maps of the heart in real time without the use of x-rays. The nonfluoroscopic system used for interventional cardiology (NOGA™, Biosense Webster) is an integrated electromechanical catheter-based cardiac mapping technique that, similarly, uses a location sensor incorporated into a mapping catheter to allow automatic and simultaneous acquisition of the catheter tip electrograms and its continuous 3-dimensional location coordinates. The mapping system acquires the location of the catheter tip electrode (throughout the cardiac cycle) together with its local electrogram (throughout the cardiac cycle) and reconstructs 3-dimensional, dynamic electromechanical maps of the heart in real time, without the use of x-rays or injection of contrast media. System Components The mapping and navigation system comprises a miniature passive magnetic field sensor (location sensor), an external ultralow magnetic field emitter (location pad), and a processing unit (CARTO) (Figure 1). The location sensor is integrated into a regular electrophysiological deflectable tip catheter (Navistar™, Webster, CA). The location pad is located beneath the patient table and generates an ultralow magnetic field (0.05 to 0.5 G). The emitted field possesses well-known temporal and spatial distinguishing characteristics that "code" the mapping space

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coded and incorporated into the endocardial surface reconstruction (red indicating the earliest activation times, purple the latest). To correct for patient movement, a second locatable catheter is used (usually placed externally on the patient's back). The mapping system subtracts the location of the mapping catheter from the simultaneous location of the reference probe, thus compensating for any motion of the patient. It is also possible to use an internal reference probe such as a locatable reference catheter positioned in a secure place within one of the cardiac chambers (e.g., the coronary sinus). In this case, the internal reference probe corrects Figure 1. The CARTO unit (A) connected to the junction box (B), through which the locatable for motion of the patient as well as the catheter is connected. The CARTO unit is also heart (primarily respiratory movements). connected to the ultralow magnetic field generOnce sufficient anatomical data are ator (location pad, C). acquired, the operator may navigate the around the patient's chest. The sensing of catheter without using fluoroscopy. the magnetic field by the location sensor Data may be acquired during the mapenables determination of the location and ping procedure to delineate anatomical orientation of the sensor in 6 degrees of structures, local electrophysiological propfreedom (X, Y, Z and roll, yaw, and pitch). erties, or a combination of both. At each location, before data are added to the map, the stability of the catheter-endocardial Mapping Procedure contact is tested by comparing the distance between consecutive end-diastolic CARTO locations (location stability, mm) and the A locatable mapping catheter is intro- time difference between consecutive duced under fluoroscopic guidance and is LATs (LAT stability, ms). The reconpositioned inside the heart to be mapped. struction is updated in real time with the The mapping system determines the loca- acquisition of each new site. From the data acquired at each site, tion and orientation of the mapping several maps are generated that include: catheter, gated to a fiducial point in the cardiac cycle. The tip electrograms are 1. Activation map: Describes the disrecorded throughout the entire cardiac persion of activation times in the cycle and are automatically associated with mapped region. Red indicates the the catheter tip location. The mapping proearliest activation whereas purple cedure involves sequentially dragging the indicates the latest activation. catheter tip over the endocardium and 2. Propagation map: A dynamic acquiring multiple tip locations, together graphic representation of the actiwith their respective electrograms. The set vation wavefront throughout the of gated catheter tip locations is used to cardiac cycle (active sites are colored red and quiescent sites blue). reconstruct the 3-dimensional endocardial surface. The LAT at each site, determined 3. Voltage maps: Graphic representation of the peak-to-peak voltage from the intracardiac electrograms, is color-

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of the local electrograms is recorded. Low voltages are colored in red. 4. Geometrical maps: These maps present graphic representations of the mapped anatomy (of the corresponding time during the cardiac cycle). Furthermore, the operator can mark an anatomical site with a colored tag; these tags can be used in designing the trajectory of complex ablations or as markers for specific electrophysiological information. NOGA The basic NOGA mapping procedure is identical to that described for the CAKTO system. However, as catheter tip locations are continuously acquired, information regarding each the complete endocardial location trajectory throughout the cardiac cycle is acquired, in addition to the local electrogram. At each location, before data are added to the map, the stability of the catheter-endocardial contact throughout the cardiac cycle is tested by comparing the distance between consecutive location trajectories (loop stability, mm) and the time difference between consecutive LATs (LAT stability, ms). The reconstruction is updated in real time with the acquisition of each new site. From the data acquired at each site several maps are generated that include: 1. Voltage map: Depiction of the peak-to-peak voltage of the local electrograms as recorded. Low voltages are color coded in red. 2. Local shortening maps: The distance between adjacent endocardial sites throughout the cardiac cycle, usually increases during diastole and decreases during systole. The local shortening index calculates the weighted average of endocardial distances around each site. Paradoxical shortening (i.e., increased distance during systole and decreased dis-

tance during diastole) are colored red while highly contractile sites are colored purple. Inherent Limitations Catheter mapping using a roving catheter is a sequential, beat-by-beat mapping approach. This imposes 2 requirements on the mapping process: a stable rhythm and a fixed reference point. Furthermore, as the localization methodology is based on recording low level magnetic fields, large ferromagnetic materials can affect the magnetic field sensed by the sensor in the catheter (usually when large objects are in the range of 1 cm from the tip). Accuracy and Reproducibility The location capabilities of the CARTO system were tested in both in vitro and in vivo studies. In vitro testing

The reproducibility of repeated location in vitro measurements of the tip of the catheter, using different orientations at various sites, was 0.16 ± 0.02 mm) (mean ± SEM) and the maximal range of the standard deviation of the location error was 0.55 ± 0.07 mm.4 Similarly, relative distances measured by the system between alternative sites with known locations were highly accurate (mean error 0.42 ± 0.05 mm).4 In vivo testing

The location error of repeated in vivo location determination of the catheter while in contact with a single site on a porcine left ventricular (LV) endocardium was 0.54 ± 0.05 mm, with a maximal range of 1.26 ± 0.08 mm.4,5 The relative distances measured by the system while sequentially withdrawing the mapping catheter inside

NONFLUOROSCOPIC ELECTROANATOMICAL AND ELECTROMECHANICAL MAPPING a long sheath at 10-mm intervals inside the heart was also determined. The average location error was 0.73 ± 0.03 mm.5 Repeated electroanatomical activation and propagation maps during sinus rhythm and pacing were similar, and enabled accurate identification of the pacing site in all animal studies.6 Accuracy was also tested by repeatedly applying radiofrequency (RF) energy to a site on the endocardium that was tagged on the electroanatomical map.4–6 These studies indicated that the location accuracy could guide delivery of RF energy to create single and multiple lesions.7–9 Furthermore, this navigation and tagging method can be used to direct repetitive applications of RF energy to adjoining sites in order to create a long and continuous lesion. A high correlation between the computer record of the length, location and shape of linear atrial longitudinal lesions, and the actual acute histopathological findings has been demonstrated.9

calculating the end-systolic volume, and 0.29% for calculating the ejection fraction. In vivo testing

The mapping procedure was evaluated in vivo by measuring the interobserver variability in determining the LV volumes, as well as by testing the correlation between the calculated stroke volume to cardiac output measured with thermodilution.5 The interobserver error of determining end-diastolic volume, endsystolic volume, and ejection fraction were 5.9%, 7.5% and 11.3%, respectively. The capability of a NOGA map to measure myocardial viability was tested in animal models with infarcted LVs by comparing the calculated infarct locations and size to the actual pathology.11–13 Results indicated a high correlation between the measured and calculated variables. Results

NOGA

Normal Maps

The capability of the NOGA system to reconstruct chamber morphology and then to calculate its volume was tested in both in vitro and in vivo studies.

CARTO

In vitro testing

The average deviation in calculating the volume of an ellipsoid phantom was 2.23%.5,10 Gepstein et al.5 mapped 6 LV casts and compared their actual volume. The volumetric measurements correlated highly with the actual volumes (r = 0.94). A dynamic jig was used to test the system capability to acquire multiple locations throughout the cardiac cycle. Comparison of the measured data to the calculated data indicated that the system had an average error of 1.43% in calculating the end-diastolic volume, 0.69% when

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Three-dimensional electroanatomical maps of the human heart were recorded by the CARTO nonfluoroscopic mapping system. These maps disclose, for the first time, the complex relationship between cardiac chamber anatomy and electrophysiological properties (Figure 2). noga

The NOGA system enables tracking of the coordinates of endocardial sites throughout the cardiac cycle (Figure 3). With use of this feature, the twist phenomenon is easily demonstrated and quantified (Figure 3). NOGA maps describe the chamber's geometry throughout the cardiac cycle (Figure 4). The end-diastolic image is identical to that generated by the CARTO system.

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Figure 2. Three-dimensional cardiac maps. A. Three-dimensional maps of the right atrium (RA): posteroanterior (PA) view (left map) and right anterior oblique (RAO) view (right map). This map was reconstructed using 65 sampling points (gray dots on map). The right atrial geometry is depicted with the respective positions of the superior and inferior vena cava and the tricuspid annulus (SVC, IVC, and TA, respectively). The earliest site of activation, represented by the red area, is located in the posterosuperior aspect of the RA just below the junction with the superior cava vein. Activation then spreads to the rest of the endocardium with 2 preferential directions, the first on the RA roof toward the left atrium (corresponding to Bachman's bundle) and the second on the posterior RA wall toward the IVC (corresponding to the crista terminalis). B. Left anterior oblique (LAO, left map) and posteroanterior (PA, right map) views of the left atrium (LA) during distal coronary sinus pacing. The maps were reconstructed using 100 sampling points. The LA geometry is depicted with the respective positions of the superior and inferior septal and lateral pulmonary veins and the mitral annulus (SSPV, ISPV, SLPV, ILPV, and MA, respectively). The earliest site of activation, represented by the red area, is located on the anterolateral aspect of the mitral annulus. C. LAO view of 3-dimensional local activation time map of both ventricles (RV, LV) during sinus rhythm. The map was reconstructed using 125 sampling points. Note the distance between the 2 right and left ventricular endocardial surfaces, depicting the interventricular septum, in which the earliest activation sites (colored red) are visualized. Also note that the LV apex protrudes laterally more than the apex of the right ventricle. D. The LV image in C (LAO view) is tilted craniocaudally and clipped (missing the anterolateral wall). The papillary muscles (PM) in both chambers are indenting into the cavity. Note also the relatively smooth endocardial surface in the LV and the trabeculated endocardium in the RV. See color appendix.

NONFLUOROSCOPIC ELECTROANATOMICAL AND ELECTROMECHANICAL MAPPING

The maximal voltage maps (Figure 5A) indicate a rather homogeneous dispersion of endocardial voltages (usually above 1 mV); however, the area of the mitral annulus has a typical low-voltage signature in all maps. Local shortening mechanical maps in normal LVs demonstrate a typical homogenous endocardial shortening (Figure 5B).

Figure 3. Location trajectory of a single site on the left ventricular endocardium. Note the ellipsoid area captured by the closed loop. Its width relates to local twist motion, while its long axis relates to the inward-outward, systolic-diastolic, endocardial movement. Red indicates systole; white indicates diastole. See color appendix.

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Clinical Use of the Nonfluoroscopic Mapping System CARTO In the short time since its introduction to the clinical arena, diverse modes of clinical use have been reported. In the following sections, some of the investigators who have published their clinical results are listed, but other chapters in this book are referred to for specific examples of the clinical use of the CARTO system. The reported clinical applications for the CARTO mapping and navigation system have included: 1. Activation mapping to identify the insertion of an accessory pathway.14–19 2. Activation mapping for detection of focal atrial tachycardia foci.20–23

Figure 4. Left ventricular geometry (right anterior oblique view) at different times throughout the cardiac cycle. Note the correlation with left ventriculography and body surface ECG. See color appendix.

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Figure 5. A normal electrical (A) and mechanical (B) image of a healthy individual (right anterior oblique view). Panel A depicts the dispersion of maximal voltages over the endocardium (red indicating low voltage and purple indicating high voltage). Note the low voltage around the mitral annulus. Panel B depicts the local endocardial shortening of the same ventricle. Note the relatively homogenous shortening properties of the endocardium, excluding the area adjacent to the mitral valve (marked in red). See color appendix.

3. CARTO mapping to identify the location of the slow pathway during the treatment of atrioventricular nodal reentry.24,25 4. Local cardiac anatomy mapping for guiding anatomically based atrial flutter ablation procedures.16,26–30 5. Substrate mapping to identify location of intra- and interscar pathways.31 6. Several recent reports have documented the use of the CARTO system for mapping and treatment of ventricular tachycardia.31–34 7. Atrial fibrillation mapping and ablation.35

2. Use of the electroanatomical/electromechanical maps as the guiding image for navigating the catheter. 3. The ability to tag anatomical and therapy delivery sites (particularly useful for cardiac ablation and local drug therapy). 4. The capacity to re-map cardiac electrical activation relative to the location of a previous intervention. NOGA

NOGA for Hemodynamic Testing: Several investigators have used the NOGA system to map the LV for hemodynamic characteristics. They have mapped subjects with normal LV function as well as pathoThe above-mentioned clinical applicalogical cases (Figure 6). Results showed a tions used the different capabilities of the high correlation between NOGA maps and nonfluoroscopic mapping system. These echocardiographic,fluoroscopic,and radionucapabilities included: clide volumetric measurements.11,36–38 1. Real-time recording of catheter location, enabling accurate navigation of it.

NOGA for Viability Testing: NOGA maps of electromechanical coupling in

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Figure 6. A huge apical aneurysm following anteroapical myocardial infarction (left panel). This 51 year-old male was referred for electrophysiological study because of nonsustained ventricular tachycardia. NOGA accurately depicted the aneurysm, which is clearly nonviable since both local electrical activity and regional contractility are decreased (right panel). The patient later underwent successful surgical aneurysmectomy. See color appendix.

Figure 7. NOGA map of a patient with an anteroapical wall infarct. A. Maximal voltage map (red indicates the low-voltage zone, i.e., scar). B. Local shortening map (red indicates impaired wall contractility). Note the mismatch between electrical activity (preserved) and mechanical property (diminished) in the midanterior wall, indicating viable myocardium. C. Single photon emission computed tomography 201TI perfusion scan at stress and at rest. Note the fixed perfusion defects. D. Positron-emission tomography with 18fluorodeoxyglucose uptake. Note the midanterior zone is indeed viable. See color appendix. (Continued)

patients with ischemic heart disease were compared with the results of a radionuclide study.11,36,38,39 The results showed a high correlation between NOGA maps and radionuclide studies. Specifically, there was a high correlation

between the location and size of areas with reduced electrical and mechanical function (electromechanical match indicating an infarcted tissue) on a NOGA map with areas of fixed defects on the radionuclide images (Figure 7). Furthermore,

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Figure 7. (Continued)

a high correlation was also demonstrated between the location and size of areas with preserved electrical but reduced mechanical activity (electro-

mechanical uncoupling indicating hibernating tissue) on the NOGA map with areas of reversible defects on the radionuclide images.

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Figure 8. NOGA electrical map (maximal voltage, right anterior oblique view) of a patient with an old myocardial infarction and drug refractory angina. One pulse with a Ho:Yag laser was fired at the tagged areas shown on the left ventricular image. See color appendix.

Guidance of Interventional Procedures

studies that the regions close to the lased channel had an angiogenic response.

Direct myocardial revascularization

Direct myocardial injection

Catheter-based direct myocardial revascularization uses laser to create channels connecting the LV lumen to the ischemic myocardium. The combination of a fiber optic carrying Ho:Yag laser radiation into the locatable catheter enabled lasing of endocardial channels (Figure 8). The unique combination of the lasing ability via a navigated catheter enabled precise lasing to ischemic areas only, avoidance of infarct areas, and accurate tagging of the channels. The accuracy and safety of the system was proved in both animal and human studies.40–43 Haudenschild et al.44 demonstrated in animal

New directions in interventional cardiology call for the ability to directly inject drugs, factors, or genes into the myocardium. Several clinical scenarios exist in which such a therapy may be indicated. For example, in the therapy of ischemic heart disease, injection of growth factors was shown to increase angiogenesis; this can also be achieved by injecting viral vectors carrying the growth factor gene. The accuracy of the system was recently tested in animals. This study45 demonstrated that the system can guide the new locatable injection catheter (Figure 9) with the same accuracy as other systems. Furthermore,

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Figure 9. New locatable injection catheter. A. A retractable needle at the catheter tip can inject drugs, factors, viruses, and genes into the myocardium. The catheter tip is locatable with the NOGA system, thus allowing the performance of a NOGA diagnostic study, which creates the baseline map defining the targets for the angiogenic therapy (B: Chronic ischemia model in a porcine heart). C. The injection sites are readily visualized where methylene blue dye was deposited intramyocardially. See color appendix.

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the injection catheter delivered all the injected media into the myocardium. Summary The nonfluoroscopic mapping system enables, for the first time, a new integrated look into cardiac electrophysiology and electromechanics. This new technical capability provides a new dimension for diagnosing different disease states. The diagnostic capabilities of complex cardiac electrophysiological cases were improved significantly with use of electroanatomical maps, which allowed for viewing of the interrelationship between cardiac anatomy and electrical activity. For the interventional cardiologist, the system offers, for the first time, a clinically validated method for determining the electromechanical coupling of the myocardium. A series of locatable therapeutic catheters (RF for cardiac ablation, Ho:Yag laser for direct myocardial revascularization procedures, and injection catheters for the myocardial gene therapy) expand the therapeutic arm of this technology. Furthermore, these minimally invasive procedures are guided using functional electroanatomical and electromechanical maps acquired during the diagnostic part of the study. In addition, the described technology provides a highly accurate, minimally invasive procedure, while minimizing the use of ionizing radiation. The need for accurate location information is becoming more important as minimally invasive therapies evolve. The nonfluoroscopic system described herein is an initial answer to this need. The future development of different locatable catheters and new localization hardware, as well as the development of newer algorithms for analyzing the acquired information, may open new applications for cardiology and expand the scope of cardiac mapping.

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Acknowledgments: The author wishes to thank Dr. Hans Kottkamp for contributing maps from patient procedures for use in this paper. The work of Prof.-Dr. Karl H. Kuck and Dr. Andreas Keck relating to myocardial injection modalities is acknowledged. Thanks also go to Dr. I. Schwartz, Dr. B. Nevo, D. Shapiro, and Y. Kedar for their assistance in preparing the figures. Appreciation is also expressed to Dr. G. Hayam for his support and contributions to this manuscript.

References 1. Ideker RE, Smith WM, Blanchard SM, et al. The assumptions of isochronal mapping. Pacing Clin Electrophysiol 1989; 12: 456-478. 2. Fedele F, Trambaiolo P, Magni G, et al. New modalities of regional and global left ventricular function analysis: State of the art. Am J Cardiol 1998;81:49G-57G. 3. Ben-Haim SA, Osadchy D, Schuster I, et al. Nonfluoroscopic, in vivo navigation and mapping technology. Nat Med 1996;2:1393-1395. 4. Hayam Gepstein L, Ben-Haim SA. Accuracy of the in vivo determination of location using a new nonfluoroscopic electroanatomical mapping system. Pacing Clin Electrophysiol 1996;19:712. 5. Gepstein L, Hayam G, Shpun S, et al. Hemodynamic evaluation of the heart with a nonfluoroscopic electromechanical mapping technique. Circulation 1997;96: 3672-3680. 6. Gepstein L, Hayam G, Josephson ME, et al. Cardiac volumetric measurement based on a novel method for nonfluoroscopic electroanatomical mapping of the heart. Eur Heart J 1996; 17:283. 7. Gepstein L, Hayam G, Ben-Haim SA. A novel method for nonfluoroscopic catheterbased electroanatomical mapping of the heart: In vitro and in vivo accuracy results. Circulation 1997;95:1611-1622. 8. Gepstein L, Shpun S, Hayam G, et al. Accurate linear radiofrequency lesions guided by a nonfluoroscopic mapping method during atrial fibrillation. Eur Heart J 1997;18:207. 9. Shpun S, Gepstein L, Hayam G, et al. Guidance of radiofrequency endocardial ablation with real-time three-dimensional magnetic navigation system. Circulation 1997;96:2016-2021. 10. Ben-Haim SA, Osadchy D, Hayam G, et al. Accuracy of volumetric measurements of a phantom with a new nonfluoroscopic

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cardiac mapping system. J Am Coll Cardiol 1996;27:347A. 11. Kornowski R, Hong MK, Gepstein L, et al. Preliminary animal and clinical experiences using an electromechanical endocardial mapping procedure to distinguish infarcted from healthy myocardium. Circulation 1998;98:1116-1124. 12. Gepstein L, Goldin A, Lessick J, et al. Electromechanical characterization of chronic myocardial infarction in the canine coronary occlusion model. Circulation 1998;98:2055-2064. 13. Gepstein L, Hayam G, Hakim G, et al. Electromechanical mapping of the heart can delineate the presence and extent of chronic myocardial infarction in dogs. Eur Heart J 1998;19(Abstr Suppl):399. 14. Hindricks G, Kottkamp H, Brunn J, et al. A new three dimensional electromagnetic mapping technology for non-fluoroscopic catheter ablation of left-sided accessory pathways. Eur Heart J 1997; 18:205. 15. Hoffmann E, Reithmann C, Nimmermann P, et al. Electroanatomic mapping of atrial tachycardia. Eur Heart J 1997;18:207. 16. Miles WM, Engelstein ED, Krebs ME, et al. Nonfluoroscopic mapping and ablation of human atrial flutter. Circulation 1996;94:SI676. 17. Smeets JLRM, Ben-Haim S, Ben David J, et al. Non-fluoroscopic endocardial catheter mapping: First experience in patients. Pacing Clin Electrophysiol 1996; 19(4 Pt 2): 627. Abstract. 18. Worley SJ. Use of a real-time threedimensional magnetic navigation system for radiofrequency ablation of accessory pathways. Pacing Clin Electrophysiol 1998;21:1636-1645. 19. Smeets JLRM, Ben-Haim SA, Rodriguez L-M, et al. New method for nonfluoroscopic endocardial mapping in humans. Accuracy assessment and first clinical results. Circulation 1998;97:2426-2432. 20. Kottkamp H, Hindricks G, Breithardt G, et al. Three-dimensional electromagnetic catheter technology: Electroanatomical mapping of the right atrium and ablation of ectopic atrial tachycardia. J Cardiouasc Electrophysiol 1997;18:1332-1337. 21. Marchlinski F, Callans D, Gottlieb C, et al. Magnetic electroanatomical mapping for ablation of focal atrial tachycardias. Pacing Clin Electrophysiol 1998;21:1621-1635. 22. Miles WM, Engelstein ED, Krebs ME, et al. Nonfluoroscopic mapping and ablation of

atrial tachycardias. Circulation 1996;94: SI380. 23. Miles WM, Engelstein ED, Krebs ME, et al. Nonfluoroscopic mapping of atrial tachyarrhythmias: Early clinical results. Eur J Cardiac Pacing Electrophysiol 1996;6: 255. 24. Kottkamp H, Hindricks G, Fetsch T, et al. Electromagnetic high-density catheter mapping within the triangle of Koch in AV node reentrant tachycardia: Pathophysiology and implications for catheter ablation. Eur Heart J 1997;18:206. 25. Miles WM, Engelstein ED, Olgin JE, et al. Slow pathway ablation using nonfluoroscopic guidance for the His bundle localization during energy delivery. Circulation 1996;94:452. 26. Shah DC, Jais P, Haissaguerre M, et al. Three-dimensional mapping of the common atrial flutter circuit in the right atrium. Circulation 1997;96:3904-3912. 27. Nakagawa H, Jackman WM. Use of a three-dimensional, nonfluoroscopic mapping system for catheter ablation of typical atrial flutter. Pacing Clin Electrophysiol 1998;21:1279-1286. 28. Shah DC, Jais P, Coste P, et al. Three dimensional right atrial mapping of common atrial flutter: Varying distribution of double spike potential. Eur J Cardiac Pacing Electrophysiol 1996;6:255. 29. Shah DC, Jais P, Coste P, et al. Propagation in the right atrium during common atrial flutter as determined by three-dimensional mapping. Eur Heart J 1996; 17:587. 30. Wilber D, Rubenstein D, Burke M, et al. Tricuspid annular activation during atrial flutter: Insights from electroanatomical mapping. Eur J Cardiac Pacing Electrophysiol 1996;6:257. 31. Dorodtkar PC, Cheng J, Scheinman MM. Electroanatomical mapping and ablation of the substrate supporting intra-atrial reentrant tachycardia after palliation for complex congenital heart disease. Pacing Clin Electrophysiol 1998;21:1810-1819. 32. Miles WM, Engelstein ED, Krebs ME, et al. Nonfluoroscopic mapping and human ventricular tachycardia: Implications of simultaneous earliest ventricular activation at more than one site. Circulation 1996;94:SI22. 33. Nademanee K, Kosar EM. A nonfluoroscopic catheter-based mapping technique to ablate focal ventricular tachycardia. Pacing Clin Electrophysiol 1998; 21:14421447.

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34. Stevenson WG, Delacretaz E, Friedman PL, et al. Identification and ablation of macroreentrant ventricular tachycardia with the CARTO electroanatomical mapping system. Pacing Clin Electrophysiol 1998;21:1448-1456. 35. Pappone C, Oreto G, Furlanello F, et al. Clinical impact of electroanatomically guided catheter compartmentation of human atria in the treatment of paroxysmal atrial fibrillation. Giornale Italiano di Cardiologia 1998;28(Suppl 1):2936. 36. Kornowski R, Hong MK, Leon MB. Comparison between left ventricular electromechanical mapping and radionuclide perfusion imaging for detection of myocardial viability. Circulation 1998;98:18371841. 37. Keck A, Kuchler, Twisselmann T, et al. Validation of a non-fluoroscopic computerbased localization technology by echocardiography. Eur Heart J 1998;19(Abstr Suppl): 156. 38. Reisman M, Wong SC, Bogosian S, et al. The NOGA project: A new method to assess myocardial function and viability. Am Heart Assoc 71st Scientific Session, Dallas, TX; November 8-11, 1998. Circulation 1998;(Suppl 1):I-509. Abstract. 39. Corvaja N, Kornowski R, Wang XD, et al. Correlation between myocardial Biosense NOGA electro-mechanical map and radionuclide perfusion imaging. Eur Heart J 1998;19(Abstr Suppl):277.

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40. Shpun S, Hayam L, Gepstein L, et al. Percutaneous myocardial revascularization with Holmium:Yttrium-aluminum-garnet laser guided by a nonfluoroscopic navigation system in the canine left ventricle. Eur Heart J 1997;18:613. 41. Shpun S, Talpalariu J, Kerner H, et al. Accurate nonfluoroscopic guidance of direct myocardial revascularization with Holmium: Yttrium-Aluminum-Garnet laser in the canine left ventricle. J Am Coll Cardiol 1998;31(2 Suppl A):307A. Abstract. 42. Kornowski R, Hong MK, Ellahham, et al. Safety and feasibility of percutaneous direct myocardial revascularization guided by Biosense™ left ventricular mapping. Eur Heart J 1998;19(Abstr Suppl):589. 43. Kornowski R, Baim DS, Moses JW, et al. Short- and intermediate-term clinical outcomes from direct myocardial laser revascularization guided by Biosense left ventricular electromechanical left ventricular mapping. Circulation 2000; 102:1120-1125. 44. Haudenschild CC, Bastaki M, Boenigk K, et al. Angiogenesis in response to minimal laser injury in porcine myocardium. J Am Coll Cardiol 1998;31(2 Suppl A): 307A. Abstract. 45. Kornowski R, Leon MB, Fuchs S, et al. Electromagnetic-guidance for catheterbased trans-endocardial injection: A platform for intramyocardial angiogenesis therapy: Results in normal and ischemic porcine model. J Am Coll Cardiol 2000;35: 1031-1039.

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Chapter 6 Principles of Magnetocardiographic Mapping Jukka Nenonen, Dr.Tech., Juha Montonen, Dr.Tech., and Markku Mdkijarvi, MD

Introduction Measurements of electrical potential differences arising from the heart (the EGG) have established an important role in clinical diagnosis and research of the cardiac function. The same bioelectrical activity in the body that generates electrical potentials also gives rise to biomagnetic fields, which are extremely low in magnitude (0.1 to 100 pT [1 pT = 10–15 T]). The first successful detection of the magnetocardiogram (MCG) was reported in 1963.1 However, it was only after the development of superconducting quantum interference devices (SQUIDs) in the early 1970s that accurate detection of biomagnetic signals became possible.2 Until the 1990s, most MCG studies were performed with single-channel devices, by moving the system sequentially over the thorax and measuring signals from one location at time. Today, noninvasive MCG mapping recordings are carried out with multichannel systems, acquiring the signals

simultaneously over the whole chest of the patient.3,4 In addition, difficulties with skin-electrode contact, which sometimes cause problems in corresponding ECG mapping studies, are avoided. Although magnetocardiography has not yet been established as a routine clinical tool, successful MCG results have been reported in clinically important problems, such as noninvasive localization of arrhythmia-causing regions in the heart,4–7 assessment of the risk of life-threatening arrhythmias,8–11 and characterization and localization of myocardial ischemia.12,13 This chapter considers the basic principles of cardiomagnetism. We first present briefly the genesis of cardiomagnetic fields. Then, instrumentation and MCG measurements are described. Next, we give an outline of signal processing methods used in evaluating the arrhythmia risk. The basic goal in most MCG studies is to solve the restricted inverse problem, i.e., to estimate the current sources underlying measured extracardiac fields. Some

From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; ®2003.

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applications of the inverse problem in terms of equivalent sources are described. Finally, the validation of the MCG results is discussed, and future trends are outlined. Clinical applications of the MCG are presented in chapter 24 of this volume. Theory Bioelectrical Current Sources ECGs and MCGs arise primarily from electrical currents in the body associated with flow of charged ions (such as Na+, K+, and Ca2+) in the myocardium. The active source regions are usually characterized with the concept of the primary current density, jp, which leaves cellularlevel details without explicit attention. It comprises the ionic currents inside and in the vicinity of excited cells. The primary current causes an electrical potential ((), which in turn generates ohmic volume currents jv=-aV(|) everywhere in the body with the electrical conductivity of a. The total current density inside the body can be expressed as: j = jp - crV<)). By finding the primary current, we can locate the source of myocardial activity. The electrical field, E, and the magnetic field, B, induced by j can be solved from Maxwell's equations. The time variation of jp is relatively slow (below 1000 Hz), which means that the source currents and external electromagnetic fields can be treated in a quasistatic approximation (the true time-dependent terms in Maxwell's equations vanish).14 Equivalent Sources A very large number of cells must be excited to achieve a measurable signal. Thus, it is convenient to introduce a macroscopic equivalent current dipole (ECD): p = J jpdu, where the integration extends over the excited source volume v. The ECD can

be regarded as the lowest order equivalent source of the ECGs and of the MCGs.15 The role of the ECD model in solving the forward and the inverse problem in magnetocardiography is considered in the following sections. One can also derive higher order equivalent generators, such as quadripoles, by using a multipole expansion.14,15 Instead of a single-current dipole, distributed source currents associated with a propagating cardiac excitation wavefront can be represented by a large number of individual-current dipoles, or more often in terms of a uniform double layer.14 This representation consists of a sheet of dipoles with equivalent strengths, which are oriented perpendicularly to the surface of the propagation wavefront. Electromagnetic Fields It can be shown that the volume currents in an infinite homogeneous volume conductor give no contribution to the electrical potential or to the magnetic field, and the fields are solely a result of the primary currents, jp. In addition, the electrical potential and the magnetic field should be independent of each other.15 If we approximate the torso by a homogeneous semi-infinite space with a plane boundary, it is possible to derive simple analytic expressions for the magnetic field of a current dipole.16 Furthermore, radial currents do not produce any magnetic field outside the semi-infinite volume conductor. MCG is thus to a great extent selectively sensitive to tangential sources, and EGG data are required if we want to recover all components of the primary current. In reality, however, the situation is more complicated because of macroscopic interfaces of electrical conductivity. By assuming that the body is a homogeneous volume conductor, bounded by the surface, S, the surface potential, (o,and the magnetic field, B, in this case can be obtained from integral equations.17

PRINCIPLES OF MAGNETOCAKDIOGRAPHIC MAPPING In the boundary-element method, electrical potential and magnetic field are calculated from discretized linear matrix equations, which can also be extended to torso models with inhomogeneities (i.e., the lungs and intracardiac blood masses).4,17 In most numerical applications to the MCG and EGG inverse problem, the surfaces are tessellated with triangular elements. Realistically shaped geometries of each patient are usually extracted from magnetic resonance imaging data. The regions of interest (e.g., the heart, the lungs, and the thorax) must be segmented from the data first. The surfaces of these volumes are then discretized for numerical calculations. The segmentation and tessellation problems are tedious, but nearly automated computer methods have been reported.18 Instrumentation The detector that offers the best sensitivity for the measurement of the tiny cardiomagnetic fields is the SQUID (see reference 3 for the basic principles). Still, strong environmental magnetic noise, unavoidable at urban hospitals and laboratories, makes the detection of biomagnetic signals impossible without specific techniques to reject the external noise. Shielding against high-frequency electromagnetic noise is relatively easy with conducting materials such as current shields of copper or aluminum. Diminishing low-frequency magnetic noise requires

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the use of magnetically shielded rooms. Besides shielded rooms, differential gradiometric measurements have been performed to reject external magnetic noise. Most instruments use first-order wirewound axial gradiometers with relatively large coils. However, planar gradiometers, realized using integrated magnetometer chips, are more space efficient and are easier to manufacture using thin film techniques. A schematic presentation of the sensor types is presented in Figure 1. A detailed presentation of the shielding and sensor techniques can be found in reference 3. Until the 1990s, most biomagnetic measurements were performed with single-channel instruments; however, reliable localizations require mapping in several locations, and this is time consuming with only one channel. Besides, spatial features of occasional temporal variations in the signals may be lost. Fortunately, the sensitivity of multichannel sensors has been improved and the number of channels increased during the last few years. In shielded rooms, sensitivities of multichannel dc-SQUID magnetometers around 2 x 10~15 TA/Hz have been reported. State-of-the art MCG systems contain more than 60 detectors in an array covering an area of about 30 cm in diameter over the patient's chest or back. The dewar containing the sensors is attached to gantry system, which allows easy positioning of the dewar above the patient's thorax. It is often also possible to move the subject bed horizontally. The position of the dewar with

Figure 1. Sensor types. A. Magnetometer; B. first-order axial gradiometer; C. first-order planar gradiometer. In B and C, the gradiometer baseline length is h.

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respect to the patient's head or torso is typically determined by measuring the magnetic field arising from small marker coils attached to the skin and by calculating their locations with respect to the sensor array. The locations of the marker coils with respect to an anatomical frame of reference can be determined before the measurement by a 3-dimensional digitizer.

The MCG system shown in Figure 2 comprises 99 channels arranged on a slightly curved surface with the diameter of about 30 cm. The magnetic field component perpendicular to the sensor array surface (Bz) is sensed by 33 triple-sensor units, in which a magnetometer is overlaying 2 perpendicular planar gradiometers (Figure 1). Figure 2 also shows an

Figure 2. The 99-channel dc-SQUID (superconducting quantum interference devices) system for cardiomagnetic studies in the HUGH BioMag Laboratory. The sensor array covers the whole anterior chest of the supine patient, who is taken into and out of the magnetically shielded room on a wheeled nonmagnetic bed. The gantry system allows easy movement and adjustment of the position of the measurement dewar. In addition, a nonmagnetic ergometer for exercise magnetocardiography is displayed.

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Figure 3. A. Magnetocardiograms measured with the 99-channel cardiomagnetometer in a patient suffering from ventricular tachycardia. The first QRS complex indicates a normal sinus beat, followed by an abnormal extrasystolic beat after the normal T wave. The signals measured by the 33 magnetometers are shown by the blue curves. Isocontour presentations of the magnetic-field component perpendicular to the sensor array surface can be extrapolated; they are here depicted both at the beginning of B) the normal sinus rhythm beat, and C) the ventricular extrasystolic beat. The circles denote to the magnetometer sensor outlines. See color appendix.

ergometer designed to produce no magnetic noise in supine exercise MCG studies. An example of rest MCG measurements with a multichannel magnetometer is displayed in Figure 3: a sinus beat is followed by a ventricular extrasystolic beat, and morphological MCG signals as well as interpolated distributions of Bz are presented. The field of magnetocardiography may expand during the next few years with the implementation of high-temperature SQUID arrays that can be operated at the temperature of liquid nitrogen.3 The higher noise level of high-temperature SQUIDs is partly compensated by the smaller distance between the sensors and the chest, and the systems are applicable to bedside recordings.19 At present, however, the conventional low-temperature SQUIDs are easier to produce and thus less expensive than the high-temperature ones. In addition, the first commercial lowtemperature SQUID systems designed to operate without external magnetic shielding have become available.19 Such systems can be applied in a catheterization laboratory, allowing to develop new MCG

tools for online electroanatomical imaging studies. Signal Processing Time Domain Analysis In multichannel MCG and EGG studies, signal averaging is often applied to improve the signal-to-noise ratio.20 A template beat is usually selected, and cross-correlation between the template and a triggered beat yields precise timing. In addition, baseline noise and differences in amplitude between beats, calculated from user-selected channels, can be used as rejection criteria. Observation of fragmented low-amplitude MCG signals at the end of the QRS complex, corresponding to the electrical late potentials, was first reported by Erne et al.8 To detect such low-amplitude signals, as well as intra-QRS fragmentation, time domain MCG and ECG analyses use both infinite and finite response filtering. Simson21 first introduced a method based on high-pass filtering for late potential analysis. A recursive Butterworth-type

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CAEDIAC MAPPING possible notching during the averaged QRS complexes.22 In this so-called "Berlin fragmentation analysis," the filtered QRS complex is characterized by finding the number of signal extrema during the QRS (M), and by calculating the product of M and the sum of the amplitude differences between neighboring extrema, called fragmentation index S. For the signal-averaged EGG data, a mean of M and S is calculated over the 3 leads, and in the case of MCG data, a mean is calculated over the sensor locations. A binomial-filtered MCG signal is displayed in Figure 4c. Spectrotemporal Analysis

The spectral turbulence analysis, based on sequential short-time fast Fourier Figure 4. Examples of filtered magnetocardio- transforms, was developed for identifygraphic signals measured from a ventricular ing arrhythmogeneity-related features in tachycardia patient for arrhythmia risk stratifi- the ECG during the QRS complex.23 In cation. A. The signal-averaged magnetocardio- this method, short time Fourier spectra graphic trace; B. the same trace after high-pass filtering; and C. the same trace after binomial fil- are calculated over time intervals of 25 tering. D. A spectrotemporal analysis of the ms through differentiated ventricular depolarization signal at 2-ms steps. A MCG trace in A. See color appendix. resulting spectrocardiogram can be visualized in logarithmic color scale, and it is filter with a cutoff frequency of 40 Hz is described quantitatively by calculating often used. A filtered QRS complex is correlations between the spectra. For the formed as a vector magnitude in the case MCG analysis, the mean value of each of the EGG data, while for the MCG data parameter is formed over the sensor locaa corresponding signal can be obtained tions. An example spectrocardiogram is as an envelope complex in each sensor presented in Figure 4d. location using Hilbert transform. 9 An example of high-pass filtered MCGs is Repolarization Analysis presented in Figure 4B. For each filtered QRS complex, the First attempts to describe vulneraQRS duration, the duration low-amplitude signal below 1 pT or 40 uV from the offset bility for malignant arrhythmias based of the QRS complex, and the root mean on dispersion of the QT time in the mul11,24 square amplitude during the last 40 ms tichannel MCG have been reported. of the QRS complex are calculated. Other For this purpose, an algorithm to facilitate automated determination of the QTthreshold values can also be used. To detect intra-QRS changes, a non- apex and QT-end intervals has been recursive band-pass filter from 37 to 90 Hz developed.24 The analysis begins with with binomial coefficients is used to enhance averaging performed over a 1-second

PRINCIPLES OF MAGNETOCARDIOGRAPHIC MAPPING period covering the PQ segment and the next T-P interval. A baseline is selected from the T-P interval, and beats with T wave amplitude below 0.6 pT from the baseline are excluded. A visual checking of the automatically determined T-apex and T-end intervals is performed to reject possible errors made by the algorithm. The algorithm used in the time domain analysis of late fields is applied to determine the QRS onset and offset times. Finally, in the case of both QT-apex interval and QT-end interval, the QT dispersion over the measured area is calculated as the difference between the longest and shortest QT intervals. Spatial MCG Map Analysis Besides analyses of morphological MCG tracings, extraction of spatial features in MCG mappings can provide valuable information of the arrhythmia vulnerability. In the case of multichannel MCG data, the distributions of the time domain parameters,9,22 as well as the distributions of QT-apex and QT-end times,11,24 can easily be displayed as isovalue colormaps. The spatial heterogeneity in such presentations can be used as a further criterion to assess the arrhythmia risk. In addition, extrema trajectory plots revealing the spatial route of the maxima and the minima in MCG distributions have been used in arrhythmia risk evaluation.10 Stress Magnetocardiography Exercise electrocardiography is a well-standardized and widely used diagnostic and prognostic test for the evaluation of patients with ischemic heart disease. Stress magnetocardiography has also received increasing interest in recent years. Development of multichannel systems has brought MCG mapping during

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interventions technically closer to clinical feasibility. Specially designed nonmagnetic ergometers facilitate the exercise testing during MCG measurements (see Figure 2). In first MCG studies with normal subjects, significant ST segment changes were observed under physical exercise, concomitantly with a normal EGG.25 First results of the method have been published in patients with coronary artery disease, and in studies of myocardial viability. These studies are presented in more detail in chapter 24. In addition to ergometer exercise, pharmacological MCG stress testing has been applied. Brockmeier et al.26 reported that multichannel MCG data showed significantly more distinct repolarization changes than simultaneously recorded 32-lead ECG maps under pharmacological stress in 3 healthy subjects. The consequences of such findings still remain to be confirmed with patient data. Source Localization In the MCG and ECG inverse problems, the objective is to estimate the primary current density underlying the electromagnetic signals measured outside or on the surface of the body. Because the inverse problem does not have a unique solution, one must replace the actual current sources with equivalent generators that are characterized by a few parameters. Thus, in most localization studies the solution to the forward problem is needed first. The simplest physiologically sound model for the myocardial current distribution is to use the best-fitting ECD. In cardiac studies an ECD is applicable for approximating the location and strength of net primary current density confined into a small volume of tissue. Myocardial depolarization initiated at a single site spreads at a velocity of about 0.4 to 0.8 mm/ms, and

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Figure 5. A. An example of a numerical torso model, showing also the triangulated surfaces of the heart. The magnetometer sensor outlines are depicted as circles. B. Magnetocardiographic localization results at 4 successive time instants (blue dots) during the onset of an extrasystolic beat. The magnetocardiographic results are visualized on the transparent 3-dimensional heart surfaces, which can be freely rotated on the computer display. See color appendix.

the ECD can be thought to be moving along the 'center of mass' of the excitation. In practice, localization based on a single ECD is meaningful only during the first 10 to 20 ms of excitation. The ECD is a commonly used source model. It has a long and important history in clinical electrocardiography (e.g., vectorcardiography). Cohen and Hosaka27 were the first to study a single ECD when describing measured MCG maps. In later MCG studies, the ECD model was used to characterize small source regions such as His bundle activation, infarction area, the arrhythmia-causing ventricular preexcitation site in the WolffParkinson-White syndrome, and other arrhythmogenic regions.4–7 Localizations of implanted pacing catheters have also been reported.7,28 In the above-mentioned studies, accuracies of 5 to 25 mm have been reported by comparing the MCG localization results to: (1) cardiac surgery; (2) catheter ablation; (3) the results of invasive electrophysiological studies; (4) ECG localization results; and (5) physio-

logical knowledge (x-ray or magnetic resonance imaging). An example of the MCG localizations is presented in Figure 5. In addition to patient studies, the ability of the MCG to locate artificial current dipoles has been tested with physical thorax phantoms.7,29 A nonmagnetic pacing catheter in a realistically shaped thorax phantom resulted in equal accuracies of 5 to 10 mm between MCG and ECG mapping data.29 On the other hand, the same catheter in 15 patients showed significantly better localization accuracy with MCG data (7 mm) than with simultaneously recorded ECG mapping data (25 mm).28 In practice, the ECG and the MCG arise from distributed current sources. Because the use of even 2 ECDs becomes very complicated in cardiac studies, it is more convenient to solve the inverse problem for a current distribution. The quasistatic bioelectromagnetic forward problem can be written in a general form: bk=J Lj, • jp du, where bk is the MCG signal measured by the kih sensor, and Lfe is the

PRINCIPLES OF MAGNETOCARDIOGRAPHIC MAPPING lead field of that sensor. Most studies dealing with distributed source models are based on the minimum norm estimation,5,12,13 where the estimate of the primary current is constructed as a linear combination of the lead fields, ~Lk. The lead fields are provided by the forward solution in the selected torso and sensor geometry. Because even small contributions of the measurement noise make the solution ill-posed, suitable regularization techniques are needed to stabilize the solution.4,13 Recently, the MCG current density reconstruction was applied for the estimation of ischemic regions induced by ergometer exercise,12,13 and in detection of viable myocardium after chronic ischemia.13 An example of such reconstructions is presented in chapter 24. Besides single-layer sources, such as the equivalent current density above, uniform double-layer sources can be applied. The lead fields from each point on the endocardial and epicardial surfaces to the MCG/body surface potential mapping sensors are used to define the sequence of cardiac excitation during the QRS complex.30 The quality of such solutions has recently been validated with intraoperative epicardial recordings in patients undergoing open-chest surgery.31 Discussion It can be concluded from all MCG studies published so far, that for some applications the diagnostic performance of MCG is superior to the conventional ECG; however, his advantage has not yet driven clinicians to widely accept and use the method, mostly because of its high cost and low availability. Nevertheless, there is a growing interest in clinical applications of MCG to therapy and diagnosis of cardiac diseases. Invasively recorded cardiac signals, such as potentials measured during elec-

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trophysiological studies on epicardial and endocardial surfaces, provide the best gold standard for validation of the MCG inverse solutions. Even though patient populations studied by MCG before or during invasive catheterization are still relatively small, the localization studies of various cardiac arrhythmias have shown encouraging results. Multichannel systems and accurate combination of the results with cardiac anatomy have improved the accuracy to the order of 5 to 10 mm, which is sufficient to aid in planning and delivering curative therapy for arrhythmia patients. Further validation for the MCG localization accuracy has been obtained by locating artificial dipole sources, such as pacing catheters inserted into the heart during electrophysiological studies. For arrhythmia risk evaluation, MCG mapping is an attractive alternative for 2 reasons: first, the ease of measurement makes it well suited for screening purposes, and second, several approaches in the analysis can be applied to a single recording. Time domain analyses of both late fields and intra-QRS fragmentation have performed well in comparison to signal-averaged ECGs. Yet, a clear demonstration of the advantages of spectrotemporal approaches in MCG analysis has yet to be made. The analysis of QT time variability in MCG mappings has yielded promising results, but the methodologies applied are still user dependent. Exploitation of the spatial features in MCG mappings has been reported to result in improved performance when compared to electrical body surface potential mapping. Further improvement may be obtained by combining the analysis of ventricular depolarization, of spatial heterogeneity in MCG repolarization, and of short-term heart rate variability—all possible with one MCG mapping recording. To date, however, no prospective evaluation has

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been done of MCG performance, notwithstanding a few ongoing studies that aim to fill this void. The arrhythmogenic substrate is not manifested in all normal sinus rhythm recordings, and interventions may be needed during MCG to stimulate extrasystolic beats and controlled arrhythmias to locate them. Thus, MCG should be available in a catheterization laboratory, but the demand of magnetic shielding and liquid helium is in practice limiting the use of MCG mapping in guiding invasive arrhythmia localization. Fortunately, compact-size higher order magnetometer arrays operated without external shielding are available and may bring MCG mapping closer to bedside clinical practice.19,32 Despite more than 20 years of MCG research, common standards of measurement techniques, data processing, and presentation are still lacking. Suggestions for such standards are emerging, but there are large differences between sensors and their arrangement in the multichannel magnetometers. Recently, tools have been developed and tested to interpolate signal morphologies and isocontour maps that are directly comparable to studies performed in other centers.33 Finally, effective signal processing and source modeling software is going to be increasingly important to extract and combine all available functional data from the cardiac function. References 1. Baule GM, McFee R. Detection of the magnetic field of the heart. Am Heart J 1963;55:95-98. 2. Cohen D, Edelsack EA, Zimmermann J. Magnetocardiograms taken inside a shielded room with a superconducting point-contact magnetometer. Appl Phys Lett 1970; 16:278-280. 3. Braginski A, Clarke J (eds): SQUID Handbook. Berlin: Wiley VCH; 2002.

4. Hamalainen MS, Nenonen J. Magnetic source imaging. In: Webster J (ed): Encyclopedia of Electrical Engineering. Vol. 12. New York: Wiley & Sons; 1999:133-148. 5. Nenonen J, Purcell C, Horacek BM, et al. Magnetocardiographic functional localization using a current dipole in a realistic torso. IEEE Trans Biomed Eng 1991;38:658-664. 6. Fenici RR, Melillo G. Magnetocardiography: Ventricular arrhythmias. Eur Heart J 1993;14(Suppl E):53-60. 7. Moshage W, Achenbach S, Gobl W, Bachmann K. Evaluation of the non-invasive localization of cardiac arrhythmias by multichannel magnetocardiography (MCG). Int J Card Imaging 1996;12:47-59. 8. Erne SN, Fenici R, Hahlbom HD, et al. High-resolution magnetocardiographic recordings of the ST segment in patients with electric late potentials. Nuovo Cimento 1983;2D:340-345. 9. Montonen J. Magnetocardiography in identification of patients prone to malignant arrhythmias. In: Baumgartner C, Deecke L, Stroink G, Williamson SJ (eds): Biomagnetism: Fundamental Research and Clinical Applications. Amsterdam: Elsevier Science/IOS Press; 1995:606-611. 10. Stroink G, Lant J, Elliot P, et al. Discrimination between myocardial infarct and ventricular tachycardia patients using magnetocardiographic trajectory plots and isointegral maps. J Electrocardiol 1992;25:129-142. 11. van Leeuwen P, Hailer B, Wehr M, et al. Spatial distribution of QT intervals: Alternative approach to QT dispersion. Pacing Clin Electrophysiol 1996; 19:1894-1899. 12. Leder U, Pohl HP, Michaelsen S, et al. Noninvasive biomagnetic imaging in coronary artery disease based on individual current density. Int J Cardiol 1998;64:83-92. 13. Nenonen J, Pesola K, Hanninen H, et al. Current-density estimation of exerciseinduced ischemia in patients with multivessel coronary artery disease. J Electrocardiol 2001;34(Suppl):37-42. 14. Malmivuo J, Plonsey R. Bioelectromagnetism—Principles and Applications of Bioelectric and Biomagnetic Fields. New York: Oxford University Press; 1995. 15. Katila T, Karp PJ. Magnetocardiography: Morphology and multipole presentations. In: Williamson SJ, Romani G-I, Kaufman L, Modena I (eds): Biomagnetism, An Interdisciplinary Approach. New York: Plenum Press; 1983:237-263.

PRINCIPLES OF MAGNETOCARDIOGRAPHIC MAPPING 16. Erne SN. High resolution magnetocardiography: Modeling and source localization. Med Biol Eng Comput 1985;23:14471450. 17. Horacek BM. Digital model for studies in magnetocardiography. IEEE Trans Magn 1973;9:440-444. 18. Lotjonen J, Reissman PJ, Magnin IE, et al. A triangulation method of an arbitrary point set for biomagnetic problems. IEEE Trans Magn 1998;34:2228-2233. 19. Leder U, Schrey F, Haueisen J, et al. Reproducibility of HTS-SQUID magnetocardiography in an unshielded environment. Int J Cardiol 2001;79:237-243. 20. Huck M, Haueisen J, Hoenecker O, et al. QRS amplitude and shape variability in magnetocardiograms. Pacing Clin Electrophysiol 2000;23:234-242. 21. Simson MB. Use of signals in the terminal QRS complex to identify patients with ventricular tachycardia after myocardial infarction. Circulation 1981;64:235-242. 22. Muller HP, Godde P, Czerski K, et al. Magnetocardiographic analysis of the twodimensional distribution of intra-QRS fractionated activation. Phys Med Biol 1999;44:105-120. 23. Kelen GJ, Henkin R, Starr A-M, et al. Spectral turbulence analysis of the signalaveraged electrocardiogram and its predictive accuracy for inducible sustained monomorphic ventricular tachycardia. Am J Cardiol 1991;67:965-975. 24. Oikarinen L, Paavola M, Montonen J, et al. Magnetocardiographic QT-interval dispersion in post myocardial infarction patients with sustained ventricular tachycardia—validation of automated QT measurements. Pacing Clin Electrophysiol 1998;21:1934-1942. 25. Takala P, Hanninen H, Montonen J, et al. Magnetocardiographic and electrocardiographic exercise mapping in healthy

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subjects. Ann Biomed Eng 2001;29:501509. 26. Brockmeier K, Schmitz L, Chavez J, et al. Magnetocardiography and 32-lead potential mapping: Repolarization in normal subjects during pharmacologically induced stress. J Cardiovasc Electrophysiol 1997;8: 615-626. 27. Cohen D, Hosaka H. Magnetic field produced by a current dipole. J Electrocardiol 1976;9:409-417. 28. Pesola K, Nenonen J, Fenici R, et al. Bioelectromagnetic localization of the pacing catheter in the heart. Phys Med Biol 1999;44:2565-2578. 29. Fenici R, Pesola K, Makijarvi M, et al. Non-fluoroscopic localization of an amagnetic catheter in a realistic torso phantom by magnetocardiographic and body surface potential mapping. Pacing Clin Electrophysiol 1998;21:2482-2491. 30. van Oosterom A, Oostendorp TF, Huiskamp GJ, ter Brake M. The magnetocardiogram as derived from electrocardiographic data. Circ Res 1990;67:1503-1509. 31. Oostendorp T, Pesola K. Non-invasive determination of the activation sequence of the heart based on combined ECG and MCG measurements. In: Nenonen J, Ilmoniemi R, Katila T (eds): Biomag 2000, Proc. 12th Int. Conf. on Biomagnetism. Espoo, Finland: Helsinki University of Technology; 2001:813-820. 32. Fenici R, Brisinda D, Nenonen J, et al. First MCG multichannel instrumentation operating in an unshielded hospital laboratory for multimodal cardiac electrophysiology: Preliminary experience. Biomed Tech (Berl) 2001;46(Suppl 2):219-222. 33. Burghoff M, Nenonen J, Trahms L, Katila T. Conversion of magnetocardiographic recordings between two different multichannel SQUID devices. IEEE Trans Biomed Eng 2000;47:869-875.

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Chapter 7 Fast Fluorescent Mapping of Electrical Activity in the Heart: Practical Guide to Experimental Design and Applications Igor R. Efimov, PhD, Martin Biermann, MD, and Douglas Zipes, MD

the effects of electrical stimuli on cellular electrical activity. Optical recordings of transmembrane Optical mapping of electrical activity in the heart with use of imaging tech- potentials can be performed in a wide range niques based on voltage-sensitive dyes of spatial resolutions from the subcellular has become an increasingly common level to the whole heart. The response time research tool in basic cardiac electro- of fast voltage-sensitive dyes lies in the physiology. This was prompted by the microsecond range and the temporal resofailure of conventionally used intra- or lution of the technique can potentially extracellular recordings to provide high- exceed that of conventional microelectrode resolution spatiotemporal maps of elec- recordings. Progress in modern computer trical activity. Despite a century of technology permits simultaneous optical evolution, conventional techniques have recordings from multiple sites, providing failed to work in at least 2 key areas of high-resolution spatiotemporal maps of research: the study of the role of spa- electrical activity. In this chapter we review current tiotemporal organization of repolarization in arrhythmogenesis and the study of technological approaches developed in the Introduction

This project was supported the by American Heart Association Northeastern Ohio affiliate Grant-in-Aid 9806201 and grant E01 HL59464-01A1 from the National Heart, Lung, and Blood Institute of the National Institutes of Health (Dr. Efimov), the Hermann C. Krannert Fund, grant PHS P50 HL52323 from the National Heart, Lung, and Blood Institute of the National Institutes of Health, and the research fellowship grant (Bi 583/1-1) from the Deutsche Forschungsgemeinschaft, Bonn, Germany (Dr. Biermann). From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; ®2003.

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area of fluorescence imaging and describe the new cardiac fluorescence imaging systems that have been developed by our research laboratories. History The possibility of recording dynamic changes in the transmembrane potential of excitable cells by optical means was first suggested in 1968 by a group of neuroscientists, Lawrence B. Cohen and co-workers,1 who discovered potential-dependent changes in the intrinsic optical properties of squid giant axons. It took nearly a decade until the first optical action potentials were recorded from giant axons2 and mammalian hearts by means of voltage-sensitive dyes.3 The first cardiac application of this method was the localization of pacemaker activity in embryonic heart preparations in 1981.4 While in the 1980s optical mapping of the heart was mostly restricted to a few noncardiac electrophysiology laboratories, widespread application of these techniques to problems unsolvable by other means began in the 1990s.5-15 Is optical mapping a technique for solving a problem, or do we have to invent problems for the technique? What is its niche in cardiac electrophysiology? One of the pioneers of fluorescent methods in neurophysiology, B.M. Salzberg, predicted that voltagesensitive dyes "could, we believe, provide a powerful new technique for measuring membrane potential in systems where, for reasons of scale, topology, or complexity, the use of electrodes is inconvenient or impossible."16 Based on our current experience in cardiac electrophysiology, Salzberg's list must be extended to recordings of action potentials in the presence of external electrical fields during stimulation and defibrillation, which were impossible with extracellular and intracellular electrodes. After decades of innovations and development, conventional electrode techniques have been brought to the limits of

perfection. The intracellular microelectrode technique still represents the gold standard for recording transmembrane action potentials, and only recently has the optical method approached comparable signal-to-noise ratios (SNRs). The chief disadvantage of the microelectrode technique lies in the impossibility of maintaining stable recordings over longer time periods from more than 2 or 3 sites, especially if the preparation is moving. Similarly, monophasic action potential recordings can be maintained only at a few sites for short periods.17 Thus, spatiotemporal mapping by either method requires the use of a roving probe, limiting the application of either method to the study of periodic activation patterns. While extracellular contact and noncontact multielectrode arrays are excellent for recording activation maps from the heart in vitro, they also remain the only techniques for in vivo mapping. Problems do, however, exist concerning the precise interpretation of the electrogram data,18,19 the determination of repolarization times being particularly unreliable.20 Thus, optical mapping is the only technique capable of recording high-resolution maps of cardiac repolarization.21 Finally, optical mapping is the only method that allows uninterrupted and artifact-free recordings of the transmembrane potentials during pacing stimuli11-13 and defibrillation shocks.7,22,23 Most reviews of the application of voltage-sensitive dyes in electrophysiology24-26 focused almost entirely on neuroscience. The only review on cardiac optical mapping was recently presented in a comprehensive book edited by Rosenbaum and Jalife.27 As highlighted by this book, optical mapping has become one of the major tools in studies of cardiac electrophysiology, significantly contributing to our exploration of various fundamental problems. Widespread application of these techniques was enhanced by the development of new

FAST FLUORESCENT MAPPING OF ELECTRICAL ACTIVITY IN THE HEART technologies during the last several years. Before then, setting up imaging systems for multisite fluorescence mapping of electrical activity in cardiac preparations posed a major technological challenge. Among other problems, it required the custom design of the imaging detector itself, a task which was beyond the capabilities of most research institutions. In this chapter we therefore not only review current technological approaches developed in the area of fluorescence imaging, but also describe the design and implementation of our new cardiac fluorescence imaging systems. These, in our opinion, represent the best current solutions for macroscopic mapping of cardiac electrophysiological activity in terms of image quality, cost and labor of setting up, and ease of operation. Physical Principles of Fluorescent Recordings The general mechanism underlying fluorescence is the absorption of photons of certain energy by a fluorescent compound, which is then excited from the ground state to an unstable energy-rich state. When the compound falls back to an intermediate lower energy level, the compound fluoresces by emitting a photon of a lower energy than the exciting photon. The wavelength of a photon being a function of its energy, the emitted light always has a longer wavelength than the exciting light, the so-called Stoke's shift. Potential-sensitive fluorescence can be the consequence of one of several mechanisms, which are related to voltage gradientdependent intra- and extramolecular rearrangements. Cohen and Salzberg,28 introduced a simple classification of voltage-sensitive dyes into 2 groups, fast and slow dyes, based on their response times and presumed molecular mechanism of voltage sensitivity. Only the fast probes are used in cardiac electrophysiology, due

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to their capability of following electrical responses on a time scale of microseconds.10,29 The precise mechanisms underlying the voltage-dependent spectroscopic properties of fast voltage-sensitive dyes are still not fully understood. According to the electrochromic mechanism, one of the most commonly accepted theories,25 the spectral shift in a chromophore's properties is voltage dependent if 2 conditions are met: (1) the light photon-produced excitation of the chromophore molecule from the ground to an excited state is accompanied by large shift in electronic charge, and (2) the vector of the intramolecular charge movement is oriented in parallel to the electrical field gradient. If the charge movement in the dye molecule occurs perpendicular to the cellular membrane of a cardiomyocyte, a dye's fluorescence will then be sensitive to the transmembrane potential. An alternative theory is the solvatochromic mechanism, which is related to electricalfield-induced reorientation of the dye molecule.30 Dye molecules experience a change in the polarity of the lipid environment during reorientation produced by the voltage gradient. Therefore, energy needed for excitation from the ground state to the first excited state is released during transition in the opposite direction and will be voltage dependent. This dependency causes the spectral voltage dependence of the chromophore. Designers of voltage-sensitive dyes had to solve several problems: 1. finding a chromophore that is capable of producing the largest movements of charges during quantum transition from the ground to the first excited state, and therefore the largest measurable spectral changes with a change in the external electrical field; 2. assuring the possibility of delivering dye molecules to the cellular membrane;

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3. assuring the proper orientation of the dye molecule perpendicular to the membrane; 4. maximizing duration of stay of the dye molecules in the desired position; 5. minimizing photobleaching of the dye; 6. minimizing side effects of the dye on the preparation in the presence and absence of light.

60 seconds, presumably on the basis of the slower translocation of di-8-ANEPPS into the cell. The price for the improved lipophilic properties of di-8-ANEPPS is however a decrease in the water solubility; this necessitates the use of surfactants, such as pluronic (F127, BASF Corporation, Mount Olive, NJ), which may not be free of toxic side effects.34 In a series of Langendorff-perfused and superfused heart preparations, we were able to achieve superior SNRs with di-8In tests of more than 1500 different ANEPPS and pluronic as compared to dicompounds, several useful classes of chro- 4-ANEPPS in only 2 of 8 experiments, mophores have emerged, including mero- while in the other 6 experiments the SNR cyanine, oxonol, and styryl dyes. Styryl was significantly worse. In our opinion, dyes represent the most popular family of di-4-ANEPPS remains the dye of choice dyes, RH-421, di-4-ANEPPS, and di-8- for whole-heart and tissue preparations. The SNR in stained cardiac prepaANEPPS being the most important members of this family. The spectroscopic rations is dependent not only on the dye properties of these dyes have been shown itself, but also on its mode of delivery. to change linearly with membrane poten- Salama32 noted that the SNR in optical tial changes in normal physiological range action potentials in frog hearts stained of transmembrane voltages in axons31 by injection of merocyanine-540 into the and heart.32 The orientation of the mole- aortic root during Langendorff perfusion cules of these dyes in the cell membrane was 10-fold higher than in similar prepais assured by the presence of lipophilic rations stained by superfusion with bath and hydrophilic groups at opposite ends of solution containing the voltage-sensitive the molecule. While the hydrophilic, neg- dye. In the rat papillary muscle, Miiller atively charged sulphonyl group anchors et al.35 were able to demonstrate by fluothe dye molecule in the aqueous extra- rescence microscopy that superfusion cellular space, the highly lipophilic hydro- with 10 to 25 umol/L di-4-ANEPPS stained carbon chains at the other end of the no more than 1 or 2 surface cell layers, molecule hold it within the bilayer lipid whereas preparations stained by arterial perfusion are expected to show a more or membrane. The stability of the position of dye less homogeneous distribution of dye molecule in the membrane can be throughout the wall. The precise depth improved by increasing the length of of optical recordings in preparations the hydrocarbon tails as was done in the stained by perfusion has been the subject ANEPPS family (di-4-ANEPPS, di-8- of debate. Model calculations by Salama32 ANEPPS, di-12-ANEPPS, nomenclature based on the depth of field of the optical described by Loew33). In single-layer cell system predicted a depth of 144 um. culture preparations, Rohr and Salzberg10 Direct measurements by Knisley36 in a demonstrated a significantly retarded tapering wedge of tissue showed that the decay of the amplitude of optical action intensity of optical action potentials potentials in preparations stained with ceased to increase if the thickness of the di-8-ANEPPS as compared with di-4- tissue was larger than 300 (um.Based on ANEPPS on continuous illumination for measurements of the absorption coefficient

FAST FLUORESCENT MAPPING OF ELECTRICAL ACTIVITY IN THE HEART of myocardium for the excitation and emission spectrum, Girouard et al.37 predicted that 95% of the signal energy originates from a tissue depth of 500 (um or less. Our recent observations in the rabbit atrioventricular nodal preparation38 are in agreement with that of Girouard et al.37 Pharmacological Effects of Voltage-Sensitive Dyes The fact that the application of voltagesensitive dyes has been demonstrated to cause side effects on preparations calls for careful control experiments in in vitro studies. So-called photodynamic damage or phototoxic effects have been documented under intense illumination, with alterations of electrical activity in neurons39 and in isolated cardiac myocytes.40 In the isolated cardiac myocytes, 1 minute of illumination with 1 W/cm2 will first cause a gradual depolarization of the membrane resting potential and a decay of the action potential amplitude. This is followed by afterdepolarizations, occasionally triggered activity, and finally cell death. Recently, the voltage-sensitive dye RH421 has been shown to increase the contractility of isolated rat cardiac cells and Langendorffperfused hearts.41 Similar effects were observed during staining with di-4ANEPPS and di-8-ANEPPS. The low level of light used in this study (<1 mW/cm2) suggests that a mechanism different than the formation of free radicals may be involved. Another pharmacological side effect of potential-sensitive styryl dyes is vasoconstriction. This effect was first described for the dye RH-414 by Grinvald et al. in 198442 and has recently been observed for di-4-ANEPPS. Concentrations above 2.5 umol/L led to a reproducible increase in the intravascular resistance of isolated arterially perfused wedge preparations of the canine's left ventricle (LV) in the absence of light (M.B., unpublished observations).

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The exact mechanism of these effects of voltage-sensitive dyes remains unknown. Formation of free radicals, sensitized by the dye molecules in presence of photons, has been implicated in causing the phototoxic effect,43 and the use of radical scavengers (antioxidants) has been shown to reduce the phototoxic effect in isolated cardiac cells.40 The voltage-sensitive dye may directly interact with voltage-gated channels, perhaps L-type Ca2+ or K+ channels, and may alter the conductivity and time-dependent gating of these channels. This hypothesis is supported by data obtained in bilayer preparations, which suggested an interaction of voltage-sensitive dyes with a number of channels. Rokitskaya and coworkers44 showed that RH421 increased the dissociation constant of gramicidin in bilayer preparations, and proposed that this resulted from modification of the dipole potential of the bilayer membrane by RH421. Data from several laboratories have suggested an interaction of RH421 with Na+,K+-ATPase45-47 though there is yet no consensus regarding the mechanism(s) underlying this effect. Frank et al.47 implicated an RH421-induced change in membrane fluidity in the inhibition of the hydrolytic activity of the Na+,K+-ATPase. Fedosova and coworkers,46 on the other hand, proposed an electrostatic mechanism of interaction between potential-sensitive styryl-based dyes (including RH421) and Na+,K+-ATPase. Finally, RH421 has been shown to interact with the water-soluble protein ribulose-1,5bisphosphate carboxylase/oxygenase as well as with polyamino acids (tyrosine, lysine, and arginine residues).47 Mechanical Motion Artifact Faithful optical recordings of cardiac action potentials require immobilization of the preparation, unless the study is limited to the analysis of action potential upstrokes only.9,10 Movement artifacts in

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optical action potentials tend to be most pronounced during the action potential plateau when contraction reaches its peak amplitude, and during the action potential downstroke when relaxation sets in, while action potential upstrokes are generally well preserved. Movement artifacts can occur due to one of several mechanisms. First, physical movement of the tissue from the field of view of one detector element into the field of view of a neighboring element may lead to artifactual recordings of transmembrane potentials from different but neighboring cells in different phases of the action potential. This kind of motion artifact tends to be most pronounced at the edges of the preparation. Second, action potentials can be distorted due to the modulation of light scattering by mechanical contraction, a phenomenon that can be observed in monolayer cell cultures even in the absence of gross movement of cells across the photodetector array.10 There are 3 basic approaches for coping with motion artifacts: (1) mechanical immobilization; (2) use of motion-artifactinsensitive signal analysis algorithms; and (3) pharmacological immobilization. Preparations can be mechanically immobilized by restricting the movement of the muscle between the walls of the tissue chamber and/or pressure pads32,48 or by stretching the tissue.32 While mechanical immobilization is free of pharmacological side effects on the electrical activity of the preparation, this technique has several limitations. This method can only be used in preparations with moderate amplitude of contractions, otherwise the pressure needed to eliminate movements can cause ischemia. This technique has only been successfully applied to guinea pig heart preparations.14,21 In addition to external compression of the heart by pressure pads, Girouard et al14 used endocardial cryoablation,49 which leaves a thin epicardial

rim of viable muscle attached to a noncontracting core of dead myocardium.50 Despite the efforts, mechanical immobilization does not fully eliminate motion artifacts in all channels even in a guinea pig heart. Therefore, special signal processing techniques based on maximum of second derivative of the optical action potential21 are usually employed to measure action potential duration instead of standard criteria such as action potential duration measured at 90% repolarization. The disadvantage of this technique is, however, that it requires very good SNRs in the optical recordings. Pharmacological methods of producing mechanically quiescent preparations may significantly affect electrical activity. Several methods are being used: perfusion with low calcium,3,12 Ca channel blockers,7 2,3-butanedione monoxime (BDM),51 and, most recently cytochalasin D (cyto D).52-54 Low calcium solutions and calcium channel blockers are rarely used in fluorescent recordings of transmembrane voltage because of their effect on crucial calcium-dependent cellular processes.3,37,55 BDM, also known as diacetyl-monoxime (DAM), has effects on a variety of channels13,56-58 and gap junctions.59 The effect of BDM on action potential duration is species dependent. A shortening of action potential duration in rabbits,60 sheep,57 guinea pig,57 and dog hearts is observed, while action potential duration increases in the rat heart.58 This species dependence is the consequence of the reduction of both the calcium current and net potassium outward current, 2 antagonistic currents with opposing influences on action potential plateau development. Thus, even though the above pharmacological methods effectively suppress contractions of the cardiac muscle, they significantly alter the electrical activity of cardiac cells, limiting their usefulness in studies related to repolarization and requiring carefully designed control experiments. Cyto D, an

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Figure 1. Optical recordings of action potentials. An epicardial fluorescence recording from a Langendorff-perfused rabbit heart stained with di-4-ANEPPS (top trace) is juxtaposed with a simultaneous bipolar electrogram from the apex of the same heart. Fluorescence was excited at 520 ± 45 nm, collected above 610 nm, amplified, filtered at 500 Hz, and sampled at 1000 Hz. The light was turned off before the end of the recording to demonstrate the amount of background fluorescence in fluorescence signals without direct current-offset subtraction.

actin filament disrupter, has recently been shown to block contraction without affecting action potential shape or duration in isolated rat myocytes in concentrations between 4 and 40 u.mol/L,52 in isolated canine right ventricular (RV) trabeculae in concentrations of 80 umol/L,53 and in perfused LV wedges at concentrations of 20 to 25 umol/L,54 and may thus be a promising new excitation-contraction uncoupling agent for optical mapping studies of cardiac repolarization. Approaches to Experimental Design Every engineering approach to the design of an optical system must address

one major problem: improving the SNR in the optical recordings at the required spatial and temporal resolution. This is achieved by decreasing system noise and/or improving the amplitude of the signal. An example of a raw optical signal is shown in Figure 1. Cellular depolarization during an action potential causes a reduction in fluorescence of 1% to 10% of the total fluorescence signal. Background fluorescence, which thus accounts for up to 99% of the fluorescence signal, is caused by accumulation of dye in nonexcitable cells, in the inner layer of the lipid membrane, or in the lipid membranes of intracellular organelles across which there is no potential change during excitation.10 To reconstruct the intracellular action potential it

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is therefore necessary to (1) subtract the background fluorescence; (2) invert the signal; and (3) normalize the signals to uniform amplitudes assuming a homogeneity of action potential amplitudes in the imaged area of the preparation. Noise

There are 3 major sources of noise present in optical recordings: shot noise, dark noise, and extraneous noise. Shot noise is the natural fluctuation in the number of photons detected by a photodetector caused by the quantumstatistical nature of photon emission and detection. Thus, shot noise occurs even in the presence of an ideal noise-free light source and ideal noise-free detector and cannot be eliminated. Shot noise is estimated by the root mean square deviation of the number of photons hitting a photodetector per unit time and is equal to the square root of their number. A typical tungsten lamp filament (1800°C) emits 1014 photons/s. Only a small fraction of these, at best about 1010 photons/s,61 will reach the photodetector, because of significant losses in the illumination optics with their narrow-band excitation filter and dichroic mirror and because of losses in the imaging optics with their long-pass emission filter. The number of photons detected by the photodetector will finally be reduced by the quantum efficiency of the photodetector, which is defined as the number of photoelectrons per photon. Of the 2 types of photodetectors used for optical recordings, photodiodes and photocathodes, the former have quantum efficiencies of 0.8 to 1.0 and the latter of only 0.15. Thus, photodiodes have a significant advantage over photocathodes, potentially yielding a nearly 6 times higher SNR. Dark noise is the noise signal emitted by a photodetector in the absence of light. Photodiodes tend to have much higher

dark noise than photocathodes, which despite their much higher quantum efficiency limits their utility at low light intensities such as in single-cell measurements, in which case the dark noise can be comparable to or even higher than the shot noise. Thus, photocathodes may yield higher SNRs than photodiodes at very low light levels, and a direct experimental comparison between the 2 detectors is required in each case. Extraneous noise is caused by noise sources in the laboratory environment. The following measures serve to cut extraneous noise down to acceptable levels: using a dark or direct current (DC)-lightilluminated room to eliminate stray light from noisy sources, using an antivibration table to isolate the set-up from mechanical vibration generated in the building and the rest of the experimental set-up, using a Faraday cage to reduce radiofrequency noise, grounding equipment to a common isolated ground to eliminate 50- or 60-Hz noise picked up by ground loops, using a low-noise light source, and isolating power supplies and amplifiers and from computing equipment. Light source and filters

Three types of excitation light sources are used in optical recordings: tungsten lamps, arc lamps, and lasers. The choice of a light source will depend on the required spatial resolution of the optical recordings. Arc lamps and lasers are typically used in micrometer-scale measurements, while tungsten lamps are most commonly used in macroscopic preparations, in which a resolution of hundreds of microns is sufficient. Lasers can provide intense illumination, which can be easily and rapidly delivered to a small spot. However, lasers have a 1% to 5% variation in the beam intensity, which is comparable to the average signal intensity recorded from most

FAST FLUORESCENT MAPPING OF ELECTRICAL ACTIVITY IN THE HEART voltage-sensitive dyes.62 Ratio-calculation feedback signal processing techniques have been applied during recordings to eliminate signal noise related to laser light intensity variability.62,63 Recent models of sapphire lasers appear to overcome this limitation. They demonstrate remarkable stability and intensity of the light. However, they are available in a limited number of wavelengths, which usually do not correspond to excitation wavelengths of most of the dyes. In addition, these lasers have a very high cost. While the intensities of arc lamps have been reported to exceed 50 to 100 times those of tungsten lamps,39 this large difference in intensity cannot be directly translated into better SNRs, as arc lamps have a significant intensity only at distinct narrow lines of the spectrum. Unfortunately, the excitation frequency of the most efficient dyes which produce the largest fractional change in fluorescence, such as di-4-ANEPPS and RH421, do not overlap with spectral lines of arc lamps. Since the absorption peaks of most of the voltage-sensitive dyes are relatively wide, the lower light output of tungsten lamps at a given spectral line can be compensated by the use of wider band-pass excitation filters. Tungsten lamps remain the most popular light source used for optical recordings. They provide a very stable output of light over a very wide range of the spectrum without the sharp peaks observed in the arc lamps' spectra. The choice of excitation filters with half-width of 90 nm can provide significant improvement in signal quality over narrow-width filters. We recorded action potentials from a Langendorff-perfused rabbit heart (n = 3) stained with di-4-ANEPPS using several narrow- and wide-band excitation filters. There was no significant difference in signal quality observed between the 2 narrow-band filters (520 ± 10 nm and 540 ± 10 nm), while recordings with

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520 ± 45 nm and 545 ± 30 nm yielded 3.1 and 2.4 times better SNR, respectively (I.E., unpublished observations). The increase in excitation bandwidth did not result in a measurable increase in phototoxicity or photobleaching.22 Photodetector design

Three engineering solutions have been used in cardiac electrophysiology for optical multiple-channel recording systems over the last 2 decades: photodiode arrays (PDAs),64 laser scanner systems,65 and charge coupled device (CCD) cameras.51 They differ not only by their way of collecting the fluorescent output of the specimen but also by the way the excitation light is delivered. PDAs have been used for optical mapping studies in neurophysiology and cardiology since 1981.66,67 With quantum efficiency above 0.8,61 photodiodes are the most sensitive sensors for medium to large light intensities, their main drawback being the size of their dark current, which may limit their usefulness at very low light intensities as in neurophysiological applications. Photodiodes are packaged in arrays of 100, 144 or 256, and 464 or more, and each photodiode will record from a large enough surface area of the preparation to receive enough photons per unit time for the accurate representation of an optical action potential. Each recording channel requires independent signal conditioning before analog-to-digital (A/D) conversion by the computer. Subtraction of background fluorescence in optical action potentials is possible on a per-channel basis either before or after A/D conversion, which can be with a resolution of 12 bit or more at sample rates in excess of 1 kHz. CCD cameras were introduced into optical mapping to achieve higher spatial resolutions and avoid the complications of setting up PDA systems. However the

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accuracy of mapping data with CCD cameras is limited by a number of factors: (1) The SNR in signals from CCD cameras is usually very poor, each small pixel being hit by very few photons per sample interval. Only a few CCD cameras allow the so-called binning of pixels, i.e., aggregating several pixels into one, in order to improve SNRs. (2) The time resolution is generally 16.7 ms, which is the National Television Standards Committee standard, 4 ms in the faster cameras and 1 ms only in dedicated high-speed cameras. (3) Many cameras do not allow subtraction of the background fluorescence, and if they do, it is a uniform offset potential subtracted off all channels. (4) Amplitude resolution is usually only 8 bits, so that assuming a fractional fluorescence of the dye of 10% per 100 mV, rarely more than 10 to 20 out of the 256 gray levels will be available for encoding an action potential. Recently, Wikswo and colleagues13 presented details of a new mapping system, in which a cooled CCD camera system was able to achieve a single pixel signal-to-noise rate of 5 to 10 at a spatial resolution of 128 x 127 pixels and 1.2 ms with 12-bit A/D conversion. Several new CCD cameras are very promising. Yet, they still yield significantly to photodiode arrays in signal quality. Laser scanning systems represent an entirely different approach, which has been described in detail elsewhere.68 The output of a single laser is acousto-optically deflected to scan some 100 sites of the whole preparation. The fluorescence emitted by each site at the time of illumination is collected by a single photodiode, which thus sequentially records the optical signals from all sites scanned by the laser beam. The advantages of laser scanning systems over PDAs are that they can cover a wider area and that flatness of the preparation is not an optical requirement. The time resolution, however, is limited to some 1 ms per 100 scanning sites.

The major disadvantage of the laser scanning techniques is the considerable photobleaching at the light intensity levels required for reconstruction of optical action potentials, which will result in a significant decrease of the level of fluorescence on a beat-to-beat time scale, necessitating a recalibration of the signals.69,70 There was no statistically significant decrease in fluorescence on the same time scale through photobleaching in experiments using a tungsten lamp in conjunction with a photodiode array detection system.22 The main drawback of photodiode arrays was that until recently the difficulties in building PDA-based mapping systems were formidable, as there were no ready-made products on the market. This approach could only be used by a few groups with access to advanced engineering resources; however, the situation has changed with the advent of the Hamamatsu C4675 photodetector (Hamamatsu City, Japan), a 256-element photodiode array manufactured in a compact enclosure complete with 256 current-to-voltage converters, with the availability of affordable bioamplifiers (~$30 per channel) originally developed in the laboratory of Dr. Lawrence B. Cohen of Yale University61 and with powerful multichannel A/D conversion boards for personal computers. An improved version of the 64channel bioamplifiers was independently developed by Innovative Technologies (Germantown, MD), in collaboration with Dr. Efimov's laboratory. While previous optical mapping systems had to be custom designed from the ground up, most hardware components can now be bought. In the following text we describe the set-up of our current mapping systems, which were originally developed in the Department of Cardiology at the Cleveland Clinic Foundation and then further improved in close collaboration with the Krannert Institute of Cardiology.

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C4675 PDA detector, which combines a 16 x 16 element photodiode array (chip The core of the optical mapping size 17.45 x 17.45 mm2 with 256 square system (Figure 2) is the Hamamatsu 0.95 x 0.95 mm2 photodiodes spaced 0.15 mm apart) with 256 current-to-voltage converters in a single compact (136 x 136 x 154 mm) enclosure. The optical system is built around a central beamsplitter cube (Oriel Corp., Stratford, CT) bearing the illumination system, a bellows apparatus with the imaging lens, a ground glass screen and reticule for focusing the imaging optics, and the above photodetector. The bellows, beamsplitter cube, and photodetector are all mounted on an optical rail (Nikon, Torrance, CA) born by a ball-bearing boom-stand (Diagnostic Instruments, Sterling Heights, MI), which permits easy readjustments of the detector in all 3 dimensions, including a change in orientation between vertical and horizontal preparations.15,22 The excitation light is produced by a 250-W quartz R tungsten halogen lamp (Oriel Corp.) powFigure 2. Experimental set-up. Fluorescence in ered by a low-noise DC power supply cardiac preparation stained with the potentialsensitive dye di-4-ANEPPS is excited by the (Oriel Corp. or Power-One, Camarillo, CA). light of a 250-W direct-current-powered tung- After cooling by means of a cold mirror, sten lamp. After passing a cold mirror and an which lets the infrared spectrum pass into electrical shutter controlling the light beam, the a finned heat sink (Oriel Corp.), the light beam is made quasimonochromatic by means path is controlled by an electronic shutof an infrared filter and a 520 ± 45 nm interference filter. It is then deflected into the light path ter (Oriel Corp.), which only opens for a of the illumination optics by means of a dichroic few seconds during each scan. The light mirror and finally focused on the preparation by beam is made quasimonochromatic by the imaging lens. Light emitted from the prepa- passing it through an infrared filter (KGl, ration is focused on the light-sensitive area of a Schott Glass, Duryea, PA) and 520 ± 45 nm 16 x 16 element photodiode array (PDA) by the imaging lens after passing the dichroic mirror interference filter (Omega Optical, Batand filtering with a long-pass (>610 nm) colored tleboro, VT). A 585-nm dichroic mirror glass filter. The photocurrents of all 256 photo- (Omega Optical) held in the beamsplitdiodes are converted into voltages by 256 first- ter cube deflects the excitation light into stage amplifiers integrated in the housing of the the imaging lens, which then focuses the PDA. The signals are then amplified and filtered in parallel by a 256-channel second-stage ampli- excitation light onto the preparation. Our fier and finally multiplexed and analog-to-digital original optical design had kept the illuconverted together with 8 additional instrumen- mination optics separate from the imagtation channels by a PC-based data acquisition ing optics, using a liquid light guide and analysis system. To focus the optics on the (Oriel Corp.) to direct the illumination preparation, a front-surfaced mirror replaces the dichroic mirror, deflecting the light from the light onto the preparation. A comparison preparation on a ground-glass reticule bearing of the illumination systems showed that a scale 1:1 outline of the photodiode array. epi-illumination through the imaging PDA System

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optics produced 3 times better SNRs and signal amplitudes as well as improved homogeneity of illumination. Since then, the use of the liquid light guide has been discontinued. The imaging optics consist of a Nikon photographic lens attached to the central beamsplitter cube (Oriel Corp.) by the means of an f-mount adapter ring (Newport, Irvine, CA) and a pair of Nikon bellows. The magnification of the system is set by expanding or shrinking the bellows, while focusing is performed by adjusting the distance between the imaging lens and the preparation. In imaging mode, the fluorescent light emitted from the preparation passes the dichroic mirror without reflection and is then filtered by a long-pass filter (>610 nm, Schott) before hitting the sensing area of the 16 x 16 photodiode array. In focusing mode, the dichroic mirror inside the beamsplitter cube is replaced by a front-surfaced plain mirror that deflects the image of the preparation onto a ground glass reticule, which bears a 1:1 representation of the outline of the photodiode array at precisely the same distance from the center of the beamsplitter cube as the photodetector. We compared several Nikon lenses: 85 mm f/1.4 AF Nikkor; 50 mm f71.2 Nikkor, 50 mm f/1.4 Nikkor, and 28 mm f/2.8 AF Nikkor. Table 1 summarizes range of field of view seen by the detector and by a single photodiode (pixel resolution), as well as the corresponding working distances for different expansion states of the bellows.

The photocurrent produced by each photodiode was first-stage amplified by its own low-noise operational amplifier inside the compact housing of the Hamamatsu C4675 detector (feedback resistors 10 to 100 MQ, resulting in a gain of 107 to 108 V/A). Increasing the first-stage amplification has been reported to improve SNRs in optical signals71; however, increasing the feedback resistors in the C4675 camera reduced its frequency response from 15 kHz to 1.5 kHz, which may be undesirable in some applications. The outputs of the first-stage amplifiers were connected to 256 second-stage amplifiers, four 64-channel cards, developed at and available from Yale University, which offer DC coupling and AC coupling with several time constants (short time constant for DC-offset subtraction, time constant of 30 seconds during data acquisition). A computer-driven transistor-transistor logic pulse is used to reset the second-stage amplifiers immediately before data acquisition in order to remove the DC offset of the optical signals caused by background fluorescence. Signals were filtered by Bessel filters with a cutoff frequency of 500 to 2000 Hz, depending on the sampling rate of the data acquisition system. The signals were fed to a multiplexer and an A/D converter board (DAP 3200/ 415e, Microstar Laboratories, Bellevue, WA). Sampling was performed at a rate of 1000 to 2000 frames/s. The sampling rate S and the cutoff frequency fc of the filters were always kept to satisfy Nyquist criterion (S = 2 fc) in order to avoid aliasing

Table 1 Field of View and Working Distances for Different Nikon Photographic Lenses At 0 To 200 mm Expansion of the Bellows Lens F/1.250mm F/1.4 50 mm F/1.4 85 mm F/2.8 28 mm

Field of View of Photodiode Array (mm) Single Photodiode 4.0-18 3.8-19 6.0-31 5.5-11

250-1120 230-1190 380-1940 340-690

Working Distance (mm)

5-46 13-49 70-200 2-10

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stimulator into the mapping system using a PC-TIO-10 timer board (National Instruments, Austin, TX), which provides 10 programmable 16-bit counters. The front end of our data acquisition system and data analysis package was programmed under Lab VIEW 5.0 for Windows NT, and integrates a fully featured programmable stimulator, control of the Computer-controlled instrumentation interface, mapping system, and an extensive data data acquisition and analysis processing, visualization, and analysis One of the biggest problems in map- package. Supported stimulation protocols include ping system design is the development of user-friendly and efficient software for burst and continuous pacing S1 with up to 3 premature pulses (S2, S3, S4) out of the data acquisition and analysis. While the chief problem in electrical first output channel and a single pulse mapping is optimizing system perfor- triggered on any previous pulse out of a mance to allow continuous data acquisi- second output channel for cross-field stimtion to a high-capacity storage medium ulation or control of another device, such such as a hard disk or a digital tape over as an external stimulator. Any number of long time intervals, in optical mapping, it mapping protocols can be defined and is also essential to provide integrated and stored by the user. For each mapping proeasy-to-use control over the complex instru- tocol, the user can select the name of the mentation. To reduce problems of photo- protocol, the state of the electrical shutter toxicity and/or photobleaching, the exposure during mapping (open or shut), the screen of preparations to intense light must be color during mapping (normal or black to kept to a minimum. Data are typically eliminate stray light from the computer only acquired in short bursts of a few sec- monitor during mapping), the mapping onds, and care must be taken that data script (full array for the entire set of chanacquisitions are not repeated unneces- nels, external for the instrumentation sarily. An optical mapping system control channels only), the matrix that will be unit must (1) precisely synchronize the used to display the data in the analysis initiation of a mapping sequence with the program, the shutter open and close times stimulation of the preparation; (2) pro- relative to the last S1 stimulus of the train, vide pulses for the control of the electri- the width and end of the sample-and-hold cal shutter of the light source, for the DC pulse to the amplifiers, the start of the offset subtraction of the signal amplifiers data acquisition relative to the last S1 and the start of the data acquisition by stimulus, the duration of the data acquithe A/D boards; (3) allow the fast logging sition, the trigger mode, and the sample of data to disk; and (4) give the operator interval in microseconds. Whenever a near-instant feedback on the quality of mapping sequence is initiated, the Labthe logged data so that the mapping VIEW timer program calls an external sequence can be repeated if necessary. 32-bit command-line executable written The purpose of the first requirement under Borland C++ 5.0 using the DAP for is to control stimulation as well as all Windows tool kit (Microstar Laboratories), other components of the mapping system which loads the appropriate data acquiby the same timing device. We therefore sition script into the data acquisition chose to integrate the programmable board's memory, initiates the data logging errors introduced by digitization of the signals. Each frame included 256 optical channels and 8 instrumentation channels including surface electrogram, stimulation and defibrillation triggers, and aortic pressure, which were stored on a hard disk for off-line analysis.

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by the board, and saves the data file to disk. Each data file contains an ANSI header, which not only contains the mapping protocol, time, and date of the recording, but the complete designation of the stimulation protocol as well. Once the data file has been written to disk, the LabVIEW data acquisition program notifies the data analysis program, which then displays the entire mapping data file on the screen for near on-line analysis of the data so that an informed decision can be made on whether the quality of the data is appropriate or the mapping sequence must be repeated. Analysis algorithms include various filtering, calibration, reconstruction of activation, repolarization, and action potential duration maps, subtraction of 2 corresponding responses in order to visualize the differences between them caused by electrical shocks, etc. Experimental preparations

Experiments can be performed in vitro on Langendorff-perfused wholeheart preparations and isolated superfused or coronary-perfused atrial and ventricular preparations in rabbits, dogs, guinea pigs, rats, and humans. The precise details of animal preparation protocols are published elsewhere.15,22 For staining, a stock solution of 5 mg di-4-ANEPPS (Molecular Probes, Inc., Eugene, OR) is prepared in 4 mL dimethyl sulfoxide (DMSO, Fisher Scientific, Pittsburgh, PA), and is stored frozen at +4°C. After gentle rewarming immediately before the experiment, a 100 uL Hamilton glass syringe is filled with the stock solution, which is then gradually injected at a rate of less than 1:1000 into an injection port (Radnoti Glass, Monrovia, CA) above the bubble trap of the perfusion system by means of a custom-built infusion pump. The method of gradual hand injection of dye into the injection port in the bubble trap21 is less cumbersome but less repro-

ducible. Recordings started 5 to 15 minutes following the staining procedure. Levels of optical signals and SNRs decrease over time, presumably because of the translocation of the voltage-sensitive dye molecules into the cell interior, as is illustrated in Figure 3. We monitored the SNR in all 256 channels during a 3-hour mapping experiment in a Langendorff-perfused rabbit heart. The SNR was defined as the ratio between optical action potential amplitude and peak-to-peak noise amplitude as measured during diastolic intervals, while the mean value and standard deviation were as calculated from all 256 recordings. Using a single exponential function approximation (SNR = SNR0 e-t/t) we estimated on half-life of the signal i = 105.2 ± 31.2 minutes. Restaining was done in some preparations, when needed. Areas of Application of the Mapping Technique Two-dimensional submillisecond mapping of transmembrane polarization during defibrillation shocks in intact heart

Optical recording of electrical activity is the only method to resolve questions related to the interaction of strong electrical fields with excitable cells, conventional electrode recordings being distorted by large-amplitude artifacts. With use of fluorescent mapping techniques, we have been able to optically record changes in transmembrane voltage with a high SNR from all 256 channels. This has permitted us to acquire 2-dimensional patterns of shock-induced polarization with submillisecond resolution. Figure 4 shows upstrokes from 2 optical action potentials recorded from the same 650 x 650 (um area of RV epicardium. As can be seen, however, recordings during defibrillation shocks have much faster components22 than normal propagated responses, the frequency content of which

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Figure 3. Decline of signal-to-noise ratios over time. The figure plots mean ± SD of the signal-tonoise ratios in all 256 fluorescence recordings from the epicardial surface of a Langendorff-perfused rabbit heart stained with di-4-ANEPPS versus time. The preparation was kept in a darkened room and was illuminated only during data acquisition for 1 to 2 seconds at a time.

has been reported to be limited to 150 Hz.37 Therefore, faster sampling is required. In our experiments signals were conditioned by a low-pass 1-kHz Bessel filter and sampled at 2 kHz without any additional filtering by software, in order to preserve the frequency content. The reason for a slower rise time of propagated responses is that optical action potentials represent spatially integrated responses of cells confined within the 3-dimensional field of view of each photodiode, which extends some 100 to 500 urn in depth.21,36,37 Thus, optical action potential upstroke rise times are a function of both conduction velocity and the size of the field of view.32 Figure 5 presents several examples of typical cellular responses during biphasic shocks applied in the plateau phase of a propagated action potential. A data

scan included the last normal propagated action potential before the shock (left waveform on trace A) and the action potential altered by the shock (right waveform on trace A). Panels B through E show 2 such action potentials from different regions of the mapping array that are superimposed and aligned by their upstrokes. As can be seen, postshock action potential prolongation is strongly dependent on the polarity of the shock and its timing in respect to the phase of the action potential. Figure 6 shows the entire mapping array with all 256 potential waveforms recorded from an 11 x 11 mm2 area of the LV anterior wall of a rabbit heart during application of a +100/-50 V, 8/8 ms biphasic shock 90 ms after the onset of the action potential in the middle of the array. Recordings such as these cannot be

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node in isolated preparations of the rabbit right atrium.15 The structural complexity of the right atrial preparation, especially in the nodal areas, makes it difficult to apply conventional electrode mapping techniques. While in unipolar electrograms the distinction between local and distant events is often impossible, bipolar electrograms represent complex spatial derivatives of the underlying spatiotemporal substrate of electrical activity.20 Since the precise field of view of both types of electrodes is unknown, interpretation of the electrograms remains challenging and requires verification by means of simultaneous microelectrode recordings.73 Like electrograms, optical recordFigure 4. Optical action potential upstrokes of a ings represent the integrated electrical propagated response during epicardial pacing (circles) and a response induced by an 8-ms 100-V response of cells from a volume of tissue, monophasic electrical shock (squares) indicated the 2-dimensional extent of which is preby the time bar. The data was amplified after direct cisely defined, even though the exact current-offset subtraction, filtered at 1 kHz, sampled depth of the recording is less certain. We at 1.9 kHz (sample interval 528 (us), and reprehave previously suggested that doublesents the summated response of the 500 x 500 um region of subepicardial myocardium within the field component optical action potentials of view of one photodiode. observed in proximal AV node and area of the bundle of His may result from spatial achieved by any other known method. As averaging of electrical activity from sevcan be seen, cellular responses recorded eral layers.15 With use of microelectrode at different locations with respect to the recordings, we were recently able to demonelectrode can differ dramatically in ampli- strate that optical recordings from the tude and polarity of cellular polarization.22 proximal part of the AV nodal area carry Most importantly, these experimental inscriptions of both superficial layers of techniques allowed to link together sev- transitional cells and deeper layer of comeral important parameters, which are pact nodal and nodal-His (NH) regions.38 responsible for arrhythmogenesis during Figure 7 illustrates this finding by juxtafailed defibrillation shocks, such as vir- posing 2 optical traces with conventional tual electrode dispersion of transmem- bipolar surface electrograms recorded brane polarization followed by dispersion from the interatrial septum and the of repolarization, and new wavefront for- bundle of His during 2 basic beats and a mation followed by formation of phase sin- single premature beat. Activation rapidly spread from the stimulation site on the gularities and reentry.72 interatrial septum toward the AV nodal area and finally conducted at the bundle Conduction from the sinoatrial node to and of His. The optical trace recorded from through the atrioventricular node the distal node shows multicomponent We recently demonstrated a nonra- responses. While the first component (T) dial spread of activation from the sinoa- in every beat represents activation of trial (SA) node to the atrioventricular (AV) superficial transitional cells, in which

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Figure 5. Optical recordings during defibrillation-strength shocks. Traces A through E were recorded from the same 650 x 650 um area of the left ventricular epicardium 8 mm from the defibrillation electrode, which was positioned close to the septum of the right ventricular cavity in a Langendorffperfused rabbit heart. This area corresponds to the "virtual electrode" area.22,72 Optical signals were amplified, filtered at 1 kHz, and sampled at 2 kHz. No additional software filtering was applied. During stimulation at 300 ms from the right ventricular apex, shocks were applied during the action potential plateau. Trace A shows a sequential record of a normal action potential followed by the action potential during and after application of the shock, while in traces B through E the 2 action potentials are superimposed, being aligned on their upstrokes. Traces B and C were recorded during shocks applied 100 ms after the upstroke, while traces D and E were recorded during shocks applied 50 ms after the upstroke. Traces B and D were recorded during +150/-100 biphasic shocks applied 100 and 50 ms after the upstroke, while traces C and E were recorded during biphasic shocks of the opposite polarity 100 and 50 ms after the upstroke. As can be seen from the recordings, the action potential prolongation through biphasic defibrillation shocks is strongly dependent on both timing and polarity of the shock.

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Figure 6. Map of 256 optical action potentials recorded from anterior epicardium during application of a +150/-100-V biphasic defibrillation shock applied during the plateau phase of cardiac action potentials in a Langendorff-perfused rabbit heart. The gray rectangular area indicates the position of the distal defibrillation electrode inside the right ventricular cavity. The data show dramatic differences between recordings performed near to and far from the electrode. The bottom traces represent enlarged views of the highlighted recordings.

conduction velocity is fast, the second component (NH) denotes activation of intranodal structures of the NH region. The increased delay between the first and the second components during conduction of the premature beat represents slowing of the intranodal conduction, which is also

indicated by the His electrogram. It should be mentioned that relative amplitudes of the 2 components in the optical trace do not represent differences in action potential amplitudes, but rather the relative contribution of different amounts of tissues from transitional and NH regions

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Figure 7. Three-dimensional (multilayer) imaging of the conduction through the atrioventricular node. The right atrium was opened through the tricuspid valve (TV), right atrial appendage, and superior vena cava (SVC), the atrial preparation containing the sinus node (SN), crista terminalis (CrT), fossa ovalis (FO), inferior vena cava (IVC), coronary sinus (CS), intra-atrial septum (IAS), atrioventricular node (AVN), and the bundle of His (His). The preparation was paced at the site marked with the asterisk at IAS with a basic cycle length S1S1 = 300 ms with a premature stimulus applied with a delay S1S2 = 170 ms. Bipolar electrograms were recorded from I AS (triangle) and bundle of His (black circle). Two optical traces from the septum (black box) and the distal AVN (gray box) are shown. All data were filtered at 50 Hz and sampled at 1 kHz. The optical data were then additionally filtered by a software-implemented 50-Hz low-pass Bessel filter.

and different relative amplitudes of the 2 components can be seen in different areas of the proximal node.38 Transmuml dog heart preparations

While a comparatively large number of optical mapping studies have investi-

gated the time course and spread of endocardial and/or epicardial repolarization, the study of transmural repolarization had previously been thought to be beyond the reach of optical mapping methods.74 We recently applied the Yan and Antzelevitch technique75,76 of isolated arterially perfused wedges of the LV wall to optical

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mapping of transmural repolarization in the canine heart. A diagonal branch of the left anterior descending artery was cannulated with a thin (<1 mm) polyurethane tube and perfused with cardioplegic solution. After unperfused tissue was trimmed off with a razor blade, all the cut vascular branches were ligated and the resulting 20 x 10 x 15 mm (length times width times wall thickness) arterially perfused tissue wedge was transferred into a vertical glass tissue chamber, in which it was gently held against the front window by means of a piston in the rear of the chamber. After an initial equilibration period of 1 hour, preparations were stained with di-4-ANEPPS (Figure 8A) and then immobilized by perfusion with 25 umol/L cyto D for 15 minutes to eliminate motion artifacts (Figure 8B). A repolarization map during endocardial stimulation at a rate of 2000 ms is shown in Figure 8C. Repolarization times were determined by the second derivative criterion,21 which closely agree with the measurements of action potential duration at 90% repolarization determined using floating microelectrodes in control experiments. The mean optical action potential duration in that preparation was 197 ± 15 ms (mean ± SD) with a maximum action potential duration of 215 ms, a finding that may suggest (in agreement with Anyukhovsky's in vivo mapping study77) that the action potential lengthening observed in the midmyocardial layers at long cycle lengths in isolated superfused dermatome slice preparations may not occur to the same extent in well-coupled arterially perfused or in vivo preparations. Application to human heart

We were the first to apply optical mapping to preparations of the human heart.78 Right atrial appendages obtained from patients undergoing bypass surgery at the Cleveland Clinic Foundation were stained

by superfusion with 20 umol/L di-4-ANEPPS for 10 to 15 minutes resulting in peak-topeak SNR of 7.1 x 1.8 (mean ± SD) in all 256 traces from each of the 4 preparations. Preparations were stimulated at a cycle length of 1000 ms at a site a few millimeters above the field of view. Optical action potentials were filtered at 500 Hz and sampled at 1 kHz per channel from 256 recording cites. A typical optical action potential is plotted in the top panel of Figure 9, while the bottom panel shows an isochronal map of endocardial activation. The isochronal map of endocardial activation clearly indicates a nonuniform spread of conduction with maximum propagation velocity along the length of 2 trabeculae, with the 2 activation fronts meeting only at the lower edge of the mapping array. Similar inhomogeneous conduction was observed in the remaining 3 preparations. Conclusions Fluorescent imaging methods developed and refined in a number of laboratories5-15 have proved to be valuable tools extending the frontiers in cardiac electrophysiology. We believe that recent technological advances will make these experimental techniques available for a larger number of research institutions; however, future development of the fast fluorescent mapping will be necessary in order to resolve several limitations and challenges. A major limitation of current techniques of potential imaging is the lack of absolute calibration. Unlike many ratiometric fluorescent probes for calcium imaging, voltage-sensitive dyes can only provide relative information about changes in transmembrane voltage. Although the changes in the absolute amount of fluorescence excited at one wavelength linearly depends on transmembrane voltage of the viewed cells, accurate calibration has so far been impossible because the

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Figure 8. Transmural optical action potentials from an arterially perfused wedge preparation of the anterior wall of the canine left ventricle. Optical action potential maps from the same transmural wedge preparation are shown both before (A) and after (B and C) perfusion with the new electromechanical uncoupling agent cytochalasin D (cyto D) during endocardial stimulation at cycle lengths of 1000 ms (A and B) and 2000 ms (C). Each plot represents the full wall thickness of the left ventricular wall with epicardial traces at the top and endocardial traces at the bottom. Action potentials were sampled at 1000 Hz and low-pass filtered by software at 100 Hz. In A and B, optical potentials are scaled to their full peak-to-peak amplitude. In C, potentials are normalized to an identical action potential amplitude and aligned with their upstrokes in order to facilitate comparison of action potential shapes and durations; the vertical bars indicate the time of repolarization determined by the second derivative criterion and the contour map represents the resulting action potential durations in milliseconds. In the absence of electromechanical uncoupling (A), motion artifacts lead to pronounced distortion of nearly all optical action potentials from the surface of the preparation, precluding the determination of repolarization times. Motion artifacts were almost completely eliminated after application of cyto D (B and C), which unlike 2,3-butanedione-monoxime, has no significant effects on action potential shape or duration in canine myocardium.51-53 Action potential duration during endocardial stimulation at 2000 Hz (C) was 197 ± 15 ms (mean ± SD). See text for further details. Mapping data courtesy of J. Wu, PhD, Krannert Institute of Cardiology.54

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Figure 9. Optical mapping of a superfused preparation of the human right atrial appendage. The top panel shows optical tracing from one of the channels, both unfiltered and after digital filtering with a low-pass frequency of 50 Hz. The bottom panel shows an isochronal map of activation with contours drawn at 2-ms intervals.

number of cells contributing to the signal is unknown. Montana et al79 demonstrated that measurements of fluorescence ratios excited at 2 wavelengths can provide such information. They have shown that the ratio between di-4-ANEPPS fluorescence levels excited at 440 and 505 nm linearly depends on transmembrane potential in a lipid vesicle in the range from -125 mV to +125 mV. Similarly, the ratio of di-8-ANEPPS fluorescence from N1E-115 neuroblastoma

cell excited at 450 and 530 nm linearly depends on transmembrane voltage. Similar efforts are now being made to measure the absolute changes in membrane potentials in the heart. Another interesting area of application is the simultaneous recording of voltage and other intracellular and extracellular characteristics using combined staining with voltage-sensitive dyes and other fluorescent probes. Efimov et al.80

FAST FLUORESCENT MAPPING OF ELECTRICAL ACTIVITY IN THE HEART demonstrated that voltage and cytosolic calcium can be measured from the same Langendorff-perfused guinea pig heart stained with the voltage-sensitive dye RH421 and the [Ca2+]-sensitive dye Rhode2. These 2 dyes have close excitation frequencies, but different emission spectra. Therefore, the only change needed in recording the apparatus was the replacement of the emission filter. A recent report by Fast and Ideker81 presented results of such simultaneous measurements of intracellular calcium and voltage in the cell culture. References 1. Cohen LB, Keynes RD, Hille B. Light scattering and birefringence changes during nerve activity. Nature 1968;218:438-441. 2. Davila HV, Salzberg BM, Cohen LB, Waggoner AS. A large change in axon fluorescence that provides a promising method for measuring membrane potential. Nat New Biol 1973;241:159-160. 3. Salama G, Morad M. Merocyanine 540 as an optical probe of transmembrane electrical activity in the heart. Science 1976; 191:485-487. 4. Kamino K, Hirota A, Fujii S. Localization of pacemaking activity in early embryonic heart monitored using voltage-sensitive dye. Nature 1981;290:595-597. 5. Davidenko JM, Kent PF, Chialvo DR, et al. Sustained vortex-like waves in normal isolated ventricular muscle. Proc Natl Acad Sci USA 1990;87:8785-8789. 6. Rosenbaum DS, Kaplan DT, Kanai A, et al. Repolarization inhomogeneities in ventricular myocardium change dynamically with abrupt cycle length shortening. Circulation 1991;84:1333-1345. 7. Dillon SM. Optical recordings in the rabbit heart show that defibrillation strength shocks prolong the duration of depolarization and the refractory period. Circ Res 1991;69:842-856. 8. Knisley SB, Hill BC. Optical recordings of the effect of electrical stimulation on action potential repolarization and the induction of reentry in two-dimensional perfused rabbit epicardium. Circulation 1993;88(5 Pt 1):2402-2414.

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9. Fast VG, Kleber AG. Microscopic conduction in cultured strands of neonatal rat heart cells measured with voltage-sensitive dyes. Circ Res 1993;73:914-925. 10. Rohr S, Salzberg BM. Multiple site optical recording of transmembrane voltage (MSORTV) in patterned growth heart cell cultures: Assessing electrical behavior, with microsecond resolution, on a cellular and subcellular scale. Biophys J 1994;67: 1301-1315. 11. Windisch H, Ahammer H, Schaffer P, et al. Optical multisite monitoring of cell excitation phenomena in isolated cardiomyocytes. Pflugers Arch-Eur J Physiol 1995;430:508-518. 12. Neunlist M, Tung L. Spatial distribution of cardiac transmembrane potentials around an extracellular electrode: Dependence on fiber orientation. Biophys J 1995;68:2310-2322. 13. Wikswo JP, Lin S-F, Abbas RA. Virtual electrodes in cardiac tissue: A common mechanism for anodal and cathodal stimulation. Biophys J 1995;69:2195-2210. 14. Girouard SD, Pastore JM, Laurita KR, et al. Optical mapping in a new guinea pig model of ventricular tachycardia reveals mechanisms of multiple wavelengths in a single reentrant circuit. Circulation 1996;93:603-613. 15. Efimov IR, Fahy GJ, Cheng YN, et al. High resolution fluorescent imaging of rabbit heart does not reveal a distinct atrioventricular nodal anterior input channel (fast pathway) during sinus rhythm. J Cardiovasc Electrophysioll997;8:295-306. 16. Cohen LB, Lesher S. Optical monitoring of membrane potential: Methods of multisite optical measurement. In De Weer P, Salzberg BM (eds): Optical Methods in Cell Physiology. New York: Wiley InterScience; 1986:71-100. 17. Franz MR. Method and theory of monophasic action potential recording. Prog Cardiovasc Dis 1991;33:347-368. 18. Berbari EJ, Lander P, Geselowitz DB, et al. The methodology of cardiac mapping. In Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping. Mount Kisco, New York: Futura Publishing Co.; 1993:63-77. 19. Biermann M, Shenasa M, Borggrefe M, et al. The interpretation of cardiac electrograms. In Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping. Mount Kisco, New York: Futura Publishing Co.; 1993:11-34.

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Probes. Boca Raton: CRC Press; 1988: 137-199. 33. Loew LM. Voltage-sensitive dyes: Measurement of membrane potentials induced by DC and AC electric fields. [Review]. Bioelectromagnetics 1992;(Suppl 1):179— 189. 34. Cohen LB, Salzberg BM, Davila HV, et al. Changes in axon fluorescence during activity: Molecular probes of membrane potential. J Membr Biol 1974;19:l-36. 35. Muller W, Windisch H, Tritthart HA. Fast optical monitoring of microscopic excitation patterns in cardiac muscle. Biophys J 1989;56:623-629. 36. Knisley SB. Transmembrane voltage changes during unipolar stimulation of rabbit ventricle. Circ Res 1995;77:12291239. 37. Girouard SD, Laurita KR, Rosenbaum DS. Unique properties of cardiac action potentials recorded with voltage-sensitive dyes. J Cardiovasc Electrophysiol 1996;7: 1024-1038. 38. Efimov IR, Mazgalev TN. High-resolution three-dimensional fluorescent imaging reveals multilayer conduction pattern in the atrioventricular node. Circulation 1998; 98:54-57. 39. Grinvald A, Hildesheim R, Farber 1C, Anglister L. Improved fluorescent probes for the measurement of rapid changes in membrane potential. Biophys J 1982;39: 301-308. 40. Schaffer P, Ahammer H, Muller W, et al. Di4-ANEPPS causes photodynamic damage to isolated cardiomyocytes. Pflugers Arch 1994;426:548-551. 41. Cheng Y, Van Wagoner DR, Mazgalev TN, et al. Voltage-sensitive dye RH421 increases contractility of cardiac muscle. Can J Physiol Pharmacol 1998;76:1146-1150. 42. Grinvald A, Anglister L, Freeman JA, et al. Real-time optical imaging of naturally evoked electrical activity in intact frog brain. Nature 1984;308:848-850. 43. Grinvald A, Segal M, Kuhnt U, et al. A. Real-time optical mapping of neuronal activity in vertebrate CNS in vitro and in vivo. In De Weer P, Salzberg BM (eds): Optical Methods in Cell Physiology. New York: John Wiley & Sons; 1986:165198. 44. Rokitskaya TI, Antonenko YN, Kotova EA. Effect of dipole potential of a bilayer lipid membrane on the gramicidin channel

FAST FLUORESCENT MAPPING OF ELECTRICAL ACTIVITY IN THE HEART dissociation kinetics. Biophys J 1997;73: 850-854. 45. Clarke RJ, Schrimpf P, SchoneichM. Spectroscopic investigations of the potentialsensitive membrane probe RH421. Biochim Biophys Acta 1992;1112:142-152. 46. Fedosova NU, Cornelius F, Klodos I. Fluorescent styryl dyes as probes for Na,KATPase reaction mechanism: Significance of the charge of the hydrophilic moiety of RH dyes. Biochemistry 1995;34:1680616814. 47. Frank J, Zouni A, van Hoek A, et al. Interaction of the fluorescent probe RH421 with ribulose-1,5- biphosphate carboxylase/oxygenase and with Na+,K(+)-ATPase membrane fragments. Biochim Biophys Acta 1996;1280:51-64. 48. Efimov IR, Ermentrout B, Huang DT, Salama G. Activation and repolarization patterns are governed by different structural characteristics of ventricular myocardium: Experimental study with voltagesensitive dyes and numerical simulations. J Cardiouasc Electrophysiol 1996;7:512530. 49. Schalij MJ, Lammers WJ, Rensma PL, Allessie MA. Anisotropic conduction and reentry in perfused epicardium of rabbit left ventricle. Am JPhysiol 1992;263(5 Pt 2):H1466-H1478. 50. Allessie MA, Schalij MJ, Kirchhof CJ, et al. Experimental electrophysiology and arrhythmogenicity, anisotropy and ventricular tachycardia. Eur Heart J 1989;10(Suppl E): 2-8. 51. Davidenko JM, Pertsov AV, Salomonsz R, et al. Stationary and drifting spiral waves of excitation in isolated cardiac muscle. Nature 1992;355:349-351. 52. Undrovinas Al, Maltsev VA. Cytoskeleton disruption results in electromechanical dissociation in rat ventricular cardiomyocytes. J Am Coll Cardiol 1997;29:404A405A. 53. Biermann M, Rubart M, Wu J, et al. Effects of cytochalasin D and 2,3-butanedione monoxime on isometric twitch force and transmembrane action potentials in isolated canine right ventricular trabecular fibers. J Cardiovasc Electrophysiol 1998;9: 1348-1357. 54. Wu J, Biermann M, Rubart M, Zipes DP. Cytochalasin D as excitation-contraction uncoupler for optically mapping action potentials in wedges of ventricular myocar-

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cHAPTER 8

Precision and Reproducibility of Cardiac Mapping Martin Biermann, MD, Martin Borggrefe, MD, Robert Johna, MD, Wilhelm Haverkamp, MD, Mohammad Shenasa, MD, and Gunter Breithardt, MD

Introduction A chief objective of cardiac mapping is the analysis of activation wavefronts emerging from structures involved in the genesis of tachyarrhythmias. Accurate localization of such structures is a prerequisite for the understanding of the pathophysiological mechanisms underlying tachyarrhythmias, for the evaluation of antiarrhythmic drugs, and for the development and targeting of nonpharmacological interventions such as catheter ablation or antitachycardia surgery. A limited number of studies have dealt with the precision and reproducibility of cardiac mapping using either a single handheld "roving" electrode guided by visual grid systems or more sophisticated analog or digital multichannel mapping systems. Analysis of electrograms can be based on visual inspection and manual assignment of activation times or on automated analysis.

As computerized multichannel mapping systems are now common in both experimental and clinical settings, an understanding of how digital signal recording and processing influence the observed data and the accuracy of interpretations upon which clinical decisions and research conclusions are based is highly relevant. The purpose of this chapter is to review the currently available information from experimental and clinical studies on the precision, accuracy, and reproducibility of the various approaches for cardiac activation mapping. Definition of Terms If the performance of a given method of measurement is to be evaluated, the terms reproducibility and repeatability, for which there seem to be no uniformly accepted definitions, are frequently used. According to Bland and Altman,1'2 repeatability is

Supported by a Research Grant from the Deutsche Forschungsgemeinschaft, DFG-Projekt Br 759/1-2. From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; c2003. 157

158 CAEDIAC MAPPING activation times from all recording sites are displayed in the format of a colorcoded or isochronal activation map, which indicates the spatiotemporal spread of activation across the mapped region(s) of the heart. Other forms of cardiac mapping focus on different aspects of cardiac electrical activity, such as repolarization maps and action potential duration maps to characterize cardiac repolarization (see chapter 7). The traditional mode of open-chest or intraoperative cardiac activation mapping first applied by Lewis and Rothschild4 in 1915, henceforth called roving-probe mapping, employed a single hand-held probe to sequentially record electrograms from different epicardial or endocardial sites during a stable cardiac rhythm such as sinus rhythm, stimulated rhythms, or monomorphic ventricular tachycardia (VT). To identify the various recording sites, most investigators used some kind of visual grid system defined by anatomical landmarks such as the atrioventricular (AV) groove or the course of the Basic Mapping Techniques coronary arteries for epicardial mapping Cardiac mapping can be defined as or the location of the septum, of the base a method by which cardiac potentials of the heart, and of the papillary muscles recorded directly from the heart or from the for endocardial mapping.3'5-12 Signals body surface (body surface mapping) or are displayed on oscilloscope screens or recorded indirectly by other means (optical recorded on paper charts or other media mapping, magnetocardiography, nuclear and activation times are determined visuphase mapping) are spatially depicted as a ally or "manually" by the electrophysiolfunction of time in an integrated manner.3 ogists in relation to a periodically occurring The most common type of cardiac time reference. The main drawbacks of mapping is activation mapping. Extra- this approach are that it is very time concellular electrograms are recorded from suming, that it is impossible to map apethe epicardial or endocardial surface of riodic rhythms such as polymorphic VTs the heart during catheter studies, during and ventricular fibrillation (VF), and that open-heart surgery, or in the experimen- it is difficult to relocate the probe to a tal setting. Each electrogram from each previous recording site with perfect accurecording site is then analyzed for times racy. For intraoperative and experimenof local activation of the myocardium in tal mapping, roving-probe mapping has the vicinity of the recording electrode in therefore been superseded by multichanrelation to a fixed time reference such as nel mapping systems developed since the a surface EGG or the output of the cardiac 1970s. Instead of a single roving probe, stimulator (see chapter 2). Finally, the arrays of recording electrodes—epicardial

inversely related to the variation in independent measurements replicated under identical conditions, i.e., the same subject, the same observer, the same method, and the same experiment. In contrast, reproducibility relates to the variation between 2 test results in the same subjects under differing conditions,2 such as different methods of measurement, different observers, or different times or places of measurement, independent of the agreement of the 2 test results with the true value of the measured parameter. Reproducibility can be quantified by calculating "limits of agreement" representing confidence intervals within which 2 test results will agree with a specified probability.1 Precision can be defined as the degree of exactness to which a measured or estimated parameter is expressed, whereas accuracy relates to the degree of correctness of a measurement or an estimate of a parameter in relation to its true value.

REPRODUCIBILITY OF MAPPING 159 sock or plaque arrays or endocardial balloon arrays inserted through a ventriculotomy or atriotomy—are brought into direct contact with the heart and electrograms from all recording sites are recorded simultaneously. While some early multichannel mapping systems used dedicated analog circuitry for on-line display of the activation sequence,13 all other systems digitize the analog signals and employ computers for the display and analysis of the electrogram data. Closed-chest or catheter mapping was first introduced for the preoperative study of cardiac arrhythmias but has, since the introduction of high-frequency catheter ablation of supra ventricular and ventricular arrhythmias, become the mainstay of the nonpharmacological management of tachyarrhythmias. Conventional catheter-based mapping technology is very much like open-chest roving-probe mapping limited by 2 main shortcomings: first, only a few electrograms from separate endocardial sites can be acquired concomitantly. Second, it is difficult to define and measure the spatial relationship of different endocardial sites with a conventional biplane fluoroscopy unit. Several new technologies have been clinically evaluated in recent years: (1) the basket catheter, (2) electroanatomical mapping, and (3) noncontact mapping. The basket catheter is composed of 8 splines, each bearing 8 equally spaced electrodes and assembled into a cage-like structure, which can be expanded and brought into contact with the endocardium of the heart chamber under study. The catheter allows the synchronous registration of 64 unipolar or 56 bipolar electrograms.14,15 Mapping accuracy with the catheter is limited by the difficulty of achieving satisfactory wall contact of all 64 electrodes and by the large interelectrode spacing, the spatial resolution being insufficient to allow catheter ablation of focal tachycardias

without further conventional mapping. The technique has been successfully applied for mapping ventricular16 and atrial tachyarrhythmias.17,18 For electroanatomical mapping with the CARTO™ system (Biosense Webster, Diamond Bar, CA), 3 magnetic coils attached to the examination table generate magnetic field gradients, which are picked up by a sensor incorporated into the tip of a conventional steerable bipolar mapping catheter. This set-up allows the measurement of the position and orientation of the catheter tip relative to a reference field sensor with a spatial accuracy of <1 mm.19 Sophisticated computer hardware and software relate electrograms with their precise locations in 3-dimensional space and allow the construction of isochronal, isopotential, and voltage maps. Data acquisition is sequential and requires a sustained stable and hemodynamically tolerated heart rhythm19,20; it has been applied for mapping of atrial21 and ventricular tachyarrhythmias.22 For noncontact mapping using the Ensite™ System (Endocardial Solutions, Inc., St. Paul, MN), a 64-electrode balloon array catheter with an inflated size of 1.8 x 4.6 cm is positioned within the heart chamber under study. The catheter emits a 5.6-kHz locator signal that allows tracking of the tip position of a conventional mapping catheter, and the shape of the heart chamber is reconstructed by sweeping this mapping catheter across the endocardium. Based on an inverse solution to Laplace's equation, 3360 'virtual' endocardial electrograms are reconstructed from the 64 far-field signals recorded by the balloon catheter. The accuracy of the algorithm has been validated both in vitro23 and in vivo.24 The reconstruction can be achieved based on data acquired during a single tachycardia cycle and can thus be used to map polymorphic, nonsustained, and hemodynamically unstable arrhythmias.

160 CAEDIAC MAPPING As quantitative data on the performance of these new systems are still lacking, the following discussion focuses on the precision and reproducibility of openchest intraoperative and experimental mapping; further studies will be needed to demonstrate that the advantages provided by the new catheter mapping studies translate into better clinical outcomes.

When evaluating different methods of measurement in biological systems, the challenge is not only to arrive at reliable estimates of the variation between test results under differing conditions but to decide how reproducible and accurate a test must be in order to be experimentally or clinically useful. For targeting surgical interventions, a spatial resolution of 10 mm seems adequate, and based on the assumption of a longitudiBasic Problems of Analysis nal conduction velocity in myocardium of Precision and Reproducibility of 0.5 mm/ms,25 activation time assignof Electrocardiographic Signals ment would have to be accurate within ±10 ms26; however, higher apparent conIndependent of the instrumentation, duction velocities that occur in 2-dimencardiac activation mapping includes many sional measurements when an activation steps between data acquisition (recording) front meets the mapped surface at an and the construction of an isochronal map angle27 may result in more stringent and the final interpretation of an activa- requirements for temporal accuracy. For tion sequence (Table 1). Each step has a targeting of other types of interventions potential bearing on the resulting acti- that have a smaller lesion size, such as vation maps and their interpretation. In high-frequency and laser lesions, it is necthe following discussion, the available map- essary for spatial and temporal accuracy ping techniques are compared in terms to be significantly higher. If the aim was of the reproducibility of the resulting acti- to resolve microreentrant circuits 5 mm vation times and interpretations of the in diameter, electrode locations would have mapped arrhythmias. to be known within 1 mm.27 A different Table 1 Steps Involved in Activation Mapping Step in Activation Mapping Selection of patient or of experimental model Induction of the arrhythmia of interest Recording of electrograms Localization of recording sites Signal conditioning Signal logging Signal analysis Map generation Interpretation of activation sequence

Critical Issues Appropriateness of selection Reproducibility and stability of induced arrhythmia(s) Type of electrode and/or electrode array(s), motion and other artifacts Reproducibility and accuracy of (re)localization Amplification, filtering, digitization Continuous or burst recording, reliability of storage media Interpretation of signals, visual or automated analysis, context (in-)dependent interpretation, verification Manual or automatic, interpolation, adequacy of spatial sampling (density of recording sites, extent of mapped area) Scientific conclusions, targeting of therapeutic intervention (s)

REPRODUCIBILITY OF MAPPING approach for determining the required accuracy in mapping studies was suggested by Pieper et al.,28 who added normally distributed random errors to activation times in an activation map of sustained VT in a human subject recorded with a 119-channel sock electrode array. While maps were still interpretable at a standard deviation (SD) of the artificially induced timing error of 3 ms, epicardial breakthrough and activation sequence were severely distorted at an SD of 6 ms, so that Pieper et al.28 demanded that activation time assignments in multichannel mapping systems be accurate within an SD of 3 ms. Selection of Patient or Experimental Model Any successful mapping study in the clinical field requires appropriate patient selection for the planned therapeutic intervention, e.g., the inducibility of hemodynamically tolerated sustained monomorphic VTs in patients intended for high-frequency catheter ablation of ventricular tachyarrhythmias. In experimental mapping, the chosen animal model and preparation must be relevant for the type of research question under study. For example, the study of ventricular tachyarrhythmias in the chronic phase after myocardial infarction generally requires hearts of a certain size such as the canine heart, while conduction in the AV node region can be studied in smaller hearts. While these issues are pertinent to the eventual success of any cardiac mapping study, they are beyond the scope of this chapter and are not discussed further. Induction of the Arrhythmia of Interest

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intervention or of scientific investigation be reproducible. This is essential if the effect of interventions is to be studied in animal models or during catheter ablation or antitachycardia surgery in humans. Should an arrhythmia not be reproducible, the absence of such an arrhythmia or the appearance of another type of arrhythmia after a therapeutic or investigational intervention may be due to chance rather than to the intervention itself. Reproducibility of the type of induced VT

Multiple morphologies of VT are common in open-chest canine models. Recordings from limb leads may be inadequate to assess whether an induced VT is identical to a previous episode. Bardy et al.29 assessed the reproducibility of induction of specific types of VT in a canine model 3 days after occlusion of the left anterior descending coronary artery followed by reperfusion. A type of VT was considered as reproducible if 2 or more episodes of monomorphic VT were obtained with similar rates, limb lead tracings (leads I, II, III), and epicardial maps based on 27 simultaneously recorded bipolar epicardial electrograms. To provide a method of distinguishing various types of VT without the necessity of a complete epicardial map, 5 of these 27 epicardial electrodes were selected. The same types of VT could be as accurately identified in these 5 electrograms as in the epicardial activation maps of the entire electrode array if activation times and electrogram morphology were analyzed. Thus, a limited number of leads can be used to differentiate between various types of VT in the canine model.29 Beat-to-beat stability

Any successful mapping study further requires that the arrhythmia that is the object of the planned therapeutic

Sequential roving-probe mapping and simultaneous multichannel mapping

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both assume that consecutive beats are identical or that a single beat is representative of ventricular activation during subsequent cycles. It is generally assumed that this is the case in normal myocardium during sinus rhythm; however, in abnormal ventricular myocardium as after myocardial infarction, abnormal activation patterns with regionally slow conduction may demonstrate beat-to-beat irregularities (Wenckebach-like pattern, 2:1 conduction pattern, etc.). Such a variation in the activation pattern may become especially important during VT, either at the onset or prior to termination when a clear-cut variation in cycle length can frequently be observed. In polymorphic VT, changes in local activation times can be expected on a beat-to-beat basis. Our experience from intraoperative mapping in patients with VT suggests that during sinus rhythm or regular paced rhythms electrograms are generally relatively stable.30 In a series of experiments in normal open-chest dogs, we performed epicardial mapping with a 240-pole epicardial plaque electrode (35 x 35.5 mm) during cathodal stimulation from the center of the plaque electrode at twice diastolic threshold. Data were acquired with a computerized multichannel mapping system developed at the University of Maastricht (The Netherlands),31 at 30 minutes after initial placement of the plaque electrode and 8 hours later. Activation times of subsequent beats were reproducible within 0.5 ± 0.9 ms (mean ± SD; range -4 to +5 ms) and within 3.6 ± 4.2 ms (range -13 to 22 ms) between the 2 sets of measurements 8 hours apart.32 To define the periodicity of epicardial and transmural ventricular activation during sustained monomorphic VT, Branyas et al.33 assessed epicardial and transmural ventricular electrograms during 6 consecutive cycles of VT in 10 patients during intraoperative mapping of sus-

tained monomorphic VT. Bipolar electrograms were recorded simultaneously using sock and needle electrodes from up to 96 epicardial and 156 transmural sites. Electrograms were analyzed for local activation time, duration, and morphology. The isochronal activation maps during VT were reproducible in each patient. The mean beat-to-beat variations in local epicardial and transmural activation times were only 1.7 ± 1.7 ms and 2.0 ± 1.9 ms, respectively.33 These and other results suggest that the analysis of a single beat of VT provides reliable information on ventricular activation in patients with sustained monomorphic VT, supporting both the technique of single-cycle activation mapping and of roving-probe mapping of subsequent cycles. Recording of Electrograms Types of recording electrodes

Electrograms can be recorded in multiple ways, by means of a single unipolar "different" electrode against a distant reference electrode (unipolar electrograms), as the difference in potential between 2 closely adjacent electrodes (bipolar electrograms), or by special pressure or suction electrodes (monophasic action potentials).34 Morphology and amplitude of the recorded electrograms depend on (1) the type of normal or abnormal depolarization responsible for the electrical potential and on local myocardial characteristics such as ischemia or infarction; (2) the orientation of the activation wavefront in relation to myocardial fiber direction25; (3) the distance between the source of the potential and the recording electrode(s); (4) the size, configuration, and interpolar distance of the recording electrode(s)35; (5) the orientation of the wavefront in relation to the

REPRODUCIBILITY OF MAPPING poles of a bipolar electrode; (6) the conducting medium in which electrograms are recorded36; and (7) other factors. Unipolar electrograms reflect both local and distant cardiac activity, the contribution of distant events decreasing in proportion to the square of the distance from the recording electrode (see chapter 2). The advantages of unipolar electrodes lie in their capability to record activation fronts from all directions with equal sensitivity, and in the fact that only one lead is required per recording channel, which is an important consideration in the design of multichannel mapping systems. Bipolar electrograms have the advantage of emphasizing local events over distant events, the contribution of which decreases in proportion to the third power of the distance, which aids in the specific detection of local activation for the purpose of activation mapping. Their disadvantage, however, is their directional insensitivity to activation fronts that are parallel to the poles of the electrode. To avoid this, coaxial electrodes or multipolar electrodes have been suggested, which however still cannot detect activation fronts parallel to the plane of the electrodes (see chapter 2). Instead of using coaxial electrodes, it is possible to calculate "virtual" bipolar electrograms as the difference between the potential of a unipolar electrode and the mean potential of the surrounding electrodes in an array of unipolar electrodes, provided these are evenly spaced. Recording artifacts

The correct and timely identification of artifactual electrograms is of crucial importance in any system of cardiac mapping and may have major influence on the final interpretation of an activation sequence, e.g., whether a fractionated electrogram represents a motion artifact or local activation in an assumed zone of

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slow conduction. While roving-probe mapping allows the instant identification and correction of artifactual recordings during on-line analysis of the recorded signals, identification of artifacts in multiple-channel mapping systems tends to be more difficult and is often possible only after completion of the experiment. Typical recording artifacts induced at the myocardium/electrode interface include the following: (1) polarization of electrodes, which can cause slow shifts of the baseline of the signals; (2) local myocardial injury resulting from inappropriate pressure by recording electrodes or after insertion of intramural "plunge" electrodes resulting in ST segment elevation in local unipolar electrograms37; (3) motion artifacts, which are often rhythmic and repeating simulating fractionated electrograms38,39 or which can be sudden shifts of potential that may be misinterpreted as activations by computer algorithms; and (4) poor contact between electrode and myocardium leading to heavier weighing of far-field effects and increased 50- or 60-Hz noise.40 Accurate Localization of Recording Sites As the object of cardiac mapping is the analysis of the spatiotemporal sequence of cardiac events, the accurate localization of the recording sites in relation to anatomical structures is of great concern, for targeting therapeutic interventions and for purposes of investigations correlating cardiac signals and local microanatomy.41 While in roving-probe mapping the chief problem is to relocalize individual recording sites with reasonable accuracy in repeated recordings during or after interventions, the main difficulty in multichannel mapping by means of electrode arrays is to identify the accurate location of an individual recording channel in

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relation to the heart, especially when several electrode arrays are used simultaneously (e.g., an epicardial sock array and an endocardial balloon array) or sequentially (e.g., epicardial sock and plaque electrodes). Unless altering cardiac contraction patterns during different rhythms cause movement of the electrode array across the heart, recording electrodes can be assumed to maintain a constant position in relation to the heart at least within each sequence of recordings. Assuming a high stability of the biological signal, reproducibility is then governed by the performance of the visual or computer-based analysis of the electrograms. The reproducibility of activation time measurements in roving-probe mapping aided by visual grid systems has been examined in a number of studies.30,42 Reproducibility of Activation Times of Visual Grid Systems for Roving-Probe Mapping For epicardial roving-probe mapping in a canine model of chronic myocardial infarction, Spielman et al.12 reported that measurements of activation times by a custom-designed interval counter were reproducible within 2 ms in repeated measurements from the same epicardial or endocardial locations. Abendroth et al.30 assessed the reproducibility of local activation times in epicardial mapping in 10 patients during open-heart surgery before cardiopulmonary bypass. Epicardial electrograms were recorded by means of a tripolar handheld probe (interpolar distance 1.5 mm). To compensate for the directional insensitivity of bipolar electrograms (s.a.), the 3 bipolar signals from each pair of poles of the tripolar electrode were summed after rectification. Activation times were determined by measuring the time of onset of

the complex based on the first elevation from baseline by more than 45°, using the signal from a multipolar electrode on the anterior wall of the right ventricle (RV) as a time reference. During the same paced atrial rhythm, activation times were measured consecutively at 75 epicardial sites defined by the crossing points in a visual grid system that included anatomical landmarks such as the left anterior descending artery. After the first series of 75 electrograms had been recorded, the recording was repeated in an attempt to obtain electrograms from the same 75 points. Mean activation time values at each site were calculated from at least 5 consecutive cycles. A representative example of the absolute differences in local epicardial activation times during repeated measurements is presented in Figure 1. In all 10 patients, the mean difference between the first and second measurements was 8 ms (range 4-12 ms) in the anterior wall, 6 ms (range 3—11 ms) in the left lateral wall, and 7 ms (range 5-15 ms) in the posterior wall.30 Blomstrom et al.42 analyzed the reproducibility and the beat-to-beat variability of local activation time measurements in 20 intraoperative atrial and ventricular mapping studies. Signals were recorded with a single hand-held roving-probe electrode at 53 predefined points in a visual grid system of the ventricular epicardium in 5 patients, at 15 predefined points on the atrial epicardium in 4 patients, and at 10 right atrial endocardial points in a single patient, all patients undergoing antitachycardia surgery. Each intraoperative mapping procedure was performed twice in each patient. The beat-to-beat variability of activation times ranged between 3.9 and 5.2 ms in 95% of the ventricular recordings, but was 11.2 ms in the atrial epicardial and 14.7 ms in the atrial endocardial measurements. The mean differences between

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Figure 1. Representative example of the absolute differences in local epicardial activation times of repeated measurements during intraoperative mapping in a patient with coronary heart disease but without prior myocardial infarction.30

the first and second measurements of activation times were 2.2 ± 5.6 ms (mean ± 2 SD) in the RV anterior wall, 1.2 ± 6.8 ms in the RV posterior wall, -0.8 ± 4.4 ms in the left ventricular (LV) anterior wall, -2.8 + 5.3 ms in the LV wall, and 0.1 ± 5.6 ms in all ventricular wall regions, but -1.5 ± 21.0 ms (!) in the atrial epicardium and 3.9 ± 33.8 ms (!) in the atrial endocardium.42 Cowan et al.43 evaluated the reproducibility of activation and repolarization time measurements in intraoperative monophasic action potential mapping during atrial paced rhythms in 30 patients. Measurements were performed by means of a special hand-held roving probe bearing a central blunt spike electrode surrounded by a concentric ring electrode. To test the reproducibility of activation time and repolarization time measurements, monophasic action potentials from 10 LV and 2 posterior RV sites were

recorded in sequence 4 times with a different starting point on each occasion. The 95% confidence interval for an estimate of epicardial activation time based on a single measurement was ±10 ms but ±21 ms for an estimate of repolarization time, so that Cowan et al. stressed the need for repeated measurements of repolarization times at the same sites if meaningful differences in repolarization times between sites were to be detected.43 These studies indicate that the differences between repeated activation time measurements by means of roving probes can be expected to lie within 5 to 10 ms for ventricular epicardial mapping, which is probably acceptable for most clinical applications during antitachycardia surgery but can be much larger for mapping in the atria. In each study, however, repeated measurements from some sites differed to a remarkable degree, which would not be acceptable for most

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research applications such as the study of drug effects. The main reason for the poor reproducibility of some measurements seems to be difficulty in retrieving the same recording sites, especially in areas that are difficult to reach such as the LV posterior wall.

(r = 0.93) with a rate of 1.1% of activation detection errors per 10% of maximum allowable calibration factor error. Activation detection was more susceptible to calibration-induced errors during VT than during sinus rhythm or epicardial pacing.44

Signal Conditioning

Digitization of electrogram data

Electrograms represent low-amplitude, high-impedance biological signals which must be amplified before they can be displayed on oscilloscope screens, recorded on paper charts or magnetic tape, or digitized for computer processing. It is essential that amplifiers do not lead to distortion of the signals and be calibrated to a uniform gain. In order to reduce the amount of noise picked up by the leads from the recording electrodes, it is desirable to locate preamplifiers and amplifiers as close as possible to the mapped heart. To evaluate the effect of nonuniformity of amplifier gains on the performance of an automatic activation detection algorithm, we repeated automatic activation detection in unipolar epicardial mapping data from chronically infarcted dog hearts after introducing random errors in the calibration factor files used to compensate errors in linear gain between the recording channels. The random errors were normally distributed with an SD of half the maximum allowable calibration factor error. Activations were detected by means of the maximum downslope algorithm with a fixed slope threshold. Detection errors were defined as either false-positive or false-negative activation detections or as differences of more than 5 ms in assigned activation times. Based on the analyses of the same 1440 electrograms with calibration factor files with maximum allowable calibration errors between 1% and 50%, the rate of faulty activation detections increased approximately linearly

In order to eliminate the need for expensive custom-designed analog hardware for the storage, retrieval, display, and analysis of mapping data, nearly all modern multichannel mapping systems digitize the electrogram data before further processing. This raises a number of important issues. Digitization is a form of data reduction. When an analog waveform is digitized, the analog-to-digital (A/D) converter takes a sample of the waveform at a constant rate called the sampling rate. The amplitude of the analog waveform at the time of sampling—which could theoretically be expressed with infinite precision—is translated into a finite discrete number, usually a binary number ranging from -128 to +127 (8-bit A/D conversion), -512 to +511 (10-bit), or -2048 to +2047 (12-bit), representing the full dynamic input range of the A/D converter (typically ±2.5 V or ±10 V). All other information contained in the analog waveform is discarded. If the dynamic range of the A/D converter is exceeded, the A/D converter saturates and potentially important information is irretrievably lost. The sampling theorem states that if a periodic waveform is band limited and sampled at a rate (called the Nyquist rate) that is equal to at least twice its highest component frequency, it can be reconstructed exactly without loss of information through sampling. The following aspects of the sampling theorem deserve special mention:

REPRODUCIBILITY OF MAPPING l.The sampling theorem assumes that sampling is performed with infinite precision, whereas any A/D converter samples with finite precision, introducing so-called digitization noise. 2. It is not possible to identify from measured frequency spectra of cardiac waveforms an upper frequency limit that includes only the physiologically significant waveform and excludes only noise, except by arbitrary choice.45 3. If a signal is sampled below its Nyquist rate, so-called aliasing occurs, in which the upper frequencies are not simply lost but contribute in an unpredictable way to the corruption of important waveform parameters.46 To prevent aliasing, electrogram signals need to undergo low-pass antialias filtering before A/D conversion at a cut-off frequency that is lower than half or even a quarter of the sampling rate. As the amount of data to be handled by a computerized multichannel mapping system increases in direct proportion to any increase in sample rate, the optimum sample rate that still preserves the pertinent features of the electrocardiographic waveforms has long been a subject of intense debate. Even though the cost of computer memory and storage media has been steadily decreasing, system performance is still an issue in systems that are to allow continuous acquisition of mapping data over extended periods. An attractive approach for reducing the amount of data to be retained for processing could be adaptive sampling, in which the sampling interval is not constant but changed according to a specified algorithm that stores samples with a higher rate when the waveform is changing rapidly and otherwise discards nearly redundant samples.47 While adaptive

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sampling has been successfully implemented in some commercially available data acquisition systems for cellular electrophysiology, the computational price for adaptive sampling in the context of multichannel mapping systems is most probably too high, increasing system complexity and potentially even degrading system performance.46 Sample rates for digitization

In a classic paper, Barr and Spach45 examined the optimum sample rates for recording the whole variety of cardiac waveforms, including ventricular electrograms, using both quantitative and qualitative criteria. They defined as the minimum sample rate the lowest rate that allowed the complete analog waveform to be reproduced from the retained digital samples within the prevailing noise level, thus basing their definition on waveform reconstruction rather than on frequency analysis. The minimum sample rates recommended were 500 Hz for ECGs from the body surface, 1000 Hz for unipolar and bipolar ventricular electrograms, and 15,000 Hz for signals from the cardiac conduction system; however, Barr and Spach pointed out that twice or more than these minimum sample rates were desirable to allow for accurate measurement and correct interpretation of unexpectedly complicated waveforms. Purkinje spikes in endocardial unipolar electrograms in particular were invisible at sample rates below 5000 Hz. Interestingly, none of the interpolation algorithms used for the reconstruction of the waveforms (linear, quadratic, and sin x/x) performed significantly better than the others.45 Other studies have investigated the effect of lower sample rates on the performance of activation detection algorithms in ventricular electrograms. Kaplan et al.48 evaluated the performance of computerized

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activation detection algorithms in bipolar epicardial and endocardial electrograms from 4 chronically infarcted dogs at an original sample rate of 1000 Hz and at reduced sample rates of 500,250, and 125 Hz. Using the maximum derivative criterion for activation detection, the criterion performed similarly at sample rates of 1000, 500, 250, and 125 with root mean square errors of the activation times in relation to the "best guesses" of 9.8, 9.0, 8.2, and 8.8 ms, respectively. When using a different morphological template-matching algorithm for the determination of activation times, reducing the sample rate degraded the performance of the algorithm with root mean square errors of 1.1, 2.9, 4.2, and 6.2 ms, respectively.48 Rosenbaum et al.26 sampled bipolar electrograms from 61 epicardial electrodes (interpolar distance 4 mm) during sustained monomorphic VT in 10 chronically infarcted dogs at 1000, 500, 250, and 125 Hz. Activation times were then assigned to the onset of the electrogram's earliest rapid deflection by trained observers aided by a computer algorithm that preselected times of activation based on the maximum absolute slope of the signal. At the full sample rate of 1000 Hz, 95% of activation times were reproducible within 9.5 ms and the mean difference in activation times between repeated assignments at the full sample rate was 2.6 + 0.1 ms (mean ± standard error of the mean). Reductions in sample rate to 500, 250, and 125 Hz increased this difference to 3.2 ± 0.2 ms (n.s.), 3.3 ± 0.2 ms (n.s.), and 6.5 ± 0.3 ms (P< 0.0005). The authors concluded that only digitization at 125 Hz significantly impaired activation time measurements in their animal model.26 Pieper et al.28 recorded unipolar and bipolar (interpolar distance 5 mm) electrograms intraoperatively with an epicardial sock electrode array from 4 patients with VT and 4 patients with Wolff-Parkinson-White (WPW) syndrome

at a sample rate of 1000 Hz. The signals were interpolated to a sample rate of 10,000 Hz and then resampled at lower sample frequencies between 2000 Hz and 200 Hz. Activation times were determined in the resampled signals by means of computer algorithms using the minimum slope criterion in unipolar electrograms and the peak amplitude criterion in bipolar electrograms, and the mean differences between these activation times and the "best guesses" of activation times were calculated. Based on a relatively strict criterion that the mean difference +1 SD be smaller than 4 ms, the minimum sample rates for bipolar signals was 500 Hz and for unipolar signals was 625 Hz.28 These results were in agreement with those of an earlier study of theirs, which had found that a greater bandwidth was necessary for accurate activation detection in unipolar electrograms despite their having lower signal power in the higher frequencies than bipolar electrograms.46 Based on these results it is probably safe to recommend sample rates for electrogram acquisition of 1000 Hz or more. Dynamic range of digitization

Another issue, which has not been examined by systematic studies, is the dynamic range of A/D conversion required for optimum resolution of electrogram signals. While some multiple-channel mapping systems employed A/D conversion at 8 bits31,46 or 10 bits,49 12-bit A/D conversion26,48,50,51 can now be regarded as standard, at least for intraoperative mapping in humans. As is illustrated in Figure 2, saturation of A/D converters by the electrogram signals leads to loss of information in the digitized signal that can result in critical misassignments of activation times. If a uniform amplifier gain is selected so as to avoid saturation in all signal channels, A/D conversion at a resolution of 8 bits

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Figure 2. Influence of amplifier saturation on electrogram analysis. In the upper panel, local activation was identified at 75 ms, whereas at a 3 times higher amplification (lower panel), local activation was identified earlier (55 ms) since part of the relevant signal was cut off because of saturation of the amplifier.

may not be enough to resolve low-amplitude fractionated activity for adequate interpretation of the signals, especially in periinfarction zones and in the human heart, which has a higher amount of epicardial fat.51 Ideker et al.51 therefore suggested that the minimum amplitude resolution in multiple-channel mapping systems should be 12 bit with automatic gain adjustment for each recording channel, or else 16 bit. In this context it is worth noting that 16bit A/D conversion does not increase storage requirements, as it is customary to encode 12-bit signals by 2-byte digits for

ease and speed of handling of information in computer memory. Signal Logging A mapping system with 256 channels sampled at 1000 Hz and 12-bit resolution will generate 29 MB of data per minute. While burst recording of brief episodes of arrhythmias can be an acceptable solution for some applications such as optical mapping, continuous recordings of arrhythmias over longer periods

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continue to pose major challenges in hardware design and optimization. Digital mapping systems may stream data to computer hard disks or to tape drives. While high-end digital tape backup systems allow the storage of redundant information to tape so that data can be fully recovered even if read errors from tape occur, systems that use modified tape drives such as video cassette recorders do not necessarily have this capability. To evaluate the frequency of read errors from video tape in the mapping system developed at the University of Maastricht,31 we repeatedly read mapping data (240 channels sampled at a resolution of 8 bits and 1000 Hz) of the same beat during sinus rhythm after forwarding and rewinding the tape. Read errors occurred randomly but at reproducible locations in the data set with a frequency of 1 error per 64,000 bytes of data or once per 270 ms of electrogram data. For comparison, a typical digital data storage-2 tape drive such as the Hewlett-Packard C1533 A (Palo Alto, CA) is specified to have an uncorrectable bit error rate of less than 1 bit in 4 x 1012 bytes. Even though the video tape logging system was able to identify read errors through the inclusion of a 9th parity bit in the information written to tape returning a uniform amplitude of-128 in case of all read errors, the analysis software misinterpreted this value as a sharp negative spike in the signal that was then falsely detected as a local activation by the activation detection algorithm.44 This example illustrates the importance of testing all aspects of a mapping system's performance before reaching scientific conclusions. Signal Analysis The chief object of signal analysis in activation mapping is the assignment of times of activation to each recorded electrogram, which are to represent instances of

local depolarization of myocardium within the field of view of the recording electrode. Activation times are then calculated as the time intervals between the assigned times of activation and a fixed or recurring time reference such as the output signal of a cardiac stimulator or an electrogram signal from a fixed reference electrode. The problem of activation detection in cardiac mapping is that it is not possible to conclude with absolute certainty from extracellular signals to intracellular events and that the gold standard for detecting activation in myocardium, a recording of the transmembrane potential either by an intracellular impalement or means of a voltage-sensitive dye (see chapter 7), is not available. Theoretical issues, which have already been addressed in chapter 2, include: (1) which morphological features in the extracellular signals best correspond with times of local activation in local transmembrane potentials? Proposed criteria for unipolar electrograms are the criterion of maximum downslope, and for bipolar electrograms the criteria of maximum amplitude, maximum absolute slope, or the point of symmetry in the waveform. (2) Which quantitative and/or qualitative criteria allow the accurate identification of local activation versus distant activation fronts in the extracellular signals? A common practice in activation detection in unipolar electrograms is the use of a slope threshold, which must be exceeded for local activation to be assumed. Confounding issues include the fact that recorded maximum negative slopes are not only highly dependent on the local characteristics of the myocardium35,52 but also on electrode geometry,35 so that results of experimental studies cannot easily be generalized. Manual electrogram analysis

As long as there are no reliable quantitative criteria for activation detection

REPRODUCIBILITY OF MAPPING in extracellular electrograms, mapping studies are forced to accept the activation time assignments by trained observers as best guesses of the underlying activation fronts. The method by which an observer strives to resolve ambiguities in the analysis of the signals is the inclusion of contextual information in the analysis of individual electrogram signals.40 This includes information on myocardial properties such as refractoriness and anisotropy, on the experimental protocol including the placement of the electrodes and the presumed cardiac rhythm (e.g., sinus rhythm versus VT or an AV reentrant tachycardia over an accessory pathway), on the presence or the absence of local pathology such as myocardial infarction, on the electrogram morphology in the same channels during different rhythms, and, most importantly, on time course and morphology of the neighboring electrograms. Electrogram features associated with local excitation generally show a temporal shift between neighboring electrogram channels due to conduction of the impulse across the heart, whereas far events and motion artifacts tend to affect neighboring channels synchronously.53 Considering the importance of these "manual" activation time assignments as de facto gold standard for activation detection in extracellular signals, surprisingly few studies have addressed the problem of the intra- and interobserver reproducibility of these manual interpretations even when these were used as standards for evaluating the performance of automatic activation detection algorithms.40,49,54 It also is of note that while some studies allowed observers to see the entire mapping array and adjust activation time assignments in the context of neighboring electrograms ("contextdependent" analysis), observers in other studies were presented electrograms in random order without access to contex-

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tual information ("context-independent" analysis). In their study on the reproducibility of intraoperative mapping, Abendroth et al.30 found that manual activation time measurements on paper chart recordings of bipolar electrograms based on the 45° criterion (see chapter 2) were reproducible within 3.0 ± 2.0 ms (mean ± SD) by the same observer and 5.0 ± 3.0 ms by different observers. Rosenbaum et al.26 evaluated the reproducibility of context-independent manual activation time assignment in maps of 61 epicardial and endocardial bipolar electrograms recorded from 10 chronically infarcted dogs during VT. Four observers were presented the electrograms in random order on repeated occasions no fewer than 7 days apart. Activation times in the 610 electrograms by the same observers were reproducible within 2.6 ± 3.6 ms (mean ± SD), with approximately 87% of all assignments being reproducible within 5 ms and 95% within 10 ms. Based on these assignments, the location of the epicardial breakthrough site varied by 1.0 ± 1.2 cm between trials. There were no significant differences in intraobserver variability of the results, and activation time assignments by the 4 observers were reported to be very similar although they were not quantified.26 Simpson et al.55 determined the reproducibility of context-independent activation time assignments in 24 mapping data sets of 117 simultaneously recorded bipolar electrograms from mongrel dogs during regular rhythms (sinus rhythm, n = 6; ventricular pacing, n = 6) and VF (n = 12). Activation times were manually assigned through context-independent analysis by 3 observers. In case of regular rhythms, all 3 observers agreed on more than 98% of all detected activations with a detection error rate of approximately 0.05 errors per second of

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electrogram data; however in case of VF, observers agreed only on 90% of all activations with a mean error rate of 0.8 errors per second.55 We evaluated the intraobserver reproducibility of manual activation time assignments of context-dependent interpretations and context-independent interpretations of electrogram data performed 1 week apart. The database included 30 sets of 240 unipolar electrograms recorded by means of a 240-pole sock electrode from 10 chronically infarcted dogs during sinus rhythm, ventricular pacing, and VT (n = 10 each). To perform activation time assignments, the observer was aided by a mouse-driven interactive routine seeking the local minimum of slope in the unipolar waveform. A detection error was defined as a false-positive or false-negative detection in a given electrogram channel or as a difference in activation time of more than 5 ms. The reproducibility of either interpretation method was very similar, with error rates between repeated contextdependent and context-independent analyses by the same observer of 9.1% and 9.0%, respectively. Interestingly, the agreement between the context-dependent and context-independent analyses of each set of interpretations was higher than the reproducibility of either analysis mode, with a mean error rate between both modes of 7.4%. To test interobserver variability, context-dependent analysis of the VT maps was performed by 4 observers on 2 occasions 1 week apart. Mean error rates for repeated analysis of the data by the same observers was 7.9% whereas the overall error rate between the analyses of the 4 observers was 11.6%, the intraobserver reproducibility thus being higher than the interobserver reproducibility. Of the total of 663 contested activation assignments between the observers, 81% concerned the interpretation of deflections as local or distant activations especially in the border zones of myocardial

infarctions, 14% were due to apparent observer errors, 4% were due to differing activation time assignments to signals with multiple deflections, and 1% were due to disagreement on classifying a deflection as artifactual. Despite the substantial differences in the annotation of individual electrogram channels, there was practically no disagreement on the timing of individual responses with root mean square errors between differing activation assignments below 1 ms, and the overall interpretation of the activation sequence was nearly identical in all analyses in the study with locations of first and last epicardial activation never differing by more than a single electrode distance.44 Based on these limited data, activation time assignments tend to be reproducible within some 5 ms in approximately 90% of all electrograms on repeated analyses by the same and by different observers. There seem to be no significant differences in the interpretation of the overall activation sequence, although major differences are to be expected in the detailed analysis of individual electrograms during chaotic rhythms such as VF and of fractionated activity in the border zones of myocardial infarctions. Automated electrogram analysis

Purely manual electrogram analysis is very time consuming.50 As time is very limited both during heart surgery and during experiments, numerous investigators have attempted to automate the process of electrogram analysis by implementing activation detection algorithms either through hardware using dedicated analog circuits or through software using digital technology. As long as there are no reliable quantitative criteria for activation detection in cardiac extracellular signals, a useful activation detection algorithm should at

REPRODUCIBILITY OF MAPPING the very least achieve optimum agreement with manual activation time assignments made by experienced electrophysiologists, which in the absence of more dependable information represent the de facto standard for electrogram analysis. Since the first description of an automatic activation detection algorithm using the criterion of maximum negative slope by Ideker et al.,56 numerous studies have investigated the performance of such algorithms under a variety of conditions. All studies employed databases of electrograms to which standard activation time assignments were made and/or verified by the investigators, either manually or aided by automatic or semiautomatic interactive detection algorithms. Important differences between the studies include (1) the algorithms under evaluation; (2) number and type of electrogram data (e.g., human intraoperative data, data from noninfarcted or infarcted canine models, unipolar or bipolar electrograms, endocardial or epicardial electrograms, regular rhythms or tachycardias); (3) the inclusion44 or exclusion40 of electrograms without detectable local activation; (4) contextdependent40,44,49,54 or context-independent analysis26,55 to build the reference database of activation time assignments; and (5) the use of the same49,54 or of an independent44 database of electrograms for evaluating the results of an optimization process. Automatic activation detection in unipolar electrograms

Nearly all activation detection algorithms for unipolar electrograms proceed according to the following logic: the program will search the electrogram for time instances with a negative slope that is lower than a certain threshold slope (slope criterion). If the slope criterion is met at more than one time instance within a specified time window, the time

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instance with the most negative slope is chosen as the time of local activation (time criterion). Differences between activation detection routines include the algorithm used for slope calculation, the threshold slope, the precise implementation of the time criterion, and the inclusion of other criteria. Slope Calculation: The ability of the available slope calculation algorithms (2-, 3- or 5-point Lagrange algorithms or data fit algorithms) to yield accurate estimates of slope in the analog waveform prior to A/D conversion is a function of many variables, including sample rate and resolution and the prevailing noise levels in the signal.57 Data fit algorithms act as a low-pass filters, which may be a useful property in the presence of excessive digitization or other noise.57 On the other hand, slope algorithms that in effect average slope over several milliseconds will hinder the sensitive detection of the brief minima of slope that occur in the course of an activation front, the width of which has been estimated to be between 0.235 and 2 ms.25 In a series of studies, Pieper et al.28 compared the effect of different slope algorithms on the performance of activation detection algorithms in unipolar electrograms. Three-point and 5-point Lagrange algorithms were the least susceptible to reductions in the sample rate from 2000 to 625 Hz and to random initiation of sampling. In a second study, Pieper et al.40 evaluated the effect of slope calculation algorithms on the temporal accuracy of automatic activation detection. The electrogram database included bipolar and unipolar recordings from a 119-pole epicardial sock electrode from 9 patients (5 with WPW syndrome, 4 with VT) recorded during sinus rhythm and ventricular pacing. Signals without detectable activation were excluded. Using manual reference annotations by 3 observers as a standard,

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detection errors were defined as differences in activation time of 10 ms or more. Activation detection based on the 5-point data fit, the 5-point Lagrange, and the 3point Lagrange algorithms had lower rates of detection errors than based on the 2-point algorithm, with error rates of 4.4%, 5.3%, 5.6% versus 7.0% in the WPW patients and 5.0%, 6.1%, 6.1% versus 7.8% in the VT patients.40 Masse49 compared the performance of activation detection algorithms for unipolar electrograms based on manual context-dependent activation time assignments in a database of 3452 epicardial and endocardial electrograms recorded from 8 patients with WPW syndrome and 13 patients with VT during sinus rhythm, stimulation, and tachycardias. With optimized slope thresholds, slope calculation by means of a 7-point differentiating filter with 60-Hz notch filter characteristics performed better than slope calculation with the 2-point algorithm only in case of signals with prominent 60-Hz noise, so that this approach was not recommended by the investigator.49 We evaluated activation detection algorithms for unipolar electrograms based on manual context-dependent activation time assignments in 6 recordings from a 240-pole sock electrode in 6 chronically infarcted dogs made during sinus rhythm (n = 2), ventricular pacing (n = 2), and VT (n = 2). At optimum parameters for both slope criterion and time criteria, slope calculation by the 3-point Lagrange algorithm yielded a minimum error rate of 6.2% versus 7.2% on slope calculation by the 2point algorithm. Slope calculation across wider time windows than 3 ms increased the activation detection error rate.44 Blanchard et al.58 found that a 17-point second-order data fit algorithm yielded the most stable determination of activation times in unipolar RV epicardial electrograms from dogs after the RV isolation

procedure. As that study excluded the problem of activation detection by means of a slope criterion, the relevance of the finding is, however, doubtful. Slope Thresholds: Most algorithms use a single "fixed" slope threshold that is uniform for all channels to distinguish between local and distant activation in unipolar electrograms. Fixed slope thresholds recommended in the literature range between -0.2 and 2.5 mV/ms, as has already been discussed in chapter 2. An alternative approach, first introduced by Witkowski and Corr59 for activation detection in bipolar electrograms, is the use of "statistical" thresholds that are calculated for each channel based on the distribution of slope values in that channel.49,54 Some activation detection algorithms dispense with slope thresholds altogether and simply search for the maximum absolute slope in the analysis window,60 which will however lead to an unacceptable rate of falsepositive activation time assignments in electrogram leads without discernible activation. While most of the evidence concerning optimal fixed slope thresholds has already been discussed in chapter 2, the following studies that published performance data on their algorithms deserve special mention. Masse et al.49,54 were the first to apply receiver operating characteristics (ROC) analysis to the performance evaluation of an activation detection algorithm. Optimum performance was defined as the point of equal sensitivity and positive predictive accuracy in detecting activation. Automatic signal analysis was rerun with a variety of different fixed and statistical threshold slopes. Optimum performance of the algorithm occurred at a fixed threshold slope of -0.2 mV/ms and a statistical threshold slope of mean -3 SD.54 With the first, the program made

REPRODUCIBILITY OF MAPPING a total of 1609 false detections (895 false negative, 714 false positive) out of 8050 activations, yielding a false detection rate of 20% or 0.45 false detections per second of electrogram data; with the latter, the program yielded very similar results, with 1605 detection errors (829 false negative, 776 false positive). We systematically evaluated the performance of activation detection algorithms for unipolar electrograms based on the aforementioned database of six 240-channel epicardial recordings from chronically infarcted dog hearts during sinus rhythm, ventricular pacing, and VT. A detection error was defined as a false-positive or false-negative detection of activation or a difference in activation times of more than 5 ms in respect to manual context-dependent activation time assignments by the investigators. The first version of the activation detector, which used a complicated statistical threshold slope algorithm, yielded error rates of 27% or higher nearly independent of the choice of threshold parameters, the main problem being false-positive detections of activation. We then modified the algorithm by introducing a fixed slope threshold. Instead of ROC analysis for a single parameter, a numerical optimization program was developed and applied to automatically search for the best combination of threshold and other parameters that resulted in the lowest number of detection errors in respect to the reference database of electrograms. Best performance of the algorithm was achieved using a 3-point Lagrange slope algorithm and at a fixed threshold slope of-1.8 mV/ms, yielding an error rate of 6.2% in respect to the database of 1440 electrograms used for the optimization and 9.6% (9.2% for sinus rhythm, 10.0% for pacing, 9.5% for VT) in respect to an independent database of 7200 electrograms. The latter error rate was of the

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same order of magnitude as the error rates between repeated context-dependent or -independent analyses by the same or by different observers (s.a.).44,61 Time Criteria: While Ideker et al. used the above-described time criterion with a time window width of 50 ms in which to search for the event with the absolute minimum of slope as the time point for the activation time assignment, Masse49 included a refractory period after each activation detection in their algorithm in which signal analysis was skipped. After systematically varying window widths between 40 and 200 ms and refractory periods between 80 and 200 ms, they found a time window of 160 ms and a refractory period of 80 ms optimal, though these settings had very little effect on overall program performance. Using a time criterion similar to Ideker's, we found a time window width of 15 ms optimal for analysis of ventricular pacing data as the program would otherwise ignore activations following stimulator spikes. For sinus rhythm and VT data, time windows between 20 and 60 ms were optimal. The effect of the time criterion was very limited, with an increase in error rate for all 3 rhythms from 6.2% to 8.1% with the least optimal time window setting.44 Additional Criteria: Pieper et al.62 suggested a criterion for the identification of noisy signal channels that calculates the SD of the signal from the mean in each channel, excluding times within ±30 ms of detected activation time and rejects the signal if the SD exceeds certain thresholds. In our above-mentioned study, we evaluated an amplitude threshold algorithm that would require electrograms to have a minimum nadir-to-peak amplitude for activation time assignments to be made.

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The additional amplitude criterion reduced error rates marginally from 6.2% to 5.1%. This was, however, most likely because of observer bias toward assigning activation times to electrograms with larger amplitudes in the electrogram database so that an amplitude criterion cannot be recommended on the basis of these results.44 This finding only applies to unipolar electrograms; an amplitude criterion did prove to be useful for preventing undesired activation time assignments to noisy low-amplitude signals in optical mapping (M. Biermann, unpublished observation). Likewise, filtering of signals did not improve the performance of the activation detection algorithm for unipolar signals,44 although filtering is often indispensable noise reduction in optical mapping signals. An algorithm that may significantly improve activation detection performance is linear interpolation across stimulus artifacts as is illustrated in Figure 3. Automatic activation detection in bipolar electrograms

While there is near universal agreement on using the criterion of maximum

negative slope for activation detection in unipolar electrograms, the criteria for activation detection in bipolar electrograms have been subject to debate. Suggested criteria include: (1) the maximum amplitude and the maximum absolute amplitude of the bipolar electrogram; (2) the maximum absolute slope of the bipolar electrogram; (3) the first elevation of the electrogram of more than 45° from the baseline (45°); (4) the baseline crossing with the steepest slope; and (5) morphological algorithms that search for the point of symmetry in the bipolar waveform. As the results of studies have already been discussed in chapter 2 and in the section on A/D conversion of electrograms in this chapter, it will suffice to cite only the most pertinent results. Paul et al.63 compared the results of activation detection in unipolar and bipolar electrograms recorded from a 41button sock electrode during sinus rhythm in 5 dogs. Activation detection by means of the maximum negative slope criterion in unipolar electrograms and by the peak and maximum absolute slope criteria in bipolar electrograms yielded very similar times of earliest epicardial activation

Figure 3. Improving automatic activation detection by interpolation across stimulus artifacts. Shown is an original epicardial electrogram (gray trace; ampfitude scale in mV, time scale in ms) recorded during unipolar epicardial stimulation from the surface of the canine left ventricle. Without interpolation, the activation detection algorithm fails to recognize the steep downstrokes of the electrogram at 47 and 347 ms as local activation of myocardium because the downstrokes of the preceding stimulus artifacts have a more negative slope. This can be avoided if the mapping program discards all samples in the trace recorded during stimulation and bridges the gap in signal by linear interpolation (solid trace).68

REPRODUCIBILITY OF MAPPING after the onset of the QRS complex of 7.9 ± 2.5 ms, 8.3 ± ms, and 9.5 ± 3.5 ms (differences not significant) and localized the first epicardial breakthrough to the same electrodes. Activation detection using the 45° criterion led to a significantly earlier activation time of-2.4 ± 1.7 ms and to a different apparent location of epicardial breakthrough.63 Using a database of 36 recordings from 119-pole sock electrode from 5 WPW syndrome and 4 VT patients during sinus rhythm and ventricular pacing, Pieper et al.40 compared the peak, baseline crossing with the steepest slope and maximum absolute slope criteria and a morphological algorithm that searched for the point of symmetry in the bipolar waveform in terms of consistency of agreement with manual context-dependent activation time assignments based on a baseline crossing with the steepest slope criterion. Electrograms without discernible activations were excluded. The morphological algorithm and the peak algorithms performed best with error rates (differences in activation times of 10 ms or more) of 1.7% and 3.0% in WPW patients and 6.2% and 6.2% in VT patients, respectively. The mean differences in activation times after the exclusion of detection errors ranged between 1.7 ± 1.6 ms (mean ± SD) and 2.1 ± 1.8 ms and were thus considered negligible. Simpson et al.64 repeatedly evaluated a complex morphological algorithm for activation detection in bipolar electrograms. In respect to a database of 49,717 activations during sinus rhythm and 61,174 activations during VF, the rate of false-positive detections with differences in activation times of 13 ms or more and false-negative detections was reported to be 23.5% for sinus rhythm and 51.6% for VF. In a second study, the authors optimized the threshold parameters of the program by means of ROC analysis aiming at maximum detection sensitivity. Using a database of 2808 electrograms (702 each

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during sinus rhythm and pacing and 1404 during VF; length of recording 200 ms) the program made false-negative detections in 1% in the sinus rhythm data and in 10% in the VF data, with false-positive detections occurring at a rate of 1.0 and 2.4 per second of electrogram data, respectively. Based on the length of the recordings of 200 ms, this would translate in false detection rates of 1 in 5 channels and 1 in 2 channels, respectively. As has been mentioned above, the detection error rates between activation time assignments by 3 different electrophysiologists were 0.05 and 0.8 errors per second for either rhythm, so that the agreement between different observers was significantly higher than between the program and the observers.55 Overall performance of detection algorithms

As can be seen from the above discussion, automatic activation detection in unipolar and bipolar electrograms is fraught with numerous difficulties, the most substantial of which is the lack of reliable quantitative criteria for detecting local activation and the susceptibility of detection programs to artifacts in the electrogram signals. A precise quantitative comparison between the studies that published performance data on the various algorithms is hindered by their use of different performance measures such as errors per activation or errors per data channel. Compared with manual activation time assignments made by electrophysiologists, a well-optimized activation detection program can nevertheless be expected to yield accurate activation detections in some 80% to 90% of all electrograms, depending on the underlying rhythm and the quality of the mapping data. At the present state of technology, automated activation detection algorithms can then serve the following purposes: (1) to help investigators focus their

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attention on the detailed analysis of signals in the region of interest during the near-online analysis of data in the course of a clinical study or an experiment, and (2) to make signal interpretation by observers more consistent and reduce observer mistakes. Unfortunately, due to the imperfections of the available algorithms it must still be considered necessary to review the raw electrogram data in all channels for the purpose of a scientific investigation.26,40,44,51 Map Generation and Interpretation of Activation Sequence The concluding steps in cardiac activation mapping are the generation of activation maps and the interpretation of the spatiotemporal activation sequence. As these steps are closely interrelated, they are discussed in the same section. Activation sequences can be presented in a variety of map formats. The simplest and most flexible format is that of a signal map, which displays sections of the original tracings with or without superimposed activation markers. Colorcoded activation maps represent activation times using a predefined color spectrum. Isochronal activation maps use isochrones connecting points with equal activation times, similar to contour lines connecting geographical points of equal elevation in a topographical map. Vector maps of activation encode activation sequences in the format of arrows, the length and direction of which encode the local apparent conduction velocity at each grid point of the map. Common to these mapping techniques are a number of assumptions that have been discussed in detail by Ideker et al.27 and Berbari et al.65: 1. The location of each recording site can be determined with acceptable accuracy. This assumption includes

both the accuracy of (re)localizing the recordings sites on the heart, which has been discussed above in respect to roving-probe mapping, and the accuracy of the 2-dimensional graphic display to represent the 3-dimensional spatial distribution of recording sites in their proper spatial relationship. If this first assumption is violated, maps may reveal spurious foci of activation.27,44 2. A single discrete activation time must exist at each recording site. This assumption is often violated in activation mapping if 2 activation fronts occur within the field of view of a recording electrode as during fractionated activity in peri-infarction zones or during fibrillatory rhythms, in which it may be conceptually difficult to divide the cardiac activation process into nonoverlapping sequences.27 3. Activation times from each recording site can be determined with acceptable accuracy.27 This assumption has been discussed in depth in the preceding sections, the necessary temporal accuracy being mainly a function of the prevailing conduction velocity and the required spatial accuracy. 4. The locations of the measurement sites must be sufficiently close together. The spatial accuracy required by a mapping study depends on its aim, e.g., to target a therapeutic intervention or to resolve the components of a reentrant circuit in an animal model. Some criteria can help decide whether the density of recording sites in the region of interest is sufficient to support the intended interpretation of the activation sequence. A simple criterion is the comparison of the lesion size of the intended therapeutic intervention such as operative resection,

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Figure 4. Isochronal mapping of one cardiac cycle during normal sinus rhythm using either each electrode (upper panel) or every second electrode (lower panel). See text for details.

cryo-, high-frequency, or laser ablation lesions with the density of recording sites during the mapping procedure. Another criterion is to decide whether the conclusions drawn from an activation map can still be supported after redrawing the map with a reduced number of recording sites.27 Figures 4 and 5 show epi-

cardial maps recorded from a chronically infarcted dog heart with a 240pole sock electrode during sinus rhythm and during figure-of-8 VT. Whereas the information on epicardial breakthrough sites, the infarction zone in the anterior wall, and latest epicardial activation is preserved in the sinus rhythm maps

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Figure 5. Isochronal mapping of one cycle of induced ventricular tachycardia using epicardial mapping data based on all electrodes (upper panel) and a reduced electrode density (every second electrode, lower panel). The presumed reentry pathway in the upper panel seems to represent a figure-of-8 pattern, whereas with a reduced electrode density any speculations about the pattern of reentry are impossible (lower panel).

REPRODUCIBILITY OF MAPPING both at full and at 50% reduced electrode density (Figure 4), suggesting that the density of recording sites was adequate, the figure-of-8 VT appears to have a focal point of origin at 50% reduced electrode density (Figure 5), indicating that electrode density is marginal for the intended interpretation of a reentrant pattern. However, neither criterion suffices to assure the completeness of a map, as is illustrated in a high-density epicardial plaque electrode recording during VT in Figure 6. Despite the nearly identical activation pattern at either recording site density, both maps are incomplete as the cardiac cycle is insufficiently depicted. 5. Isochrone-generation algorithms assume that the surface to be mapped is continuous and locally autocorrelated, i.e., that any value on the surface can be found by interpolation between neighboring measurement points; however, these assumptions are violated if wavefronts break through from below or terminate abruptly in an infarction zone.65 Ambiguities arise especially in case of locally varying conduction velocities and in the distinction between very slow conduction and conduction block. One step toward resolving at least some of these ambiguities would be the application of more advanced contour drawing algorithms that would not interpolate activation times across regions without detectable activation and across regions of conduction block.65 Map formats that obviate the need for at least some of these assumptions include potential maps, which display the spatial distribution of potential either in

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a color-coded format or through isopotential lines, and derivative maps,66 which similarly encode the first derivative of potential. A disadvantage of potential maps and derivative maps is that a sequence of maps is needed to display a single activation sequence, which is the reason why activation maps are still the most popular format for the purpose of printed documentation. If displayed as a movie sequence on a computer screen, they allow a very quick visual assessment of the underlying activation pattern without a need for indepth signal analysis, a technique that was first used by Parson et al.67 in their analog intraoperative mapping system.

Conclusion

Many ambiguities exist in the interpretation of electrogram signals, the limiting factor being the lack of precise quantitative criteria for identifying local activation of myocardium in extracellular signals. Thus, despite major advances in signal processing and computing technology, the effective gold standard for the interpretation of extracellular signals remains their visual analysis by the trained observer, which takes into account clinical or experimental information and the context of the neighboring signals. We believe that in addition to more quantitative pathophysiological information on local and far-field effects in extracellular signals, a major element in the quest to improve reproducibility and reliability of electrogram analysis by human observers will be the development of more intuitive and less restrictive user interfaces, which allow multiple simultaneous user-definable views on the mapping data in conjunction with more flexible options for annotating the data, as is illustrated in Figure 7.

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Figure 6. High-density electrode array recording from the anterior wall of a chronically infarcted canine heart with activation maps based on 100% and 50% electrode density. Despite almost identical activation pattern, both maps are incomplete as the cardiac cycle is insufficiently depicted.

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Figur e 7. Multimoda l signa l analysi s with athird-generatio n mappin g program . Signal analysi s can bemad emor econsisten atn d ma pgenerao ti nca n b esimplifie dthroug h mappni gsoftwar etha a t low s multipl e simultaneou s view s o fth e dat a in avariet yo format f s in a nonmoda l graphi c interface . The screensho t shows "Universa l Mapping, " the f i rts author's mappni g program f o r viewin g and annotatin g unipola r an d bipola r electrogra m and optica l mappn i g data . Simultaneou s windows displa y a ma po fal l signal s (right) , an enlarge dvie wo th f ecurrentl yselecte dtrac e (righ tbottom) , an enlarge d view of th e neighborin g traces ("close-up " window), color-coded map(s ) fo activatio n and/or repolarizatio n times ("color maps") , a movin g dsi pa l y of potentia l rofirs t derivativ e of potentia l ("movie " window) , ad n contour map s of activatio n or repolarizatio n times , action potentia l durations , potential or firs t derivativ e of potentia l ("contour " window ; shown si an isoderivativ e map of th e ventricual r tachycardi a ta the time fo f i r st epicardia l breakthrough) . A mouse click on an y data channe l n i an ywindo wwil l shif th t efocu st oth e n i tende d channel , an d an ychang et oth eannotatio n ofa signa l channel such as a different activation time assignment will instantly be reflected in all other windows.68

Lewi s , T Rothschil d MA. eTh excitator y p r o c ess ni t eh dog's h e a. r tPta rII. Th e ventricles . Philos Trans R Soc LondBBiol 1.B l a JM nd A,l t m aDnG Statistica . ml e t h Sci 191250;168:12-6 . f ods o assessin r a g rge e m ebne ttw e et w n o 5.D u r D r re Roo , JPs Epicardia . excitatio l n m e t h o sd f clinica o l m e a s u r e. m e Lancet nt of the ventricles in a patient with Wolff19863;33:073-10 . P a r k i n s o n - Weh si ty n d r oem( t y pe B) . Cir2. Altman DG. PracticalStatistics for Medical culation 215-35:1967; . Research.London : Chapma nan H dal 1;991 . 6. Fontain G e Guiraudon , G Fran , Rk e, alt . 3. Gallaghe r JJ , Kasell J, Sealy W C, et al. Epicardia lmappin gan dsurgica lt r e a t m e nt Epicardial mapping in the Wolffinis xcase soresistan f ventricula t tachy r Parkinson-White syndrome. Circulation cardi no arelate t t cdoornaro artery easdi y . 1978;57:854-866.

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16. Yoshida Y, Hirai M, Murakami Y, et al. Localization of precise origin of idiopathic ventricular tachycardia from the right ventricular outflow tract by a 12lead ECG:A study of pace mapping using a multielectrode "basket" catheter. Pacing Clin Electrophysiol 1999;22:17601768. 17. Scaglione M, Riccardi R, Calo L, et al. Typical atrial flutter ablation: Conduction across the posterior region of the inferior vena cava orifice may mimic unidirectional isthmus block. J Cardiovasc Electrophysiol 2000;11:387-395. 18. Zrenner B, Ndrepepa G, Schneider MA, et al. Mapping and ablation of atrial arrhythmias after surgical correction of congenital heart disease guided by a 64electrode basket catheter. Am J Cardiol 2001;88:573-578. 19. Gepstein L, Hayam G, Ben Haim SA. A novel method for nonfluoroscopic catheterbased electroanatomical mapping of the heart. In vitro and in vivo accuracy results. Circulation 1997;95:1611-1622. 20. Gepstein L, Evans SJ. Electroanatomical mapping of the heart: Basic concepts and implications for the treatment of cardiac arrhythmias. Pacing Clin Electrophysiol 1998;21:1268-1278. 21. Weiss C, Willems S, Rueppel R, et al. Electroanatomical mapping (CARTO) of ectopic atrial tachycardia: Impact of bipolar and unipolar local electrogram annotation for localization the focal origin. JInterv Card Electrophysiol 2001;5:101-107. 22. Yano K, Keida T, Suzuki K, et al. Catheter ablation of idiopathic left ventricular tachycardia with multiple breakthrough sites guided by an electroanatomical mapping system. JInterv Card Electrophysiol 2001;5:211-214. 23. Gornick CC, Adler SW, Pederson B, et al. Validation of a new noncontact catheter system for electroanatomic mapping of left ventricular endocardium. Circulation 1999; 99:829-835. 24. Schilling RJ, Peters NS, Davies DW. Simultaneous endocardial mapping in the human left ventricle using a noncontact catheter: Comparison of contact and reconstructed electrograms during sinus rhythm. Circulation 1998;98:887898. 25. Roberts DE, Hersh LT, Scher AM. Influence of cardiac fiber orientation on wavefront voltage, conduction velocity, and

REPRODUCIBILITY OF MAPPING 185 tissue resistivity in the dog. Circ Res 1979;44:701-712. 26. Rosenbaum DS, Kaplan DT, Wilber DJ, et al. The precision of electrophysiological mapping: Localizing depolarization wave fronts from digital extracellular electrograms and the role of data sampling rate. J Cardiovasc Electrophysiol 1990;1: 2-14. 27. Ideker RE, Smith WM, Blanchard SM, et al. The assumptions of isochronal cardiac mapping. Pacing Clin Electrophysiol 1989;12:456-478. 28. Pieper CF, Blue R, Pacifico A. Influence of time of sampling onset on parameters used for activation time determination in computerized intraoperative mapping. Pacing Clin Electrophysiol 1991;14:21872192. 29. Bardy GH, Smith WM, Ungerleider RM, et al. Identification of reproducible ventricular tachycardia in a canine model. AmJCardiol 1984;53:619-625. 30. Abendroth RR, Ostermeyer J, Breithardt G, et al. Reproducibility of local activation times during intraoperative epicardial mapping. Circulation 1980;62:75-79. 31. Hoeks APG, Schmitz GML, Allessie MA, et al. Multichannel storage and display system to record the electrical activity of the heart. Med Biol Eng Comput 1988;26: 434-438. 32. Breithardt G, Shenasa M, Biermann M, et al. Precision and reproducibility of isochronal electrical cardiac mapping. In: Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping. Mount Kisco, New York: Futura Publishing Co.; 1993: 35-49. 33. Branyas NA, Cain ME, Cox JL, Cassidy DM. Transmural ventricular activation during consecutive cycles of sustained ventricular tachycardia associated with coronary artery disease. Am J Cardiol 1990;65:861-867. 34. Franz MR. Method and theory of monophasic action potential recording. Prog Cardiovasc Dis 1991;33:347-368. 35. Witkowski FX, Penkoske PA, Kavanagh KM. Mapping of ventricular fibrillation: Technical considerations. In: Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping. Mount Kisco, New York: Futura Publishing Co.; 1993:79-84. 36. Green LS, Taccardi B, Ershler PR, Lux RL. Epicardial potential mapping: Effects of conducting media on isopotential and iso-

chrone distributions. Circulation 1991;84: 2513-2521. 37. Durrer D, Van der Tweel LH. Excitation of the left ventricular wall of the dog and goat. Ann N YAcad Sci 1957;65:779-803. 38. Gallagher JJ, Kasell JH, Cox JL, et al. Techniques of intraoperative electrophysiologic mapping. Am J Cardiol 1982; 49:221-240. 39. Wit AL, Josephson ME. Fractionated electrograms and continuous activity: Fact or artifact. In: Zipes DP, Jalife J (eds): Cardiac Electrophysiology and Arrhythmias. Orlando: Grune & Stratton, Inc.; 1985:343352. 40. Pieper CF, Blue R, Pacifico A. Activation time detection algorithms used in computerized intraoperative cardiac mapping: A comparison with manually determined activation times. J Cardiovasc Electrophysiol 1991;2:388-397. 41. De Bakker JMT, Van Capelle FJL, Janse MJ, et al. Reentry as a cause of ventricular tachycardia in patients with chronic ischemic heart disease: Electrophysiologic and anatomic correlation. Circulation 1988;77:589-606. 42. Blomstrom P, Blomstrom-Lundqvist C, Berglin-William-Olson E, Olson SB. The variability and reproducibility of atrial and ventricular activation times during cardiac mapping. New Trends Arrhythmias 1989;5:9-18. 43. Cowan JC, Griffiths CJ, Hilton CJ, et al. Epicardial repolarisation mapping in man. Eur Heart J 1987;8:952-964. 44. Biermann M. Die Reproduzierbarkeit des kardialen elektrophysiologischen Mappings. Untersuchungen uber die manuelle und automatische Auswertung unipolarer Elektrogramme. Munster: Westfalische Wilhelms-Universitat Munster; 1994. 45. Barr RC, Spach MS. Sampling rates required for digital recording of intracellular and extracellular cardiac potentials. Circulation 1977;55:40-48. 46. Pieper CF, Lawrie G, Roberts R, Pacifico A. Bandwidth-induced errors in parameters used for automated activation time determination during computerized intraoperative cardiac mapping: Theoretical limits. Pacing Clin Electrophysiol 1991;14: 214-226. 47. Blanchard SM, Barr RC. Comparison of methods for adaptive sampling of cardiac electrograms and electrocardiograms. Med Biol Eng Comput 1985;23:377-386.

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48. Kaplan DT, Smith JM, Rosenbaum D, Cohen RJ. On the precision of automated activation time estimation. Comput Cardiol 1988:101-104. 49. Masse S. Detection optimale des temps d'activation d'electrogrammes epicardiques et endocardiques chez I'homme. Universite de Montreal, 1989. 50. Pieper CF, Parsons D, Lawrie GM, et al. Design and implementation of a new computerized system for intraoperative cardiac mapping: Experience during surgery. JApplPhysiol 1991;71:1529-1539. 51. Ideker RE, Smith WM, Wolf P, et al. Simultaneous multichannel cardiac mapping systems. Pacing Clin Electrophysiol 1987;10:281-292. 52. Page PL, Cardinal R, Savard P. Sinus rhythm mapping in a canine model of ventricular tachycardia. Pacing Clin Electrophysiol 1988;11:632-644. 53. Anderson KP, Walker R, Ershler PR, et al. Determination of local myocardial electrical activation for activation sequence mapping. A statistical approach. Circ Res 1991;69:898. 54. Masse S, Savard P, Shenasa M, et al. Performance of the automatic detection of local activation times on unipolar cardiac electrograms in man. IEEE Eng Med Biol 1988; 10th Ann. Int. Conf.:112. 55. Simpson EV, Ideker RE, Cabo C, et al. Evaluation of an automatic cardiac activation detector for bipolar electrograms. Med Biol Eng Comput 1993;31:118-128. 56. Ideker RE, Smith WM, Wallace AG, et al. A computerized method for the rapid display of ventricular activation during the intraoperative study of arrhythmias. Circulation 1979;59:449-458. 57. Marble AE, Mclntyre CM, HastingsJames R, Hor CW. A comparison of digital algorithms used in computing the derivative of left ventricular pressure. IEEE Trans Biomed Eng 1981;28:24-529. 58. Blanchard SM, Smith WM, Damiano RJ, et al. Four digital algorithms for activa-

tion detection from unipolar epicardial electrograms. IEEE Trans Biomed Eng 1989;36:256-261. 59. Witkowski FX, Corr PB. An automated simultaneous transmural cardiac mapping system. Am J Physiol 1984;247: H661-H668. 60. De Bakker JMT, Janse MJ, Van Capelle FJL, Durrer D. An interactive computer system for guiding the surgical treatment of life-threatening ventricular tachycardias. IEEE Trans Biomed Eng 1984;31: 362-368. 61. Biermann M, Shenasa M, Haverkamp W, et al. Improving automatic activation detection in epicardial electrograms by numerical optimization. Eur Heart J1994; 15(Suppl):212. 62. Pieper CF, Glenn P, Parsons D, Pacifico A. Current source density electrode array improves spatial resolution for intraoperative mapping. Pacing Clin Electrophysiol 1991;14:711. Abstract 63. Paul T, Moak JP, Morris C, Garson A. Epicardial mapping: How to measure local activation? Pacing Clin Electrophysiol 1990;13:285-292. 64. Simpson EV, Ideker R, Smith WM. An automatic activation detector for bipolar cardiac electrograms. IEEE Eng Med Biol 1988; 10th Ann. Int. Conf.: 113-114. 65. Berbari EJ, Lander P, Scherlag BJ, et al. Ambiguities of epicardial mapping. JElectrocardiol 1991;24:16-20. 66. Ershler PR, Lux RL. Derivative mapping in the study of activation sequence during ventricular tachyarrhythmias. Comput Cardiol 1987:623-624. 67. Parson I, Mendler P, Downar E. On-line cardiac mapping: An analog approach using video and multiplexing techniques. Am J Physiol 1982;242:H526-H535. 68. Biermann M, Zipes DP. A new mappingsystem independent analysis program for extracellular electrograms and optical action potentials. Eur Heart J 1997; 18: 38A. Abstract.

Chapter 9

The Ideal Cardiac Mapping System Raymond E. Ideker, MD, PhD, Patrick D. Wolf PhD, Edward Simpson, MS, Eric E. Johnson, MD, Susan M. Blanchard, PhD, and William M. Smith, PhD

This chapter was written for the first edition of this book. It deals almost exclusively with electrical mapping systems. Many techniques that were predicted to improve cardiac mapping systems by the turn of the century have not been implemented. Examples include the ability to record action potentials and ion concentrations from tens of thousands of sites transmurally. While optical mapping techniques can record these variables from numerous sites on the epicardium or endocardium,1 attempts are just beginning to obtain optical recordings transmurally.2,3 Another of our projections that was overly optimistic was the number of electrodes from which simultaneous recordings would be made. We estimated this number to be 3000 by 1995 and 30,000 by 2000. In 2002, the largest operational electrical mapping system of which we are aware can record from approximately 1700 electrodes simultaneously. In this

area also, greater strides have been made in optical than in electrical mapping. Optical mapping systems that utilize charge coupled devices can record from more than 100,000 locations.4 One prediction that has been fulfilled is the automatic detection of activations and the identification and quantification of activation fronts.5 While the ideal cardiac mapping system is not yet available, we remain optimistic that many of the capabilities discussed in this chapter will become available in the foreseeable future. When the editors asked us to discuss the ideal cardiac mapping system, we were delighted because it meant we could have the fun of imagining what such a mapping system should do without having to put forth the effort of actually trying to build it. If technology continues to advance at its present rate, we believe that most of the features of the ideal mapping system we will describe can be

This work is supported in part by the National Institutes of Health research grants HL-28429, HL-33637, HL-42760, HL-44066, and HL-40092; and the National Science Foundation Engineering Research Center grant CDR-8622201 From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; c2003. 187

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implemented by the end of the century. channels feasible around the turn of the Any exceptions will be specifically noted. century, using commercially available comIn the short term, the ideal mapping ponents. Simultaneous recordings from system is easy to describe. It is the same 30,000 electrodes will allow the entire epidescription that we imagine Moe, Harris, cardium to be mapped with electrodes less and Wiggers6 would have given after con- than 2 mm apart or will allow the epistructing their 3-channel string gal- cardium or endocardium bordering an vanometer mapping system in 1941, that infarct to be mapped with a plaque greater we would have given 15 years ago when than 16 cm2 containing electrodes 25 um we built our first 32-channel mapping apart. Such close electrode spacing would system,7 and that we would give now just alleviate much of the uncertainty involved after completing our 528-channel map- with interpolation between more widely ping system.8 The ideal mapping system spaced electrodes; it would provide less is the one that has just a few more elec- ambiguous information about complex actitrodes and that can process the data just vation sequences that occur during fibrillaa little faster than the mapping system tion10,11 and during fractionated activation you now have. Everyone who has ever through regions of patchy infarctions.12 mapped has had the experience of wishThe use of thousands of electrodes ing they had electrodes in some part of will require us to change the way in which the heart where they do not have any or the recordings are processed. It will no wishing they had electrodes more closely longer be possible to inspect all electrospaced in the region where something grams manually to verify that a single particularly interesting looks like it might activation time has been properly selected for each beat of each electrode recording.13 be happening.9 Long-term changes are difficult to If 1 second is necessary to verify a single predict because it is impossible to foresee activation in a single channel, then the what technological breakthroughs will analysis of a single beat will require about occur in the meantime. If we ignore these 1 hour for 3000 electrodes and about 1 day possible breakthroughs and just extrapo- for 30,000 electrodes. Most of the processes late using current rates of improvements of data analysis and display will have to in processor speeds and storage capabili- be automated to make practical the use of ties, the possibilities are still very exciting. a 3000- or 30,000- channel mapping system. In the long term, we think the number of Instead of static pictures of isochronal electrodes will continue to increase. From maps, animations of the recorded poten1940 to 1980 the number of electrodes tials or the temporal or spatial derivatives increased by an order of magnitude from of the potentials will probably be dis3 to approximately 30, while from 1980 played as shown in Figure 1.14,15 These to 1990 they increased another order of two types of displays have the advantage magnitude to approximately 300. We pre- that they show all of the recorded data or dict that within a few years, the number a transform of the data, while isochronal of electrodes will increase another order of maps show only a single time point during magnitude to approximately 3000. Yet each complex.16 For example, during a another order of magnitude larger in the single cycle of an arrhythmia, a tracing number of electrodes, 30,000, would be may contain 2 complexes (Figure 1), while desirable. Processors and buses continue an isochronal map would display only the to get faster and data storage devices con- time of the fastest unipolar downslope. tinue to increase in capacity at a rate of An animation of the spatial derivatives of progress that should make this many the potentials will show both complexes.

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Figure 1. Selected frames of an animated computer display of the temporal derivative of the potentials for 121 simultaneously recorded electrodes during one cycle of ventricular fibrillation in a pig. The electrodes are in an 11 x 11 grid with an interelectrode distance of 280 um. The colors represent the derivative of the potential in units of volts per second as indicated by the scales at the bottom of the figure. One electrode recording was inadequate because the electrode wire was broken; the black square in each frame represents this electrode. The vertical line in the tracing below each frame, recorded from one electrode, indicates the time of each frame. The location of this electrode is indicated by the arrow in frames A and E. The first complex in the electrode recording arises from an activation front that enters the upper left corner of the grid and then blocks about one third of the way across the grid (frames A through D). The second complex arises from activation of the remainder of the grid by 2 fronts that enter the lower left and upper right corners of the grid and then collide (frames E through H). See color appendix.

Sock and plaque electrode arrays containing thousands of electrodes will probably be constructed using microelectronic techniques.17 Connectors containing hundreds or thousands of contacts will be necessary to connect such electrode arrays to the data acquisition system because the use of smaller connectors will be impractical. For example, 1200 DB25 connectors would be required for 30,000 unipolar electrodes. With 30,000 channels, the technology for placing the electrodes, rather than the mapping system, will become the limiting

factor. A 30,000-channel mapping system would provide enough electrodes so that the entire 3-dimensional volume of the ventricle could be mapped with 6000 plunge electrodes either 2 mm or 3 mm apart and with electrodes spaced every 2 mm along each plunge needle. However, insertion of 6000 plunge needles into the heart is probably impractical, even if the electrodes are made very small, light, and flexible using microelectronic techniques and the insertion technique involves inserting many plunge needles as a unit simultaneously. It is likely that some of

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these needles would pierce small arteries, causing ischemia and altering intrinsic cardiac electrical properties. An alternative method for determining the 3-dimensional patterns of cardiac electrical activity is to solve an inverse volume conductor problem similar in some respects to the inverse problem of electrocardiography in which epicardial potentials are estimated numerically from body surface potentials.18 This form of the inverse problem of electrocardiography has a unique solution because all of the electrical sources are contained within the epicardium so that no sources are within the volume bordered by the body surface and the epicardium. For the inverse problem, we are proposing that the potentials recorded from the epicardium and endocardium, as well as from some intramyocardial electrodes on plunge needles, would be used to calculate the potential distribution throughout the heart volume. This inverse problem is much more difficult than the inverse problem of electrocardiography previously mentioned because electrical sources exist and must be identified within the heart volume. This inverse problem does not have a unique solution and we do not know whether a feasible solution is possible. A major problem with acquiring thousands of channels of signals is the analysis and comprehension of a huge amount of data. The ideal mapping system should be capable of displaying the data in a form in which the information can be readily grasped. This will probably entail animated 3-dimensional graphic displays of all or of a part of the heart that can be rotated or sliced through at any desired angle (Figure 2.15 Ideally, such displays should be available almost immediately after the data are recorded. Data acquisition should be continuous through the time the electrodes are in place so that any event of interest will be captured.9 Real time analysis of the data

should be possible so that certain interventions, such as delivering a shock, will be possible when a particular set of predefined criteria occurs. To minimize noise, the initial amplification stage of the ideal mapping system should be as close as possible to the recording site, possibly on the electrode apparatus itself.19 Digitization, data compression, and some analysis may also be performed at this site to decrease storage and computational requirements of the external mapping system. Multiplexing of these data will decrease the number of connectors referred to previously. It should be able to verify that each electrode recording is adequate, for example by monitoring electrode impedance and noise levels. The mapping system should be able to identify the location of each electrode and how it moves throughout the cardiac cycle, possibly by ultrasonic crystals attached to the electrodes or by impedance imaging.20 The ideal mapping system should be able to record the potentials generated by pacemakers and defibrillators and to calculate the potential gradient of the electrical stimuli at each electrode.21 Besides the times of activation, the mapping system should also be able to determine other cardiac electrophysiologic variables,22 such as the action potential duration and the refractory period of the tissue adjacent to each electrode.23 It should also be able to determine the transmembrane potential at each electrode, perhaps by using either optical mapping24 or small monophasic action potential electrodes.25 It should be able to pace or ablate from any recording electrodes. The ideal mapping system should also be able to measure nonelectrophysiological variables such as sodium, potassium, and calcium concentrations, pH, and oxygen levels.26 It should also be able to determine certain anatomical features, such as whether the electrode is in viable

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Figure 2. Potentials generated throughout a canine heart by a defibrillation shock. A representation of a cut volume of the heart is shown in which each cube corresponds to a 2 mm x 2 mm x 2 mm block of myocardial tissue and is colored to indicate potential in that block. The most negative potentials are indicated by violet and the most positive are indicated by orange. Heart geometry was obtained by magnetic resonance imaging. The depth of the cut plane can be varied interactively by the investigator along any of the 3 axes. The cubes containing Xs indicate the locations of those recording electrodes exposed in this cut plane. Values in the other cubes are obtained by interpolation. See color appendix.

myocardium, old infarct scar, or acutely necrotic tissue, thus allowing identification of the location and extent of infarction.27 The ideal mapping system should be small, inexpensive, easy to use, and self calibrating. With a few exceptions,28-30 multichannel computer-assisted mapping studies have been performed by placing electrodes acutely in patients or animals who are under general anesthesia and have their chests open. Electrodes should be developed that can be introduced into a ventricular cavity via a percutaneously

inserted catheter and that can record from many known sites on the endocardium simultaneously.31 Such an electrode array would probably lead to improved ablative therapy for ventricular tachycardia and might allow the development of ablative techniques to treat those patients with recurrent ventricular fibrillation in whom the first cycle of the arrhythmia arises from the same cardiac region during different episodes of fibrillation. Much exciting progress has been made in developing animal models of sudden cardiac death.32-35 Multichannel mapping

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tropically and is guided by fiber orientasystems using chronically implanted election in guinea pig hearts. Circ Res 1995; trodes and telemetry are needed so that 77:784-802. the mechanism of the spontaneous onset 4. Baxter WT, Davidenko JM, Loew LM, of arrhythmias can be investigated in conet al. Technical features of a CCD video scious, unfettered animals. camera system to record cardiac fluorescence data. Ann Biomed Eng 1997;25: Thus, taking a broad view, we think 713-725. the ideal mapping system could (1) record 5. Huang J, Rogers JM, KenKnight BH, cardiac electrical activity, potential graet al. Evolution of the organization of dients created by defibrillation shocks, epicardial activation patterns during transmembrane potentials, action potenventricular fibrillation. J Cardiac Electrophysiol 1998;9:1291-1304. tial durations, refractory periods, ion con6. Moe GK, Harris AS, Wiggers CJ. Analycentrations, and tissue health (i.e., normal sis of the initiation of fibrillation by elecand infarcted), simultaneously and contrographic studies. Am J Physiol 1941; tinuously from 30,000 electrodes; (2) 134:473-492. determine electrode locations and the 7. Ideker RE, Smith WM, Wallace AG, et al. A computerized method for the rapid disadequacy of electrode recordings; and (3) play of ventricular activation during the display the results in an animated 3intraoperative study of arrhythmias. Cirdimensional display almost immediately culation 1979;59:449-458. after the data are acquired. We would be 8. Wolf PD, Rollins DL, Blitchington TF, interested in borrowing such a mapping et al. Design for a 512 channel cardiac mapping system. In: Mikulecky DC, Clarke AM system as soon as someone builds it. (eds): Biomedical Engineering: Opening Taking a more practical view, the New Doors. Proceedings of the Fall 1990 ideal mapping system is the one that Annual Meeting of the Biomedical Engiaccomplishes the job at hand. The ideal neering Society. New York: New York Uniclinical mapping system may be different versity Press; 1990: 5-13. than the ideal experimental mapping 9. Ideker RE, Smith WM, Wolf PD, et al. Simultaneous multichannel cardiac mapsystem. In the final analysis, the ideal ping systems. Pacing Clin Electrophysiol mapping system is the system that effi1987;10:281-292. ciently obtains the diagnosis, identifies 10. Witkowski FX, Penkoske PA. Activation the region, or tests the experimental patterns during ventricular fibrillation. In: Jalife J (ed): Ann. N YAcad Sci, V. 591, hypothesis that one is interested in, even Mathematical Approaches to Cardiac if it only requires recording a single cycle Arrhythmias. New York: The New York from a single electrode. Academy of Sciences; 1990:219-231.

References 1. Girouard SD, Laurita KR, Rosenbaum DS. Unique properties of cardiac action potentials recorded with voltage-sensitive dyes. J Cardiac Electrophysiol 1996;7: 1024-1038. 2. Baxter WT, Pertsov A, Berenfeld 0, et al. Demonstration of three-dimensional reentry in isolated sheep right ventricle. Pacing Clin Electrophysiol 1997;20:1080. Abstract. 3. Kanai A, Salama G. Optical mapping reveals that repolarization spreads aniso-

11. Johnson EE, Idriss SF, Melnick SB, et al. Conduction block during ventricular fibrillation in pigs mapped with closely spaced electrodes. Circulation 1991;84: 11-499. Abstract. 12. Gardner PI, Ursell PC, Fenoglio JJ Jr., et al. Electrophysiologic and anatomic basis for fractionated electrograms recorded from healed myocardial infarcts. Circulation 1985;72:596-611. 13. Smith WM, Ideker RE. Computer techniques for epicardial and endocardial mapping. Prog Cardiovasc Dis 1983;26:15-32. 14. Ershler PR, Lux RL. Derivative mapping in the study of activation sequence during ventricular tachyarrhythmias. In: Ripley KL (ed): Proc. Comput Cardiol. Washington, DC:

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IEEE Computer Society Press; 1986: 26. Richards DA, Cody DV, Denniss AR, et al. Ventricular electrical instability: A pre623-624. dictor of death after myocardial infarc15. Palmer TC, Simpson EV, Kavanagh KM, tion. Am J Cardiol 1982;51:75-80. et al. Visualization of bioelectric phenomena. CRCCrit Rev Biomed Eng 1992; 20:355-372. 27. Cabo C, Wharton JM, Wolf PD, et al. Activation in unipolar cardiac electrograms: 16. Ideker RE, Smith WM, Blanchard SM, A frequency analysis. IEEE Trans Biomed et al. The assumptions of isochronal carEng 1990;37:500-508. diac mapping. Pacing Clin Electrophysiol 28. Spach MS, Barr RC. Ventricular intra1989;12:456-478. mural and epicardial potential distribu17. Mastrototaro JJ, Massoud HZ, Pilkington tions during ventricular activation and TC, et al. Rigid and flexible thin-film mulrepolarization in the intact dog. Circ Res tielectrode arrays for transmural cardiac 197 5;37-.243-257. recording. IEEE Trans Biomed Eng 1992; 29. Spach MS, Barr RC. Analysis of ventric39:271-279. ular activation and repolarization from 18. Rudy Y, Messinger-Rapport BJ. The inverse intramural and epicardial potential disproblem in electrocardiography: Solutions tributions for ectopic beats in the intact in terms of epicardial potentials. CRC Crit dog. Circ Res 1975;37:830-843. Rev Biomed Eng 1988; 16:215-268. 19. Wise KD, Najafi K. Microfabrication tech- 30. Spach MS, Barr RC, Lanning CF, et al. Origin of body surface QRS and T wave niques for integrated sensors and microsystems. Science 1991;254:1335-1342. potentials from epicardial potential distributions in the intact chimpanzee. Cir20. Wolf PD, Smith WM, Pilkington TC. culation 1977;55:268-278. Determination of cardiac geometry from impedance measurements. In: Kim Y, 31. Taccardi B, Arisi G, Macchi E, et al. A new intracavitary probe for detecting the Spelman FA (eds): Proc Annu Int Conf site of origin of ectopic ventricular beats IEEE Eng Med Biol Soc Piscataway, NJ: during one cardiac cycle. Circulation 1987; Institute of Electrical and Electronics Engineers, Inc.; 1989:1239-1240. 75:272-281. 21. Wolf PD, Rollins DL, Smith WM, et al. A 32. Schwartz PJ, Billman GE, Stone HL. Autonomic mechanisms in ventricular cardiac mapping system for the quantifibrillation induced by myocardial tative study of internal defibrillation. In: Harris G, Walker C (eds): Proc Annu ischemia during exercise in dogs with healed myocardial infarction: An experInt Conf IEEE Eng Med Biol Soc Piscataway, NJ: Institute of Electrical and imental preparation for sudden cardiac death. Circulation 1984;69:790-800. Electronics Engineers, Inc.; 1988:217-218. 22. Simpson EV, Ideker RE, Cabo C, et al. 33. Parker GW, Michael LH, Entman ML. An Evaluation of an automatic cardiac actianimal model to examine the response to vation detector for bipolar electrograms. environmental stress as a factor in Med Biol Eng Comput 1993;31:118-128. sudden cardiac death. Am J Cardiol 1987;60:9J-14J. 23. Haws CW, Lux RL. Correlation between in vivo transmembrane action potential 34. Patterson E, Holland K, Eller BT, et al. durations and activation-recovery interVentricular fibrillation resulting from vals from electrograms: Effects of interischemia at a site remote from previous ventions that alter repolarization time. myocardial infarction. A conscious canine Circulation 1990;81:281-288. model of sudden coronary death. Am J 24. Dillon S, Morad M. A new laser scanning Cardiol 1982;50:1414-1423. system for measuring action potential 35. Li HG, Jones DL, Yee R, et al. Electropropagation in the heart. Science 1981; physiologic substrates associated with 214:453-456. pacing-induced heart failure in dogs: 25. Franz MR. Method and theory of monoPotential value of programmed stimulaphasic action potential recording. Prog tion in predicting sudden death. J Am Coll Cardiouasc Dis 1991;33:347-368. Cardiol 1992; 19:444-449.

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Part 3 Mapping in Experimental Models of Cardiac Arrhythmias

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Chapter 10 The Role of Myocardial Architecture and Anisotropy as a Cause of Ventricular Arrhythmias in Pathological States Nicholas S. Peters, MD and Andrew L. Wit, PhD

It has been recognized that abnormalities in conduction of the cardiac impulse are an important cause of arrhythmias since Mayer1 and Mines2 performed their classic experiments on circulating excitation in rings of excitable tissue. These investigators showed the important relationships between the conduction velocity, path length, and recovery of excitability that are the prerequisites for reentrant excitation. The reentrant impulse must continuously encounter excitable tissue as it propagates through the reentrant pathway. Regions of this pathway that have already been excited must have adequate time to recover before they can be excited again. This adequate time is often provided by slower than normal conduction of the reentrant impulse in part or all of the pathway.3 Conduction of the cardiac impulse is dependent on both the active membrane properties of cardiac cells (depolarization phase of the action potential) and the

passive properties determined by resistive discontinuities to cytoplasmic current flow determined in part by the distribution of intracellular (gap) junctions.4 Most investigations on reentry until the mid 1970s focused on abnormalities in the depolarization phase of the action potential as the primary cause of the altered conduction needed for reentry. This culminated in the concept of the slow response action potential, which Cranefield5 proposed was the major cause of reentrant arrhythmias. Although gap junctions were, at this time, known to be the low-resistance pathways between the cytoplasmic compartments of the cardiac myocytes, receiving a lot of attention from experimental electrophysiologists for their central role in myocardial conduction,6 gap-junctional coupling did not play a prominent role in the development of concepts on the mechanisms of reentrant arrhythmias up to this time.

Supported by Program Project Grant HL 30557 and by Grant R-37 HL 31393 from the National Heart Lung and Blood Institute, National Institutes of Health. From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; c2003. 197

198 CARDIAC MAPPING In 1959, Sano et al.7 reported that conduction properties in myocardium were different in different directions, the definition of anisotropic conduction. In 1976, Clerc8 extended these observations, showing that the more rapid conduction in the direction of the long axis of myocardial fiber bundles that is characteristic of heart muscle was associated with a lower intracellular resistivity in this direction and that the slower conduction transverse to the long axis of the myocardial fiber bundles could be attributed to a greater intracellular resistance in this direction. However, it was not until Spach and his colleagues published a series of papers between 1979 and 1989 9-16 that the anisotropic properties of cardiac muscle and the myocardial structure, i.e., architecture, that determines these properties were considered to be an important cause of slow activation that is instrumental in the occurrence of reentrant arrhythmias. In broad terms, the influence of tissue architecture on conduction is determined by the size and shape of individual myocytes, and by the quantity, 3dimensional distribution, and physiological behavior of the gap junctions. There is also some influence of the properties of the extracellular space in which current flows during action potential propagation. The role of gap junctions in influencing myocardial conduction properties in health and disease has developed from simple concepts to a more complex subject as research has demonstrated progressively increasing complexity of gap-junctional form and function. Understanding in this area has been greatly advanced in recent years by elucidation of the morphological, physiological, and molecular aspects of these junctional resistances. In this chapter we discuss the role of the anisotropy of ventricular muscle as a cause of ventricular arrhythmias in pathological conditions, and the evidence indicating how alterations in

myocardial architecture, particularly involving the gap junctions, that occur in these conditions may underlie the arrhythmogenesis. Anisotropic Conduction in Normal Ventricular Myocardium Conduction in normal ventricular myocardium is anisotropic, meaning that conduction properties vary depending on the direction in which they are measured relative to the structure of the myocardium. In normal ventricular myocardium, conduction in the direction parallel to the long axis of the myocardial fiber bundles is about 3 times more rapid than that transverse to the long axis. The anisotropy is considered to be uniform because it is characterized by "an advancing wavefront that is smooth in all directions relative to the orientation of the long axis of the myocardial fiber bundles."11 Uniform anisotropy is exemplified by the conduction properties of normal septal ventricular muscle shown in Figure 1A. The muscle in the diagram was stimulated in the center (pulse symbol) and activation spread away from this site in all directions. In the direction of the longitudinal axis of the fibers (from top to bottom) the isochrones are widely spaced indicating rapid conduction, in this case conduction velocity is 0.51 m/s. There is a relatively broad area of fast conduction with an elliptic shape of the isochrones that is characteristic of uniform anisotropy.12 In the direction transverse to the long axis (to the right and to the left), the isochrones are spaced close together indicating slower conduction; conduction velocity is 0.17 m/s in this example. As the direction of propagation changes between these 2 axes, the apparent conduction velocity changes monotonically from fast to slow, another characteristic of uniform anisotropy.10

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Figure 1. Characteristics of uniform anisotropic conduction in ventricular muscle. The excitation sequence in A was constructed from extracellular electrograms recorded at 100 sites on the endocardial surface of the right ventricular septum. The isochrones showing the activation pattern are characteristic of uniform anisotropy. The extracellular waveforms in B were recorded at the sites indicated by the dots on the activation map. The electrogram with the solid trace was recorded from a region of transverse propagation while the electrogram indicated by the dashed trance was recorded from a region of longitudinal propagation. The effects of different directions of propagation on the upstroke of the action potential are shown in C. The direction of propagation at the single transmembrane recording site was altered from longitudinal (dashed upstroke) to transverse (solid upstroke). Reproduced from reference 12, with permission.

The slow conduction in the direction transverse to the longitudinal fiber axis occurs despite action potentials with normal resting potentials and upstroke velocities. Spach et al.10 have described that the differences in conduction velocity based on direction of propagation are accompanied by changes in the action potentials. Their experiments have shown that when going from fast longitudinal conduction to slow transverse conduction, the rate of depolarization during the upstroke of the action potential (Vmax) increases and the time constant of the foot of the upstroke decreases without any change in the resting potential as illustrated in Figure 1C; the upstroke that is dashed was recorded from a cell during longitudinal propagation while

the upstroke indicated by the solid line was recorded from the same cell during transverse propagation. These characteristics are opposite to the changes in the action potentials associated with slowing of conduction when the membrane currents are altered (such as by resting membrane depolarization) where slow activation is associated with slowing of the rate of depolarization during the upstroke of the action potential17,18; however, results of other studies do not agree with these observations. Studies on anisotropic propagation in tissue cultures of rat neonatal myocytes have not shown a difference in upstroke velocity dependent on direction of impulse propagation relative to the long axis of the cells, although the cultures demonstrated prominent

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anisotropic conduction patterns.19,20 This controversy must be solved by further investigation. Slow conduction in the transverse direction is also associated with a decrease in the amplitude of the extracellular electrogram showing that there is a decrease in the extracellular current flow as a result of the increased axial resistivity. In uniform anisotropic tissue, the extracellular unipolar waveform has a large-amplitude, smooth biphasic positive-negative morphology during propagation in the fast longitudinal direction, which is shown in Figure 1B by the dashed lines, and a low-amplitude smooth triphasic (negative-positive-negative) morphology in the transverse direction, which is shown by the solid line. The initial negativity of the electrogram in the transverse direction is a reflection of distant activity rapidly propagating along the longitudinal axis.9 Myocardial Architecture Underlying Uniform Anisotropic Conduction in Ventricular Muscle The mechanism for anisotropic propagation in normal ventricular myocardium with rapid conduction in the longitudinal direction and slow conduction in the transverse direction is related to the myocardial architecture, that is, the size and shape of the myocardial cells and bundles and the distribution of their interconnections. Ventricular muscle cells are elongated, about 120 um long and 20 um in diameter, but are irregular in shape with multiple step-like, transversely oriented sites of interconnection between adjacent cells called intercalated DISKS.21,22 The intercalated disk The intercalated disks incorporate the closely opposed plasma membranes

of 2 adjacent cardiac myocytes to form the specialized intercellular junctions that facilitate the coordinated interaction of the cardiac myocytes to form whole muscle fibers and to integrate myocardial function.23 They maintain mechanical, metabolic, and electrical coupling of the abutting myocytes.21,22,24 Thin section electron microscopy reveals the ultrastructural arrangement of the intercalated disk, and how the cells abutting at the disk interdigitate such that their paired plasma membranes follow the contour of the myofibril ends21 (Figure 2). The disk steps back and forth across the cell terminal, alternating between longitudinal and transverse orientations with respect to the long axis of the myocytes.21 The size of intercalated disks varies, spanning up to 20 or more myofibrils transversely and the length of several sarcomeres longitudinally. In ventricular myocardium, large intercalated disks exist at the ends of the myocytes, with smaller disks, each orientated transversely, along the length of the cell. Each myocyte in human ventricular myocardium has a mean of 11.6 intercalated disks.25 Specialized intercellular junctions comprise the intercalated disk. Two of these junctions, the fascia adherens and the desmosome, are referred to as anchoring junctions and are concerned with intercellular adhesion.21 The other specialized intercellular junction in the intercalated disk is the gap junction, which is responsible for intercellular communication, forming low-resistance pathways for passive flow of ions, and small molecules between cells.6 These junctions are the focus in this chapter of the discussion on myocardial architecture and arrhythmogenesis, since the distribution of gap junctions and their physiological properties have important influences on conduction and anisotropy. The arrangement of the 3 specialized junction types in the intercalated disk

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Figure 2. Micrographs illustrating the characteristic architecture of myocardial cellular interconnections. A. Light micrograph of longitudinally sectioned ventricular myocardium (stained with toluidine blue), showing the darkly stained transversely orientated intercalated disks (arrows) traversing at sites of intercellular abutment. B. Low-power thin section electron micrograph of a single intercalated disk, illustrating the step-like structure of convoluted (plicate) regions interspersed by the less conspicuous and less convoluted, more longitudinally orientated interplicate regions containing some of the larger gap junctions (arrows, see text) which appear as fine, electron-dense membranes. C. Higher power electron micrograph of a small intercalated disk illustrating the long stretch of gap-junctional membrane at the edges (periphery) of the disk (arrows). D. High-power electron micrograph of part of a gap junction, showing the pentalaminar structure of the closely apposed cell membrane lipid bilayers. A x450, B x10,000, C x25,000, D x100,000. B and D reproduced from reference 107, with permission. C reproduced from reference 25, with permission.

differs between the regions of transverse and longitudinal orientation.21 Fasciae adherents are confined predominantly to the transverse regions, which are highly convoluted, or plicate. Gap junctions are located predominantly in the longitudinal nonplicate (interplicate) regions, although small gap junctions are also present among the adhering junctions of the plicate regions (Figure 3). Desmosomes may exist in both regions. Although typically found in the intercalated disk of ordinary working ventricular myocardium as described, gap junctions are not necessarily confined to disks in all parts of the heart; the specialized myocytes of the atrioventricular node, for example, have

no identifiable intercalated disks even though gap junctions are present.21,26,27 The gap junctions Structure of Gap Junctions: Gap junctions are specialized regions of close interaction between the sarcolemmata of neighboring myocytes in which clusters of transmembrane channels bridge the paired plasma membranes (for reviews see references 6 and 28 through 33). Figure 4A is a 3-dimensional diagrammatic representation of the structure of a gap junction, spanning the 2-component cell membranes as defined by the model of Makowski et al.34'35 Gap junctions are relatively

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Figure 3. Diagrammatic representation of the intercalated disk region of a cardiac myocyte, at which an abutting myocyte would interact with complementary disk features (compare with Figure 2). Note the step-like structure of the disk, with longitudinal (interplicate) and transverse (plicate) regions. The specialized intercellular junctions of the disk have a specific pattern of distribution (see Figure 2). Fasciae adherents (not shown) are located in the plicate regions. Gap junctions (indicated by solid ovals) occur predominantly in the interplicate regions (larger junctions) with smaller junctions in the plicate regions. Desmosomes (not shown) exist in both regions.

inconspicuous at thin section electron microscopy because there is little electron-dense material on the cytoplasmic side of the membranes, which are separated by a gap of only 2 to 3 nm (Figure 2D).36 The technique of freeze-fracture enables electron microscopic examination of detail of gap junctions viewed face on in the plane of the membrane,37,38 with visualization of the densely packed protein particles responsible for the physiological properties of these membrane specializations (Figure 5). The integral proteins of the gap junction, called connexins, exist in hexameric units called connexons, each of which possesses a 1.5- to 2-nm central pore and makes contact with another connexon in the opposing membrane (Figure 4). The

Figure 4. Model of the connexin43 gap junction. Gap junctions consist of clusters of channels (a). Each channel consists of a pair of connexons, one contributed from each plasma membrane. The connexon consists of 6 connexin molecules (connexins 43, 40, and 45 in mammalian myocardium, singly or in combination). Each connexin molecule has 4 transmembrane regions, with the amino and carboxy termini situated on the cytoplasmic side of the membrane (b).

pair forms a complete channel linking the cytoplasmic compartments of the abutting cells, providing a low-resistance pathway for the passage of ions and small molecules (up to 1 kd) 39-41 and for electrical propagation.42 Using ultrarapid freezing techniques to capture gap-junctional organization as it is in the myocardium of the living animal, freeze-fracture electron microscopy shows the clusters of connexons comprising a gap junction containing ordered hexagonal arrays with connexonfree aisles of membrane separating each array (Figure 5). The results of gap junction modeling suggest that this channel arrangement lowers the access resistance, and thus increases the conductance, of a

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Figure 5. Electron micrograph of freeze-fracture replica showing the densely packed connexon particles (arrow) of an entire junctional plaque. x100,000. Reproduced from reference 25, with permission.

gap junction compared with a denser junctional channels on electrical propagamass of an equal number of channels.43 tion are likely to be of biological relevance, Furthermore, these same authors sug- they remain unproven. It is likely, howgest that the peripheral ring of large gap ever, that both the physiological properjunctions characteristic of intercalated ties of the channels and their distribution disks in ventricular myocardium lies influence conduction. directly in the path of the depolarizing The connexins are a multigene family membrane potential as it passes along the of conserved proteins, different members lateral sarcolemma of abutting myocytes, of which are expressed in different cell enhancing longitudinal conduction veloc- types, tissues, and species.30,32,33,44,45 Gap ity and the degree of anisotropy of prop- junctions and connexins occur in all mulagation.43 Although the influences of these ticellular tissues in the body, and throughultrastructural arrangements of gap- out the animal kingdom from the sponges

204 CARDIAC MAPPING to humans.44 Transcripts for 14 different connexin molecules have now been identified in mammals.30,32,33,45-47 This family of connexin isoforms possesses varying degrees of molecular homology and similarity of topology within the cell membrane (Figure 4B), 44 but the domains of the connexin molecule that lie on the cytoplasmic side of the membrane are the regions of greatest difference in sequence and length between the connexin species and appear to be the main determinants of the differences in biophysical properties between gap junctions composed of the specific connexins.44 The conventional nomenclature for a connexin (Cx) is with a suffix referring to its molecular weight, ranging from 26 kd (Cx26) to 50 kd (Cx50). Qualitative data on the differential expression of the different principal connexin isoforms (connexins 43, 40, 45) in the mammalian heart including humans have confirmed that connexin43 is the most abundant connexin in ordinary atrial and ventricular myocardium. Moderate amounts of connexin45 are also found in both tissue types. Connexin40 is present in large amounts in the atria but not in the ventricles.48-51 It has been reported that working subendocardial ventricular myocardium expresses connexin40 (although less than in atrial tissue) and that this expression can be upregulated by hypertensive hypertrophy.52 Although there remains some debate about connexin expression in the His bundle and bundle branches, with interspecies differences complicated by conflicting data likely to be attributable to difficulty locating these tissues with certainty, there is general consensus that in the conducting tissue of the human heart, connexin40 is present only in the His bundle, and not in the bundle branches. This finding is consistent with the emergence of connexin40 as a potential candidate gene for mutations responsible for a hereditary conduction tissue disease.53 In other mammalian

species, connexin40 may be expressed in the proximal bundle branches in addition to the His bundle. Sinus and atrioventricular nodal gap junctions are small and sparse,27,54,55 making connexin detection difficult and variable using the different immunohistochemical techniques employed (described below). Although it is therefore difficult to reach a general conclusion, these nodal tissues are reported to contain mainly connexin45, with lower levels of connexin40 and, in the atrioventricular node but not the sinus node of human hearts, connexin43.49 Other connexins including connexin37,56 connexin46,57 and a connexin-related protein, MP70,58 are not thought to be expressed to any significant level in cardiac myocytes, and are not considered further in this chapter. Physiological Properties of Gap Junctions: In general, gap-junctional membrane is several orders of magnitude more permeable than nonfunctional plasma membrane, as indicated by a relative high junctional conductance, and therefore provides lowresistance pathways for current flow between myocardial cells.6 Early studies using molecular probes to investigate the nature of gap-junctional coupling demonstrated that the aqueous pore in the gapjunctional channel, which is the cause of the low resistance, is 1.5 to 2 nm in diameter.59 There is also evidence that gapjunctional channels show selectivity to diffusion of molecules, depending on their size and, possibly, their charge.60 The cation/anion selectivity ratio for rat connexin43 gap-junctional channels has been estimated as close to 1.0, indicating that these channels are nonselective aqueous pores.60 Connexin45, however, has marked cation selectivity.61 There also appears to be connexin-specific, size-dependent permeability of molecules,62 although there is no apparent relationship between molecular size permeability and ion selectivity.63 The physiological impact of these

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observations is likely to be significant, and tary conductance of connexin40 is 150 to they clearly demonstrate that gap-junc- 200 pS,73 a value higher than that of contional channels cannot be considered to nexin43 that possibly contributes to the be simple passive conduits between cyto- high conduction velocity of the His-Purkinje plasmic compartments of adjacent cells. tissue, in which there is abundant conThe permeability of a gap junction (or nexin40. Connexin45 shows low values for junctional conductance) is determined by unitary conductance (36 pS, 22 pS).33 To the number of junctional channels, the pro- add to this complexity, it has become apparportion of these channels that are in the ent that connexons, at least in vitro, may open state, and the permeability (conduc- be composed of a single connexin species tance) of the open channel. Other factors (homomeric) or several connexin species that affect gap-junctional conductance, in (heteromeric), and a gap-junctional chanaddition to the permeability of the open nel may be made up of 2 connexons that channels (single-channel conductance) and are identical (homotypic) or not (hetthe proportion of channels that are open, erotypic). Although little is known of the are the total surface area of the gap junc- occurrence of heterotypic and heteromeric tion and the number of junctional channels channels in naturally occurring gap juncper unit area of junction. These factors can tions, there is evidence for the potential vary among tissue types and are affected physiological properties of such channels in vitro.74"76 Of importance, is the inabilby disease, as is discussed later. The gap-junctional channels are gated ity of connexin40 connexons to interact and can exist in an open or closed state. with connexin43 connexons to make funcThe proportion of channels that are in an tional heterotypic channels.76"78 This is open state and the permeability (conduc- the only example of incompatibility among tance) of each channel have an important the cardiac connexins. Thus, the myocytes influence on the gap-junctional perme- of the connexin40-bearing regions of the ability or gap-junctional conductance. The conducting system are electrically isopermeability of the gap junction channel lated from the surrounding ordinary in the open state, or unitary conductance, myocardium expressing connexin43, preis determined by the connexin protein that cluding dissipation of electrical current forms the channel. The different connexin and possibly increasing the efficiency of conisoforms have different unitary conduc- duction and the coordination of ventricular tances. The conductance of a single con- activation. The transjunctional voltage may also nexin43 channel in its main conductance state is of the order of 40 to 60 PS.51, 63-65 influence gap-junctional conductance by Although it was originally believed that its effect on the number of channels in the unitary conductance of a single chan- the open state. In large gap junctions connel does not vary and that the channel taining many cell-to-cell channels, both functions in an all-or-nothing manner, the instantaneous (immediately after either open or closed,4,6 it is now recog- applying an intercellular potential difnized that channels may also have one or ference) and steady-state current-voltage several subconductance states.66-69 Two relationships tend to be linear.79-81 When minor conductance states of 20 to 30 pS only a few open channels are present, and 70 to 100 pS have been reported under although the instantaneous relationship various experimental conditions.70-72 The may be linear, the steady-state currentphysiological, and potential pathophysi- voltage relationship progressively falls to ological, significance of this, however, near zero with increasing voltage.82'84 remains to be elucidated. The main uni- This observation reflects so-called voltage

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dependence of channel gating causing a progressive closure of channels, a phenomenon that is not apparent when measuring the net conductance of larger junctions,80 and is of unknown physiological significance. In the case of connexin43, conductance starts to decrease when the transjunctional voltage exceeds -40 mV.82 This value is similar to that of connexin40,73 and indicates that these connexins are only weakly voltage sensitive. By contrast, connexin45 is highly voltage dependent, with an opening probability of only -10% at -40 mV. It has been suggested that, although the physiological role of connexin45, which has low conductance and is present at low levels, is uncertain, its strong voltage dependence may serve to electrically isolate diseased myocytes in which there are persistent disturbances of resting and/or action potential with respect to healthier neighboring cells.32 Prolonged depolarization, for example, as may occur in ischemic regions, may result in a degree of electrical isolation of tissue containing connexin45 junctions. The conductance of gap-junctional channels may change under different physiological or pathophysiological conditions and these changes may have a profound influence on conduction. Two possible mechanisms for changes in conductance of gap junctions are changes in the proportion of channels in the open state and alterations in expression of channels, either numbers or their constituent proteins (connexins). There are a number of changes to the chemical and electrical environment that can alter the number of channels in the open state and that might influence conduction. We only mention a few of these influences. Gapjunctional conductance is reduced by elevated intracellular [Ca2+]85,86 and by low intracellular pH (below 6.5).87,88 Singlechannel studies have shown the effects of Ca2+ and pH to result from a change in opening probability, the single-channel

conductance remaining unchanged.64,89,90 There is also an interrelationship between intracellular calcium and pH. Although changes of pH within the physiological range (7.4 to 6.5) may not directly affect junctional coupling, they may modulate the response to changes in [Ca2+],80 the lower the pH, the greater the Ca required to reduce junctional conductance to its half maximum value. pH may become an important direct influence at the low pH associated with hypoxia and ischemia in diseased myocardium.91 Connexin45 is considerably more sensitive to pH than connexin43, showing near complete closure of connexin45 connexons (due to an opening probability of ~0) at pH 6.3. Internal longitudinal resistance through the intracellular pathway in myocardium increases substantially in myocardium deprived of oxygen.92,93 Although this phenomenon may be mediated predominantly by changes (decrease) in intracellular pH and calcium concentrations (increase) that accompany hypoxia, there is evidence that there may also be early ischemic and hypoxic changes in connexon configuration in gap junctions.94-96 These changes may represent a stage of altered gap junction function before the irreversible changes heralding the onset of irreversible cell damage (recently reviewed by Peters97 and discussed later in this chapter). Arachidonic acid, as well as nonesterified fatty acids and long-chain acyl carnitines that may accumulate during myocardial ischemia, are capable of closing intercellular gap-junctional channels in cardiac myocytes.98-101 Doxyl stearic acids (with a fatty acyl chain), but not stearic acids, rapidly and reversibly reduce junctional conductance to very low levels without affecting the unitary conductance.98-102 In addition to the rapid changes in gap-junctional conductance that occur through gap-junctional gating discussed above, the turnover of connexins is a highly dynamic and presumably flexible

MYOCARDIAL ARCHITECTURE AND ANISOTROPY AND VENTRICULAR ARRHYTHMIAS process that might influence gap-junctional physiology. Gap-junctional plaques as a whole appear to remain relatively constant and stable, but observed halflives of the order of 3 hours50,103 for connexin43 and connexin45 indicate that although relatively slow when compared with changes brought about by changes in gap-junctional gating, turnover of connexins might be altered in certain pathophysiological states. Therefore, disease processes acting at one or more sites of the genetic control and expression of junc tional proteins may influence the conductance properties of gap junctions by affecting connexin turnover, influencing gap-junctional size, distribution, and composition. In addition, post-translational modification of gap-junctional proteins may affect gap-junctional conductance. In cardiac myocytes, connexin43 is present in at least 2 phosphorylated forms and in one unphosphorylated form. The native protein is post-translationally modified by the addition of phosphate groups onto serine residues.32 Phosphorylation appears to reduce unitary conductance of connexin43 gap junctions by increasing the proportion of channels from the main conductance state of about 50 pS, to the lower conductance of 20 pS. Dephosphorylation induces a third conductance state of 70 pS.71,72,104 Connexin45 is a phosphoprotein,78'105 and, like connexin43, has multiple conductance states depending on the degree of phosphorylation. Pathophysiological processes that can influence phosphorylation are therefore predicted to alter current flow through gap junctions and to affect cardiac conduction. Gap-junctional Organization in Ventricular Myocardium and its Relationship to Anisotropic Conduction: With the advent of the molecular biological techniques used to determine the amino acid sequence of what was, at that time, the only identified mammalian myocardial connexin, connexin43,46

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antibodies were generated in the early 1990s to selected sequences from the connexin43 molecule. One such antipeptide antibody raised in rabbits against residues 131 to 142 of the connexin43 molecule58,106,107 (part of the cytoplasmic loop Figure 4) has been used to characterize mammalian ventricular myocardial gap junction organization by incubating tissue sections with this antibody, followed by a second incubation with a fluorosceinlabeled swine antirabbit antibody. The result is fluoroscein label localized to the sites of connexin43 concentration, which, in combination with the technique of confocal laser scanning microscopy, has permitted highly sensitive immunofluorescent imaging of gap junctions in isolated cardiac myocytes (Figure 6C), and through thick slices of intact cardiac tissue (Figure 6A). By this approach, mapping of spatial patterns of gap junction distribution and quantification of connexin expression levels from the digital images so acquired has been accomplished27,106-110 through volumes of myocardium that are sufficiently large to make them relevant to overall electromechanical function of the tissue and the heart as a whole. Furthermore, by comparative measurement of connexin content in health and disease,25,111,112 and by applying similar immunohistochemical approaches to localizing the other cardiac connexins, the relative abundance of each of the myocardial connexins has been determined.27,48,113 The novel insights so derived have been highly complementary to the ultrastructural data generated by electron microscopic techniques. As discussed above, ultrastructural studies have revealed that in ordinary mammalian ventricular myocardium, gap junctions are confined almost exclusively to the well-developed intercalated disks and exist therein as ovoid or irregular plaques, measuring up to 2 (im or greater in diameter25,38,107 and containing up to

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Figure 6. Confocal immunolocalization of connexin43 gap junctions in human left ventricular myocardium. A. A single optical section of longitudinally sectioned myocardium revealing each individual gap junction domain as a separate spot grouped within intercalated disks. B. An optical projection series from transversely sectioned myocardium showing a number of intercalated disks viewed enface, showing the labeled gap-junctional population within each. C. Higherpower transverse section showing 2 intercalated disks in which there is the characteristic arrangement of a peripheral ring of large gap junctions and smaller central domains (see Figure 2C). D. Optical projection series through a single isolated ventricular myocyte labeled for connexin43, showing the positions of the intercalated disks, as clusters of transversely oriented labeled gap junctions. A x700, B x850, C x3500, D x1000.Reproduced from reference 25, with permission.

several thousand connexons. The inter- ters of fluorescent label confined to the locaplicate (longitudinally orientated) regions tions of the intercalated disks (Figure 6), of the intercalated disk in canine ventri- and show a similar distribution to that cle contain large gap junctions,25,109 with observed in the working ventricular smaller gap junctions interspersed among myocytes of other mammalian species.115,116 the anchoring junctions in the plicate Gap junctions within the disk in the regions (Figure 3). Particularly large gap human heart are typically organized as a junctions have been observed at the peripheral ring of larger junctions (mean periphery of the disk25,109 (Figures 3 and long axis length 0.67 |im), with smaller 6C). Consistent with these observations, junctions centrally (0.36 u,m), and occupy confocal images of immunolabeled human a mean surface area of 0.005 )j,m2/u,m3 ventricular myocardium reveal that from myocyte volume (Figure 6).25 6 years of age,114 connexin43-containing The pattern of gap junction distribugap junctions are organized in a well- tion in normal myocardium is believed to ordered pattern visible as multiple clus- be one major determinant of the normal

MYOCARDIAL ARCHITECTURE AND ANISOTROPY AND VENTRICULAR ARRHYTHMIAS uniformly anisotropic pattern of electrical propagation, by which conduction parallel to the fiber orientation is up to 3 times more rapid than transverse to it.115'116 Furthermore, the pattern of coupling of myocytes and, in particular, the adequacy of the side-to-side connections, is important in determining that a wavefront shows homogeneous propagation,19 loss of which promotes the formation of reentrant circuits causing arrhythmias (described later). Normal gap-junctional organization may therefore require precise control, with the possibility that arrhythmias may arise if the normal gap junction distribution is disrupted. In mature human ventricular myocardium, the cells have an average of 11.6 intercalated disks.25 In canine ventricular (subendocardial) myocardium, each ventricular muscle cell is connected to approximately 11 to 12 other muscle cells.114"122 As a result of the intercalated disks that exist at sites along the length of cardiac myocytes, gap-junctional connections between the cells occur between both the ends of cells and the sides of cells; in canine ventricular myocardium, 29% of cells are connected to each other only side to side, while 34% are connected only end to end. The remaining cells are connected in both the end-to-end and side-to-side direction, such that approximately half of all connections are side to side and half are end to end.117'122'123 Therefore, with respect to gap-junctional coupling in ventricular myocardium, activation wavefronts can conduct equally well between individual cells in both the longitudinal and transverse directions because there are approximately equal numbers of connections. However, in the transverse direction a wavefront will encounter more gap junctions than over an equivalent distance in the longitudinal direction because cell diameter is much less than cell length (23 p,m versus 122 |um in the study of Saffitz et al.123)

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and, therefore, the wavefront must traverse more cells transversely per unit distance. The resistivity of gap-junctional membrane, although several orders of magnitude lower than non-gap-junctional plasma membrane, is several orders of magnitude higher than the cytoplasmic intracellular resistivity. In the longitudinal direction, the cytoplasmic and gapjunctional components to the total impedance are about equal124; however, there is a greater resistance transversely than longitudinally because of the increased number of gap junctions and, therefore, slower transverse conduction. Myocytes in different cardiac tissues are interconnected with tissue-specific 3dimensional patterns that appear to determine their conduction properties. In the studies on canine ventricular myocardium referred to in the preceding paragraph,117'122 although the ratio of cell length to width measured in isolated canine ventricular myocytes is approximately 6.1:1, the side-to-side and end-toend distribution of connections between the irregularly shaped myocytes means that in whole tissue, the length-to-width ratio of cell profiles traversed by the lines on a grid orientated parallel and perpendicular to the long cell axis is only 3.4:1.117'122 This ratio, derived from the packing geometry, is consistent with the degree of anisotropy of conduction as measured in ventricular myocardium, and illustrates the importance of considering both the distribution of gap-junctional connections and the packing geometry in correlating architecture with conduction properties. In contrast to these data from ventricular myocardium, the crista terminalis of the right atrium has most connections in the end-to-end direction, with few side-to-side connections, and has anisotropic conduction properties that are distinctly different from those of the left ventricle (LV). Longitudinal conduction is considerably more

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rapid in the crista terminalis than in the to extracellular space and cellular couventricle, but transverse conduction in pling that cardiac myocytes, as discussed the crista terminalis is very slow, with a previously, are coupled almost exclusively longitudinal-to-transverse velocity ratio at only a few intercalated disks, which of approximately 10:1 compared with 3:1 may exist on parallel side branches of the main cell long axis. Although at other in ventricular myocardium.11 A further level of complexity in the points along their length and circumferrelationship between tissue structure and ence, adjacent myocytes may be partially propagation involves the properties of the separated from the adjacent cells by extracellular space. In some regions of extracellular space or tissue, there may the ventricles, particularly the papillary still be sites of approximation and juncmuscles, connective tissue septa subdi- tional interaction between them. It is vide the myocardium into unit bundles, a therefore not possible to determine structural feature that also contributes whether adjacent cells are coupled to each to the anisotropic conduction properties of other from histological examination of ventricular muscle. Such bundles are only a single, or even several, tissue seccomposed of 2 to 30 cells surrounded by tion^)—complete sectioning through the a connective tissue sheath. Within a unit entire cell concerned, in the manner of bundle, cells are tightly connected or cou- Hoyt et al.,117 is necessary. Fine connecpled to each other through the gap junc- tive tissue septae, per se, may not signiftions in the intercalated disks, as icantly alter intercellular coupling or described above. All the cells of a unit conduction velocity through the tissue bundle are connected to each other within unless they divide points of potential gapthe space of 30 to 50 [im down the length junctional intercellular contact. Gap juncof a strand.125 As a consequence of the tions occupy only a small proportion of the many intercellular connections, the myo- cell surface, so even a substantial increase cytes in a unit bundle are activated uni- in connective tissue septation may have formly and synchronously as an impulse little effect on electrical coupling. Although the definition of uniform propagates along the bundle. The unit bundles are also connected to each other. anisotropy is an advancing wavefront Unit bundles lying parallel to each other that is smooth in all directions relative to are connected in a lateral direction at the long axis of the myocardial fiber bunintervals in the range of 100 to 150 |im.125 dles, this is based on the characteristics As a consequence of this structure, the of activation at a macroscopic level where myocardium is better coupled in the the spatial resolution encompasses numerdirection of the long axis of its cells and ous myocardial cells and bundles. Because bundles (because of the high frequency of of the irregularities in cell geometry and the gap junctions within a unit bundle) irregular distribution of the gap junctions than the direction transverse to the long of normal ventricle as described in the axis (because of the low frequency of previous paragraphs, activation at a interconnections between the unit bun- microscopic level with a spatial resolution dles). This is reflected in a lower axial comparable to individual cells is, in fact, resistivity in the longitudinal direction quite irregular. This irregularity has been than in the transverse direction in car- shown in the computer model of Spach and diac tissues that are composed of many Heidlage127 that approximates the architecture of LV epicardium (Figure 7) with plibundles.8,126 It is necessary to bear in mind when cate junctions connecting the cells in the interpreting any of the studies relating longitudinal direction, and interplicate

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Figure 7. Diagram showing the electrical equivalent circuit of 3 interconnected myocytes in a 2dimensional computer model. Each membrane patch corresponds to a 10 [o,m x 10 jam intracellular segment. In each patch, during depolarization, there is an outward capacitive current (lc) in parallel with an inward ionic current (llon). Intracellularly, the patches are interconnected with low resistances (r,) to produce an isotropic sheet. The boundaries between cells (diagonal lines) are represented by the absence of cytoplasmic interconnections. Electrical coupling between myocytes is achieved through 3 resistors that represent 3 types of gap junctions: plicate, interplicate, and combined plicate. Reproduced from reference 127, with permission.

and combined plicate junctions connecting the cells in the lateral direction (70% of gap-junctional conductance in the interplicate areas and 30% in the plicate areas). In this model, during both longitudinal and transverse conduction, plane waves did not occur at a microscopic level because of the disruption of the excitation wave by the irregularly located cell boundaries and the associated irregular distribution of the gap junctions. Steplike increases in activation time in the longitudinal direction occurred in the regions of end-to-end connections between cells, corresponding to the irregular distribution of the plicate junctions and large lateral jumps in activation times occurred between cells in the transverse direction at the lateral borders of the cells. Nonuniformities at a microscopic scale have also been shown by Fast and Kleber and colleagues19,20 in a tissue culture preparation of neonatal myocytes, in which optical dyes permitted the con-

struction of high-resolution excitation maps. Nonuniformity was most evident in the transverse direction. Discontinuous transverse propagation was amplified by the presence of intercellular clefts between cells and in regions where nonmyocyte cells (fibroblasts) were coupled to myocytes and acted as a current sink.20 Such clefts and nonmyocyte cells are present in normal myocardium, and therefore influence conduction but may become more of a factor in diseased myocardium, leading to nonuniform anisotropy (see next section). Normal Myocardial Architecture, Uniform Anisotropy, and Arrhythmogenesis In general, the normal ventricles are not prone to the occurrence of reentrant arrhythmias except for ventricular fibrillation. The spiral wave reentrant mechanism that has been suggested to be an

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important cause of fibrillation can occur in a homogeneous medium and is not dependent on anisotropy. However, myocardial fiber orientation (anisotropy) has been shown to be an important factor in determining the patterns of activation during the electrical induction of ventricular fibrillation.128 The Effects of Cardiac Disease on Anisotropy and Arrhythmogenesis

tions, in addition to the more gross associated structural disruption caused by fibrosis and the formation of connective tissue septa. It has been shown in myocardial cultures that sites of inhomogeneity of gap-junctional distribution represent sites for nonuniform propagation and block of conduction in the direction transverse to the myocardial fiber axis.20 Myocardial ischemia and hypertrophy cause alterations in gap-junctional organization25,111,129 and connexin expression25,52,129 that might have similar effects.

Changes in the characteristics of anisotropic propagation from uniform Ischemic Heart Disease: Effects anisotropy to nonuniform anisotropy can lead to the occurrence of arrhythmias. on Ventricular Gap Junctions, This association was first described in the Anisotropy, and Arrhythmogenesis atrium by Spach et al.11,13 Nonuniform The principal cause of the mortality anisotropy is defined as the maintenance and morbidity in ischemic heart disease of tight electrical coupling between cells is myocardial contractile dysfunction and in the longitudinal direction but disruparrhythmias. A mechanism that may be tion of the smooth transverse pattern common to dysfunction of both the coorcharacteristic of uniform anisotropy dination of contraction and the cardiac described above. Propagation transverse rhythm is the reduced velocity and the to the long axis in nonuniformly anisotropic altered patterns of conduction known to myocardium occurs in a markedly irreg11,13 occur in myocardial ischemia. In ischemic ular sequence or "zigzag conduction." A marker for nonuniform anisotropic con- heart disease, changes in the electroduction is fractionation of the waveforms physiological properties of the myocardium of the extracellular electrograms caused associated with arrhythmogenesis are by the zigzag transverse propagation. The dependent on such variables as the duranonuniformity of anisotropic conduction tion of ischemia, the degree of ischemia, in atrial myocardium described by Spach and the presence or absence of infarction and Dolber13 resulted from disruption of and cell death. Throughout this range the lateral gap-junctional connections by of pathophysiological settings, there is evithe formation of connective tissue septae dence to implicate gap-junctional derangeduring aging, while longitudinal coupling ment and anisotropy of conduction in the by gap junctions was maintained. Trans- mechanism of the ensuing electromechanformation of the anisotropic conduction ical dysfunction. properties from uniform to nonuniform is also associated with the occurrence of Effects of acute ischemia ventricular arrhythmias associated with The arrhythmias that occur within the some pathological conditions. The structural basis for nonuniform anisotropic first hour after the onset of severe ischemia conduction in the ventricles in arrhythmo- caused by an acute coronary artery occlugenic disease states may relate directly to sion are associated with depolarization alterations in the organization of gap junc- of the membrane resting potential and

MYOCARDIAL ARCHITECTURE AND ANISOTROPY AND VENTRICULAR ARRHYTHMIAS partial inactivation of sodium channels resulting from the membrane depolarization.130 The changes in Na+ channel function contribute to slowing of conduction and the occurrence of reentrant excitation. Much of the depolarization can be attributed to an increase in extracellular potassium accumulation. In addition, there are important changes in the functional properties of the gap-junctional connections that are likely to contribute to the very slow conduction characteristic of acutely ischemic myocardium. In a computer model, Quan and Rudy131 have shown that by decreasing membrane conductance for Na+, conduction velocity can only be reduced to about one third of its normal value, whereas by increasing the degree of cellular uncoupling it can be reduced by a factor of 20 before block occurs. Internal longitudinal resistance and resistivity of the cell-to-cell conduction pathway (gap-junctional resistance) increase substantially in myocardium deprived of oxygen by 10 to 15 minutes.92,93,132-134 Since gap-junctional conductance is impaired by such factors as an increase in intracellular Ca2+ and H+ concentration (see previous section), one might expect a priori that myocardial hypoxia, by inducing these cytoplasmic changes, would reduce cellular coupling and therefore slow conduction. Furthermore, long-chain acyl carnitines, lipid metabolites that accumulate within minutes of the onset of ischemia and which are highly arrhythmogenic, markedly reduce gap-junctional conductance.101 On the other hand, in that ATP depletion may result in dephosphorylation of connexin43 and may increase coupling,70,73,104 at least partial compensation for these uncoupling ionic changes may be expected. When hypoxia is caused by occlusion of a coronary vessel, extracellular resistance (comprising both intravascular and interstitial resistance) immediately increases by

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approximately 30%.92 This increase is related to collapse of the vascular bed. Thereafter, there is a slow further increase in extracellular resistance most likely caused by osmotic cell swelling135 and the consequent reduction of the extracellular space. The intracellular resistance remains unchanged during the initial phase of ischemia; this phase may vary from 12 to 23 minutes. Afterward, rapid cellular uncoupling occurs to cause an increase in intracellular resistance.92 During the initial phase, when intracellular coupling resistance remains unaltered, a small decrease in conduction velocity of about 15% can be expected as a consequence of the increase in extracellular resistance, assuming that action potential upstroke remains unchanged (which it does not). The rapid cellular uncoupling that occurs later causes conduction to become slow and discontinuous and eventually leads to complete conduction block. Actual conduction velocity measured in acutely ischemic papillary muscles is lower than predicted by cable theory and this is due to the changes in action potential upstroke characteristics and excitability. So far, no precise quantitative assessment has been made of the relative contributions of the changes in both passive and "active" electrical properties to conduction velocity, but a recent report suggests that an alteration in coupling is an important factor.124 The changes in coupling resistance are expected to alter the anisotropic properties of the ventricular myocardium. Since transverse propagation occurs over many more gapjunctional connections than longitudinal conduction, slowing of conduction transversely would be expected to be more severe than longitudinally and to become much more irregular.136 This sudden increase in intracellular resistance after about 20 minutes of ischemia in physiological studies92 can be correlated with the results of early ultrastructural studies that showed alterations

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of gap junction membranes after 20 to 30 minutes of ischemia,94,136 and suggested that morphological changes of gap junctions, particularly with respect to the packing of the junctional channels, may contribute to the uncoupling process. The quantity of gap-junctional membrane does not seem to change.96 This led to the widely held view that uncoupling agents such as hypoxia and ischemia may act via conformational change of the connexons within the cell membrane lipid bilayers, thereby closing the junctional channels.137 There have been many attempts to correlate gap-junctional morphology with functional state.41,94,138-141 Despite a lack of consensus, considerable knowledge of gap junction organization has arisen from these freeze-fracture studies, which might be summarized by the predominant (but not unanimous139) view that the connexons in the functioning gap junctions of living ventricular myocytes typically exist in multiple small hexagonal arrays, becoming randomly distributed and then compacted (with a higher overall density in a homogeneous hexagonal pattern) in conditions under which uncoupling may occur. More extensive uncoupling after 60 minutes of hypoxia is accompanied by a 45% reduction in gap-junctional membrane and reduction in the total number of intercellular gap-junctional channels. Thus, cellular uncoupling heralds the onset of irreversible damage and coincides with the second phase of extracellular K+ accumulation.142,143 Chronic Myocardial Ischemia: Chronic myocardial ischemia is also associated with arrhythmogenesis that may involve alterations in intercellular coupling and anisotropy. There is a substantially reduced connexin43 gap-junctional surface area in recurrently ischemic, but not infarcted, human ventricular myocardium.25 Structurally well-preserved LV biopsies of

recurrently ischemic myocardium have been obtained from patients with triplevessel coronary artery disease and demonstrable regional ischemia at preoperative assessment who were undergoing bypass operations.25 Despite a normal pattern of gap-junctional distribution in a normal number of intercalated disks per myocyte, and a normal mean density of packing of the constituent connexons, a 47% reduction in connexin43—gap junction surface area was detected in the ischemic samples, translating to a significant reduction in connexin43 expression per myocyte.25 The intercalated disk counts in ventricular myocardium from the human ischemic hearts were normal, and the distribution of long-axis lengths of labeled gap junctions also showed no difference from the normal, suggesting that the reduction in gap-junctional content in the ischemic myocardium occurs as a result of reduction in the numbers of all sizes of junctions within each intercalated disk. Therefore, the basic architecture of intercellular abutments is not altered significantly. Thus, although ultrastructural studies have suggested that gap-junctional surface area is reduced in the setting of ischemia persisting to the point of irreversible damage and destruction of the cells (infarction),96 this stage is, by definition, not reached in patients with recurrent or persistent ischemia without infarction. A reduced gap-junctional connexin43 content is a possible anatomical factor in the pathogenesis of electromechanical dysfunction and arrhythmias in the chronically ischemic heart providing the potential for slowing and nonuniformity of conduction, without invoking the gross alterations consequent on infarction and ensuing fibrosis as the explanation. Gap Junction Organization in InfantRelated Myocardium: The Infant Border Zones: Reentry is the primary mechanism of ventricular tachycardias (VTs) late after

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myocardial infarction (>72 hours).130 The region in which reentry occurs in infarcted ventricles has been localized to the surviving borders of healing or healed myocardial infarcts.116,130,144,145 The site of origin of arrhythmias at these later periods depends on the location of the surviving myocardial cells in and around the infarcted region that form the infarct border zone. The electrophysiological mechanisms causing these arrhythmias (functional or anatomical reentry) depend on the anatomical arrangement of the surviving myocardium and the alterations in their membrane function. In that reentry requires an electrophysiological substrate that includes regions of slow conduction and block, the demonstration of abnormal conduction in infarct border zone myocardium with normal, or near normal, action potential depolarization phases130,146,147 indicates that structural remodeling of the constituent myocytes, and in particular the changing balance of longitudinal and transverse connections between them that influence the anisotropic properties, may be an important determinant of arrhythmogenesis.148 Patients with sustained VT often have a large area of solid, homogeneous infarct, which may include the septum and which extends around much of the circumference of the ventricular cavity, from the anterior to the lateral wall.149 The solid infarct may be transmural in that it extends from the subendocardium into the subepicardium. On the subepicardial surface over solid infarct, there is patchy infarct and surviving subepicardial muscle, the epicardial border zone in which reentrant circuits sometimes occur. Older infarcts are composed of larger areas of patchy infarct and thinner epicardial border zones. Surviving subepicardial muscle cells can be found even in ventricular aneurysms. Similarly, in canine models of myocardial infarction caused by com-

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plete occlusion of a main coronary artery such as the left anterior descending (LAD), a narrow layer of epicardial muscle (epicardial border zone) survives over the region of solid transmural infarction.116,150,151 Muscle fibers on the epicardial surface of transmural anteroseptal canine infarcts caused by permanent occlusion of the LAD near its origin survive because they still receive blood flow from epicardial branches of the circumflex artery or from collaterals of the LAD that anastomose with the patent circumflex. A redistribution of coronary blood flow from necrotic endocardial layers to surviving epicardial ones may also maintain their viability.152 A narrow "ribbon" consisting of Purkinje fibers, and sometimes ventricular muscle, survives between the subendocardial surface of the solid infarct and the ventricular cavity in both human and experimental infarcts—the subendocardial border zone.149,153,154 Subendocardial scarring and trapping of myocardial fibers in a subendocardial scar may also extend into areas around the periphery of infarcts. This is most often the site of origin of VT in human infarcts.155 In some hearts, bundles of myocardial fibers extend from the subendocardial border zone or lateral border zone, deeper into the subendocardium, and also into midmyocardial regions of the solid infarct, forming subendocardial or intramural conducting pathways.130,145 Tracts of subendocardial and intramural muscle bundles may sometimes form reentrant circuits. The epicardial border zone is an important site of arrhythmia origin in healing experimental (canine) infarcts.116,151,156 Although there is some evidence that clinical arrhythmias sometimes arise in this region,157 it is probably not the primary site of origin of most clinical VTs. The microscopic anatomy (architecture) of the border zone of surviving epicardial muscle in canine infarcts has important influences

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on impulse conduction in this region that cause arrhythmias in the experimental model. The microscopic anatomy also changes with time as the infarct heals, causing time-dependent changes in conduction properties. The influences of architecture on conduction as determined in this model can be applied to border zones in human infarcts and provide information relevant to the origin of clinical tachycardias. The surviving muscle fibers in the epicardial border zone are arranged parallel to one another during the healing phase of myocardial infarction (first week after coronary occlusion). The long axis of the muscle fiber bundles is perpendicular to the LAD and extends from the coronary artery toward the lateral LV and apex,116,158 the same orientation as epicardial muscle fibers in the noninfarcted anterior LV.126 The muscle fibers may be either tightly packed together, as they are in the normal subepicardium, or they may be separated by edema that is commonly seen in a healing infarct (Figure 8). The parallel orientation forms an anisotropic structure that has important influences on conduction properties that may cause reentry. The intracellular ultrastructure of the surviving muscle fibers is mostly normal except for the accumulation of large amounts of lipid droplets that may be a reflection of changes in metabolism of the cells. However, a striking change in structure of this region is an abnormal distribution of gap-junctional interconnections among cells that occurs by 4 days after coronary occlusion.159 Although the surviving myocytes in the border zone adjacent to necrotic cells have normal histological features, they have varying degrees of disruption of gap junction distribution, as shown by immunolabeling of connexin43 (Figure 9), similar to that which has also been described in healed human infarcts (described in the next section). Connexin43 is distributed

around the entire cell surface, with a large amount located along the lateral membrane (Figure 9). This disturbance occurs early after infarction as a primary pathophysiological response of these cells, prior to the physical disruption of intercellular connections by extrinsic fibrotic scarring and distortion that occurs later in the healing and healed phases (see next section). The disturbed gap-junctional pattern is most prominent immediately abutting the necrotic tissue, and extends through the border zone toward the epicardial surface to a distance of up to 840 (im from the interface with the necrotic myocardium. In most regions disturbed gap-junctional distribution does not extend throughout the full thickness of the epicardial border zone, the distribution of connexin43 in myocytes closest to the epicardial surface and most distant from necrotic tissue being in the normal transversely oriented pattern describing the locations of the normal intercalated disks (partial-thickness gap-junctional disarray) (Figure 10). In thinner regions of the epicardial border zone, however, the layer of disturbed gap-junctional distribution extends throughout the entire thickness of the surviving epicardial border zone, all the way to the epicardial surface (Figure 10). The arrangement of the muscle fiber bundles and the alterations in intercellular connections as indicated by the altered connexin43 distribution are both associated with nonuniform anisotropic properties of conduction in the canine epicardial border zone and related to the mechanisms of arrhythmogenesis.116'148 The nonuniform anisotropic properties are illustrated in Figure 11 (left panel), which shows a map of impulse propagation in the epicardial border zone of a 4day-old canine infarct when the border zone was stimulated in the center of the anterior wall of the LV. Rapid activation toward the margin of the anterior LV

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Figure 8. Photomicrographs of the parallel oriented surviving muscle fibers in the epicardial border zone of a healing canine infarct (4 days old). In some regions fibers are widely separated (A), while in others (B), they are more closely packed together. Reproduced from reference 116, with permission.

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Figure 9. Confocal micrographs of connexin43-labeled canine infarct epicardial border zone myocardium, 4 days after left anterior descending artery ligation. A. The necrotic infarct (inf) is free of label, but the surviving myocytes abutting the infarct show grossly abnormal connexin43 gap junction distribution with label distributed all around the cell borders. B. A transmural section showing orderly, predominantly transversely orientated arrays of label abutting the epicardium at the top of the micrograph, contrasting with the abnormal longitudinal arrays in the myocytes abutting the label-free infarct beneath. Note the frequently observed absence of label along the border of the myocytes immediately abutting the infarct. A x700, B x300. Reproduced from reference 159, with permission.

Figure 10. Schematic representation of connexin43-immunolabeled epicardial border zone of 4day-old canine infarct showing the distinction between partial-thickness (to left of diagram) and fullthickness (to right of diagram) disturbance of connexin43 gap-junctional distribution (gj disarray). Reproduced from reference 159, with permission.

bounded by the LAD and toward the apex of the lateral left ventricle (LL) is indicated by the widely spaced isochrones. This is the direction of the long axis of the myocardial fiber bundles. Transverse to the long axis, activation is very slow and irregular as indicated by the closely bunched isochrones. Electrogram fractionation is also associated with the transverse activation.

The mechanism of the nonuniform anisotropy is uncertain. During the first 4 days after coronary occlusion, it is not a result of increased connective tissue associated with infarction,159 which is only evident at later times (see below). It is possible that the edema formation "pulls apart" muscle bundles, disrupting some of the gap-junctional connections and preferentially influencing transverse conduction.

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Figure 11. Left: Activation map of the epicardial border zone of a 4-day-old canine infarct during pacing. Each small number is the activation time at a site where an extracellular electrogram was recorded. Isochrones are drawn at 10-ms intervals and labeled with larger numbers. Margins of the recording electrode array at the left anterior descending coronary artery (LAD), base, apex, and lateral left ventricle (LL) are labeled. The border zone was stimulated in the center (pulse symbol) and activation spread toward all margins of the electrode array (arrows). Rapid activation occurred in the direction of the long axis of the myocardial fiber bundles (toward the LL and LAD margins) while very slow, nonuniform propagation occurred transverse to the long axis (toward the base and apex). Right: The activation pattern in the epicardial border zone of the same heart during sustained ventricular tachycardia induced by programmed stimulation. Two reentrant wavefronts rotate around parallel lines of functional block (thick black lines) that formed in regions of nonuniform transverse propagation (see left panel). This figure-of-8 reentrant pattern is indicated by the arrows.

The role of increased connexin43 location along lateral aspects of the cells would seem to contradict the electrophysiological findings of decreased transverse conduction; however, the functional properties of the lateral connexins have not yet been determined (discussed later). There is a relationship between regions of nonuniform anisotropic conduction and the occurrence of reentrant circuits that cause sustained VT in the canine model of infarction.116 Reentrant circuits in the epicardial border zone that cause arrhythmias are functional; they can be induced to form by programmed stimulation which initiates tachyarrhythmias. The circuits form when an appropriately timed stimulated premature impulse blocks in the epicardial border zone.116,151,156 The mechanism for block may involve anisotropic properties of this region: preferential conduction block of premature impulses in

the longitudinal direction in nonuniformly anisotropic myocardium,14 although there is also evidence for an increased refractory period at the site of block.156 There is also a relationship between regions of nonuniform anisotropic conduction and the location of the functional lines of block of stable reentrant circuits that cause sustained VT. These are not the same lines of block that occur during premature stimulation. The right panel of Figure 11 is an activation map of the reentrant circuit that formed to cause a sustained VT initiated by programmed ventricular stimulation in the same heart in which the nonuniform anisotropic activation pattern was described in the left panel of Figure 11. The map illustrates activation occurring in a figure-of-8 reentrant pattern.156 The reentrant wavefront propagates around 2 lines of functional conduction block indicated by the thick

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black lines. These lines of block formed in the regions of very slow transverse propagation resulting from the nonuniform anisotropic conduction characteristics of this area. The exact mechanism for block has not been determined but there appears to be failure of transverse propagation during the tachycardia. Sometimes very slow conduction also occurs across parts of the lines of block (pseudoblock) transverse to the long axis of the myocardial fibers.116 The formation of long lines of stable functional block and the slow activation around the ends of the lines of block that occurs transverse to the long axis of the fiber bundles are necessary for the occurrence of sustained tachycardia. When these block lines do not form, perhaps because of insufficient nonuniform anisotropy, unsustained VT or ventricular fibrillation, but not sustained tachycardia, occur. Functional reentrant circuits in which lines of block form in regions of nonuniform anisotropy and in which much of the slow activation necessary for reentry to occur is caused by slow nonuniform anisotropic conduction around the ends of the lines of block, is called anisotropic reentry.148 Although the mechanisms for the formation of the functional lines of block in anisotropic reentrant circuits has not yet been completely elucidated, the relationship between the regions in which they form and the microscopic anatomy of those regions may be relevant.159 Stable reentrant circuits associated with sustained, monomorphic VT in the canine model of infarction occur in the very thin areas of the epicardial border zone where the altered distribution of connexin43 extends throughout its full thickness. Boundaries between these regions with full-thickness abnormalities and adjacent regions that have more layers of surviving cells, and abnormal connexin43 distribution extending only part way through the epicardial border zone, are the locations

of the functional lines of block in the reentrant circuits.159 This relationship is shown in Figure 12. The top panel is an activation map of a figure-of-8 reentrant circuit in the epicardial border zone of a 4-day-old canine infarct. The bottom panel shows the location of full-thickness gap-junctional disturbances (circles) and partial-thickness gap-junctional disturbances (X's) determined by fluorescent antibody techniques. The junction of the full-thickness and partial-thickness disturbance correlates with the location of the functional lines of block of the reentrant circuit. Therefore, transverse conduction is decreased in these regions, leading to the occurrence of the lines of block. Although much has been written on the role of fibrosis and increased connective tissue septation in promoting nonuniformity of propagation,11,13,122,160 an abnormal pattern of gap junction distribution in the absence of fibrotic scarring may be an important factor in the substrate for reentry after infarction. The mechanism by which the change in gapjunctional distribution causes nonuniform anisotropy and influences the location and characteristics of the reentrant circuit in this experimental model, possibly by defining the location of the lines of functional block, has yet to be determined. The abnormal redistribution of gap junctions to the lateral interfaces between myocytes might be expected to enhance (rather than impair) side-to-side coupling, thereby improving transverse conduction and reducing anisotropy. However, the presence of connexin labeling in immunohistochemical studies, the observation of morphologically recognizable gap junctions using electron microscopic techniques, or the detection of mRNA for a particular connexin must be interpreted with some caution, as these findings do not necessarily signify the presence of intact or functional gap

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Figure 12. Top: Map of activation times of a stable reentrant circuit during sustained ventricular tachycardia in the epicardial border zone of a 4day-old canine infarct. The activation times (in ms, small numbers) are shown, as are lines of isochronal activation, at 10-ms intervals (larger numbers). The lines of functional block are shown by the thick black lines. Arrows point out the activation pattern. Bottom: Map of the distribution of fullthickness disturbance of gap junction organization (o = fullthickness disturbance, x = partial-thickness disturbance) in the same 6 x 6 cm square of the epicardial border zone. The area of full-thickness gap junction disarray coincides approximately with the common central pathway of the reentrant circuit, and the borders between full-thickness and partial-thickness gap junctional array locates the line of functional block. Reproduced from reference 159, with permission.

junctions nor do they indicate the func- dosis, increased intracellular calcium) tional status of the gap junctions. Growing that may act to uncouple the border zone knowledge of the multiplicity of connexin myocytes. Furthermore, there is a conisoforms that exist in normal mammalian centration-dependent stimulation of conmyocardium 54,113 raises the additional nexin43 mRNA and protein expression, question of alterations of the relative albeit in cultured fibroblasts,161 which expression of these connexins in disease could also have an important pathophysstates.52,129 Presently, it is unknown iological role in the early healing phase whether such changes take place in the following infarction. At present, the funcsetting of myocardial ischemia or infarc- tional status of the gap junctions in these tion; but if they do, alterations in cou- border zone cells is unknown, but one pospling characteristics would be expected sibility is that the redistribution of gap to accompany them. The redistribution junctions may be a compensatory response of connexin43 in the epicardial border to substantial metabolic uncoupling in zone should, however, be considered in this tissue. Metabolic uncoupling may the context of the likely metabolic dis- reduce conductance more in the transturbances of ischemic tissue (hypoxia, aci- verse direction than longitudinal,115, 162

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and the redistribution of gap junctions to the lateral cell borders may incompletely counteract the consequent tendency to enhanced anisotropy of conduction. A similar concept of compensatory alterations of gap junctions in pathological tissue has previously been hypothesized as a possible explanation for the alterations of connexin isoform expression in myocardial hypertrophy.52 It would therefore seem possible that, under pathological conditions, myocytes may be capable of modulating the nature of their coupling to neighboring cells by varying the organization and composition of gap junctions. If the zone of full-thickness gap-junctional disturbance was characterized by selective impairment of transverse coupling, myocardial conduction between the lines of functional block (the common central pathway) would show enhanced anisotropy, with an even greater tendency than normal for this myocardium to support longitudinal conduction more than transverse (Figure 12). However, as a propagating wavefront through the common central pathway reaches the end of the region of full-thickness disturbance, its outer edges would tend to encounter myocardium with only a partial-thickness disturbance of junction distribution and with better transverse coupling particularly in the more superficial cell layers. Propagation would therefore be improved transversely, and start turning laterally. Once transverse conduction in this direction extended beyond the line of full-thickness disturbance, propagation would then occur longitudinally (in the opposite direction) through the excitable tissue lateral to, and previously protected from depolarization by, the enhanced anisotropy defining the lines of functional block. With the curvature of the limit of full-thickness gapjunctional disturbance at the other end of the lines of functional block, the parallel wavefronts in the outer pathways will, by the same mechanism, start to propagate

transversely and turn medially to coalesce, thus defining the other end of the line of functional block (Figure 12). Remodeling of experimental and human infarct structure continues as the infarcts heal, leading to further changes with time. In particular, the deposition of connective tissue and the formation of the scar can distort the normal relationship of the surviving myocardial fiber bundles.158 This in turn influences conduction characteristics. Figure 13 illustrates these changes in the epicardial border zone of canine infarcts; however similar changes occur in the subendocardial border zone or in regions where fiber bundles penetrate into the solid core of the infarct in both experimental and human infarcts. The muscle cells become trapped in the dense scar tissue formed from the adjacent infarct. In some regions myocardial fibers can become markedly separated from each other along their length.122, 146, 158 In a quantitative study on healed canine infarcts, it was found that there is a concomitant reduction in the number of cells to which each myocyte is connected, from 11.2 in normal tissue to 6.5 in the fibrotic infarct border zone, associated with a greater reduction of predominantly side-to-side cell interconnections than end-to-end.122 Connections of cells in primarily side-to-side apposition were found to be reduced by 75% whereas primarily end-to-end connections was reduced by 22%.122 Overall, these results are consistent with the hypothesis that there is reduced cellular coupling with a disproportionate increase of resistivity in the transverse direction, thus enhancing anisotropy. In canine LVs up to 10 weeks after infarction, the surviving myocardium adjacent to the healed infarct has smaller and fewer gap junctions per unit length of intercalated disk and per unit myocyte sectional area.122 A selective reduction of the larger junctions results in a decrease in the proportion of total gap junction in the interplicate segments of the intercalated disk.122

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Figure 13. Surviving epicardial border zone muscle fibers in a 2-week-old canine infarct (A and B) and in a 2-month-old canine infarct (C and D). In A, the thin surviving rim (arrows) consists of several layers of ventricular muscle cells between the epicardium and the granulation tissue of the healing infarct. These surviving cells are separated by fibrous tissue, especially adjacent to the infarct. The parallel orientation of the myocardial fibers is maintained. At high magnification (B), these myocardial cells are shown to be intact with distinct cross striations. In C, the disorganization of the surviving myocardial cells in the thin rim at 2 months is evident. The cells are widely separated and disoriented because of ingrowth of fibrous tissue from the adjacent infarct. At high magnification (D), the myocardial cells have distinct cross striations and central nuclei. The bars represent 50 jam. Reproduced from reference 158, with permission.

Immunohistochemical examination of connexin43 gap-junctional membrane in the border zone of healed human infarcts has demonstrated the occurrence of gap-junctional reorganization in addition to the reduced transverse connections described above; altered gap junction distribution occurs in surviving myocytes up to 700 [im from the interface with the fibrotic infarcted tissue (Figure 14).111 Within this border zone region, compar-

atively few labeled gap junctions are organized into discrete, transversely orientated intercalated disks, and many are spread longitudinally over the cell surface. The dispersed connexin43 is still located at cell-to-cell appositions in regions where they are not disrupted by the encroaching scar tissue (Figure 14A). Individual junctions and groups of junctions, though often maintaining intercellular gap-junctional contact, are displaced so that discrete

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Figure 14. Confocal micrographs of longitudinally sectioned connexin43-immunolabeled human ventricular myocardium from the border of a healed infarct. A. At lower power, showing the infarct scar (s) with no labeling, the highly disrupted gap-junctional distribution within about 700 jim of the scar and the normal appearances more distant from the scar (top left corner). x210. B. At high power, showing myocytes traversing densely fibrotic scar (no labeling). There is profuse label along the length of these attenuated and degenerated but viable cells. x810. Reproduced from reference 108, with permission.

intercalated disk zones are less clearly defined (Figures 14 through 16). This latter feature is most evident in healed partial-thickness myocardial infarction, in which the demarcation between scar and myocardium is least discrete, with greater interdigitation of these tissues. The disruption of the gap junction distribution in the infarct-related tissue is possibly due to a redistribution of the preexisting population of junctions rather than the cells producing an entirely new, modified population.111 In addition, some junctional contacts are entirely disrupted, and

intracytoplasmic junctions, previously reported as a feature of degenerating myocytes in a variety of cardiac pathological conditions,163 are seen and are likely to contribute to the dispersed immunolabeling pattern observed by confocal microscopy.111 The electrophysiological effects of the anatomical structure of healed infarct border zones are striking. The reduced connections among fiber bundles described above lead to slow activation. Detailed measurements of activation patterns and transmembrane potentials in isolated,

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Figure 15. Localization of the abnormally distributed connexin43 illustrated by a confocal optical section series of images taken at 1-jim steps through a human infarct border zone myocyte, showing connexin43 label along the edge of the cell in (i) (arrow), with a progressive change in successive slices to show the upper surface of the cell in (v) and the other edge of the cell in (vi), thus describing the surface of the myocyte. Images such as these suggest that this abnormally distributed label is at or near the cell surface. x1100. Reproduced from reference 97, with permission.

superfused preparations of the epicardial border zone from healed canine infarcts

plified in the left panel, in which conduction velocity in the direction of the arrows have illustrated these conduction prop(in the direction of the long axis of the erties, which are also expected to occur in muscle fibers) is 20 to 40 cm/s. (The other regions of healed infarcts with a isochrones are much more widely spaced, similar anatomy.146 Figure 17 compares reflecting the faster conduction velocity.) activation of the epicardial border zone The very slow conduction velocity occurs in a healed infarct (2 months, right panel) in the border zone of healed infarcts (right with activation in a healing infarct (5 panel) despite the normal transmembrane days, left panel). In the 2-month infarct, potentials recorded at most sites as exemactivation moving in the directions indi- plified by the record of the action potencated by the arrows is very slow, as shown tial above the map. The slow activation is by the close bunching of the isochrones. In therefore dependent on the structural altersome regions it takes the activation wave ations that occur as the infarct heals rather 10 ms to move a distance of 0.5 mm, a con- than on abnormalities in transmembrane duction velocity of 0.05 cm/s. This is much potentials.146 In regions in which there is slower than in healing infarcts as exem- no longer parallel orientation of muscle

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Figure 16. Thin section electron micrograph showing intact gap-junctional membrane (arrows) between cell processes of 2 highly degenerated myocytes in a human infarct border zone, showing amorphous cytoplasm and an absence of contractile apparatus. x40,000. Reproduced from reference 111, with permission.

bundles, there no longer exist the welldefined anisotropic properties seen in healing infarcts, that is, conduction is slow in all directions rather than just transverse to the long axis of parallel organized muscle bundles. These same structural features found in the epicardial border zone of canine infarcts, regions of sparse, poorly connected myocardial fibers in disarray, and regions of parallel oriented bundles of fibers disconnected in the transverse direction, also occur in the epicardial and endocardial border zone of human infarcts and are expected to affect conduction properties in the same way that they do in the experimental infarcts. In studies on infarcted human papillary muscle, de Bakker et al.164 mapped impulse propaga-

tion in the thin bundles of muscle fibers that coursed through the infarct scar on the subendocardial surface. They found that the parallel oriented fiber bundles formed conduction pathways insulated from each other by the connective tissue septa except at occasional sites along the length where they were interconnected. Conduction velocity along each of the tracts, parallel to the long axis of the fiber bundles, was rapid, 0.79 m/s (probably because action potentials may be normal and longitudinal connections are not disrupted), but transverse activation was very slow because of the sparse interconnections among the bundles. These conduction pathways can form reentrant circuits.

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Figure 17. Activation maps of small regions of epicardial border zones from a 5-day-old canine infarct (left) and a 2-month-old infarct (right). Action potentials were recorded at each of the sites shown by the dots to construct the map. An electrogram was recorded at the site indicated by the larger stippled circle. Representative action potentials and electrograms are shown above each map. The arrows and isochrones show the direction of propagation. The distance scale for each panel is shown below; note that the scale is twice as large for the 5-day-old infarct than the 2-month-old infarct. Reproduced from reference 146, with permission.

Altered Expression of Connexin43— A General Occurrence in Chronic Myocardial Disease? An alteration in connexin43 gap junctions is not confined to the ischemic heart. Ischemic hearts can have associated hypertrophy,165 and the effects of hypertrophy alone have been assessed by examination of nonischemic hypertrophied myocardium obtained from patients undergoing surgical replacement of a stenosed aortic valve. Quantitative confocal microscopy revealed a 40% reduction per unit volume of connexin43 compared to control heart tissue.25 The reduction appears to occur by cells

maintaining an approximately constant gap junction complement per cell while undergoing considerable increase in size. Decreased connexin43 levels have also been reported by quantitative immunoblotting of myocytes isolated from transgenic hypertensive rats.52 A marked (54%) downregulation of connexin43 mRNA and protein expression occurs in the LVs of patients with idiopathic dilated cardiomyopathy and severely impaired LV function, which, unlike the ischemic heart, in which there is a concomitant upregulation of connexin40 mRNA, shows no change in connexin40 mRNA or protein levels.166 Infection of cultured myocytes with Trypanosoma cruzi,

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the unicellular parasite responsible for Chagas' disease (the most common cause of heart disease in South America), is reported to lead to reduced levels of immunocytochemical staining of connexin43 gap junctions.167 Altered expression of connexin43 in the heart may therefore prove to be a general feature of diverse chronic myocardial diseases. Further, that this feature is common to ischemic heart disease, hypertrophy, and Chagas' disease, all of which have an arrhythmic tendency, supports the possibility of an association between connexin43 levels and conduction disturbances. If this is the case, possible alterations in the quantity and spatial distribution of myocardial connexins and gap junctions along with changing connexin phenotypes and post-translational modification (e.g., phosphorylation states) may modulate the effects of altered connexin43 expression. The development of models for investigating the effects of overexpression and underexpression (or complete ablation) of connexin expression168 in genetically altered animals will provide a useful tool with which such fundamental questions can be investigated.

Summary and Conclusions

In the complex world of arrhythmogenesis and the electrical myocyte networks that underlie both normal and abnormal conduction, there has been progress in understanding the relevance of tissue architecture and the hardware for communication in the form of the gapjunctional channels. Myocardium has the potential for substantial remodeling of its gap-junctional network to become a fundamental component of the anatomical substrate for arrhythmogenesis. Little is yet known, however, of the language of communication via these networks and

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structure on reentrant circuits in the epicardial border zone of subacute canine infarcts. Circ Res 1988;63:182–206. Hoyt RH, Cohen ML, Saffitz JE. Distribution and three-dimensional structure of intercellular junctions in canine myocardium. Circ Res 1989;64:563-574. Dolber PC, Beyer EC, Junker JL, Spach MS. Distribution of gap junctions in dog and rat ventricle studied with a doublelabel technique. J Mol Cell Cardiol 1992;24:1443-1457. Beyer EC, Kistler J, Paul DL, Goodenough DA. Antisera directed against connexin43 peptides react with a 43-kd protein localized to gap junctions in myocardium and other tissues. J Cell Biol 1989;108:595–605. Yancey SB, Biswal S, Revel J-P. Spatial and temporal patterns of distribution of the gap junction protein connexin43 during mouse gastrulation and organogenesis. Development 1992; 114:203–212. Fromaget C, El Aoumari A, Gros D. Distribution pattern of connexin43, a gap-junctional protein, during the differentiation of mouse heart myocytes. Differentiation 1992;51:9-20. Luke RA, Saffitz JE. Remodeling of ventricular conduction pathways in healed canine infarct border zones. J Clin Invest 1991;87:1594-1602. Saffitz JE, Davis LM, Darrow BJ, et al. The molecular basis of anisotropy: Role of gap junctions. J Cardiovasc Electrophysiol 1995;6:498–510. Cooklin M, Sheridan DJ, Fry CH. Investigation of conduction velocity changes in myocardial hypertrophy: Role of altered junctional impedance. Circulation 1995;92:I503. Sommer JR, Dolber PC. Cardiac muscle: Ultrastructure of its cells and bundles. In: Paes de Carvalho A, Hoffman BF, Lieberman M (eds): Normal and Abnormal Conduction in the Heart. Mt. Kisco, New York: Futura Publishing Co.; 1982:127. Roberts DE, Hersh LT, Scher AM. Influence of cardiac fiber orientation on wavefront voltage, conduction velocity and tissue resistivity in the dog. Circ Res 1979;44:701-712. Spach MS, Heidlage JF. The stochastic nature of cardiac propagation at a microscopic level. Electrical description of myocardial architecture and its

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application to conduction. Circ Res 1995; 76:366-380. 128. Chen PS, Cha YM, Peters BB, Chen LS. Effects of myocardial fiber orientation on the electrical induction of ventricular fibrillation. Am J Physiol 1993;264: H1760-H1773. 129. Peters NS, del Monte F, MacLeod KT, et al. Increased cardiac myocyte gapjunctional membrane early in renovascular hypertension. J Am Coll Cardiol 1993;21:59A. 130. Wit AL, Janse MJJ. The Ventricular Arrhythmias of Ischemia and Infarction. Electrophysiological Mechanisms. Mt Kisco, New York: Futura Publishing Co.; 1993. 131. Quan W, Rudy Y. Unidirectional block and reentry of cardiac excitation: A model study. Circ Res 1990;66:367-382. 132. Hiramatsu Y, Buchanan JW, Knisley SB, Gettes LS. Rate-dependent effects of hypoxia on internal longitudinal resistance in guinea pig papillary muscles. Circ Res 1988;63:923–939. 133. Ikeda K, Hiraoka M. Effects of hypoxia on passive electrical properties of canine ventricular muscle. Pflugers Arch 1982; 393:45-50. 134. Streit J. Effects of hypoxia and glycolytic inhibition on electrical properties of sheep cardiac Purkinje fibres. J Mol Cell Cardiol 1987; 19:875–885. 135. Tranum-Jensen J, Janse MJ, Fiolet JWT, et al. Tissue osmolality, cell swelling and reperfusion in acute regional myocardial ischemia in the isolated porcine heart. Circ Res 1981;49:364–381. 136. McCallister LP, Trapukdis S, Neely JR. Morphometric observations on the effects of ischemia in the isolated perfused rat heart. JMol Cell Cardiol 1979;11:619–630. 137. Unwin PNT, Zampighi G. Structure of the junction between communicating cells. Nature 1980;283:545-549. 138. Dahl G, Isenberg G. Decoupling of heart muscle cells: Correlation with increased cytoplasmic calcium activity and with changes in nexal ultrastructure. J Membr Biol 1980;53:63–75. 139. Deleze J, Herve JC. Effect of several uncouplers of cell-to-cell communication on gap junction morphology in mammalian heart. J Membr Biol 1983;74: 203-215. 140. Green CR, Severs NJ. Gap junction connexon configuration in rapidly frozen myocardium and isolated intercalated disks. J Cell Biol 1984;99:453-463.

141. Shibata Y, Page E. Gap junctional structure in intact and cut sheep cardiac Purkinje fibers: A freeze-fracture study of Ca2+-induced resealing. J Ultrastruc Res 1981;75:195-204. 142. Kleber AG, Cascio WE. Ischemia and Na+/K+ pump function. In: Rosen MR, Palti Y (eds): Lethal Arrhythmias Resulting from Myocardial Ischemia and Infarction. Boston: Kluwer Academic Publishers; 1989:77-90. 143. Cascio WE, Yan GX, Kleber AG. Passive electrical properties, mechanical activity, and extracellular potassium in arterially perfused and ischemic rabbit ventricular muscle. Effects of calcium entry blockade or hypocalcemia. Circ Res 1990;66:1461-1473. 144. Josephson ME, Horowitz LN, Farshidi A, Kastor JA. Recurrent sustained ventricular tachycardia. 1. Mechanisms. Circulation 1978;57:431-440. 145. DeBakker JMT, Coronel R, Tasseron S, et al. Ventricular tachycardia in the infarcted, Langendorff-perfused human heart: Role of the arrangement of surviving cardiac fibers. J Am Coll Cardiol 1990;15:1594-1607. 146. Gardner PI, Ursell PC, Fenoglio JJ Jr, Wit AL. Electrophysiologic and anatomic basis for fractionated electrograms recorded from healed myocardial infarcts. Circulation 1985;72:596–611. 147. Myerburg RJ, Gelband H, Nilsson K, et al. Long-term electrophysiological abnormalities resulting from experimental myocardial infarction in cats. Circ Res 1977;41:73–84. 148. Wit AL, Dillon SM, Coromilas J, et al. Anisotropic reentry in the epicardial border zone of myocardial infarcts. Ann NYAcad Sci 1990;591:86–108. 149. Bolick DR, Hackel DB, Reimer KA, Ideker RE. Quantitative analysis of myocardial infarct structure in patients with ventricular tachycardia. Circulation 1986;74:1266–1279. 150. Wit AL, Allessie MA, Bonke FIM, et al. Electrophysiologic mapping to determine the mechanism of experimental ventricular tachycardia initiated by premature impulses. Experimental approach and initial results demonstrating reentrant excitation. Am J Cardiol 1982;49:166185. 151. Mehra R, Zeiler RH, Gough WB, ElSherif N. Reentrant ventricular arrhythmias in the late myocardial infarction

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period. 9. Electrophysiologic-anatomic correlation of reentrant circuits. Circulation 1983;67:11–24. Hirzel HO, Nelson GR, Sonnenblick EH, Kirk ES. Redistribution of collateral blood flow from necrotic to surviving myocardium following coronary occlusion in the dog. Circ Res 1976;39:214–222. Friedman PL, Fenoglio JJ Jr, Wit AL. Time course for reversal of electrophysiological and ultrastructural abnormalities in subendocardial Purkinje fibers surviving extensive myocardial infarction in dogs. Circ Res 1975;36:127–144. Fenoglio JJ Jr, Pham TD, Harken AH, et al. Recurrent sustained ventricular tachycardia: Structure and ultrastructure of subendocardial regions in which tachycardia originates. Circulation 1983; 68:518–533. Josephson ME. Clinical Cardiac Electrophysiology. Techniques and Interpretations. Philadelphia: Lea and Febiger; 1993. El-Sherif N. The figure 8 model of reentrant excitation in the canine postinfarction heart. In: Zipes DP, Jalife J (eds): Cardiac Electrophysiology and Arrhythmias. New York: Grune & Stratton; 1985:363-378. Littmann L, Svenson RH, Gallagher JJ, et al. Functional role of the epicardium in postinfarction ventricular tachycardia. Observations derived from computerized epicardial activation mapping, entrainment, and epicardial laser photoablation. Circulation 1991;83:1577– 1591. Ursell PC, Gardner PI, Albala A, et al. Structural and electrophysiological changes in the epicardial border zone of canine myocardial infarcts during infarct healing. Circ Res 1985;56:436–451. Peters NS, Coromilas J, Severs NJ, Wit AL. Disturbed connexin43 gap junction

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distribution correlates with the location of reentrant circuits in the epicardial border zone of healing canine infarcts that cause ventricular tachycardia. Circulation 1997;95:988–996. Saffitz JE, Hoyt RH, Luke RA, et al. Cardiac myocyte interconnections at gap junctions-role in normal and abnormal electrical conduction. Trends Cardiovasc Med 1992;2:56–60. Doble BW, Kardami E. Basic fibroblast growth factor stimulates connexin-43 expression and intercellular communication of cardiac fibroblasts. Mol Cell Biochem 1995;143:81–87. Balke CW, Lesh MD, Spear JF, et al. Effects of cellular uncoupling on conduction in anisotropic canine ventricular myocardium. Circ Res 1988;63:879-892. Buja LM, Ferrans VJ, Maron BJ. Intracytoplasmic junctions in cardiac muscle cells. Am JPathol 1974;74:613-648. de Bakker JMT, van Capelle FJL, Janse MJJ, et al. Slow conduction in the infarcted human heart. "Zigzag" course of activation. Circulation 1993;88:915–926. Anversa P, Beghi C, Kikkawa Y, Olivetti G. Myocardial response to infarction in the rat. Morphometric measurement of infarct size and myocyte cellular hypertrophy. Am J Pathol 1985;484:492. Dupont E, Kaprielian R, Yeh H-I, et al. Connexin messenger ribonucleic acid expression in the healthy and diseased human heart. Eur Heart J 1996; 17(Abstract):600. Campos De Carvalho AC, Tanowitz HB, Wittner M, et al. Gap junction distribution is altered between cardiac myocytes infected with Trypanosoma cruzi. Circ Res 1992;70:733-742. Reaume AG, De Sousa PA, Kulkarni S, et al. Cardiac malformation in neonatal mice lacking connexin43. Science 1995; 267:1831-1834.

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Chapter 11 The Figure-of-Eight Model of Reentrant Ventricular Arrhythmias Nabil El-Sherif, MD, Edward B. Caref PhD, and Mark Restivo, PhD

Reentrant excitation is an important mechanism for ventricular tachyarrhythmias. A better understanding of this mechanism can provide a basis for improved management. Reentrant excitation occurs when the propagating impulse does not die out after complete activation of the heart, as is normally the case, but persists to reexcite the atria or ventricles after the end of the refractory period. Reentrant excitation can be subdivided into circus movement excitation and reflection. In circus movement reentry, the activation wavefront encounters a site of unidirectional conduction block and propagates in a circuitous pathway before reexciting the tissue proximal to the site of block after expiration of its refractory period. In this chapter, circus movement reentry is discussed with special reference to the figure-of-8 model of reentry first described in the canine postinfarction model.1 Classification of Circus Movement Reentry Circus movement reentry can be classified into anatomical and functional

types. This classification is based primarily on the nature of the central obstacle around which the circulating wavefront propagates. A combination of functional and anatomical obstacles is sometimes necessary for the initiation of circus movement reentry. Anatomical or Ring Models of Reentry In the anatomical model of reentry, the reentrant pathway is fixed and anatomically determined. The earlier models of circus movement consisted of rings of cardiac and other tissue obtained from various animals including mammals (Figure 1).2–5 In the intact heart, excitable bundles isolated from surrounding myocardium can form anatomical rings for potential circus movement. Examples include circus movement involving normal atrioventricular (AV) conducting bundles and AV accessory pathways,6 circus movement involving the His bundle branches or Purkinje network,7 and circus movement

Supported in part by Veterans Administration Medical Research Funds to NES and MR. From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; e2003. 237

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Figure 1. Anatomical (ring) models of reentry. A. Mines' diagram of a ring preparation composed of the auricle and ventricle of the tortoise, in which he observed reciprocating rhythm. Both connections between auricle and ventricle could transmit an excitation wave. During reciprocating rhythm, the 4 portions of the preparation marked V1, V2, A1, A2 contracted in that order. From Mines GR. J Physiol 1913;46:349–382. B. A proposed mechanism for reentry in a Purkinje-muscle loop. The diagram shows a Purkinje fiber bundle (D), which divides into 2 branches, both connected distally by ventricular muscle. Circus movement will develop if the stippled segment (A-B) is an area of unidirectional conduction. An impulse advancing from D would be blocked at A but would reach and stimulate the ventricular muscle at C by way of the other terminal branch. The excitation from the ventricular fiber would then reenter the Purkinje system at B and traverse the depressed region at a slow rate so that by the time it arrived at A, the site would have recovered from refractoriness and would again be excited. From Schmitt FO, Erlanger J. Am J Physiol 1928/1929;87:326. C. Diagram of a possible reentrant pathway, partly through bundles of surviving myocardial fibers embedded in the fibrous tissue of an old myocardial infarct. The main bundle bifurcates and gives rise to 2 possible exits toward the larger subendocardial muscle mass. Reproduced with permission from deBakker JMT, Van Capelle FJL, Janse MJ, et al. Circulation 1988;77:589–606. Copyright 1988, American Heart Association. D. Diagram of the initiation of circus movement in a ring model, emphasizing the importance of unidirectional block. A properly timed stimulus (*) will block in one direction because of nonhomogeneous refractoriness (stippled zone) but will continue to conduct in the ring in the other direction. A circus movement will be established if the returning wavefront finds that the site of unidirectional block has recovered excitability, thus permitting conduction to proceed uninterrupted.

using surviving myocardial bundles in a postinfarction scar.8 It is important to remember, however, that an anatomically determined pathway that can potentially support reentry does not automatically create circus movement. A critical functional perturbation of part of

the pathway must take place before a circus movement is initiated. Central to the initiation of a circus movement in an anatomical ring is the development of unidirectional block. Here, a stimulus blocks in one direction because of nonhomogeneous electrophysiological properties, but

FIGURE-OF-8 MODEL OF REENTRANT VENTRICULAR ARRHYTHMIAS it continues to conduct in the other direction. A circus movement is established if the returning wavefront finds that the site of unidirectional block has recovered excitability, permitting conduction to proceed uninterrupted. Thus, it is clear that in an anatomically predetermined circuit, a significant functional component exists and can be modulated, for example, by pharmacological agents. Surgical interruption of an anatomical ring-like circuit can be accomplished by cutting (or ablating) at any point along the ring. For example, in the ring circuit that uses an AV accessory pathway, the anatomical substrate consists in large part of pathways of excitable bundles that are not connected to adjacent atrial and ventricular myocardium. The circuit can be interrupted with ease by interrupting conduction of either the normal AV pathway or, as is currently the practice, the accessory AV pathways.

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model,1 the anisotropic model,16 and the spiral wave model17,18 (Figure 2). Nature and Characteristics of the Functional Obstacle of the Reentrant Circuit

The nature of the functional arc of block during the initiation of a reentrant circuit has been investigated by several groups. Allessie et al.11 were the first to show that differences in refractory periods of atrial fibers at adjacent sites can result in functional block if premature stimulation is applied to the site of shorter refractoriness. Later, Gough et al.12 showed that circus movement developed around arcs of functional conduction block in the surviving epicardial layer overlying canine ventricular infarction owing to spatial inhomogeneity of refractoriness. The latter could be due to differences in active membrane properties of adjacent fibers that affect their depolarization or repolarization characteristics, or due Functional Models of Reentry to discrete differences in intercellular Functional reentrant circuits can connections. Depression of active membrane propdevelop in the interconnected syncytium of myocardial bundles in the atria or ven- erties of myocardial fibers is a major tricles. Central to the development of a determinant of functional conduction functional circus movement is the cre- block and slowed conduction, leading to ation of a functional barrier of conduc- circus movement reentry in the acute tion block. The nature of this functional phase of myocardial ischemia.19,20 Within barrier has been investigated in some minutes of coronary artery occlusion, the detail during the initiation of circus move- cells in the center of the ischemic zone ment. Functional conduction block initi- show progressive decrease in resting ating a circus movement can be caused by membrane potential, action potential (1) abrupt changes in cardiac geometry9; amplitude, duration, and upstroke veloc(2) decremental conduction leading to ity.19 After a brief initial shortening, the propagation failure10; (3) regional differ- refractory period begins to lengthen even ences in refractory periods11–13; or (4) dif- though action potential duration continferences in conduction properties relative ues to shorten.19 El-Sherif et al.21 and to fiber orientation.14 The last 2 mecha- Lazzara et al.22 used the term postreponisms have received wider attention. The larization refractoriness to indicate that models of functional reentrant circuits at certain stages of ischemia, the memthat have been widely investigated are brane may remain inexcitable even when the leading circle model,15 the figure-of-8 it has been completely repolarized. Such

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Figure 2. Functional models of reentry. Leading Circle Model: diagrammatic representation of this model in isolated left atrium of the rabbit. The central area is activated by converging centripetal wavelets. Reproduced with permission from Allessie MA, Bonke FIM, Schopman FJG. Circ Res 1977;41:9–18. Copyright 1977, American Heart Association. Figure-of-8 Model: activation map (in 20-ms isochrones) of a figure-of-8 circuit in the surviving epicardial layer of a dog 4 days after ligation of the left anterior descending artery (LAD). The circuit consists of clockwise and counterclockwise wavefronts around 2 functional arcs of conduction block that coalesce into a central common front that usually represents the slow zone of the circuit. From El-Sherif N. In: Zipes D, Jalife J (eds): Cardiac Electrophysiology and Arrhythmias. New York: Gruneand Stratton; 1985:363-378. Anisotropic Model: activation map and schematic representation of the reentrant circuit during sustained monomorphic ventricular tachycardia in a thin epicardial layer frame obtained by an endocardial cryotechnique in a Langendorff-perfused rabbit heart. A single-loop reentry forms around a functional arc of conduction block. From Brugada J, et al. Pacing Clin Electrophysiol 1991 ;14:1943–1946. Spiral Wave Model: activation map of spiral wave activity in a thin slice of isolated ventricular muscle from a sheep heart (right panel). Isochrone lines were drawn from raw data by overlaying transparent paper on snapshots of video images during spiral wave activity (left panel not from same experiment). Each line represents consecutive positions of the activation front recorded every 16.6 ms. From Krinsky VL, et al. Proc R Soc Lond 1992;437:645–655.

increases in refractory period can exceed the basic cycle length, at which point 2:1 responses occur.19 The marked dependence of recovery of excitability on the resting potential in partially depolarized ischemic myocardial cells is probably the most

important determinant for the occurrence of slow conduction and conduction block in the acute phase of myocardial ischemia.20 The depressed upstroke of ischemic action potentials is the result of a depressed fast Na+ current.23 The postrepolarization

FIGURE-OF-8 MODEL OF REENTRANT VENTRICULAR ARRHYTHMIAS refractoriness in depolarized cells has been attributed to delayed recovery from inactivation of the fast Na+ current.24 In the subacute (healing) phase of myocardial ischemia (1 to 7 days after infarction), depressed membrane properties of surviving myocardial fibers bordering the infarction continue to be a major determinant of conduction abnormalities underlying circus movement reentry. Intracellular recordings from the surviving "ischemic" epicardial layer of 3 to 5 days postinfarction canine heart show cells with various degrees of partial depolarization, reduced action potential amplitude, and decreased upstroke velocity.25–27 Full recovery of responsiveness frequently outlasts the action potential duration reflecting the presence of postrepolarization refractoriness. In these cells, premature stimuli could elicit graded

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responses over a range of coupling intervals. Slowed conduction, Wenckebach periodicity, and 2:1 or higher degrees of conduction block could easily be induced by fast pacing or premature stimulation (Figure 3). Isochronal mapping studies have shown that both the arcs of functional conduction block and the slow activation wavefronts of the reentrant circuit develop in the surviving electrophysiological abnormal epicardial layer overlying the infarction.28–30 Studies from the same laboratory using high-resolution mapping of activation and refractory patterns have shown that functional block is necessary for both the initiation and sustenance of reentrant excitation and that the functional block necessary for initiation of reentry is due to abrupt changes in refractoriness occurring over distances of 1 mm or less (Figures 4 and 5).13 The abrupt

Figure 3. Recordings from a dog with a 3-day-old infarction, illustrating action potential characteristics in ischemic epicardium. The sketch of the preparation shows 2 intracellular recordings (X and Y) and a close bipolar recording (1) from the infarction zone (stippled area). Ischemic cells had decreased upstroke velocity, reduced action potential amplitude, and a variable degree of partial depolarization. The 2 cells were recorded 5 mm apart in the infarction zone but showed significant difference in their resting potential. The resting potential of the Y cell was only slightly reduced (-80 mV), but it still had a poor action potential. The preparation was stimulated at a cycle length of 290 ms, which resulted in a Wenckebach-like conduction pattern. Note that the pacing cycle length exceeded the action potential duration of the cells, suggesting that refractoriness extended beyond the completion of the action potential (i.e., postrepolarization refractoriness). Reproduced with permission from El-Sherif N, Lazzara R. Circulation 1979;60:605–615. Copyright 1979, American Heart Association.

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Figure 4. High-resolution determination of spatial refractory gradients and their relationship to the arc of functional conduction block from a 4-day-old canine infarction. A high-density bipolar electrode plaque with 1 -mm interelectrode spacing was positioned on the epicardial surface at the site of the arc of block induced by premature stimulation (S2) as determined from an earlier low-resolution sock electrode array. The plaque was oriented with the electrode rows perpendicular to the arc. The figure illustrates 5 bipolar electrograms recorded successively at 1-mm distance (a to e). The values of the effective refractory period in milliseconds at each site are shown. The arrows indicate the end of the effective refractory period relative to S1 activation at each site. The S1,and S2 activation maps are shown on the right. The asterisk on the S1, map denotes the site of stimulation. During S1, sites a to e were activated sequentially within a 12-ms interval (conduction velocity of 42 cm/s). During S2, conduction between sites a and c was relatively slow compared with S1,. Conduction block developed abruptly between sites c and d. Sites d and e were activated 65 ms later by the wavefront that circulated in a clockwise direction around one end of the arc of block. The site of conduction block coincided with a 35-ms abrupt increase in the effective refractory period between sites c and d. Note that the arc of block was parallel to the left anterior descending artery (LAD), represented by the broken line. Reproduced with permission from Restivo M, Gough WB, El-Sherif N. Circ Res 1990;66:1310–1327. Copyright 1990, American Heart Association.

changes in refractoriness did not seem to be related to specific geometric characteristics or anisotropic conduction properties of the ischemic myocardium. Action potential recordings from surviving myocardial bundles from hearts with chronic (healed) myocardial infarction (MI) have shown a wide spectrum of configurations. Some studies have shown normal action potential characteristics of surviving myocardial bundles from hearts in which circus movement reentry could

be initiated,8,31 whereas other studies have shown various degrees of depressed action potentials.32 In the former situation, reentrant excitation is explained primarily on the basis of the nonuniform anisotropic properties of the surviving myocardial bundles in a healed infarction scar. However, a combination of functional and anatomically determined reentrant circuits in those hearts, similar to original examples of ring model reentry, cannot be ruled out.

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Figure 5. Five successive bipolar electrograms (A-E) recorded at 1 -mm distance across an arc of functional conduction block induced by S2 stimulation (right panel). The layout of the high-density plaque was similar to that shown in Figure 4, but the recordings were obtained from a different experiment. The effective refractory period (ERP) at each site is shown, and the arrows indicate the end of the ERP relative to ST activation. Abrupt conduction block occurred during S2 stimulation between sites B and C and coincided with an abrupt increase in ERP of 25 ms. The asterisks indicate the electrotonic deflection recorded in electrograms C and D distal to block. The amplitude of the electrotonic deflection diminished with the distance from the site of block. A graphic illustration of the distribution of ERP across an 8-mm distance is shown on the left. Reproduced with permission from Restivo M, Gough WB, El-Sherif N. CircRes 1990;66:1310–1327. Copyright 1990, American Heart Association.

Primarily through the work of Spach et al.,14 anisotropic discontinuous propagation was shown to produce all of the conduction disturbances necessary for circus movement reentry without the presence of spatial differences in refractoriness. The safety factor of propagation of early premature impulses was shown to be dependent on fiber orientation, with unidirectional block occurring during propagation along the long axis of the fibers and slowed conduction persisting across the fibers, thus setting the stage

for reentrant excitation. The slower conduction in the transverse direction is due higher axial resistivity, which may be partly explained by fewer shorter gapjunctional contacts in a side-to-side direction.33 The normal uniform anisotropic conduction properties of the myocardium may be altered further after ischemia and can be markedly exaggerated in healed infarcts. Electrical uncoupling and increase of extracellular resistance resulting in reduced space constant have been shown in slowly conducting regions of chronically

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infarcted canine myocardium.34 Spach et al.35 suggested that both spatial inhomogeneity of refractoriness and anisotropic conduction properties may contribute to one model of reentrant excitation in the canine atria. The combination of spatial inhomogeneity of refractoriness and anisotropic conduction properties may be applicable to other models of reentry. A possible example is the experimental model of functional "anisotropic" reentry in which circus movement could be induced in a thin layer of normal myocardium obtained by endocardial cryotechnique in a Langendorffperfused rabbit heart.16 The nature of the central arc of functional block during sustained reentry has been more difficult to investigate. In a report by Dillon et al.,36 it was suggested that most of the central barrier around which circus movement orients in the surviving epicardial layer of the postinfarction canine heart was in fact a line of pseudoconduction block resulting from very slow conduction along the longitudinal fiber axis. However, high-resolution recordings of the central arc of block in this model13 as well as in other atrial models of functional reentry 37,38 clearly showed the presence of a discrete finite zone of functional conduction block explained by the bidirectional invasion of this zone by the opposing activation wavefronts on either side of the central barrier (Figure 6). Electrotonic conduction in this zone could create a constant functional block around which circus movement is oriented. The factors that determine the location and orientation of the central functional barrier during sustained reentry are not well defined and may be related to refractoriness or anisotropic differences. The length of the central functional barrier is also of interest. A circuit with a very small central barrier (i.e., a central core of functional refractory tissue) is typical of the leading circle model of reentry15

and resembles the vortex-like waves or spirals that have been demonstrated in a number of excitable media39 and in normal isolated ventricular muscle.17,18 However, in most of the functional circuits that have been mapped in vitro or in vivo, including the original leading circle model15 and the model of reentry in the Langendorff-perfused rabbit heart,16 the central obstacle was shown to consist of an arc of block of some finite length rather than a confluent central vortex. Topology of Functional Circus Movement The topology of functional circus movement is of both theoretical and practical importance. The typical functional circus movement in a syncytium has a figure-of-8 configuration consisting of clockwise and counterclockwise wavefronts around 2 functional arcs of block that coalesce into a central common front that commonly represents the slow zone of the circuit.1 This zone is the most vulnerable part of the circuit and the site at which pharmacological agents or ablative procedures can selectively modulate the circus movement. On the other hand, a single reentrant functional loop can also develop in a syncytium. However, it usually develops contiguous to an anatomical barrier. The most typical example of the development of a single functional reentrant loop has been shown by Schoels et al.38 in a study of circus movement atrial flutter in the canine postpericarditis model (Figure 7D). In this model, the majority of atrial flutter is due to a single-loop circus movement. During the initiation of a single reentrant loop, an arc of functional conduction block extends to the AV ring, forcing activation to proceed only as a single wavefront around the free end of the arc before breaking through the arc at a site close to the AV ring. Activation continues as

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Figure 6. Recordings of high-density bipolar electrode array at multiple locations (I to IV) along a continuous arc of functional conduction block during a figure-of-8 sustained monomorphic ventricular tachycardia (top panel). In this and subsequent maps, epicardial activation is displayed as if the heart was viewed from the apex located at the center of the circular map. The perimeter of the circle represents the atrioventricular junction. Activation isochrones are drawn at 20-ms intervals. Arcs of functional conduction block are represented by heavy solid lines. Arrows indicate wavefront direction during sustained ventricular tachycardia. Both arcs are oriented approximately parallel to the longitudinal axis of the epicardial muscle fibers. A portion of this activation map is shown in the lower right panel. The shaded rectangles represent the column for each array location. Electrograms recorded in proximity to the arc of block show split electrograms composed of 2 discrete potentials separated by a variable isoelectrical interval: one deflection represents local activation, the other deflection is an electrotonic potential reflecting activation recorded 1 mm away. The interval between the 2 deflections is greatest at the center of the arc (locations II and III), where the difference in isochronal activation, by whole ventricle mapping, is greatest. The interval between the 2 deflections is less as the reentrant impulse circulates around the end of the arc (location I). The electrographic characteristics of functional conduction block are the same at location IV, which indicates that the arc of functional conduction block was longer than that predicted by the whole ventricle mapping technique. The 2 deflections in electrogram d at location III may both represent electrotonic potentials. Reproduced with permission from Restivo M, Gough WB, El-Sherif N. Circ Res 1990;66:1310–1327. Copyright 1990, American Heart Association.

a single circulating wavefront around an arc of block in proximity to the AV ring or around a combined functional/anatomical obstacle with the arc usually contiguous with the inferior vena cava. Spontaneous38 or pharmacologically induced40 termination

of single-loop reentrant circuit occurs when conduction fails in a slow zone and the arc of block rejoins the AV ring. A critical analysis of the leading circle or anisotropic models of reentry15,16 shows that they may be a special modification of

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Figure 7. The leading circle model of reentry. A. and B. Isochronal maps of activation of a premature beat (S2) and the first reentrant beat (A1) from an in vitro preparation of atrial myocardium of the rabbit. The arcs of functional conduction block are represented by heavy solid lines. Isochrones are drawn at 5-ms intervals. Note that the properly timed premature stimulus resulted in a continuous arc of functional conduction block; the activation front circulated around both ends of the arc, coalesced, and then broke through the arc to reexcite myocardial zones on the proximal side of the arc. This resulted in splitting of the original arc into 2 separate arcs. Modified with permission from Allessie MA, Bonke Fl, Schopman FJG. Circ Res 1976;39:168–177. Copyright 1976, American Heart Association. B. A circulating wavefront continued around one of the arcs. However, the second arc of block shifted its site and joined the edge of the preparation so that the second circulating wavefront was aborted. If the preparation in B was inserted into the in situ heart, the second aborted circulating wavefront would be activated, thus resulting in a figure-of-8 reentrant pattern. C. Diagram of the leading circle model. Reproduced with permission from Allessie MA, Bonke FIM, Schopman FJG. Circ Res 1977;41:9–18. Copyright 1977, American Heart Association. D. Isochronal map of atrial epicardial activation during circus movement atrial flutter in the canine sterile pericarditis model. The atria are displayed in a planar projection as though separated from the ventricles along the atrioventricular ring and incised on the inferior bodies of both atrial appendages from the atrioventricular ring to their tips. The shaded area represents the orifices of the atrial vessels. The figure shows a single-loop reentrant circuit around a central obstacle composed of a functional arc of block (heavy solid line) and an anatomical obstacle (the orifice of the inferior vena cava). Reproduced with permission from Schoels W, Gough W, Restivo M, El-Sherif N. Circ Res 1990;67:35–50. Copyright 1990, American Heart Association.

the figure-of-8 model (Figure 7A-C). Thus, a figure-of-8 pattern may be the basic topology of a functional reentrant circuit in the interconnected syncytial structure of the atria and ventricles.1 The long arcs

of functional conduction block that sustain large reentrant circuits in the canine postinfarction ventricle and the small arcs of functional block that sustain small reentrant circuits in the leading circle or

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anisotropic models may represent 2 ends of a spectrum of the same electrophysiological phenomenon. Induced Versus Spontaneous Circus Movement Reentry Figure-of-8 reentrant excitation in the canine postinfarction heart may occur "spontaneously" during a regular cardiac rhythm41 but is commonly induced by premature stimulation. Induction of reentry by premature stimulation depends on the length of the arc of functional conduction block and the degree of slowed conduction distal to the arc induced by premature stimulation.28,29 A premature beat that successfully initiates reentry results in a longer arc of conduction block or slower conduction compared with one that fails to induce reentry (Figure 8). When a single premature stimulus (S2) fails to initiate reentry, the introduction of a second premature stimulus (S3) may be necessary. S3 usually results in a longer arc of conduction block or slower conduction around the arc. The slower activation wavefront travels around a longer, more circuitous route, thus providing more time for refractoriness to expire along the proximal side of the arc of unidirectional block. Reexcitation of this site initiates reentry. The beat that initiates the first reentrant cycle, whether it is an S2 or an S3, results in a continuous arc of conduction block. The activation front circulates around both ends of the arc of block and rejoins on the distal side of the arc of block before breaking through the arc to reactivate an area proximal to the block. This results in splitting of the initial single arc of block into 2 separate arcs. Subsequent reentrant activation continues with afigure-of-8activation pattern, in which 2 circulating wavefronts advance in clockwise and counterclockwise directions, respectively,

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around 2 arcs of conduction block. During a monomorphic reentrant tachycardia, the 2 arcs of block and the 2 circulating wavefronts remain fairly stable. On the other hand, during a pleomorphic reentrant rhythm, both arcs of block and the circulating wavefronts can change their geometric configurations while maintaining their synchrony. The development of multiple asynchronous reentrant circuits usually ushers the onset of ventricular fibrillation.29,42 Reentrant activation spontaneously terminates when the leading edge of both reentrant wavefronts encounters refractory tissue and fails to conduct. This results in coalescence of the 2 arcs of block into a single arc and termination of reentrant activation. For reentry to occur during regular cardiac rhythm, on the other hand, the heart rate should be within the relatively narrow critical range of rates during which conduction in a potentially reentrant pathway shows a Wenckebach-like pattern.41 During a Wenckebach-like conduction cycle, a beat-to-beat increment in the length of the arc of conduction block or the degree of conduction delay occurs until the activation wavefront is sufficiently delayed for certain parts of the myocardium proximal to the arc of block to recover excitability and become reexcited by the delayed activation front. A Wenckebach-like conduction sequence may be the initiating mechanism for repetitive reentrant excitation (e.g., a reentrant tachycardia) or may result in a single reentrant cycle in a repetitive pattern, giving rise to a reentrant extrasystolic rhythm (Figure 9). The majority of reentrant circuits in the canine postinfarction model develop in the surviving epicardial layer and can be viewed as having an essentially 2dimensional configuration. However, reentrant circuits can also be identified in intramyocardial41,43 or subendocardial locations.29 The latter location is of special

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Figure 8. Epicardial isochronal activation maps during a basic ventricular stimulated beat (S-,), initiation of reentry by a single premature stimulus (S2), and sustained monomorphic reentrant ventricular tachycardia (VT). A representative ECG is shown in the lower right panel. The recordings were obtained from a dog 4 days after ligation of the left anterior descending artery (LAD). Site of ligation is represented by a double bar. The outline of the epicardial ischemic zone is represented by the dotted line. During S1, the epicardial surface was activated within 80 ms, with the latest isochrone located in the center of the ischemic zone. S2 resulted in a long, continuous arc of conduction block within the border of the ischemic zone. The activation wavefront circulated around both ends of the arc of block and coalesced at the 100-ms isochrone. The common wavefront advanced within the arc of block before reactivating an area on the other side of the arc at the 180-ms isochrone to initiate the first reentrant cycle. During sustained VT, the reentrant circuit had a figure-of-8 activation pattern in the form of a clockwise and counterclockwise wavefront around 2 separate arcs of functional conduction block. The 2 wavefronts joined into a common wavefront that conducted between the 2 arcs of block. The sites of the 2 arcs of block during sustained VT were different to various degrees from the site of the arc of block during the initiation of reentry by S2 stimulation. The lower right panel illustrates the orientation of myocardial fibers in the surviving ischemic epicardial layer perpendicular to the direction of the LAD. The arrow represents the longitudinal axis of propagation of the slow common reentrant wavefront during a sustained figure-of-8 activation pattern, which is oriented parallel to fiber orientation and perpendicular to the nearby LAD segment. Modified from Assadi M, et al. Am Heart J 1990; 119:1014–1024.

interest because it may be comparable to reentrant circuits described in the surviving subendocardial muscle layer in the heart of patients with chronic ML44 This underscores the fact that, depending on

the particular anatomical features of the infarction and the geometric configuration of ischemic surviving myocardium, reentrant circuits can be located in epicardial, subendocardial, or intramyocardial zones.1

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Figure 9. Isochronal maps of a reentrant trigeminal rhythm. Epicardial activation maps as well as selected electrographic recordings from a dog 4 days after infarction in which a reentrant trigeminal rhythm developed during sinus tachycardia are shown. During sinus rhythm at a cycle length of 325 ms there was a consistent small arc of functional conduction block near the apical region of the infarct and relatively slow activation of nearby myocardial zones (map 1). The activation pattern, however, was constant in successive beats reflecting a 1:1 conduction pattern. Spontaneous shortening of the sinus cycle length to 305 ms resulted in the development of a single reentrant beat after every second sinus beat. During the reentrant trigeminal rhythm, the epicardial activation map of the first sinus beat showed the development of a longer arc of functional conduction block compared with the one during sinus rhythm at a cycle length of 325 ms (map 2). The activation front circulated around both ends of the arc of block but was not sufficiently delayed on the distal side of the arc of block. On the other hand, the activation map of the second sinus beat showed more lengthening of the arc of block at one end but more characteristically a much slower conduction of the 2 activation fronts circulating around both ends of the arc of block (map 3). The degree of conduction delay was sufficient for refractoriness to expire at 2 separate sites on the proximal side of the arc, resulting in 2 simultaneous breakthroughs close to the ends of the arc, thus initiating reentrant excitation. The leading edge of the 2 reentrant wavef ronts coalesced but failed to conduct to the central part of the epicardial surface of the infarct—that is, to areas that were showing slow conduction during the preceding cycle. This limited the reentrant process to a single cycle (map 4). It also resulted in recovery of those myocardial zones in the central part of the infarct, allowing the next sinus beat to conduct with a lesser degree of conduction delay, thus perpetuating the reentrant trigeminal rhythm. Analysis of the 2 electrograms recorded from each of the 2 reentrant pathways (B and C, and D and E, respectively) shows a characteristic 3:2 Wenckebach-like conduction pattern. The figure illustrates the complexity of conduction patterns in ischemic myocardium and the presence of a zone of dissociated conduction. This is represented by site F, which was showing a 2:1 conduction pattern during the 3:2 Wenckebach cycle and reentrant trigeminal rhythm described earlier. From El-Sherif N, et al. J Am Coll Cardiol 1985;6:124–132. Reprinted with permission from the American College of Cardiology.

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lating excitation and to identify the critical site along the reentrant circuit at which interruption of reentrant excitation could be successfully accomplished.45 Spontaneous termination of figure- These studies have demonstrated that of-8 sustained monomorphic reentrant reentrant activation could be successfully ventricular tachycardia (SMVT) always interrupted when cooling or cryoablation occurs when the 2 circulating wavefronts was applied to the part of the slow block in the central common pathway common reentrant wavefront immedi(CCP). Distinct electrophysiological chan- ately proximal to the zone of earliest reacges consistently precede spontaneous tivation (Figure 12). At this site, the termination of stable SMVT. Two basic common reentrant wavefront is usually mechanisms of spontaneous termination narrow and is surrounded on each side have been observed: (1) acceleration of by an arc of functional conduction block. conduction occurs in parts of the reen- On the other hand, localized cooling of trant circuit and is associated with slow- the site of earliest reactivation commonly ing of the conduction and finally conduction failed to interrupt reentry. The common block in the CCP. Acceleration of conduction reentrant wavefront usually broke through occurs in the last few cycles of VT both at the arc of functional conduction block to the outer border of the arcs of functional reactivate other sites close to the origiconduction block in the "normal" myocar- nal reactivation site without necessarily dial zone and at the pivot points to the changing the overall reentrant activation entrance to the CCP (Figures 10 and 11). pattern. Intraoperative detailed endocardial When acceleration of conduction was and epicardial mapping during VT in compensated on a beat-to-beat basis by an patients with previous myocardial infarct equal degree of slowing in the CCP, there and ventricular aneurysm has demowas no discernible change in the cycle nstrated the presence of a figure-of-8 length of the VT in the ECG. In some reentrant circuit in the endocardial episodes, the termination of the original region. A figure-of-8 reentrant circuit reentrant circuit was followed by the could be interrupted by electrical fulgudevelopment of a different, slower reenration of the region encompassing the trant pathway that lasted for one or a few 46 common reentrant wavefront. Recently, cycles prior to termination. (2) The activation wavefront in the CCP abruptly 3-dimensional electromagnetic mapping broke across a stable arc of functional has been used to localize the optimal site conduction block, resulting in premature for radiofrequency ablation of postinactivation of the CCP and conduction farction sustained VT. Macroreentrant circuits with 1 or 2 loops rotated around block. a protected isthmus bound by 2 approximately parallel conduction barriers of Interruption of a Figure-of-8 either a functional line of block, a scar area, or the mitral annulus. The same Reentrant Circuit critical isthmus could be shared by more Reversible cooling or cryoablation of than one VT morphology. Radiofrequency localized areas of the figure-of-8 reentrant ablation performed across the critical circuit in the canine postinfarction heart isthmus prevented the recurrence of VT was used to prove the presence of circu- in 90% of cases.47 Electrophysiological Mechanisms of Spontaneous Termination of Figure-of-8 Reentry

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Figure 10. Epicardial isochronal activation maps of the last 4 cycles of the ventricular tachycardia (VT) shown in Figure 11. Isochrones are represented by closed contours at 10-ms intervals; arcs of functional block are represented by heavy solid lines. The position of the left anterior descending coronary artery is represented by the dashed lines. The electrode sites of the electrograms shown in Figure 11 are represented by solid circles. The tachycardia was maintained by a stable clockwise wavefront at the anterolateral border of the ischemic zone, while the location and configuration of the counterclockwise wavefront at the apex varied from beat to beat. However, both wavefronts joined into a central common pathway (CCP), where conduction was significantly slowed. The maps illustrate gradual acceleration of conduction at the outer border of the arcs of block and the pivot points to the entrance to the CCP. During the last VT cycle (V_T), conduction block developed in the CCP with termination of reentry.

Entrainment, Termination, or Acceleration of Figure-of-8 Reentrant Tachycardia by Programmed Stimulation In thefigure-of-8reentrant circuit, the 2 arcs of conduction block and the slow

common reentrant wavefront are functionally determined and cycle length dependent. A tight fit exists at certain locations during the reentrant tachycardia, with the circulating wavefront closely following the refractory tail of the previous revolution. This is particularly significant in the zone of the slow common reentrant wavefront.

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ulated termination of reentrant tachycardia, the stimulated wavefront must arrive at the area with the longest refractoriness in the zone of the slow common reentrant wavefront before refractoriness expires, thus resulting in conduction block. If this area is strategically located between the 2 arcs of functional conduction block, reentrant excitation is terminated. The 3 factors that determine whether the stimulated wavefront can reach this zone in time for conduction block are (1) the cycle length of stimulation; (2) the number of stimulated beats; and (3) the site of stimulation (Figures 13 and 14).48 The optimal method for stimulated termination of reentry is to apply a critically coupled single stimulus to the proximal side of the slow common reentrant wavefront that conducts prematurely to the strategic zone for conduction block. The stimulus can only result in local capture and does not have to conduct to the rest of the ventricles (i.e., concealed conduction). When a single stimulated waveFigure 11. Surface ECG lead and selected elecfront fails to terminate reentry, one or trograms along the clockwise wavefront shown in Figure 10. Spontaneous termination of sus- more subsequent wavefronts may suctained monomorphic ventricular tachycardia (VT) ceed. However, the stimulated train must is illustrated. Numbers are in milliseconds and be terminated after the beat that interillustrate the shortening of the VT cycle length rupts reentry. Otherwise, a subsequent prior to termination at different electrode sites as shown in Figure 10. The vertical bars at the top stimulated beat could reinitiate the of the figure delineate the time intervals of the 4 same reentrant circuit or induce a difactivation maps shown in Figure 10. ferent circuit. The new circuit could have a shorter revolution time, resulting in The reentrant circuit conduction time is tachycardia acceleration and occasiondetermined by the area with the longest ally degeneration into ventricular fibrilrefractoriness in the zone of the slow lation. Overdrive termination of reentry common reentrant wavefront. It is safe requires both a critical cycle length of to assume that during reentrant tachy- stimulation and a critical number of cardia, the duration of refractoriness in beats in a stimulated train. Otherwise, the zone with the longest refractoriness the stimulated train could establish a probably cannot shorten any further. This new balance of refractoriness and conis not the case, however, with the rest of duction velocity in the reentrant paththe reentrant pathway. A stimulated way. This could perpetuate the reentrant wavefront at a cycle length shorter than process at the shorter cycle length of the the tachycardia cycle length can still con- stimulated train, resulting in entrainment, duct in these zones. In other words, these and spontaneous reentry would resume zones have a gap of excitability. For stim- on termination of the train. Studies of

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Figure 12. Interruption of a figure-of-8 reentrant tachycardia in the epicardial layer overlying 4-dayold canine infarction by cryothermal techniques. The control activation map is shown on the left (VT), and the map of the last reentrant beat before termination on the right (VT-CRYO). Selected epicardial electrograms are at the bottom. The position of the cryoprobe is represented by the shaded circle. The reentrant circuit was interrupted by reversible cooling of the distal part of the common reentrant wavefront (site H). During control, the conduction time between the proximal electrode site G and the more distal site H was 33 ms. Before termination of the tachycardia, an incremental beat-to-beat increase of the conduction time between sites G and H occurred, associated with equal increases in the tachycardia cycle length. When conduction block developed between the 2 sites, the reentrant circuit was terminated and electrogram H recorded an electrotonic potential but no local activation potential. This was represented on the isochronal map by an arc of conduction block (heavy solid line) that joined the 2 separate arcs of conduction block into one. Modified from Assadi M. Am Heart J 1990; 119:1014–1024.

the effects of programmed stimulation on figure-of-8 reentrant tachycardia illustrate the significance of the site of stimulation and emphasize the need for more precise localization of the slow zone of reentry and the direction of the activation front in this zone in the clinical setting.

Effects of Modulation of Spatially Nonhomogeneous Refractory Patterns on the Initiation of Reentry Evidence of the role of spatially nonhomogeneous distribution of refractoriness

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Figure 13. Electrocardiographic recording showing that a train of 4 stimulated beats (marked by vertical bars) resulted in entrainment, termination, or acceleration of a monomorphic reentrant tachycardia, depending on the cycle length of stimulation. Recordings were obtained from 4-dayold canine infarction. The numbers are in milliseconds. The time lines represent 100 ms. From El-Sherif N, et al. Pacing Clin Electrophysiol 1987; 10:341–371.

in the formation of the arc of functional conduction block in the figure-of-8 model of reentry in the canine subacute infarction heart can be obtained from at least 4 types of experiments: (1) experiments in which a short-long-short stimulated cardiac sequence is required for successful initiation of reentry49; (2) experiments in which the initiation of reentry can be prevented by changing the activation pattern of the basic stimulated beat50; (3) experiments showing the effects of adrenergic stimuli on the initiation of reentrant excitation51; and (4) data on the modulation of refractoriness by antiarrhythmic agents. Short-Long-Short Cardiac Sequence Facilitating Induction of Reentry A short-long-short cardiac sequence frequently precedes episodes of ventricular

tachyarrhythmias. Programmed electrical stimulation techniques that use such a sequence have been shown to facilitate the induction of VT52 and macroreentry within the His-Purkinje system.53 For the His-Purkinje system, a short-long-short cardiac sequence has been shown to result in differential changes in refractoriness of Purkinje and muscle fibers that could facilitate the initiation of reentry.54 For the canine postinfarction model, it was also shown that a critically coupled premature stimulus applied after a conditioning train consisting of a series of short cardiac cycles with abrupt lengthening of the last cycle of the train was more successful in inducing a reentrant ventricular tachyarrhythmia than were fixed conditioning trains of short or long cycles (Figure 15).49 The abrupt lengthening of the cardiac cycle before the introduction of a premature stimulation resulted in differential

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Figure 14. Diagram of the mechanisms of entrainment, termination, and acceleration of reentrant ventricular tachycardia by overdrive stimulation as shown in Figure 13. In each of the 3 panels, the control reentrant circuit is labeled 1 and the 4 beats of the stimulated train are labeled 2 to 5. The control circuit has a figure-of-8 configuration, and conduction in the slow zone of reentry proceeds from left to right. The heavy solid lines represent arcs of functional conduction block. Stimulation was applied at the distal side of the slow zone, as shown by the asterisks. During entrainment (left panel), the stimulated wavefront collides with the emerging slow reentrant wavefront. It then circulates and arrives earlier to the proximal part of the slow zone of reentry. This is consistently associated with a change in the conduction pattern in the slow zone, with the development of new functional arcs of block and much slower conduction in parts of this zone. However, a new equilibrium quickly develops in which successive stimulated beats, represented by cycles 3, 4, and 5, maintain the same new conduction pattern at the shorter cycle length of stimulation, thus entraining the tachycardia. On cessation of stimulation, reentry will resume as shown in cycle 6. For termination of reentry, on the other hand (middle panel), successive stimulated beats, now applied at a relatively shorter cycle compared with the entraining train, will result in gradually more conduction delay. Conduction block eventually develops at the proximal part of the slow zone of reentry, as shown in cycle 5. Right panel: The same 4-beat stimulated train is applied at a still shorter cycle length. In this case and because of the short cycle length of stimulation, the second stimulated beat represented by cycle 3 has already blocked in the proximal part of the slow zone of reentry. If stimulation is stopped at this point, the reentrant tachycardia terminates. However, if stimulation is continued, the third and fourth stimulated beats represented by cycles 4 and 5 initiate new arcs of block and different reentrant pathways so that on termination of the stimulated train, a new and possibly faster reentrant circuit will occur. From El-Sherif N, et al. Pacing Clin Electrophysiol 1987;10:341–371.

lengthening of effective refractory periods (ERPs) at adjacent sites within the border of the epicardial ischemic zone. On the other hand, those same sites showed comparable shortening of refractory periods after a train of regular short cycles and comparable lengthening of ERPs after a train of regular long cycles (Figure 16).

Epicardial sites closer to the center of the ischemic zone that showed more electrophysiological abnormality, as suggested by their longer refractory periods, showed greater dependence on the immediately preceding cycle compared with more normal sites located close to the border of the ischemic zone. This suggested that

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Figure 15. Epicardial activation maps of the premature stimulus following 3 different conditioning stimulated trains from a dog with 4-day-old myocardial infarction. Protocol A consisted of a train of 8 beats at a cycle length of 300 ms. Protocol B consisted of a train of 8 beats at a cycle length of 300 ms, with the exception of the last cycle before the premature stimulus, which was abruptly increased to 600 ms. Protocol C consisted of a train of 8 beats at a cycle length of 600 ms. The coupling interval of the premature stimulus was the same during the 3 stimulation protocols at 170 ms. During protocol A, the premature stimulus resulted in an arc of functional conduction block (heavy solid line) within the border of the ischemic zone. The activation wavefront circulated around both ends of the arc, coalesced, and reached the distal side of the arc at the 100-ms isochrone. The relatively short circulation time around the arc did not allow for refractoriness to expire proximal to the arc and for reexcitation to take place. The epicardial activation pattern of the premature stimulus during protocol C was largely similar to that during protocol A. On the other hand, during protocol B, the premature stimulus resulted in a significantly longer arc of functional conduction block by extending the arc during protocols A and C on both the septal and lateral borders of the ischemic zone. The activation wavefront circulated around both ends of the arc, coalesced, and advanced slowly to reach the distal side of the arc at the 160-ms isochrone. The longer circulation time allowed for refractoriness to expire on the proximal side of the arc and for reexcitation to take place. Reproduced with permission from El-Sherif N, Gough WB, Restivo M. Circulation 1991;83:268–278. Copyright 1991, American Heart Association.

ischemic myocardium may have less memory of the cumulative effect of preceding cycle lengths than does normal myocardium. The differential lengthening of refractory periods at adjacent sites after abrupt lengthening of the cardiac cycle resulted

in sufficiently increased dispersion of refractoriness during which a premature stimulus resulted in the development of functional conduction block between those sites. The development of longer arcs of functional conduction block with the short-long-short cardiac sequence resulted

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Figure 16. Recordings obtained from the same experiment shown in Figure 15, illustrating effective refractory measurements at selected adjacent sites proximal and distal to the arc of block during stimulation protocols A to C. During the 3 stimulation protocols, the difference in refractory periods between adjacent paired sites that spanned the arc of block was 20 ms or longer, whereas adjacent sites on the same side of the arc of block differed by less than 20 ms. The refractory periods at all sites were shortest during protocol A, showing an increment during protocol B, and generally increased further during protocol C. However, the percentage of increment in the refractory periods between protocols A, B, and C differed at sites proximal and distal to the arc. At sites proximal to the arc, the step increment in refractory period was approximately equal between protocols A and B and between protocols B and C. On the other hand, at sites distal to the arc, most of the increment in refractory period occurred between protocols A and B. This differential behavior of refractory period in response to protocol B resulted in an increased dispersion of refractoriness between adjacent sites within the border of the ischemic zone and in the development of functional conduction block between these sites. Reproduced with permission from El-Sherif N, Gough WB, Restivo M. Circulation 1991 ;83:268-278. Copyright 1991, American Heart Association.

in a longer reentrant pathway and, hence, an increased reentrant circuit conduction time. Another factor that contributed to increased reentrant circuit conduction time was further slowing of conduction

around the arc of block. The combination of these factors allowed more time for refractoriness to expire proximal to the arc of block and for the circulating wavefront to reexcite those sites to initiate reentry.

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Prevention of Reentrant Excitation by Dual Stimulation During Basic Rhythm Initiation of reentrant excitation can be prevented by changing the activation pattern of the basic stimulated beat. The spatial patterning of recovery time depends on the activation pattern of the basic beat, in addition to the spatially nonhomogeneous refractory distribution induced by ischemia. The dispersion of recovery time can be modified by stimulation at 2 ventricular sites during the basic beat. The arc of conduction block can be modified or abolished entirely by appropriate selection of the secondary stimulation site in the ischemic zone and the temporal sequencing of the paired

stimuli (Figure 17). Asynchronous dual stimulation, with preexcitation of an appropriate site in the ischemic zone, was frequently successful in preventing the initiation of reentry by a fixed coupled premature stimulus. In all instances that resulted in the prevention of reentry, the secondary site was distal to the arc of block that formed after the control S2 stimulation. The secondary site should be in an area of long refractoriness that activated late during the basic beat. Properly applied dual stimulation differentially peels back recovery time in the ischemic zone. Successful dual stimulation depended on the reduction of 2 factors: the spatial gradient of recovery time and the dispersion of recovery time across the arc. The former determines

Figure 17. Abolition of the arc of functional conduction block by dual S1 stimulation. Top (control): S1 activation occurred within 60 ms. A gradient of recovery time between the 190and 230-ms isochrones supported the formation of an arc of block during S2. Bottom (dual asynchronous stimulation): The 2 sites of stimulation, one from the right ventricle (as in control) and one from the ischemic zone distal to the arc of block, are represented by asterisks. When the dual ischemic site was preexcited by 40 ms, no 2 adjacent sites differed in recovery time by more than 20 ms. A zone of graded recovery time that could support functional conduction block was not present. An arc of conduction block did not form. In this experiment, the recovery time was computed by the sum of the activation time (stimulus artifact to response during S1,) plus the effective refractory period at each site. Reproduced with permission from Restivo M, Gough WB, El-Sherif N. Circulation 1988;77:429-444. Copyright 1988, American Heart Association.

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the extent and location of the continuous arc of conduction block and the latter determines whether areas distal to block have recovered during the premature stimulation. Reducing the difference in activation time across the arc of block to a value less than the ERP of the premature stimulus proximal to the arc is the mechanism by which dual S1 stimulation can prevent the initiation of reentry.50 Recently, the technique of biventricular pacing that involves simultaneous pacing of the right and left ventricles has gained wide acceptance. The technique is indicated in patients with depressed left ventricular function and a wide QRS complex and is primarily intended to improve the ventricular function. However, it is also expected to be antiarrhythmic by decreasing the spatial dispersion of repolarization. Effects of Adrenergic Stimuli on the Initiation of Reentrant Excitation It is well known that adrenergic autonomic activity has a role in the generation and perpetuation of cardiac arrhythmias.55,56 However, ischemia can directly affect the sympathetic innervation of the ventricles and thereby alter the effects of sympathetic tone on the ventricle.57 Transmural myocardial ischemia has been shown to disrupt sympathetic innervation of the ventricles and to cause denervation hypersensitivity.58,59 In this case, sympathetic stimulation would increase the dispersion or gradient of refractoriness between innervated and denervated regions.57 The adrenergic effects on reentrant excitation in the canine postinfarction model were investigated by Butrous et al.51 Bilateral stimulation of the ansae subclaviae preferentially improved conduction of premature beats in the normal zones. This corresponded to an improvement in excitability, as measured by a

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decrease in stimulus strength at the same premature coupling interval as control. Consequently, the ERP was preferentially shortened at normal sites but not at ischemic sites. Both of these changes contributed to a shift of the arc of functional conduction block toward more normal tissue. As a result, sites proximal to the arc of functional conduction block had more time to recover excitability and thereby were available to be reexcited by the distal activation wavefront (Figure 18). These observations are consistent with the tenet that sympathetic denervation occurred in the ischemic area, where a thin epicardial layer of myocardium survived the infarction. Another effect of preferential shortening of the refractory period in the normal zone was the extension of the arc of functional conduction block. The lengthening of the arc of functional conduction block or the de novo creation of an arc of functional conduction block in the ischemic zone can potentially facilitate the occurrence of reentrant excitation. In contrast to sympathetic stimulation, intravenous infusion of norepinephrine preferentially shortened the ERP of sites in the ischemic zone, thereby indicating that denervation hypersensitivity had occurred at those sites. The spatial dispersion of refractoriness and the arc of functional conduction block were significantly reduced in size. As a consequence, previously inducible reentrant rhythms were no longer inducible (Figure 19). Thus, sympathetic stimulation can be considered an arrhythmogenic intervention, whereas norepinephrine infusion may be considered antiarrhythmic in this model. Effects of Antiarrhythmic Agents on Figure-of-8 Reentry The pharmacological basis of antiarrhythmic drug therapy in the treatment of reentrant tachyarrhythmias has been

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Figure 18. Epicardial activation patterns and representative electrograms from a dog with 4-dayold myocardial infarction showing the effects of sympathetic stimulation (SS) on the initiation of reentrant excitation. The heart was stimulated at a basic cycle length of 400 ms (S^, and a premature beat (S2) was introduced at a cycle length of 160 ms. During control, S2 resulted in an arc of functional conduction block. Activation arrived on the distal side of the arc of block but did not reexcite areas proximal to the arc (i.e., no reentrant beat occurred). During subsequent bilateral ansae subclaviae stimulation, an S2 at 160 ms produced an arc of functional block and a reentrant beat. The upward arrows on the electrograms represent the effective refractory periods (values indicated) relative to the time of activation at 3 sites across the arc of block. During the control S2, the arc of block was located between sites b and c. During bilateral ansae subclaviae stimulation, the arc of block shifted toward the more normal myocardium (to between sites a and b). Conduction was improved proximal to the arc, as evidenced by the reduction in response interval (R^)- The R1R2 interval at site a shortened during sympathetic stimulation, but the effective refractory periods at sites a, b, and c were not changed. Therefore, S2 arrived earlier at site a, and R2 was unable to propagate directly to site b because it was still refractory (see electrograms). Consequently, the wavefront conducted around the arc of block and then excited site b. The conduction time from site a to site b provided sufficient time for site a to recover its excitability, and the wavefront was then able to reexcite site a. Thus, a reentrant beat was initiated. Reproduced with permission from Butrous GS, Gough WB, Restivo M, et al. Circulation 1992;86:247-254. Copyright 1992, American Heart Association.

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Figure 19. Epicardial activation maps of a premature stimulus (S2) during control (left) and during norepinephrine infusion (right) from a dog with 4-day-old myocardial infarction. During control S2, a long continuous arc of functional conduction block developed. The activation wavefront circulated around the arc then reexcited a site proximal to the arc to initiate a reentrant beat. The dotted line in the control map represents the area of earliest reentrant excitation. The enlarged section (bottom) is from the area of the arc of functional conduction block, with the refractory period of each respective site noted. During norepinephrine infusion, the arc divided into 2 smaller arcs. The dotted line in the norepinephrine maps represents the previous position of the arc of functional conduction block during control. The total activation time shortened during norepinephrine infusion, and there was preferential shortening of the effective refractory period at sites previously distal to the arc of functional conduction block. In this example, the average refractory period for all sites proximal to the arc of functional conduction block during control was 179 ± 12 ms. It was shortened to 161 ± 10 ms (an average shortening by 18 ms) during norepinephrine infusion. The average refractory period for all sites distal to the arc of functional conduction block during control was 249 ±15 ms. It was shortened to 195 ± 18 ms (an average shortening by 54 ms) during norepinephrine infusion. Therefore, norepinephrine decreased the gradient of refractoriness between the normal and ischemic sites. Consequently, the length of the arc of functional conduction block was reduced and shifted toward more ischemic tissue, resulting in prevention of the initiation of reentrant excitation. Reproduced with permission from Butrous GS, Gough WB, Restive M, et al. Circulation 1992;86:247-254. Copyright 1992, American Heart Association.

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guided mostly by the original observations of Mines.5 For any model of reentrant activity, the key ingredients are unidirectional block and conduction around an alternate pathway that is impaired to a degree to permit reexcitation. Drug action is usually defined by its effect on action potential characteristics (upstroke velocity and action potential duration). In the not too distant past, if one were to pick up Goodman & Gilmaris The Pharmaceutical Basis of Therapeutics,60 one would find that antiarrhythmic efficacy was based on drug action on reentry within an anatomically predetermined pathway. In the simplest sense, Class I agents were believed to cause conduction failure in the slow conduction portion of the circuit and Class III agents were believed to cause block within the circuit due to prolonged refractoriness. Besides the simplicity of the reentrant circuit topology, the primary flaw in this reasoning was that drug action was evaluated in normal tissue. Even the sophistication of the more recent Sicilian Gambit61 classification system is limited because of insufficient allowance for evaluation of drug action in diseased myocardium. The results of the Cardiac Arrhythmia Suppression Trial (CAST)62 brought attention to drug-induced proarrhythmia and, in particular, to the problems associated with the means by which the efficacy of potential antiarrhythmic drugs are determined. While there is a heightened awareness of the problems associated with antiarrhythmic drugs (especially in the treatment of ventricular tachyarrhythmias), drug development is hampered by the fact that the mechanisms of drug-induced antiarrhythmic or proarrhythmic effects still remain unclear. The induction or suppression of reentrant activation in the figure-of-8 model

involves a complex interplay of functional activation properties and changes in these properties that are associated with the pathological state of the heart. The antiarrhythmic or proarrhythmic effect of a drug can be best studied by measuring the rate-dependent effects of the agent on conduction and refractory properties of normal and ischemic zones in the heart.63-65 The proceeding section reviews the electrophysiological actions of 3 antiarrhythmic agents, flecainide, lidocaine and azimilide, in the figure-of-8 model of circus movement reentry in the subacute MI period in the dog. The working model is based on the facts that functional conduction block occurs because of differentially prolonged refractoriness between normal and ischemic zones and that tachycardiadependent slow conduction occurs in the hypoperfused ischemic layer. The hypothesis tested is that agents that promote slower conduction in the ischemic zone or extend the line of block by differential prolongation of refractoriness in the ischemic zone favor reentry. Figure 20 is a bubble diagram illustrating the antiarrhythmic (or proarrhythmic) effects of all 3 drugs. Results are from 3 different studies and no statistical comparisons were made to compare antiarrhythmic efficacy between the 3 drugs. Flecainide is a Class Ic agent whose effects are believed to be highly selective for use-dependent blockade of sodium channels. Some of the proarrhythmic effects of flecainide were known prior to CAST. Ranger et al.66 made the interesting observation that VT could be aggravated in patients by exercise, i.e., fast heart rates. We studied the rate-dependent effect of flecainide in the canine subacute infarction model. The ECGs in Figure 21 illustrate the rate-dependent aggravation of arrhythmia due to flecainide. Insight into

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Figure 20. Bubble diagrams illustrating the effects of flecainide, azimilide, and lidocaine for induction of reentry with only a single premature beat (S2). Responses were classified as follows: VT = sustained monomorphic ventricular tachycardia; NS-RA = nonsustained reentrant activity; NR = no response (less than 3 unstimulated responses). Proarrhythmic effects (upward sloping lines) are defined as the drug-induced transformation of: (1) NR to either NS-RA or VT, or (2) NS-RA to VT. Antiarrhythmic affects are the converse of above and are indicated by downward sloping lines. Horizontal movement indicates no effect of the drug on arrhythmia induction. A. Flecainide was administered as a bolus dose of 1 mg/kg given over 8 minutes, followed by a maintenance infusion of 1 mg/kg/h. For flecainide, there were 4 proarrhythmic events in 16 dogs (25%) at a basic cycle length (BCL) of 500 ms, and 10 proarrhythmic events at a BCL of 300 ms (62.5%). The proarrhythmic effect of flecainide was more prominent at a BCL of 300 ms (P < 0.005, Pearson chi-square). B. Azimilide was tested at 3 doses. It had essentially no effect at 3 mg/kg. The drug was effective in suppressing reentrant ventricular arrhythmias in 6 dogs at a dose of 10 mg/kg and 2 additional dogs at 30 mg/kg. In 2 dogs, there was a drug-induced aggravation of inducible reentrant VT at a dose of 10 mg/kg and at 30 mg/kg there was one case of drug-induced conversion of NR to NS-RA. C. Lidocaine was administered as a bolus dose of 6 mg/kg, i.v., delivered over 10 minutes, followed by a maintenance infusion of 75 jig/kg/min. For lidocaine, there were 3 proarrhythmic events in 18 dogs (16.7%) at a BCL of 500 ms, and 8 proarrhythmic events at a BCL of 300 ms (44.4%). The overall proarrhythmic effect of lidocaine showed increased tendency at a BCL of 300 ms, but the difference was not statistically significant. On the other hand, the proarrhythmic effect of lidocaine through transformation to VT was more prominent at a BCL of 300 ms compared to 500 ms and was statistically significant (P < 0.05). There were no cases of drug-induced transformation of VT to NS-RA or NR.

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Figure 21. A. Lead II ECG from an experiment during which, in the control pre-drug state, S1,S2 of 170 ms did not induce any response at basic cycle lengths (BCLs) of 300 ms or 500 ms. After flecainide infusion, no response was elicited at a cycle length of 500 ms. However, at a cycle length of 300 ms, a single extrastimuli induced a sustained monomorphic ventricular tachycardia. B. and C. Isochronal activation maps during regular pacing and premature stimulation. The maps show the epicardial ventricular surface with the perimeter representing the atrioventricular ring and the center the apex; the left anterior descending coronary artery is shown by the dotted line. In these maps each isochronal contour represents a portion of the heart that was activated within a 20-ms time frame. Panel B shows S1, activation patterns during BCLs of 500 ms and 300 ms. In control, the ventricles were activated within 80 ms. There was no evidence of conduction block in any of the maps. After flecainide, the heart activated within 80 ms at a BCL of 500 ms and increased slightly to 100 ms at a BCL of 300 ms. The S2 activation maps for an S^ coupling of 170 ms are shown in C. The upper left map shows the control pre-drug activation pattern at BCL of 500 ms. An arc of functional conduction block formed during the 40-ms isochrone. Activation reached the distal border of the arc during the 160-ms isochrone. Because the maximum activation time difference across the arc of block was only 120 ms, the impulse was unable to reenter. Following flecainide, reentry could not be induced by premature stimulation at that cycle length. The activation map was essentially the same as control. At a BCL of 300 ms, reentry was not induced during control, but following flecainide, more dramatic changes in S2 activation are apparent. Again, the premature wavefront blocked within 40 ms. Flecainide depressed conduction in the slow zone and delayed activation at the distal border of the arc of block to the 200-ms isochrone, increasing the activation time difference across the line of block to 160 ms, and the impulse was able reenter near the site of initial conduction block.

the mechanism is revealed by analysis of the isochronal activation maps in panels B and C, which show that conduction in the ischemic zone is differentially depressed (without conduction failure) relative to

the normal zone. In the S2 maps, there is little difference in the size of the line of block before or after flecainide. Because we have shown that block results from differences in ERPs between normal and

FiGURE-OF-8 MODEL OF REENTRANT VENTRICULAR ARRHYTHMIAS ischemic zones, we measured ERPs before and after flecainide. Consistent with its Class Ic action and lack of change in the lines of block, we found no statistically significant difference in ERP.63 We then

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measured the rate dependence of conduction velocity in normal and ischemic tissue using a specially designed highresolution cross electrode. The results, shown in Figure 22, show a rate-dependent

Figure 22. Rate dependence of conduction velocity in control and in the presence of flecainide. Results shown are from 6 dogs. In these graphs, conduction velocities were normalized to the longest basic cycle length (BCL) (600 ms). The effect of the drug was more pronounced for propagation in the ischemic zone relative to the normal zone and was statistically significant at the shorter BCLs in all 4 groups. Flecainide caused a rate-dependent reduction of conduction velocity of 14% (longitudinal) and 8% (transverse) for a change in basic drive from 600 ms to 250 ms in normal tissue, and a reduction of 27% (longitudinal) and 25% (transverse) for a change in basic drive from 600 ms to 250 ms in ischemic tissue.

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preferential depression of conduction in the ischemic zone. Azimilide is a Class III agent that acts by prolonging refractoriness and has little effect on conduction velocity. Figure 23 shows maps from a representative experiment in which a sustained monomorphic VT was induced by a single premature beat. A dose of 10-mg/kg azimilide slowed the VT with termination coinciding with block of the reentrant impulse within the slow common reentrant pathway. To understand the mechanism of azimilide action, we measured ERPs in this vicinity at cycle lengths of 350 ms and 600 ms, before and after drug infusion. Data shown in panel A of Figure 24 indicate that block occurred because of a prolongation of ERP at the

critical sites. The preferential prolongation of ERP in the ischemic zone by azimilide is illustrated in panel B. Recently we have shown that in the canine right atrial enlargement model of circus movement atrial flutter, both azimilide and dofetilide were 100% effective in terminating flutter and preventing reinduction. Efficacy relied on a similar mechanism of differentially prolonged refractoriness in the slow conduction component of the reentrant circuit where drug-induced termination occurred.67 Lidocaine is a familiar and widely used antiarrhythmic agent, with occasional proarrhythmic effects having been noted for years.68,69 Lidocaine is more of a mixed bag compared to the previous drugs described above. Lidocaine has

Figure 23. Isochronal activation maps of control ventricular tachycardia (VT) induction and termination of VT by azimilide. The left panel shows the map of the control VT induced by a single premature beat. The cycle length of the VT in this example was 285 ms. In this experiment, the reentrant circuit had a figure-of-8 morphology in which 2 wavefronts circulated around 2 lines of functional conduction block. Each isochronal contour line represents the boundary of a region activated by the reentrant wavefronts in successive 10-ms intervals. In this particular experiment, there was a region of necrosis extending through to the epicardial surface and is indicated in the maps by shaded region. The 2 wavefronts joined once every cycle and conducted slowly through the ischemic epicardial layer in the area bounded between the 2 lines of block (isochrones 120 through 250). The maps in the middle panel show that following a 10 mg/kg infusion of azimilide, the cycle length of the VT progressively slowed to 320 ms before druginduced termination of the rhythm. Azimilide caused a marked increase in overall conduction time within the slow conducting pathway. In the last beat (right panel; VTerm), the reentrant impulse failed to penetrate through the ischemic zone and blocked during the 470-ms isochrone between sites L and M.

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Figure 24. A. Effective refractory period (ERP) values near the site of block during termination for the example shown in Figure 23. The lines of block from the activation maps are shown with ERP values, and clearly show that conduction block occurred near sites with the greatest refractory period values in control (sites M and N). These sites also had the longest refractory period values after 10 mg/kg azimilide. B. A grouped analysis on the effect of azimilide on increasing ERP in normal and ischemic zones using 2 basic cycle lengths (BCLs). Data presented in this figure are for those sites in which ERP data was obtained in control, 10 mg/kg azimilide, and 30 mg/kg azimilide. One way analysis of variance was performed to test for significant differences between the groups. Student's paired Mest for paired data was then performed to test for differences between control ERP and ERP at both drug doses. There was a statistically significant increase in ERP by azimilide in normal and ischemic tissues at both BCLs. The drug-induced increase in ERP was statistically greater in ischemic tissue compared to normal tissue for both drug doses and at both BCLs.

been shown to completely abolish membrane responsiveness in severely ischemic cells in the epicardial border zone during the subacute phase of MI.69 Not only does lidocaine depress conduction, but it also has a prolonging effect on ERP. Similar to flecainide, lidocaine has been shown to cause a selective ratedependent decrease in conduction velocity in the ischemic zone. This is keeping with its possible proarrhythmic action.

We examined the effect of lidocaine on normal and ischemic tissue.63 The proarrhythmic effects of lidocaine, shown in Figure 25, are due not only to depressed conduction in the ischemic zone but an increased path length resulting from differential prolongation of ERP. Figure 26 shows that lidocaine could be antiarrhythmic only if ERPs in the ischemic zone were prolonged to such a degree that the reentrant impulse blocked in

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Figure 25. Proarrhythmic effect of lidocaine. Activation maps from a representative experiment illustrating the effects of lidocaine at different basic cycle lengths (BCLs). The upper left panel shows a control isochronal map of a premature beat following a drive of 29 basic beats at a BCL of 300 ms. After an S1,S2 of 180 ms, the premature activation wavefront encountered a continuous line of block within 40 ms. Conduction then proceeded retrogradely through the slow zone and blocked at 140 ms distal to the block. After lidocaine, premature stimulation at the sameS1S2coupling interval had a proarrhythmic effect. The line of block was longer compared to the control pre-drug map, and conduction within the slow zone proceeded much slower. Because of the additional delays incurred, the impulse reached the distal border of the block later than in control and reactivation occurred within 240 ms. In this experiment, a sustained monomorphic ventricular tachycardia (VT) (upper right panel) was induced. The VT cycle length was 184 ms. In the same experiment, lidocaine also had a proarrhythmic effect (nonsustained VT) at a BCL of 500 ms but no sustained VT was induced.

the common reentrant pathway. The effects of lidocaine on ERP in normal and ischemia tissue are summarized in Figure 27. In summary, in the subacute MI model, pure Class I drug action appears to be proarrhythmic with regard to reentrant substrates. This is due to a preferential depression of conduction in the

ischemic zone, a factor that favors reentry. The role of Class III or Class Ib agents is more complex. In this functional model of reentry, the obstacle around which the reentrant impulse circulates is most probably due to differential prolongation of refractoriness between normal and ischemic zones. Therefore, any agent that preferentially prolongs refractoriness

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Figure 26. Antiarrhythmic effect of lidocaine. Lidocaine exhibited antiarrhythmic behavior if there was a marked increase in effective refractory period (ERP) after drug, as illustrated in the following maps. In this example at a basic cycle length of 500 ms, a single premature beat at 200 ms (left panel) caused block and reentry within 200 ms. After lidocaine (right panel), reentry did not occur with the same premature coupling interval. The impulse coalesced at the entrance of common reentrant pathway within 120 ms, and blocked within the common reentrant pathway within 180 ms. ERPs were measured at critical sites within the common pathway. Though conduction in the vicinity of sites C and D was slowed after lidocaine, there was a marked prolongation of refractoriness at sites within the ischemic zone (C and D) relative to border sites (B) and normal sites (A). Site D could not activate because it was still refractory; block occurred between sites C and D.

may increase the length of the functional obstacle, again a factor that favors reentry. In the examples presented here, antiarrhythmic action occurs only when refractoriness is prolonged to such a degree that conduction block within the slow pathway of the circuit is possible. Because it

is impossible to predict the wide range of electrophysiological consequences of MI in all cases, antiarrhythmic drug therapy must be carefully tailored to individual situations. What may be good in one situation may result in opposite results in another.

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CAEDIAC MAPPING Figure 27. Effect of lidocaine on effective refractory period (ERP) in normal and ischemic tissue. Lidocaine increased refractoriness to a greater extent in the ischemic zone compared with the normal zone, but tne effect was rate independent. The drug-induced ERP increase (ERPlidocaine-ERPcontrol) was more pro-

nounced in the ischemic zone compared to normal. The difference was statistically significant at 300 ms (16.3 ±19.1 versus 9.8 ± 10.7; P < 0.05). Pacing threshold increased significantly after lidocaine in both normal (0.9 ± 0.7 mA versus 1 .4 ± 1.3mA; P< 0.01) and ischemic (0.8 ± 0.5 versus 1.2 ± 0.9; P < 0.01) zones, but there was no statistically significant difference in excitability between normal and ischemic zones before or after drug. Our previously published reports established that arcs of functional conduction block result from regional disparities in refractoriness along a continuous region of the ischemic border zone. Arcs of block were measured before and after lidocaine by planimetry on a 3-dimensional heart model. The lengths of the arcs of functional conduction block for all experiments were 11 .8 ± 3.9% longer after lidocaine; the effect was statistically significant (P < 0.01). The lengths of arcs of block were computed in the proarrhythmic and antiarrhythmic groups for those beats in which the activation wavefront reached the distal side of the arc. The lengthening by lidocaine was greater (14.3 ± 5.2%; P< 0.05) in the proarrhythmic group compared to the antiarrhythmic group (10.2 ± 1.7%).

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FiGURE-op-8 MODEL OF REENTRANT VENTRICULAR ARRHYTHMIAS 50. Restivo M, Gough WB, El-Sherif N. Reentrant ventricular rhythms in the late myocardial infarction period: Prevention of reentry by dual stimulation during basic rhythm. Circulation 1988;77:429-444. 51. Butrous GS, Gough WB, Restivo M, et al. Adrenergic effects on reentrant ventricular rhythms in subacute myocardial infarction. Circulation 1992;86:247-254. 52. Denker S, Lehman MH, Mahmud R, et al. Facilitation of ventricular tachycardia induction with abrupt changes in ventricular cycle length. Am J Cardiol 1984; 53:508-515. 53. Denker S, Lehman MH, Mahmud R, et al. Facilitation of macroreentry within the His-Purkinje system with abrupt changes in cycle length. Circulation 1984;69:26— 32. 54. Denker S, Lehman MH, Mahmud R, et al. Divergence between refractoriness of HisPurkinje system and ventricular muscle with abrupt changes in cycle length. Circulation 1983;68:1212-1221. 55. Zaza A, Schwartz PJ. Role of the autonomic nervous system in the genesis of early ischemic arrhythmias. J Cardiovasc Pharmacol 1985;7:8-12. 56. Szekeres L, Boros E, Pataricza J, Udvary E. Sympathetic neural mechanisms in cardiac arrhythmias. J Mol Cell Cardiol 1986; 18:369-373. 57. Inoue H, Skale BT, Zipes DP. Effects of myocardial ischemia and infarction on cardiac afferent sympathetic and vagal reflexes in the dog. Am J Physiol 1988; 255:H26-H35. 58. Kammerling JJ, Green FJ, Watanabe AM, et al. Denervation supersensitivity of refractoriness in noninfarcted areas apical to transmural myocardial infarction. Circulation 1983;67:787-796. 59. Inoue H, Zipes DP. Results of sympathetic denervation in the canine heart: Supersensitivity that may be arrhythmogenic. Circulation 1987;75:877-887. 60. Goodman AG, Goodman LS, Gilman A (eds): Goodman & Oilman's The Pharmacological Basis of Therapeutics. 6th ed. New York: Macmillan; 1980.

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61. Task Force of the Working Group on Arrhythmias of the European Society of Cardiology: The Sicilian Gambit. A new approach to the classification of antiarrhythmic drugs based on their actions on arrhythmogenic mechanisms. Circulation 1991;84:1831-1851. 62. Cardiac Arrhythmia Suppression Trial (CAST) Investigators. Preliminary report: Effect of encainide and flecainide on mortality in randomized trial of arrhythmia suppression after myocardial infarction. N Engl J Med 1989;321:406-412. 63. Restivo M, Yin H, Caref EB, et al. Reentrant arrhythmias in the subacute infarction period. The proarrhythmic effect of flecainide acetate on functional reentrant circuits. Circulation 1995;91:1236-1246. 64. Yin H, El-Sherif N, Caref EB, et al. Actions of lidocaine on reentrant ventricular rhythms in the subacute myocardial infarction period in dogs. Am J Physiol 1997;272:H299-H309. 65. Restivo M, Yin H, Caref EB, et al. Selective effect of class III antiarrhythmic agents on refractoriness determines efficacy in post infarction reentrant ventricular tachyarrhythmias (VT). Circulation 1996;94(Suppl):I-161. 66. Ranger S, Talajic M, Lemery R, et al. Amplification of flecainide-induced ventricular conduction slowing by exercise. A potentially significant clinical consequence of use-dependent sodium channel blockade. Circulation 1989;79:1000-1006. 67. Restivo M, Hegazy M, El-Hamami M, et al. Efficacy of azimilide and dofetilide in the dog right atrial enlargement model of atrial flutter. J Cardiovasc Electrophysiol 2001;12:1018-1024. 68. Krejcy K, Krumpl G, Todt H, Raberger G. Lidocaine has a narrow antiarrhythmic dose range against ventricular arrhythmias induced by programmed electrical stimulation in conscious dogs. Pflugers Arch 1992;346:213-218. 69. Lazzara R, Hope RR, El Sherif N, Scherlag BJ. Effects of lidocaine on hypoxic and ischemic cardiac cells. Am J Cardiol 1978; 41:872-879.

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Chapter 12 Demonstration of Microreentry Hasan Garan, MD

The accepted criteria for demonstrating that the mechanism of an arrhythmia is reentry include the presence of a region of unidirectional conduction block with return of an impulse to its site of origin via retrograde conduction in the previously blocked, but now recovered, pathway before the onset of the next cardiac cycle, and perpetuation of this geometry during the subsequent cardiac cycles.1 Reentry as the underlying mechanism becomes more convincing if the impulses expected to arise from reentry are eliminated by reversible or irreversible interruption of this pathway. When a wavefront satisfying these criteria travels along pathways several centimeters long, reentrant excitation in the form of a closed loop can, at least in theory, be detected directly by examining the local electrograms recorded by electrodes positioned along the pathways provided that the mapping system allows recordings dense enough to define the entire circuit.2 This finding has been termed macroreentry. Demonstration of the classic criteria for entrainment,3 such as progressive fusion during pacing at decreasing cycle

lengths from a site outside the circuit, is used to further support macroreentry as mechanism. However, if reentry is present but confined to a smaller area, e.g., 1 cm in diameter, direct demonstration of reentrant excitation underlying a clinical arrhythmia may not be possible within the resolution of the recording system. Reentry confined to such a small volume of myocardium has been termed microreentry to distinguish it from macroreentry. Definition of Microreentry It is difficult to formulate a canonical definition of microreentry based on circuit size alone since it may not be possible to determine pathway size with precision in clinical tachycardias using intracardiac recording techniques. This task, difficult enough sometimes for even obviously reentrant rhythms such as atrioventricular nodal reentrant tachycardia, may be more difficult for intra-atrial reentry and even more so for the majority of clinical reentrant ventricular tachycardias (VTs). Circuit size and geometry

From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; e2003. 275

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are likely to be complex and determined by the underlying individual pathology. Furthermore, only certain special components of the circuit may be fixed while other components may be dynamically coupled or uncoupled to the former resulting in changes in cycle length or surface QRS morphology.4 Therefore, rather than being categorical, the distinction between "microreentry" and "macroreentry" becomes a question of degree, and the circuit size may display a continuum from small to large. A more practical and clinically relevant definition of microreentry may be based on operational criteria. The tachycardia in question should manifest the accepted general characteristics of reentry rather than triggered activity or automaticity.5 Other supporting observations for reentry such as classic criteria for entrainment should be sought, although inability to demonstrate entrainment does not rule out microreentry since the excitable gap may be narrow and attempts to entrain may result in entry block. Furthermore, in microreentry, activation sequence mapping during tachycardia usually identifies an early site of origin, but in contrast to macroreentry where a circular loop of electrical activity can be demonstrated during each cardiac cycle, e.g., atrial flutter,6 the global activation pattern during a tachycardia with underlying microreentry manifests a radial, centrifugal spread away from this site of earliest activation or the "source." The practical question then becomes whether any component of microreentry, located close to the site of breakthrough, can be identified during endocardial catheter mapping. A Protected Zone with Altered Conduction Most models of reentry incorporate a zone of slow, often decremental, conduction taking place in a region that is "protected," or surrounded by nonconducting

tissue that prevents the wavefront, which is progressing unidirectionally in the area of slow conduction, from breaking through into the surrounding tissue at random sites. The absence of conduction in the surrounding tissue may be due to an anatomical barrier or, more commonly, to functionally refractory tissue.7 It is no longer generally accepted that these areas have well-defined and fixed entry and exit sites and also well-defined orthodromic and antidromic directions of conduction during arrhythmia. Such regions of slow conduction have been clearly demonstrated in animal models of experimental infarction.7-9 They may be epicardial,7 endocardial,8 or intramural (midmyocardial)8'9 in location, depending on the specific model and the technique used to create the experimental myocardial infarction. Extracellular recordings from such regions often display fractionated, low-amplitude electrograms with long duration.8,10 The electrophysiological and anatomical bases for the fractionated electrograms have been carefully investigated in a canine model.11 The fractionated electrograms correspond to regions where fibrosis during infarct healing has caused wide separation of individual myocardial fibers and has distorted their orientation.11 This distortion results in a matrix of inhomogeneous anisotropy, and even if transmembrane action potentials recorded from individual myocytes may be normal, the overall conduction in the area is abnormally slow due to its fiber geometry, and the extracellular electrograms recorded from within such a region look fractionated and prolonged.11 For the clinical cardiac electrophysiologist, the demonstration of microreentry during mapping requires (1) identification of a zone of slow conduction manifesting either fractionated and prolonged local electrograms spanning diastole and recorded from closely spaced sites or, rarely, continuouelPANDIASTOLIC ectrical

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this would be reentry in a small confined area. It has been demonstrated in an experimental canine model that in cases of midmyocardial reentry, the zone of slow conduction may remain "hidden" from the electrodes confined to the endocardial and the epicardial surfaces.8 Thus, specific location as well as the orientation of the slowly conducting pathways will determine whether they can be detected by endocardial catheters, and not all such regions can be adequately investigated by endocardial catheters. Intimately tied in with these issues is the question of Location of Microreentry whether there is a critical myocardial The electrode catheters are confined thickness necessary for observation comto the endocardial surface of the heart. patible with microreentry as defined above. Since the site of origin for clinical ischemic For example, in contrast to macroreentry, sustained VT frequently resides in a suben- microreentry is rarely, if ever, observed as docardial region,12 it is usually possible to a mechanism of clinical arrhythmia in record electrograms from such a subendo- the human atrial myocardium, and focal cardial region of abnormal conduction with atrial tachycardias almost always result the use of endocardial catheters. However, from automaticity or triggered activity.16 not all regions of slow conduction are Furthermore, it is not entirely clear subendocardial, and in a small number of whether microreentry is an arrhythmia cases of clinical VT, intraoperative record- mechanism that can occur solely in disings have convincingly demonstrated the eased myocardium, which provides the presence of epicardial reentry incorporat- substrate for nonuniform anisotropy. ing a zone of slow conduction with com- Spach at al.17 have demonstrated in superplete diastolic bridging in electrodes limited fused human atrial pectinate muscle fibers age-related obliteration of side-to-side to the epicardial surface.13,14 The orientation of the zone of slow electrical coupling between fibers resultconduction is also critical. Slow conduc- ing in reentry, even if not sustained reention taking place in a midmyocardial trant activation, arising within very small region may be difficult if not impossible areas. This mechanism results from the to detect by endocardial electrodes. Con- age-related exaggeration of the homogesider for example the mechanism of non- nous or uniform anisotropy and may be homogeneous anisotropic reentry.15 A scar categorized as microreentry due to the or a functional arc of conduction block small dimensions of the interdigitating may be located in the midmyocardium fibers involved (Figure 1).17 It is not clear if with slow transverse conduction moving such a mechanism in aging but undiseased around a fulcrum point resulting from human atrium can result in "idiopathic" the geometry of the edges,15 alternating clinical atrial tachycardia. Atrioventricuwith faster longitudinal conduction which lar nodal reentry may be the best undermay result in a discrete focus of endocar- stood clinical microreentry in the atrium. dial breakthrough and a "monoregional It had been believed that the circuit underspread" of activation from this source on lying this arrhythmia was confined to the the endocardial surface, whereas, in fact, compact atrioventricular node, but recent activity recorded from a single site, and (2) proof that these electrograms represent electrical activity in an essential component of the circuit, rather than representing activity in pathways outside the circuit, bearing no relationship to the reentry process. In other words, demonstration of microreentry requires finding a zone manifesting early, mid, and late diastolic potentials all recorded within a "reasonably small" site, and demonstrating their specificity as part of the arrhythmia mechanism.

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Figure 1. Microreentry in response to premature stimulation in superfused nonuniform anisotropic atrial bundle. Results were obtained at basic stimulus rate of 171/min; a premature stimulus was introduced every 10th beat at interval shown in box above each group of waveforms. Drawing at upper left shows locations where each waveform was recorded. Drawing at lower right shows perimeter of reentrant circuit, indicated by solid lines with arrows. Shaded region denotes surrounding areas in which 0 waveforms were measured at 16 sites; at each of these peripheral sites, local excitation occurred after that of sites confined to perimeter of reentrant circuit for the corresponding areas. From Spach et at. Circ Res 1988;62:828.

data from several electrophysiology laboratories suggest a substantially larger circuit incorporating atrial tissue with slow conduction due to anisotropy.18 Intrafascicular microreentry, as opposed to interfascicular macroreentry, has been proposed as a plausible mechanism for idiopathic left ventricular VT in the absence of myocardial pathology.19 Detection of the Region of Slow Conduction No matter where it is, microreentry has to "solve" the problem of slow conduction taking place in a small region.

In order to have "hidden" diastolic activation for a long time, sometimes few hundreds of milliseconds, taking place in such a small region, the average local velocity must be exceedingly low. There are several possible mechanisms, including nonhomogeneous anisotropy, of such a slow average conduction. It is not known whether directional differences in velocity in nonuniformly anisotropic tissue of the diseased myocardium alone may account for a very low average velocity necessary for clinical microreentry. Alternatively, a very special redundant pathway geometry may be needed. As a third mechanism, one may think of isolated but closely spaced myocardial fibers, able

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to communicate with each other by elec- from this site displays potentials throughtrotonus not altogether interrupting but out the entire cardiac cycle without any slowing continuous conduction by pro- appreciable isoelectric diastolic interval, ceeding in "jumps." Clinically used endo- thereby bridging the discrete local eleccardial electrode catheters are incapable trograms that do have such isoelectric of making a distinction among these interectopic intervals recorded from neighboring sites.22-24 Initiation of VT preceded mechanisms. Fractionated, prolonged local elec- by critical delay and fractionation in local trograms may be recorded from several electrograms bridging the interectopic disparate sites in the myocardium if the interval between the last extrastimulus underlying pathological process is wide- and the first VT complex was first demonspread. The zone of slow conduction of the strated in a canine model25 and subsegreatest interest is the one linked to the quently recorded during endocavitary earliest "presystolic" local electrogram left ventricular mapping with the use that precedes the onset of the QRS deflec- of electrode catheters during clinical tion in any surface ECG lead. Electrical VT.22 In a larger series, some form of activity in the small pathways with abnor- CEA bridging 2 consecutive QRS commal conduction participating in microreen- plexes during monomorphic VT was try is thought to contribute little, if any, demonstrated in 36% of the 56 patients to the surface ECG signal whose onset studied.23 According to the microreentry model, comes after the electrical activity exits the slow pathway and begins to rapidly electrical activity is present in some comdepolarize the bulk of the surrounding ponent of a small circuit at any moment myocardium. Thus, the distal segment or throughout the entire cardiac cycle. CEA, the site of exit of such a pathway is likely recorded by closely spaced bipolar electo be at or near the site that displays the trodes,24 rather than composite electrodes earliest discrete local electrogram in the covering a large area of the ventricular tachycardia cycle, preceding the onset of wall,25 is more likely to represent microreenthe QRS complex. The surface ECG algo- try. Furthermore, CEA should demonrithms applied to the surface QRS com- strate an organized pattern that displays plexes recorded during sustained VT and repeating and reproducibly identifiable pace mapping techniques20,21 may be used components bearing the same temporal to facilitate the detection of the approximate relation to each other during every carlocation of the earliest local electrogram, diac cycle (Figure 2).24 However, CEA, and therefore, by proximity, the approxi- even if it manifests an organized pattern mate location of the zone of microreentry. with regularly recurring, reproducible comBeyond this approximate localization, ponents, and recorded by closely spaced defining the exact site and various com- bipolar electrodes, should not be equated ponents of microreentry requires detailed with microreentry unless other confirmapping in a relatively small area of the matory electrophysiological phenomena ventricle. are shown to be present. Marked fractionation of a bipolar electrogram into multiple asynchronous high-frequency signals during pacing has been observed Continuous Electrical Activity before and interpreted as exaggerated Continuous electrical activity (CEA) is desynchronization of activation within a said to be present at a recording site when region comprising distorted heterogeneous the local bipolar electrogram recorded fibers.26

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Figure 2. The reproducible components of continuous electrical activity (CEA) during sustained monomorphic ventricular tachycardia (VT) in a canine model of experimental myocardial infarction recorded by endocardial left ventricular electrodes. The components display the same temporal relationship during each cardiac cycle.

In order to prove its close relationship to reentry in a small region, one must show that CEA is coextensive and coterminal with the tachycardia. This means, among other things, that at the recording site manifesting CEA during the tachycardia, CEA should not be present during pacing (with sinus rhythm background) at the same cycle length as the tachycardia. This however, is a necessary but not a sufficient criterion. During the arrhythmia, CEA should not only be present at the critical site of reentry, but during a train of stimuli that entrain the tachycardia

without interrupting it, its reproducible components should manifest simultaneous entrainment. Conversely, stimuli that terminate the tachycardia should interrupt the CEA first and transform it into more discrete electrograms after which no tachycardia complexes should be observed (Figure 3).24 Pacing during the tachycardia at cycle lengths shorter than the tachycardia cycle length, which captures the ventricle but encounters entry block at the site of origin of the tachycardia and therefore fails to either entrain or terminate the tachycardia,

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Figure 3. Ventricular pacing during sustained monomorphic ventricular tachycardia (VT) in the same canine model as in Figure 2. The arrowheads depict the reproducibly recurrent components of continuous electrical activity (CEA). Despite capture, the first train of stimuli neither terminates nor entrains VT (entry block) and at the same time leaves the components of CEA unaffected. The second train of stimuli transforms CEA into fractionated but discrete local electrograms (penetration of the circuit), and at the same time interrupts VT.

should not reset or alter in any way the pattern of organized CEA since there is no penetration into the area where CEA is taking place (Figure 3). After cessation of pacing, the tachycardia and the components of CEA should both continue unaffected, bearing the same temporal relationships and phase delays (Figure 3).

By contrast, extrastimuli that reset the tachycardia should also reset the components of CEA and should leave them unaffected. One of the most convincing ways to demonstrate the basic relationship between organized reproducible CEA and the tachycardia circuit is to terminate the

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Figure 4. Interruption of continuous electrical activity (CEA) with termination of ventricular tachycardia (VT) by a single subthreshold stimulus delivered at a site immediately adjacent to the site of CEA, during the refractory period of the ventricular myocardium, 85 ms after the onset of the surface ECG QRS complex.

tachycardia using a subthreshold stimulus scanning the diastolic CEA and delivered to the site of CEA. If such a stimulus is delivered during the refractory period of the tissue surrounding the protected zone, e.g., within or immediately after the QRS complex, it may still penetrate the circuit locally and terminate microreentry. If such a subthreshold stimulus terminates CEA and at the same time interrupts the tachycardia (Figure 4), the cause-effect relationship will be established. Isolated Mid-diastolic Potentials Pandiastolic CEA with reproducible components manifesting all the criteria mentioned above is a rare finding in the clinical cardiac electrophysiology labora-

tory. It is much more common during mapping to record isolated, low-amplitude, middiastolic electrograms corresponding to electrical activity in a protected zone of abnormally slow conduction, especially when mapping VT or ischemic disease (Figure 5).27-30 It is not possible to define local geometry with precision using a single mapping catheter in the left ventricle. However, fractionated, prolonged, mid-diastolic potentials may result from activity in different nonlinear components of the microreentry circuit.29,30 These potentials may thus represent specific segments of CEA associated with microreentry, all of which may not be recorded with a single bipolar electrode. As with CEA, demonstrating the specificity of these mid-diastolic signals is of greatest importance, and electrophysiological phenomena confirming

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Figure 5. Fractionated low-amplitude local electrograms identified with arrows recorded in middiastole (160 ms prior to surface EGG fiducial point) recorded during clinical sustained monomorphic ventricular tachycardia, with the use of a quadripolar exploring (Expl.) electrode. RVA = right ventricular apex.

that these potentials are recorded from pathways within the VT circuit must be sought. First, it should be demonstrated that the low-amplitude, mid-diastolic electrogram results from conduction in a pathway which gives rise to the subsequent VT wavefront, rather than resulting from secondary, late activation of a site outside the circuit from the previous VT beat. Careful examination of resetting and entrainment patterns may help to make this distinction. An extrastimulus applied during VT to a site spatially remote from the catheter recording the mid-diastolic potential, and synchronized to this middiastolic signal, that resets the VT should also reset the mid-diastolic potential with little or no change in the temporal relationship and phase delay between the mid-diastolic potential, the surface QRS complex, and the rest of the local electro-

grams (Figure 6).29 Similarly, trains of extrastimuli applied at a site far from the electrode recording the mid-diastolic potential that entrain the VT must entrain the mid-diastolic potential as well,27,30 again with no variation in the temporal relationship of this mid-diastolic component and the QRS complex of the VT which follows (Figure 7). With the geometry shown in Figure 8A, microreentry appears as a modified version of figure-of-8 mechanism.31 Prototypical figure-of-8 reentry incorporates one central pathway of slow conduction with larger, faster conducting pathways coupled to it.31 Similarly, microreentry may be considered a mechanism incorporating multiple small pathways coupled to each other, making up a small circuit with narrow exit points simulating focal mechanism with monoregional spread.

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Figure 6. Resetting of ventricular tachycardia (VT) and the mid-diastolic potential recorded from an exploring electrode (Expl.) during diastole from the right ventricular apex, remote from the site of earliest recorded electrogram. The resetting stimulus is delivered at a ventricular site outside the VT circuit.

An extrastimulus during VT delivered at a site within the microreentry circuit produces results different from the findings described above. First of all, one would expect a narrow excitable gap in a small area manifesting nearly CEA. If the paced wavefront propagates only in antidromic direction, VT is likely to terminate, possibly after a fusion beat. If the stimulus propagates in the orthodromic direction, the VT will be reset, but with no change in QRS morphology. Such a finding should be spatially (site of stimulation) and temporally (coupling interval) reproducible. Similarly, as with macroreentry, entrainment from a site within the protected zone of slow conduction should cause no fusion activation, and therefore no change in the surface QRS morphology of the entrained complexes27 (Figure 9).

This brings up another mapping-related question, namely whether entrainment with concealed fusion can be used to assess the approximate size of the reentrant circuit. The size of the area over which entrainment with concealed fusion is demonstrable, possibly with different degrees of latency, may be assessed with reasonable accuracy. This observation alone is not able to determine the actual geometry or the orientation of the pathways but may provide a rough estimate of the relative size of the tissue in which microreentry is confined. Figures 8C and 8D show schematically how pacing attempts from closely spaced sites may be used to probe the geometry of the underlying circuit. Newly available techniques of electroanatomical mapping may be superior to conventional fluoroscopy for this purpose.32

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Figure 7. "Entrainment" of the diastolic potentials recorded during the same ventricular tachycardia (VT), as shown in Figure 6. The low-amplitude, fractionated potentials recorded by the exploring electrode (Expl.) are accelerated to the pacing cycle length (300 ms) with long stimulus-to-potential interval, but without disturbing the potential-to-VT temporal relationship in the first VT return beat.

Another rare finding that may increase the specificity of mid-diastolic potentials as electrical activity recorded from within a protected zone of microreentry is the demonstration that isolated diastolic potentials march through the entire diastole in a continuous fashion, during small displacements of the recording electrode. This finding is almost as specific as pandiastolic CEA with all components recorded from a single recording site. It also identifies the proximal and the distal segments of the circuit for purposes of ablation. As the diastolic potential marches temporally from early to mid to late portions of diastole, it is assumed that the recording electrode catheter is marching spatially from proximal to middle to distal segments of the circuit, relative to the site of exit. It should be

realized, however, that this temporal-spatial correlation may be the consequence of our perception of a 3-dimensional mechanism as 2-dimensional, confined to the endocardial surface. Resetting and entrainment maneuvers may be tried at successive catheter sites for acquiring corroborating data. It is important to emphasize that local geometry cannot be defined with precision with the use of endocardial electrode catheters, even with ones incorporating multiple recording bipoles. Furthermore, the novel techniques of noncontact mapping are not likely to elucidate the components of microreentry, as it is likely to be identified with this technique as any other focal mechanism with centrifugal spread.

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Figure 8. A. Schematic diagram representing a microreentry circuit with the slowly conducting pathway depicted by a wavy line (p) and surrounded by functionally (broken line) and anatomically (hatched rectangle) refractory zones. The site of exit is marked by the letter E. B. Resetting of ventricular tachycardia (VT) and the microreentry circuit by a stimulus delivered at a site remote from the site of exit (E) from the circuit. The VT cycle number is indicated by n. C. Resetting of VT and microreentrant activation by a stimulus delivered at a site within the slow component of reentry, resulting in antidromic conduction block and orthodromic propagation. D. Termination of VT with a stimulus delivered slightly more distally in the pathway blocking both in antidromic and in orthodromic directions.

Termination of the Tachycardia -with Interruption of Conduction Within Microreentry Another method of confirming the specificity of the mid-diastolic potentials as basic to VT mechanism involves interruption of VT by interventions that

reversibly inhibit conduction in the circuit located within the protected region where these mid-diastolic fractionated potentials are recorded.33 Unlike the operating room, the electrophysiology laboratory is not a setting where reversible cooling of a recording site can readily be carried out, at least with currently available electrode catheters. Subselective cold saline

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Figure 9. Entrainment with concealed fusion during ventricular tachycardia with a cycle length of 500 ms. Pacing at a cycle length of 460 ms accelerates the tachycardia rate with no change in surface QRS morphology in surface ECG leads I, aVF, and V^ . Latency during pacing is similar to the diastolic potential-to-QRS interval during the ventricular tachycardia.

injections into coronary artery branches can be used to terminate VT, but this technique is not specific enough to serve as a probe for a microreentry circuit. Subthreshold stimuli delivered within the refractory period to the zone of slow conduction and timed appropriately to prolong the refractoriness in the slow pathway, but without resulting in a propagated beat, may be used to attempt termination of the VT. When such a subthreshold stimulus captures the local tissue that does not generate a surface vector, and fails to propagate in both the orthodromic and the antidromic directions and eliminates the mid-diastolic potential, one would expect VT to terminate if the site of stimulation is instrumental for giving rise to the following VT wavefront (Figures 8D and 10). Although a rare finding, if present and reproducible, this observation lends powerful support to the idea that a small protected zone, the site of the mid-diastolic potential recording, is within the VT circuit. It is less likely for a macroreentry or a large figure-of-8 mechanism to be affected by a subthreshold stimulus since there would be a greater chance for the participation

of alternate pathways or alternate exit sites necessary for the continuation of the tachycardia. Conclusion

Microreentry is an arrhythmia mechanism that may be difficult to define with precision in the cardiac electrophysiology laboratory. For the practical purposes of endocardial mapping, it may be reduced to the simplified concept of slow conduction in pathways confined to a small volume of myocardium protected by surrounding refractory tissue. Demonstration of microreentry then requires a search for bridging diastolic CEA or isolated, mid-diastolic, fractionated electrograms scanning diastole, whose specificity must be properly tested before their putative link to the reentry process can be established. Several questions remain. First, whether microreentry, based on unusual geometry or physiological aging, can exist in cardiac tissue without injury or disease, is an interesting question that warrants further investigation. Second, it is not clear whether microreentry can be the basis of

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Figure 10. Termination of clinical ventricular tachycardia (VT) by a subthreshold stimulus delivered to a recording site manifesting prolonged fractionated electrograms. A bipolar stimulus is delivered by the distal pair of electrodes and the local electrogram is recorded by the proximal pair on a quadripolar electrode catheter.

clinical atrial arrhythmias or can be confined to the endocardial or the epicardial layers of the ventricular myocardium rather as a 2-dimensional mechanism. If, alternatively, microreentry can occur only as a 3-dimensional mechanism in the ventricular myocardium, it may represent, as described above, the special case of a small figure-of-8 circuit, which, because of its orientation and special geometry, mimics a focal mechanism. Clarification of these issues influences the success of ablative therapies for clinical tachycardias.

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DEMONSTRATION OF MICROREENTRY 7. Mehra R, Zeiler R, Gough WB, El-Sherif N. Reentrant ventricular arrhythmias in the late myocardial infarction period. 9. Electrophysiologic-anatomic correlation of reentrant circuits. Circulation 1983;67: 11-24. 8. Garan H, Fallon JT, Rosenthal S, Ruskin JN. Endocardial, intramural, epicardial activation patterns during sustained monomorphic ventricular tachycardia in late canine myocardial infarction. Circ Res 1985;56:736-754. 9. Kramer JB, Saffitz JE, Witkowski FX, Corr PB. Intramural reentry as a mechanism of ventricular tachycardia during evolving canine myocardial infarction. Circ Res 1985;56:736-754. 10. Okumura K, Olshansky B, Henthom RW, et al. Demonstration of the presence of slow conduction during sustained ventricular tachycardia in man. Circulation 1987;75:369-378. 11. Gardner PI, Ursell PC, Fenoglio JJ Jr., et al. Electrophysiologic and anatomic basis for fragmented electrograms recorded from healed myocardial infarcts. Circulation 1985;72:596-611. 12. Josephson ME, Harken AH, Horowitz LN. Endocardial excision. A new technique for the treatment of recurrent ventricular tachycardia. Circulation 1979;60:14301439. 13. Littman L, Svenson RM, Gallagher JJ, et al. Functional role of the epicardium in postinfarction ventricular tachycardia. Circulation 1991;13:1577-1591. 14. Harris L, Downar E, Mickleborough L, et al. Activation sequence of ventricular tachycardia: Endocardial and epicardial mapping studies in the human ventricle. JAm Coll Cardiol 1987;10:1040-1047. 15. Wit AL, Dillon SM. Anisotropic reentry. In: Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping. Mount Kisco, NY: Futura Publishing Co.; 1993:127-154. 16. Wellens HJJ, Brugada P. Mechanisms of supraventricular tachycardia. Am J Cardiol 1988;62:10D-15D. 17. Spach MS, Dolber PC, Heidlage JF. Influence of the passive anisotropic properties on directional differences in propagation following modification of the sodium conductance in human atrial muscle. Circ Res 1988;62:811-832. 18. Janse MJ, Anderson RH, McGuire MA. AV nodal reentry: Part I: AV nodal reen-

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try revisited. J Cardiovasc Electrophysiol 1993;4:573-586. 19. Tai Y, Lee KL. Interfascicular macroreentry versus intrafascicular microreentry. J Cardiovasc Electrophysiol 1996;7:275. Letter. 20. Josephson ME, Waxman HL, Cain ME, et al. Ventricular activation during ventricular endocardial pacing. II. Role of pacemapping to localize origin of ventricular tachycardia. Am J Cardiol 1982;50:11-22. 21. Kuchar DL, Ruskin JN, Garan H. Electrocardiographic localization of the site of origin of ventricular tachycardia in patients with prior myocardial infarction. JAm Coll Cardiol 1989;13:843. 22. Josephson ME, Horowitz LN, Farshidi A. Continuous local electrical activity. A mechanism of recurrent ventricular tachycardia. Circulation 1978;57:659-665. 23. Brugada P, Abdollah H, Wellens HJJ. Continuous electrical activity during sustained monomorphic ventricular tachycardia. Am J Cardiol 1985;55:402-411. 24. Garan H, Ruskin JN. Localized reentry: Mechanisms of induced sustained ventricular tachycardia in canine model of myocardial infarction. J Clin Invest 1984; 74:377. 25. El-Sherif N, Hope RR, Scherlag BJ, Lazzara R. Reentrant arrhythmias in the late myocardial infarction period. 2. Patterns of initiation and termination. Circulation 1977;55:702-719. 26. Elharrar V, Foster PR, Jirak TL, et al. Alterations in canine myocardial excitability during ischemia. Circ Res 1977;40: 98-105. 27. Morady F, Fran R, Kou WH, et al. Identification and catheter ablation of a zone of slow conduction in the reentrant circuit of ventricular tachycardia in humans. J Am Coll Cardiol 1988;ll:775-782. 28. Fitzgerald DM, Friday KJ, Yeung JA, et al. Electrogram patterns predicting successful catheter ablation of ventricular tachycardia. Circulation 1988;4:806-814. 29. Garan H, Ruskin JN. Reproducible termination of ventricular tachycardia by a single extrastimulus within the reentry circuit during the ventricular effect refractory period. Am Heart J 1988; 11:546-550. 30. Kay GN, Epstein AE, Plumb VJ. Region of slow conduction in sustained ventricular tachycardia: Direct endocardial recordings and functional characterization in

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humans. J Am Coll Cardiol 1988; 11:109116. 31. El-Sherif N, Smith A, Evans K. Canine ventricular arrhythmias in the late myocardial infarction period: Epicardial mapping of reentrant circuits. Circ Res 1981;49:255-265. 32. Shpun S, Gepstein L, Hayam G, BenHaim SA. Guidance of radiofrequency

endocardial ablation with real-time threedimensional magnetic navigation system. Circulation 1997;96:2016-2021. 33. El-Sherif N, Mehra G, Gough WB, Zeiler RH. Reentrant ventricular arrhythmias in the late myocardial infarction period: Interruption of reentrant circuits by cryothermal techniques. Circulation 1983; 68:644-656.

Chapter 13 Optical Mapping of the Effects of Defibrillation Shocks in Cell Monolayers Vladimir G. Fast, PhD and Andre G. Kleber, MD

Introduction Strong electrical shocks are applied to terminate life-threatening cardiac arrhythmias such as ventricular fibrillation. Although this procedure is used in patients on a routine basis, the mechanism by which the extracellular electrical field affects the transmembrane potential (Vm) of cardiac cells and terminates fibrillation is not fully understood. From theoretical studies it has been proposed that the microscopic structure of cardiac tissue might cause Vm changes (AVm) necessary for defibrillation. Until recently, the experimental verification of this hypothesis was not possible because of the complex 3-dimensional structure of cardiac tissue and the lack of methods to measure distribution of Vm at the microscopic level. These problems were overcome with the development of a new experimental approach that combines high-resolution optical mapping of Vm and techniques for growing cardiac tissue

with defined architecture in cell culture. This chapter discusses the application of this approach to studying effects of extracellular shocks on Vm and presents a short overview of recent experimental results. Mechanisms for Vm Changes During Defibrillation Analysis of the classic cable model indicates that in a structurally continuous tissue, the Vm changes induced by a uniform extracellular field should be limited to a very small tissue area near the shock electrodes.1,2 With increasing distance from the electrodes, changes in Vm will decay exponentially and approach zero beyond 1 to 2 mm from the shock electrodes. To induce Vm changes in tissue far from the shock electrodes, a redistribution of axial current between intracellular and extracellular spaces must take place. In general, this can occur either because of the nonuniform distribution of the extracellular shock field or because of the

This work was supported by the Swiss National Science Foundation and the Swiss Heart Foundation. From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; e2003.

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spatial variation in the tissue structure. The first mechanism relates AVm, or "virtual electrodes," to the spatial derivative of the extracellular field called activating function. 3-6 Several mechanisms of the second kind have been proposed. One of them, known as the mechanism of secondary sources, suggests that changes in Vm are produced by the microscopic resistive barriers associated with cell boundaries7-10 or by larger resistive barriers associated with the vasculature, intercellular clefts, or connective tissue sheets separating cell bundles and cell layers.11-12 Another structure-dependent mechanism relates Vm changes to rotation of anisotropy axes in space.13 Combination of both structural factors (tissue anisotropy) and the nonuniform shock field can produce Vm changes via the "dog-bone" effect.14-15 Cell Culture as a Model for Studying Defibrillation Mechanisms Until recently, the effects of tissue structure on defibrillation were investigated almost exclusively in computer models.7-10,16,17 Experimental studies of the structural effects in the heart are hampered by the 3-dimensional anatomy of cardiac muscle that prevents precise correlation of Vm changes with the tissue structure, especially at the microscopic level. Also, because cardiac muscle contains structural discontinuities of multiple types, separating the effects of one individual structure from another as well as from effects of other, structureindependent, factors is extremely difficult. These obstacles can be overcome using cultures of cardiac cells. The advantage of cell cultures is that they grow as 2-dimensional monolayers and their microscopic structure can be precisely determined and correlated with electrophysiological measurements. In addition, the monolayer structure can be modified in

a desirable way using techniques for directed cell growth, which greatly facilitates structure-function studies. A number of such techniques were developed at the Department of Physiology of the University of Bern (Switzerland) that allow the growth of cell cultures in predetermined geometric patterns such as cell strands and geometric expansions,18,19 or in anisotropic patterns.20,21 Electrophysiological parameters of cell cultures such as the maximal upstroke rate of rise and the conduction velocity are quite similar to those measured in adult ventricular tissue.18,22 The differences include smaller cell size and more uniform distribution of gap junctions along the cell perimeter in cell cultures as compared to the adult tissue.20,21 Microscopic Optical Mapping of Vm in Cell Cultures The optical mapping technique was used to measure Vm changes with microscopic resolution. This technique, which involves staining of excitable tissue with a voltage-sensitive dye and measurement of the dye fluorescence with an array of photodetectors, has been widely used for multisite recordings of Vm from brain and cardiac tissue.23 We have adapted this method for microscopic measurements of Vm in myocyte cultures.20-"22,24 The key elements of this technique, a voltagesensitive dye, a light source, optical filters, objectives, and amplifiers, were selected to achieve high signal-to-noise ratio when measuring weak optical signals from a small cell membrane area. The experimental set-up for optical recordings, shown schematically in Figure 1, was built around an inverted epifluorescent microscope (Axiovert 35M, Zeiss, Germany). To optically record Vm, the voltage-sensitive dye RH-237 (Molecular Probes, Eugene, OR) was used. The excitation light was provided by a 100-W arc

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Figure 1. Schematic diagram of the optical mapping set-up. See text for detailed description.

mercury lamp, which delivers a strong light in the green range, where the RH237 dye has its absorption maximum. The light was passed through a heat filter and a band-pass exciting filter with a transmittance range of 530 to 585 nm. The light was deflected by a dichroic mirror with transmittance at greater than 600 nm, and focused on the preparation by the objective lens. The emitted fluorescent light was collected by the same objective, passed through the dichroic mirror, filtered with a low-pass filter at greater than 615 nm, and measured using a 10 x 10 photodiode array (Centronic Ltd., Surrey, England) attached to the photographic port of the microscope. Individual diodes in this array have dimensions of 1.4 x 1.4 mm2 with an interdiode distance of 0.1 mm. Microscopic objectives with magnification lOx, 20x, 40x, 63x, and lOOx (Zeiss) and numerical apertures of 0.5, 0.75, 1.3, 1.25, and 1.3, respectively, were used. Objectives with lower magnifications are also available but, because of

their small numerical apertures and poor light collecting efficiency, they did not provide enough light for the voltagesensitive measurements in cell cultures. Taking into account the additional magnification of the photographic port of 2.5x, the total optical magnification was in the range between 25x and 250x that resulted in an imaged area of 56 x 56 (im2 (lOx magnification) to 5.6 x 5.6 Jim2 (lOOx magnification) per diode. The photocurrents from the 96 diodes were converted to voltages by custom-built current-to-voltage converters. The 2 most important parameters of the converters are the gain and the bandwidth. The gain depends on the value of the feedback resistors, Rf. The bandwidth depends on the speed of operational amplifiers and, because of the presence of a stray capacitance parallel to the feedback resistors, on the Rf value as well. To faithfully reproduce a normal cardiac action potential upstroke, the system bandwidth should be greater than 1 kHz. We achieved a

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Figure 2. A. Schematic diagram of the experimental chamber showing an area of cell growth (black) on a glass cover slip (white) and shock electrodes (+ = anode; - = cathode). B. Optical recording of background fluorescence and an action potential upstroke. The fractional change of fluorescence associated with the action potential was 12% of the background level. C. Determination of shock-induced change in transmembrane potential (AVm). A stimulus was applied to induce an action potential. Action potential amplitude (APA) was measured as the difference between the fluorescence levels immediately before and after the upstroke. The Vm was measured twice: without a shock (gray line) and with a 10-ms shock applied during action potential plateau (black line). The shock-induced AVm was measured as the difference in fluorescence intensity between a linear fit of the plateau phase depicted by a thin line and the signal magnitude 5 ms after the shock onset and expressed as a percentage of APA. This linear fit corresponded to the time course of Vm without a shock.

bandwidth of ~1.6 kHz and a low noise level by using feedback resistors with a value of 100 MO and operational amplifiers OPA121 from Burr-Brown (Tucson, AZ). A typical signal at the output of a current-to-voltage converter is shown in Figure 2B. It consists of a large negative deflection associated with the beginning of light exposure and corresponding to background fluorescence. The smaller positive deflection corresponds to the upstroke of an action potential initiated by a stimulation pulse. The fractional change of fluorescence given by the ratio of the action potential amplitude (APA) and the value of the background fluorescence can vary throughout the preparation and it also changes during the course of the experiment. Immediately

after the dye staining, the fractional change can be as high as 15%. As a result of dye internalization, the fractional fluorescence change decreases down to 5% within the first hour after staining. The useful portions of optical signals typically do not exceed a few dozen millivolts, which is too small to be measured without amplification by standard 12-bit analog-to-digital (A/D) converters. However, amplifying the useful signals also amplifies the portions corresponding to the background fluorescence, which may saturate the A/D converters. To avoid this limitation, the background fluorescence was subtracted before amplification. Both background subtraction and subsequent amplification were performed using second-stage

OPTICAL MAPPING OF EFFECTS OF DEFIBRILLATION SHOCKS amplifiers with a sample-and-hold circuitry (Figure 1). Subsequently, signals were multiplexed and digitized at 12-bit resolution and sampling rate of 10 to 25 kHz for each of the 96 channels. One of the major limitations of optical mapping is the phototoxic effect of voltage-sensitive dyes. This effect is especially important in cell cultures in which a relatively small amount of light is emitted by a cell monolayer and achievement of a high signal-to-noise ratio demands loading cells with a dye at a high concentration and a strong light excitation. To limit the phototoxic effect, cells were illuminated by excitation light for only a short period, typically 80 ms. Depending on optical magnification and intensity of illumination, up to 6 measurements can be performed in these conditions without significant cell deterioration. Measurements of the Shock-Induced Vm Changes An important advantage of optical mapping over conventional recordings using electrodes is that optical signals are devoid of stimulation artifacts. This is especially important for defibrillation studies where strong artifacts created by the defibrillation shocks interfere with conventional measurements of Vm during shocks and during at least the first 20 to 50 ms after the shocks. The disadvantage of optical recordings is that the optical signals reflect only relative changes of Vm. The absolute value of optical signals depends, in addition to Vm, on density of dye staining, degree of dye internalization, and uniformity of light intensity. Combination of these factors results in a significant variability of fluorescence intensity throughout the preparation, independently of the underlying variation of Vm. The V m -independent variability of

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optical signals can be reduced somewhat by measuring the fractional changes in fluorescence. This does not, however, eliminate the signal variability completely, because the fractional change of fluorescence itself varies throughout the preparation (mainly because of nonuniform dye internalization). To eliminate the V m -independent variability, optical signals were normalized relative to the portion of optical signals, which represents the APA. To accomplish that, an action potential was initiated before each shock, the optical APA was measured, and the shockinduced changes in Vm were normalized by the APA (Figure 2C). In doing so, an assumption was made that APA does not vary throughout the imaged area. This assumption is likely to be true in the microscopic measurements when the size of an imaged area is comparable to the length of the electrotonic constant, which in cell monolayers is about 360 urn.25 This assumption might not hold true on a larger spatial scale in pathological conditions, such as ischemia, favqring nonuniform distribution of APA. To create a uniform extracellular field, electrical shocks were applied via 2 large platinum plate electrodes positioned at opposite ends of the perfusion bath (Figure 2A). The bath measured 2.2 x 2.2 cm2 and the electrode dimensions were 2 x 0.2 cm2. Monophasic truncated exponential shocks with a duration of 10 to 12 ms were delivered using custom-built shock generators. The generators were triggered by the stimulus pulse and could produce shocks at specified times during the cardiac cycle. In most of the measurements, the extracellular voltage gradient produced by the shock in the bath was measured simultaneously with the optical recordings of Vm by 2 silver electrodes with a diameter of 0.2 mm and an interelectrode distance of 3.5 mm.

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The electrodes were positioned near the mapping area and aligned with the direction of the electrical field. Effects of Cell Boundaries on AVm One of the intriguing possibilities is that changes in Vm can be created by boundaries of the individual cell. This idea was proposed in 1986 by Plonsey and Barr,7 based on theoretical studies of a 1-dimensional cable model with periodic resistive barriers. In this model, changes in Vm appear as periodic ("sawtooth") oscillations with hyperpolarization on one side of a resistive barrier and depolarization on the other side. The idea that cell boundaries account for defibrillation was attractive because this type of structural discontinuity is the most universal feature of cardiac tissue. However, the hypothesis that cell boundaries induce major changes in Vm was not confirmed experimentally.

The effect of cell boundaries on shock-induced Vm changes was investigated in narrow cultured cell strands with a width of 30 to 60 (im.26 In such strands, because of the aligning influence of the strand edges, cells were oriented along the strand axis. A uniform extracellular shock field was applied along the strand axis and changes in Vm were measured with subcellular resolution. Figure 3 shows an example of AVm/ APA measured with resolution of 6 um during application of a shock with strength of 11 V/cm. Panel A shows the phase-contrast image of cells with superimposed grid of photodiodes, and panel B illustrates selected action potentials. At all measuring sites, the cell membrane was hyperpolarized during the shock. There was no abrupt transition from hyperpolarization to depolarization as expected from secondary sources, and no significant changes in AV m / APA were found between measuring sites localized in neighboring cells.

Figure 3. Effect of cell boundaries on transmembrane potential (VJ. A. Image of cells at xlOO magnification with overlaid photodiode array grid. B. Drawing of the cell strand with outlined boundaries of individual cells and selected optical recordings of change in Vm during application of shock. Reproduced from reference 26, with permission.

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Similar results were obtained in 12 cell in Vm, the search for secondary sources preparations. shifted to larger anatomical discontinuA likely explanation for the discrepancy ities. One type of such a discontinuity in between the theory and the experiments cardiac muscle is related to the incluregarding secondary source formation is the sions of connective tissue into the effect of "lateral averaging" described for myocardial structure that interrupt the microscopic conduction.22 The mechanism continuity of the intracellular space on a of secondary source formation was origi- scale of several cell lengths or more, i.e., nally proposed from 1-dimensional models several hundred micrometers. To invesof cardiac tissue where axial current is tigate the effect of such discontinuities on forced to flow through each resistive bar- Vm, we have produced cell monolayers rier represented by an intercellular junc- with intercellular clefts of variable tion, which resulted in a large voltage dimensions.29 Uniform-field shocks were drop across this junction. In a 2-dimen- applied across clefts and the Vm changes sional tissue, however, a portion of local were measured as a function of the cleft axial current can bypass an individual length. resistive barrier and flow through cell juncFigure 4 demonstrates the effect of an tions offered by lateral cell connections. In electrical shock on Vm near an intercellucell cultures, this averaging effect might lar cleft. Panel A shows an image of the cell be especially prominent because of the monolayer and the grid indicating the posirelatively uniform distribution of gap tion of the photodiodes. The intercellular junctions along the cell perimeter.21 cleft is delineated with a dashed line. The Whether cell junctions can change Vm sub- length and the width of the cleft were stantially in adult cardiac tissue in vivo is approximately 240 |nm and 60 |im, respecnot known. On one hand, adult myocytes tively. The stimulation electrode was are longer than the neonatal cells in cul- located above the mapping area and the ture and gap junctions in the ventricular shock electrodes were located on the left myocardium tend to be more concentrated and the right sides. Panel B shows the at cell ends27 thus favoring the formation isochronal map of activation spread initiof secondary sources. On the other hand, ated by the stimulus. Panels C and D cells in the intact tissue are arranged in depict isopotential maps of the relative a 3-dimensional structure, which increases changes in Vm, AVm/APA, caused by electhe degree of intercellular connectivity trical shocks of opposite polarities. The and is predicted to reduce effects of indi- pattern of AVm/ APA distribution was convidual resistive discontinuities on Vm. sistent with the mechanism of secondary How these opposing influences interplay sources. In panel C, cells were depolarized is not presently known. Measurements of on the right side and hyperpolarized on the effects of shocks carried out recently in the left side of the cleft. The isopotential rabbit papillary muscle using a roving map contained 2 localized regions of microelectrode did not reveal secondary maximal depolarization and maximal sources at the cell boundaries,28 adding hyperpolarization adjacent to the obstasupport to the conclusions drawn from the cle, corresponding to current sources and current sinks, respectively. With the cell culture studies. reversed shock polarity (panel D), the Effects of Intercellular Clefts on AVm regions of depolarization and hyperpolarization were interchanged. After it was found that cell boundIndividual recordings illustrating aries do not produce significant changes changes in Vm across the obstacle from the

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Figure 4. Effect of an intercellular cleft on transmembrane potential (Vm). A. Image of a cell monolayer (x20 magnification) with the grid illustrating the position of the photodiodes and the dashed line outlining the position of the intercellular cleft (length = 240 (im). B. Isochronal map of activation spread initiated by stimulation from above the mapping area. Activation times are determined from the time of earliest activation within the mapping region. Isochrones are drawn at intervals of 100 us. C and D. Isopotential maps of change of Vm/action potential amplitude (AVm/APA; in %) induced by shocks of opposite polarities. Shock strength was 7.5 V/cm (A) and 8.5 V/cm (B). Gray areas depict the intercellular cleft. The outline corresponds to the boundary of the photodiode array. See color appendix. Reproduced from reference 29, with permission.

OPTICAL MAPPING OF EFFECTS OF DEFIBRILLATION SHOCKS same experiment are shown in Figure 5 (panels A and B) for opposite shock polarities. The shape of AVm traces in most cases corresponded to the truncated exponential field pulse in both depolarized and hyperpolarized areas. Panels C and D illustrate profiles of AVm/APA along the horizontal axis for the 2 shock polarities. In the first case (panel C), the profile was symmetrical with depolarization and hyperpolarization decaying within a short distance from the

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obstacle. A slightly asymmetrical voltage profile was observed when shock polarity was reversed (panels B and D). The strength of secondary sources depended on both field strength and cleft length. We defined the magnitude of secondary sources as the difference of AVm/APA measured across the middle of an obstacle, (AVm/APA)diff. For a given obstacle, the average of 2 (AVm/APA)diff values was taken that were measured in

Figure 5. Action potentials near intercellular cleft during shock application. A and B. Selected optical traces recorded across the cleft at 2 shock polarities. The numbers correspond to the photodiode locations in Figure 4C and D. C and D. Dependence of change of transmembrane potential/action potential amplitude (AVm/APA; in %) on distance. Distance 0 corresponds to the center of the photodiode 1 in Figure 4C. Reproduced from reference 29, with permission.

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response to shocks of opposite polarities. Figure 6 demonstrates the dependence of the (AVm/APA)diff on the obstacle length for 2 shock strengths. In the first group, the average shock strength was 8.5 ± 0.6 V/cm (range of 7.5-9.4 V/cm, n=14). In the second group, the average shock strength was 18.0 ± 1.0 V/cm (range of 16.1-19.3 V/cm, n=16). The (AVm/APA)diff values measured across 20 clefts are plotted in Figure 6B. The cleft lengths were in the range of 45 to 270 jam. Within this range of cleft lengths, the relation between the obstacle length and the magnitude of the secondary sources could be closely approximated by a linear fit. From these data, an estimate of the critical obstacle length necessary for direct activation of cells can be predicted. Assuming that a shock produces symmetrical changes in Vm and that cells are excited in the area of maximal depolarization (if Vm is depolarized by 25% APA above the resting level, corresponding to a depolarization of 25 mV30), cells will be directly activated when the (AVm/APA)diff value is larger than 50%. From the linear functions in Figure 6B, this estimated critical obstacle length is approximately 85 ± 8 urn for a shock strength of 18.0 V/cm and 171 ± 7 |um for a shock strength of 8.5 V/cm. Direct Stimulation by Secondary Sources To test the prediction that resistive discontinuities cause direct excitation of cardiac tissue during application of extracellular shocks, shocks were delivered during diastole and the isochronal maps of activation spread initiated by shocks were analyzed. Figure 7 illustrates direct activation of myocytes by a secondary source created by an inexcitable obstacle. Panels A and C show isochronal maps of activation spread initiated by shocks of opposite polarities, and panels B and D

show the corresponding recordings of Vm from the sites surrounding the obstacle. With one shock polarity (panel A), a small cell region adjacent to the obstacle on the left side was directly activated by the shock. This was evident from the fact that the cells in this region had the earliest activation times, and action potential upstrokes (panel B, traces 1 and 2) exhibited biphasic shapes: the shock caused initial rapid membrane depolarization (shown by arrows), which was followed by excitation. This area of earliest activation was superimposed on the region of maximal depolarization produced by the shock during the plateau phase of the action potential (Figure 4D). Away from this area the amplitude of initial depolarization decreased (traces 3 through 6). The cells on the right side of the obstacle were transiently hyperpolarized by the shock, with hyperpolarization gradually increasing toward the center (traces 7 through 10). The initial membrane hyperpolarization was followed by depolarization, which resulted from the propagating wave initiated on the left side of the obstacle. An almost symmetrically reversed activation pattern was observed when the shock polarity was reversed (panels C and D). The site of the earliest activation in this case was located on the right side, superimposed on the area of maximal depolarization during application of shock in the plateau phase (Figure 4C). The stimulating efficacy of secondary sources depended on both shock strength and obstacle length. With an average strength of 8.2 V/cm (number of obstacles, n=28), shocks of both polarities directly excited cells when the obstacle length was 196 ± 53 [im (n = 14). Shocks of only one polarity resulted in direct activation when the obstacle length was 134 ± 49 fim (n=5, P<0.05). No activation by shocks of both polarities was observed at obstacles with length of 84 ± 23 \im (n = 9, P < 0.001). Thus, the critical obstacle length necessary

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Figure 6. Strength of secondary sources as a function of obstacle length. A. Scheme illustrating the definition of the strength of secondary source, (AVm/APA)diff. B. Plot of the dependence of the difference of change of transmembrane potential/action potential amplitude (AVm/APA)diff on the obstacle length at 2 shock gradients: 8.5 V/cm (open circles) and 18 V/cm (closed triangles); R indicates the correlation coefficient of the linear regression. Reproduced from reference 29, with permission.

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Figure 7. Direct activation of cells by secondary sources. A. Isochronal map of activation spread (interval = 150 (is) initiated from a secondary source on the left side of the obstacle during the application of a shock in diastole. The location of the secondary source coincides with the location of the area of maximal depolarization observed during shock application in the plateau phase of the action potential (Figure 4D). The obstacle is shown in white. Arrows indicate the direction of activation spread. Circles with numbers represent the centers of photodiodes selected for display of optical signals (B). Activation times are determined from the time of earliest activation within the mapping region. B. Optical recordings from locations indicated in A. Arrows indicate the direct membrane depolarization produced by a shock in the area of earliest activation. C and D. Isochronal map of activation (C) and selected optical signals (D) during the application of a shock of opposite polarity. The format is the same as in A and B. Reproduced from reference 29, with permission.

OPTICAL MAPPING OF EFFECTS OF DEFIBRILLATION SHOCKS for the direct cell activation with shocks of 8.2 V/cm was between 84 and 196 |um. The estimate of critical length of 171 ± 7 ^m obtained from shock-induced changes of Vm during the plateau phase of action potential (Figure 5) falls within this range. Discontinuities with dimensions of several hundred micrometers and larger are common in ventricular myocardium. In human pectinate muscle, connective tissue septa with such dimensions were found in ventricular tissue from young individuals and much larger (up to I mm) septa were found in the aging myocardium.31 Experiments in cell cultures suggest that such inexcitable obstacles may contribute to tissue excitation and defibrillation during the application of extracellular electrical shocks in the whole heart. The importance of tissue discontinuities for stimulation and defibrillation is also supported by the results of recent experiments in canine heart where it has been shown that artificial transmural incisions in ventricular wall give rise to activation fronts during stimulation and reduce the threshold of stimulation.32 Shock-Induced AVm in Cell Strands The boundaries of cell bundles and cell layers form a different type of structural discontinuity, which might be responsible for formation of secondary sources in cardiac muscle. The layer structure might be especially important because it is present throughout the bulk of ventricular myocardium, where cardiac cells are organized into lamina of variable thickness running across ventricular walls.12 To evaluate the role of this type of structures in defibrillation, we have carried out several studies in which Vm changes were measured in linear cell strands during application of uniform-field shocks. In the first study performed on strands averaging 0.2 mm in width, it

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was shown that, in accordance with the cable theory, shocks induced a hyperpolarization in cells facing the anode and a depolarization in the cells facing the cathode.26 Contrary to the linear cable theory, however, the effects of shocks on Vm were asymmetrical. When shocks were applied during action potential plateau, the magnitude of maximal hyperpolarization measured at one boundary of a strand was much larger than the magnitude of maximal depolarization measured at the opposite strand boundary. The effect of AVm asymmetry was voltage dependent. When shocks were applied in the repolarization phase, the Vm changes were symmetrical. The asymmetry of AVm indicates a strong nonlinearity in the Vm response to a defibrillation shock. A similar nonlinear response of Vm to electrical shocks was also observed in experiments on intact cardiac muscle.33,34 Two other studies were performed to investigate the mechanism of AVm asymmetry.35,36 In the first, the Vm responses to shocks were investigated as a function of strand width and shock strength.35 It was found that depending on these parameters, 3 different types of Vm responses could be produced. Relatively weak shocks applied to narrow strands induced the simplest symmetrical type of Vm changes. Figure 8 shows an example in which a 1.9-V/cm shock was delivered across a 0.15-mm-wide strand during the action potential plateau. The shock induced a depolarization in cells facing the cathode and a hyperpolarization in the cells facing the anode. The shape of the shock-induced AVm (panel C) was very similar to the rectangular shape of the shock waveform. The map of AVm distribution (panel B) was uniform and the transition from hyperpolarization to depolarization was gradual and linear (panel D). Overall, the AVm of this type reflects the redistribution of current supplied by the shock electrodes

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Figure 8. Symmetrical shock-induced change in transmembrane potential (AVm) in cell strands. A. Image of a cell strand with mask outlining the photodiode array. B. Isopotential map of AVm (%action potential amplitude [APA]) induced by 1.9-V/cm shock. Isopotential lines are drawn at an interval of 5%APA. C. Optical recordings of Vm during action potential upstroke and shock application. The numbers correspond to the photodiodes in A. E = recording of shock field in the bath. D. Spatial distribution of AVm measured 2 ms after the shock onset. Distance 0 corresponds to the center of photodiode 1 in A. Reproduced from reference 35, with permission.

according to the predictions of the linear cable model. Two other types of Vm responses were observed as shock strength or strand width were increased. Figure 9 illustrates AVm induced by shocks with a strength of 27 V/cm (panel B, thin traces) and 39 V/cm (panel B, thick traces) in a 0.15-mm strand. In the case of the 27-V/cm shock, the AVm

distribution across the strand was time dependent. During the early phase of the shock, the AVm distribution was symmetrical, as illustrated by the AVm profile measured 0.3 ms after the shock onset (panel C, dashed line). This indicates that, similar to effect of weak shocks (Figure 8), the initial response of Vm to the stronger shock was passive and linear.

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Figure 9. Asymmetrical shock-induced change in transmembrane potential (AVm). A. Image of a cell strand with grid of photodiodes. B. Recordings of AVm during application of 27-V/cm shock (thin traces) and 39-V/cm shock (thick traces). Signals are normalized by action potential amplitude (not shown). The numbers correspond to the photodiodes in A. C and D. Profiles of spatial distribution of AVm at different times during shocks. Distance 0 corresponds to the center of the photodiode 1 in A. Reproduced from reference 35, with permission.

Soon after the shock onset, however, Vm at all points shifted toward more negative levels. As a result, the AVm distribution became asymmetrical with hyperpolarization much greater than depolarization, which is exemplified by the AVm profile at t = 3 ms. Such shifts of Vm to

more negative levels indicate a net increase of current in the outward direction, which can be caused by generation of an outward current within the strand or by a decrease of an inward current. When the shock strength was increased to 39 V/cm, a third type of AVm

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was observed (panel B, thick traces). Similar to the case with the 27-V/cm shock, very early changes in Vm were nearly symmetrical; this is illustrated by the AVm profile measured at t = 0.3 ms (panel D). At t = 3 ms, the AVm distribution became strongly asymmetrical. Later during the shock, however, the membrane potential shifted to more positive levels approaching the level measured with the weaker, 27V/cm shock. As a result of this shift, the degree of AVm asymmetry was reduced. This later shift of Vm to more positive levels indicates increased current flow in the inward direction. To determine more precisely the voltage dependence for different types of AVm, the absolute values of maximal positive and negative changes in Vm at opposite strand borders were measured 2 ms after the shock onset as a function of shock strength in the 0.15-mm strands (n = 14). These data are shown in Figure 10. The linear and symmetrical AVm (Type I) were observed when shock strength was less than ~9 V/cm and the maximal magnitudes of AV^ and AV^ were less than ~40%APA. The thin straight line extrapolates this linear dependence into areas of larger voltages. The asymmetrical AVm (Type II), that deviated from the passive linear dependence, were observed when AVm exceeded ~40%APA. The transition to AVm with hyperpolarization-induced positive shift of Vm (Type III) occurred when shock strength was increased above -27 V/cm and the maximal negative AVm exceeded ~200%APA. Mechanism of Nonlinear AVm The ionic mechanisms of nonlinear Vm responses to electrical shocks are not yet known. The AVm asymmetry with larger AV-m thanAV+mreflects an increase in the net outward current. Therefore, it was expected that AVm asymmetry was related to the flow of an outward potas-

sium current and that application of potassium channel blockers should reduce the degree of the AVm asymmetry. Contrary to this expectation, however, application of potassium channel inhibitors barium chloride (inward rectifier), dofetilide (delayed rectifier), and 4-AP (transient outward current) did not reduce the AVm asymmetry,35,36 indicating that none of these outward currents was responsible for the AVm asymmetry. Unexpectedly, it was found that the asymmetrical behavior of AVm was reversed by application of the calcium channel blocker nifedipine. Figure 11 shows the effect of nifedipine on AVm caused by a 10.8-V/cm shock (control) and by a 10.6-V/cm shock (nifedipine) in a strand 0.8 mm in width. The spatial distribution of AVm in both cases was uniform (panel B) and nonlinear (panel C) as expected from AVm in strands with width larger than electrotonic space constant (about 350 jum25). In the control, the maximal amplitudes of AV+ and AV-m were 57 and -200%APA (panel C), respectively, resulting in the asymmetry ratio of 3.5. Application of nifedipine caused a strong increase of the AV+ (to 100%APA), whereas the maximal AV^ only slightly increased (to -207%APA). The increase of AV+ by nifedipine reduced the asymmetry ratio from 3.5 to 2.1. Similar results were obtained in a total of 8 strands measuring 0.8 mm in width. Panel D presents statistical data from these measurements. Nifedipine caused a large and reversible increase of AV^ by 70% (P < 0.001), a small increase if AV~, and, therefore, a decrease of the AVm asymmetry from 2.52 ± 0.5 to 1.62 ± 0.2 (P < 0.001). The effect of nifedipine on AV^ and AVjn indicates that the AVm asymmetry was caused by the outward flow of ICa in the depolarized portions of strands. Normally, ICa is inward but it changes direction when Vm exceeds the ICa reversal potential. According to patch clamp

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307

Figure 10. Voltage dependence of transmembrane voltage (Vm) response to shocks. The graph shows the dependence of maximal positive and maximal negative changes in Vm (AVm) on shock strength in 0.15-mm strands (n = 14). The AVm were measured at the opposite strand borders 2 ms after shock onset. Thick solid lines depict 3-order polynomial fits of data. The thin solid line corresponds to the linear dependence of AVm observed at weak shocks. Vertical dashed lines separate ranges of linear and symmetrical Vm response (Type I), asymmetrical Vm response (Type II), and nonmonotonic Vm response with hyperpolarization-induced positive shift of Vm (Type III). Reproduced from reference 35, with permission.

studies, the ICa reversal potential in rat and rabbit myocytes is 45 to 50 mV.37'38 Therefore, positive AVm with magnitudes larger than 45 to 50 mV, such as shown in Figure 11, should be reduced by the outward flow of ICa, which explains how blocking of ICa with nifedipine increases AV^. The effect of AVm asymmetry might have important implications for defibril-

lation in the whole heart. During fibrillation, most of the myocardium is in the depolarized state. Therefore, the effects of defibrillation shocks on Vm are predicted to be asymmetrical, with a larger portion of myocardium undergoing negative Vm (hyperpolarization) changes than positive Vm changes (depolarization). It has been shown that an interaction between areas

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Figure 11. Effect of nifedipine on shock-induced change in transmembrane potential (AVm). A. Optical recordings of Vm from selected diodes and shock waveform taken in control and during nifedipine application in strand with width of 0.8 mm. The corresponding shock strengths were 10.8 V/cm and 10.6 V/cm. B. Isopotential maps of AVm distribution 5 ms after the shock onset. Thick lines depict the zero isoline, which separates areas of depolarization and hyperpolarization. C. Spatial profiles of AVm across the strand. D. Effect of nifedipine on optical AV+, AV~, and the asymmetry ratio AV^/AV+ in 8 strands. Shock strength was 9.3 ± 0.8 V/cm. Reproduced from reference 36, with permission.

OPTICAL MAPPING OF EFFECTS OF DEFIBRILLATION SHOCKS of hyperpolarization and depolarization might determine the success or the failure of defibrillation.39 Because the asymmetry in AVm affects the size and the shape of the areas of hyperpolarization and depolarization, it might affect the outcome of a defibrillation shock. Knowledge of ionic mechanisms involved in shock-induced AVm might provide an opportunity for pharmacological modulation of AVm and, therefore, of defibrillation efficacy.

Conclusions These studies demonstrate the feasibility of the approach that uses voltage-sensitive dyes and the optical mapping technique to study the mechanisms of shock-induced changes of Vm. Experiments carried out in cell cultures indicate that smallest resistive discontinuities related to individual cell boundaries are unlikely to play a significant role in defibrillation. Larger discontinuities in tissue structure such as intercellular clefts and strand boundaries produce substantial changes in Vm and might contribute to tissue excitation and defibrillation. Shock-induced changes in Vm are strongly nonlinear and are a result of dynamic interaction between several factors, including active and passive properties of cell membrane, myocardial structure, as well as applied electrical field.

References 1. Weidmann S. Electrical constants of trabecular muscle from mammalian heart. J Physiol (Lond) 1970;210:1041-1054. 2. Kleber AG, Riegger CB. Electrical constants of arterially perfused rabbit papillary muscle. J Physiol (Lond) 1987;385: 307-324. 3. Rattay F. Analysis of models for external stimulation of axons. IEEE Trans Biomed Eng 1986;33:974-977. 4. Rattay F. Analysis of models for extracellular fiber stimulation. IEEE Trans Biomed Eng 1989;36:676-682.

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5. Sobie E, Susil R, Tung L. A generalized activating function for predicting virtual electrodes in cardiac tissue. Biophys J 1997;73:1410-1423. 6. Muzikant A, Henriquez C. Bipolar stimulation of a three-dimensional bidomain incorporating rotational anisotropy. IEEE Trans Biomed Eng 1998;45:449462. 7. Plonsey R, Barr R. Inclusion of junction elements in a linear cardiac model through secondary sources: Application to defibrillation. Med Biol Eng Comput 1986; 24:137-144. 8. Plonsey R, Barr R. Effect of microscopic and macroscopic discontinuities on the response of cardiac tissue to defibrillating (stimulating) currents. Med Biol Eng Comput 1986;24:130-136. 9. Krassowska W, Pilkington T, Ideker R. Periodic conductivity as a mechanism for cardiac stimulation and defibrillation. IEEE Trans Biomed Eng 1987;34:555560. 10. Plonsey R, Barr RC, Witkowski FX. Onedimensional model of cardiac defibrillation. Med Biol Eng Comput 1991;29:465469. 11. Sommer JR, Scherer B. Geometry of cell and bundle appositions in cardiac muscle: Light microscopy. Am J Physiol 1985;17: H792-H803. 12. Le Grice IJ, Smaill BH, Chai LZ, et al. Laminar structure of the heart: Ventricular myocyte arrangement and connective tissue architecture in the dog. Am J Physiol 1995;38:H571-H582. 13. Trayanova N, Skouibine K, Aguel F. The role of cardiac tissue structure in defibrillation. Chaos 1998;8:221-233. 14. Wikswo J. Tissue anisotropy, the cardiac bidomain, and the virtual electrode effect. In: Zipes DP, Jalife J (eds): Cardiac Electrophysiology: From Cell to Bedside. Philadelphia: W.B. Saunders Co.; 1995: 348-361. 15. Roth B. Mechanisms for electrical stimulation of excitable tissue. Crit Rev Biomed Eng 1994;22:253-305. 16. Keener JP. Direct activation and defibrillation of cardiac tissue. J Theor Biol 1996;178:313-324. 17. Pumir A, Krinsky VI. How does an electric field defibrillate cardiac muscle? Physica D 1996;91:205-219. 18. Rohr S, Scholly DM, Kleber AG. Patterned growth of neonatal rat heart cells in culture.

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Morphological and electrophysiological characterization. Circ Res 1991;68:114-130. 19. Rohr S, Kucera JP, Fast VG, Kleber AG. Paradoxical improvement of impulse conduction in cardiac tissue by partial cellular uncoupling. Science 1997;275:841-844. 20. Fast VG, Kleber AG. Anisotropic conduction in monolayers of neonatal rat heart cells cultured on collagen substrate. Circ Res 1994;75:591-595. 21. Fast VG, Darrow BJ, Saffitz JE, Kleber AG. Anisotropic activation spread in heart cell monolayers assessed by high-resolution optical mapping: Role of tissue discontinuities. Circ Res 1996;79:115-127. 22. Fast VG, Kleber AG. Microscopic conduction in cultured strands of neonatal rat heart cells measured with voltage-sensitive dyes. Circ Res 1993;73:914-925. 23. Grinvald A, Frostig RD, Lieke E, Hildesheim R. Optical imaging of neuronal activity. PhysiolRev 1988;68:12851367. 24. Fast VG, Kleber AG. Cardiac tissue geometry as a determinant of unidirectional conduction block: Assessment of microscopic excitation spread by optical mapping in patterned cell cultures and in a computer model. Cardiovasc Res 1995;29: 697-707. 25. Jongsma HJ, van Rijn HE. Electrotonic spread of current in monolayer cultures of neonatal rat heart cells. J Membr Biol 1972;9:341-360. 26. Gillis AM, Fast VG, Rohr S, Kleber AG. Effects of defibrillation shocks on the spatial distribution of the transmembrane potential in strands and monolayers of cultured neonatal rat ventricular myocytes. CircRes 1996;79:676-690. 27. Hoyt RH, Cihen ML, Saffitz JE. Distribution and three-dimensional structure of intercellular junctions in canine myocardium. CircRes 1989;64:563-574. 28. Zhou X, Knisley SB, Smith WM, et al. Spatial changes in the transmembrane potential during extracellular electric stimulation. CircRes 1998;83:1003-1014. 29. Fast VG, Rohr S, Gillis AM, Kleber AG. Activation of cardiac tissue by extracellular electrical shocks: Formation of "secondary

sources" at intercellular clefts in monolayers of cultured myocytes. CircRes 1998; 82:375-385. 30. Luo CH, Rudy Y. A model of the ventricular cardiac action potential. Depolarization, repolarization, and their interaction. CircRes 1991;68:1501-1526. 31. Spach MS, Dolber PC. Relating extracellular potentials and their derivatives to anisotropic propagation at a microscopic level in human cardiac muscle. Evidence for electrical uncoupling of side-to-side fiber connections with increasing age. Circ Res 1986;58:356-371. 32. White J, Walcott G, Pollard A, Ideker R. Myocardial discontinuities: A substrate for producing virtual electrodes to increase directly excited areas of the myocardium by shocks. Circulation 1998;97:17381745. 33. Zhou XH, Rollins DL, Smith WM, Ideker RE. Responses of the transmembrane potential of myocardial cells during a shock. J Cardiovasc Electrophysiol 1995;6: 252-263. 34. Zhou X, Smith W, Rollins D, Ideker R. Transmembrane potential changes caused by shocks in guinea pig papillary muscle. Am JPhysiol 1996;271:H2536-H2546. 35. Fast VG, Rohr S, Ideker RE. Non-linear changes of transmembrane potential caused by defibrillation shocks in strands of cultured myocytes. Am JPhysiol 2000; 278:H688-H697. 36. Cheek E, Ideker R, Fast V. Non-linear changes of transmembrane potential during defibrillation shocks: Role of Ca2+ current. Circ Res 2000;87:453-459. 37. Yuan W, Ginsburg KS, Bers DM. Comparison of sarcolemmal calcium channel current in rabbit and rat ventricular myocytes. JPhysiol 1996;493:733-746. 38. Gomez JP, Potreau D, Branka JE, Raymond G. Developmental changes in Ca2+ currents from newborn rat cardiomyocytes in primary culture. Pflugers Arch 1994;428:241-249. 39. Efimov I, Cheng Y, Van Wagoner D, et al. Virtual electrode-induced phase singularity. A basic mechanism of defibrillation failure. CircRes 1998;82:918-925.

Chapter 14 Effects of Pharmacological Interventions on Reentry Around a Ring of Anisotropic Myocardium: A Study with High-Resolution Epicardial Mapping Josep Brugada, MD, PhD, Lucas Boersma, MD, and Maurits Allessie, MD, PhD

Introduction Reentry has been recognized as the mechanism of most clinically relevant arrhythmias. It is not surprising though that increasing efforts have been directed toward understanding the mechanisms by which different therapeutic options might modify a reentrant pathway. In this chapter we review the different mapping studies that we have performed concerning the effects of several pharmacological interventions on a simplified model of reentry around a ring of anisotropic myocardium in Langendorff-perfused rabbit hearts. Description of the Experimental Model Our experimental model consists of a ring of healthy epicardium in the left

ventricle (LV) of Langendorff-perfused rabbit hearts. Briefly, once the heart has been removed from the animal and connected to a Langendorff perfusion system, the right ventricle (RV), the interventricular septum, and the endocardial and intramural layers of the LV are destroyed by an endocardial cryoprocedure described extensively elsewhere. As a result, only a thin layer of approximately 1 mm thick of LV epicardium survives. The rest of the myocardium is completely destroyed. A cryoprobe is then applied through the epicardium and an area of approximately 20 x 10 mm is destroyed in the surviving layer. The result is a perfused ring of epicardium in the LV. We have previously shown that application of programmed electrical stimulation induces a sustained and stable reentrant rhythm around the ring.1

From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; e2003.

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Mapping Technique

and analog-to-digital converted into a single digital signal. A computerized system was then used to store, analyze, and generate color-coded activation maps.

For high-resolution mapping of epicardial excitation, a mapping system developed in the Department of Physiology at the University of Limburg in Maastricht, The Netherlands was used.2 A spoon- Characteristics of the Reentrant shaped electrode containing 248 individual Ventricular Tachycardia electrodes at regular distances of 2.25 mm allowed continuous and simultaneous Epicardial mapping showed that recording of 248 unipolar electrograms tachycardias induced during programmed covering the whole LV epicardium. The electrical stimulation were based on conindividual leads converged into a main tinuous reentry of the electrical impulse cable and were connected to an amplifica- around the ring.1 In Figure 1, an example tion unit containing 248 different amplifiers. of initiation of ventricular tachycardia (VT) After amplification and filtering (bandwidth by 2 premature beats is given. Ten selected 1 to 400 Hz), the analog signals were sam- electrograms around the obstacle and 4 actipled by multiplexors (1-kHz sampling rate) vation maps during induction of VT are

Figure 1. Ten electrograms around the ring and 4 activation maps are shown during initiation of reentrant ventricular tachycardia. The 4 consecutive activation maps are obtained during application of the last of a series of 10 stimuli given at a regular cycle of 250 ms (S1), the first premature beat (S2), the second premature beat (S3), and the first reentrant beat. Numbers indicate local activation time in milliseconds. Isochrones are drawn at 10-ms intervals. Arrows indicate direction of propagation. Double bars during S1 and S2 indicate collision of opposite wavefronts, and during S2 the double bars indicate unidirectional block. LAD = left anterior descending coronary artery. Empty area at the center of the map indicates the central anatomical obstacle.

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Figure 2. Activation maps and a single electrogram during 2 different reentrant tachycardias with cycle lengths of 136 and 195 ms, respectively. Encircled numbers indicate the position of the electrograms in the circuit. Abbreviations and symbols as in Figure 1.

shown. During S: (basic pacing at 250-ms intervals) and S2 (first premature beat given 154 ms after S^, the impulse propagated from the site of stimulation around both sides of the ring to collide in the corridor between the left anterior descending coronary artery and the obstacle. During S3 (second premature beat given 116 ms after S2), the impulse propagating counterclockwise blocked in the base of the LV. The clockwise wavefront continued to propagate and arrived at the site of unidirectional block 150 ms after block occurred. The cells proximal to the line of block already recovered their excitability and the impulse continued to propagate in a clockwise reentrant pathway. The resulting sustained VT had a regular cycle length (CL) of 128 ms. In different hearts, the CL of VT ranged between 121 and 224 ms (mean 165 ± 17 ms) and strongly depended on the size of the

obstacle. During each experiment, the tachycardia CL was very regular with a maximal variation in time of ±3 ms. In Figure 2,2 reentrant VTs with a regular CL of 136 and 195 ms are shown. Due to the anisotropic characteristics of the ventricular myocardium, conduction velocity was different in different segments of the ring depending on the angle between the fiber orientation and the direction of propagation of the impulse. In segments of the ring where the excitation wave propagated parallel to the epicardial fiber direction (base and lateral wall of the LV), longitudinal conduction velocity was approximately 60 cm/s. In segments of the ring where the excitation wave propagated perpendicular to the epicardial direction (corridor between the left anterior descending coronary artery and the obstacle), transverse direction velocity was 3 times slower (20 cm/s).

314 CAKDIAC MAPPING As is shown later in this chapter, this anisotropic distribution in conduction velocity during tachycardia plays an important role in modulating the effects of different pharmacological interventions.3-11 Effects of Pharmacological Interventions on Reentry We tested the effects of several pharmacological interventions administered during VT in this simplified model of reentry, including increasing doses of some well-known Class I antiarrhythmic drugs (propafenone and flecainide), a local anesthetic with Class I properties (bupivacaine), a new Class I drug (barucainide), a pure sodium channel blocker (tetrodotoxin), a new Class III antiarrhythmic drug (RP62719), extracellular potassium, an electrical uncoupler (heptanol), and a potassium channel opener. Table 1 shows the effects of the different interventions on the VT CL. All drugs with Class I effects (flecainide, propafenone, barucainide, and bupivacaine), the cellular uncoupler heptanol, and high concentrations of extracellular potassium significantly prolonged the VT CL and, at a given dosage, terminated

reentry in all hearts. The Class III drug RP62719 did not significantly prolong the VT CL, and reentry terminated in only 4 of 8 hearts tested. Administration of the potassium channel opener shortened the VT CL in 2 of 4 hearts, did not modify it in the remaining 2 hearts, and no VT termination was observed. Drugs or Pharmacological Interventions that Decrease the Number of Sodium Channels Available During the Phase 0 of the Action Potential: Class I Drugs, Tetrodotoxin, and Extracellular Potassium These pharmacological interventions can be discussed together since they all had similar electrophysiological effects on VT. The administration of these drugs resulted in a dose-dependent increase in CL of VT until reentry terminated. Figure 3 shows 18 electrograms around the ring and 2 activation maps during control and at the moment of tachycardia termination during the administration of 7 mg/L of flecainide. The tachycardia CL, which was 199 ms during control, prolonged to 575 ms just prior to termination. The increase in

Table 1 Effects of Pharmacological Interventions in Ventricular Tachycardia Drug

n

CL Control

Propafenone Flecainide Barucainide Bupivacaine Tetrodotoxin Potassium Heptanol RP62719 K-opener

5 5 5 8 6 10 10 8 4

142 ±12 1481115 153±11 152 ±16 167117 142 + 13 144 ±13 165117 128 ±7

CL Drug 1134198 873 1 75 433 + 46 684 + 78 7931122 774 + 378 4881118 182 ±24 11915

Termination 5/5 5/5 5/5 8/8 6/6 10/10 10/10 4/8 0/4

Ventricular tachycardia cycle length is compared during control (CL control) and after administration of the maximal dose of the drug or prior to termination of tachycardia (CL drug). All numbers are represented in milliseconds as mean ± SD. Termination means the number of tachycardias that did terminate during administration of the drug.

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Figure 3. Eighteen electrograms around the ring during control (tachycardia cycle length 199 ms) and administration of 7 mg/L of flecainide. Prior to termination, tachycardia cycle length prolonged to 575 ms. Termination of tachycardia occurred because of complete conduction block of the circulating impulse between electrograms 13 and 14.

CL was the result of a dose-dependent depression in conduction velocity around the ring. However, this depressing effect on conduction velocity was not homogenous in all segments of the ring. A predominant effect was observed in those segments in which conduction velocity was faster during control, that is, where conduction was parallel to the epicardial fiber direction. This is illustrated in Figure 4. During control (potassium 4 mmol/L), the tachycardia CL was 134 ms. Longitudinal

conduction velocity was 66 cm/s. During administration of 6, 10, and 12 mmol/L of potassium, the CL prolonged to 142, 215, and 467 ms, respectively. Longitudinal conduction velocity was depressed by approximately 5 times more than transverse conduction velocity. Thus, the first conclusion of these results is that drugs with Class I properties prolong the VT CL in a dose-dependent manner. At a given dose, reentry can no longer proceed and tachycardia terminates. This effect was

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Figure 4. Activation map during control tachycardia (potassium 4 mmol/L) and administration of 6, 10, and 12 mmol/L potassium. Cycle length increased from 134 to 467 ms. Abbreviations and symbols as in Figure 1.

common to all drugs with Class I proper- resulted in a marked prolongation in tachyties and was related to a predominant cardia CL and, at a given dose, reentry could action on longitudinal conduction velocity. no longer proceed and tachycardia terminated. However, as for drugs with Class I Cellular Uncouplers: Heptanol effects, this prolongation in tachycardia CL was the result of a predominant depression The administration of heptanol pro- on transverse conduction velocity. This is duced a dose-dependent prolonging effect illustrated in Figure 5. During control, the on tachycardia CL. As for drugs with Class tachycardia CL was 147 ms. Longitudinal I properties, increasing doses of heptanol conduction velocity was 69 cm/s and

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Figure 5. Activation map during control and administration of 1, 2, and 3 mmol/L of heptanol. Cycle length prolonged from 147 to 345 ms. Crowding of isochrones indicates a preferential effect on transverse conduction. Abbreviations and symbols as in Figure 1.

transverse conduction velocity was 21 ms. During administration of 1,2, and 3 mmol/L of heptanol, the CL prolonged to 185, 292, and 345 ms, respectively. Longitudinal conduction velocity was similar, whereas transverse conduction velocity had been drastically reduced to less than 10 cm/s. It can thus be concluded that the administration of heptanol produces a dose-dependent prolongation in tachycardia CL. This effect

is related to a predominant depression in transverse conduction velocity. Drugs that Prolong the Refractory Period of the Cells Involved in the Circuit: Class III Drugs These drugs had a completely different effect on reentry than sodium channel

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blockers or electrical uncouplers. They did not modify conduction velocity; however, by increasing the refractory period, they shortened the excitable gap. If the excitable gap became short enough to allow collision of the head of the propagating impulse with its own tail of refractoriness, termination of tachycardia occurred. Conversely, if the increase in refractoriness was not enough to close the excitable gap, tachycardia persisted. Drugs that Shorten the Refractory Period of the Cells Involved in the Circuit: Potassium Channel Openers The administration of drugs with potassium channel opener properties results in a shortening in the duration of the action potential and, thus, a decrease in the effective refractory period of the cells. In 2 of the 4 tachycardias tested (those with the shorter CL during control), administration of the drug resulted in a slight but significant shortening of the CL. This suggests that due to the short CL during control, the head of the circulating impulse propagated into partially refractory tissue. Thus, these tachycardias had no fully excitable gap. Shortening of the refractory period by the drug resulted in the creation of a fully excitable gap, allowing the circulating impulse to propagate through completely recovered tissue and thus speeding up conduction velocity. In the remaining 2 tachycardias, a fully excitable gap was already present during control and a decrease in the refractory period did not modify conduction velocity because the impulse was already propagating through totally recovered tissue. Mechanisms of Termination of Reentrant VT by Various Pharmacological Interventions In all cases, drugs with Class I properties and cellular uncouplers terminated

VT when a given dose was reached. This suggests that when the depressing effect in conduction velocity produced by these drugs reaches a certain level, the impulse can no longer propagate, and block in conduction occurs resulting in tachycardia termination. This mechanism of termination was true in the majority of cases.5 In Figure 6, an example is shown during administration of potassium and heptanol in the same heart. After administration of 4 mmol/L of heptanol, the CL prolonged to 390 ms and depression in conduction velocity was so important in the corridor between the left anterior descending coronary artery and the obstacle that the impulse could no longer proceed and tachycardia terminated. Block in conduction was observed in an area of the circuit where during control conduction velocity was slower (transverse conduction). However, in the same heart, after administration of 23 mmol/L of potassium the CL prolonged to 386 ms and depression in conduction velocity was then so important in the free wall of the LV that tachycardia terminated. Termination now occurred in an area of fast conduction (longitudinal conduction). Modification in conduction properties by these drugs was not homogeneous. In some cases we observed that the depression in conduction velocity resulted in the creation of arcs of functional conduction block not extending along the whole width of the ring. The occurrence of these arcs of functional block had some important consequences in determining the mechanism of termination of tachycardia. If conduction around one of these arcs of functional conduction block was slow enough to allow recovery of the cells situated proximal to the line of block, reentry of this area could occur and initiate an antidromic wavefront. Collision of this antidromic wavefront with the ongoing orthodromic wave might then result in termination of tachycardia. This is shown in Figures 7 and 8. During control, tachycardia had

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Figure 6. Activation maps during termination of tachycardia during administration of 4 mmol/L of heptanol and during administration of 12 mmol/L of potassium in the same heart. Control tachycardia had a cycle length of 147 ms. During administration of 4 mmol/L of heptanol, the cycle length was prolonged to 390 ms just prior to termination. Termination of reentry occurred because of complete conduction block of the impulse at the area of slow transverse conduction. During administration of 12 mmol/L of potassium the cycle length was prolonged to 386 ms just prior to termination. Termination now occurred because of complete conduction block of the impulse at the area of fast longitudinal conduction. Abbreviations and symbols as in Figure 2.

a CL of 130 ms; after administration of 30 umol/L of tetrodotoxin, CL was prolonged to 672 ms. A long arc of functional block occurred in the base and free wall of the LV. The impulse propagated around the bottom end of this arc of block and initiated propagation in the opposite direction in the circuit. However, the time difference between the orthodromic and antidromic wavefronts was not long enough to allow recovery of cells situated proximal to the block line and bidirectional

block occurred in this area. During the next cycle, the impulse again initiates an antidromic wavefront that propagates slower than in the previous cycle, and thus the time difference at the line of block is long enough to allow recovery of the cells proximal to the block. Reentry of this area occurs and the antidromic wavefront propagates around the circuit. Because the ongoing orthodromic wavefront still propagates, collision of both the antidromic and the orthodromic wavefronts results in

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Figure 7. Activation maps during control and administration of 30 mmol/L of tetrodotoxin. During control, tachycardia had a cycle length of 130 ms. During administration of 30 mmol/L of tetrodotoxin, the cycle length was prolonged to 672 ms 2 cycles prior to termination. Multiple arcs of functional conduction block are now apparent, especially a long line of block at the atrial free wall. The clockwise wavefront proceeds around this line of block and reactivates the area distal to the block in a counterclockwise direction. During the next cycle, this line of block extends toward the apex and conduction around the line results in a delay sufficient to allow recovery of cells proximal to the block. The clockwise wavefront continues to propagate and collides with the counterclockwise wavefront that results from microreentry within the macroreentrant circuit.

termination of the tachycardia. We call this phenomenon echo-wave termination.5 Echo-wave termination occurred during administration of all drugs with Class I effects, heptanol, and Class III drugs.

Implications of These Results Pharmacological interventions modify conduction properties in a reentrant pathway depending on their electrophysiological

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Figure 8. Same example as in Figure 7, now showing the 21 electrograms around the ring shown in control panel of Figure 7 by encircled numbers.

effects. Drugs that decrease the number of sodium channels available during phase 0 of the action potential and thus depress the Vmax, produce a marked slowing in conduction velocity. This results in an increase in the tachycardia CL and termination of reentry. The same effect was observed with drugs that predominantly modify the passive membrane properties, like heptanol, by increasing intercellular resistance. As has been demonstrated, however, sodium channel blockers and heptanol modify a different segment in the reentrant circuit depending on the anisotropic properties of the tissue.12 Thus, depending on the drug used, we can target our effect to a specific segment of the circuit. Heptanol cannot be used as an antiarrhythmic because of its toxic effects when administered in vivo. It is not unlikely that the development of new drugs with a similar electrophysiological effect but without toxic side effects may introduce a new generation of antiarrhythmic drugs that could be used to specifically modify the zone of slow conduction in the reentrant circuit. In our model, Class III drugs terminated only those tachycardias with a rather short excitable gap. This suggests that Class III drugs might only be effective in

terminating fast VTs based on functional rather than anatomical reentry. These results do not preclude a preventing effect on tachycardia inducibility. We have previously demonstrated that the initiation of tachycardia in this model occurred when very short coupled premature beats were applied to areas of the circuit with a short refractory period. It can be speculated that prolongation in refractoriness of the myocardium avoids the occurrence of these very short coupled premature beats, which might be the ones initiating reentry. Administration of potassium channel openers has some interesting physiological implications. It is difficult during fast tachycardias to measure the excitable gap. It is even more difficult to measure the fully and the nonfully excitable gap. In the 4 experiments we performed, shortening of the refractory period resulted in acceleration of the faster tachycardias, suggesting that a fully excitable gap was not present. On the contrary, in the 2 slower tachycardias, in which an excitable gap could be demonstrated by the resetting response, the lack of acceleration during administration of the drug suggested that the gap was fully excitable. In the future, it will be interesting to study the effects of these drugs in the so-called functional

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reentrant tachycardias in order to characterize the electrophysiological properties of these circuits. In this model, using high-resolution epicardial mapping, it could be demonstrated that areas of very slow conduction could be the result of what has been called zigzag conduction. Clinical mapping of tachycardias with a transvenous approach is far less accurate. Areas in which large conduction delays are demonstrated are usually taken as areas of conduction block. It is possible that some of them are in fact areas of zigzag conduction, absolutely necessary to the circuit, and probably the ones to be targeted during nonpharmacological treatments, especially catheter ablation. In summary, high-resolution epicardial mapping is a useful tool in the study of conduction properties of the electrical impulse. The use of more complex and sophisticated mapping systems both experimentally and clinically will probably allow recognition of new and unsuspected features during reentry. References 1. Brugada J, Boersma L, Kirchhof C, et al. Reentrant excitation around a fixed obstacle in uniform anisotropic ventricular myocardium. Circulation 1991;84:1296— 1306. 2. Brugada J, Mont LL, Boersma L, et al. Differential effects of heptanol, potassium and tetrodotoxin on reentrant ventricular tachycardia around a fixed obstacle in anisotropic myocardium. Circulation 1991; 84:1307-1318. 3. Brugada J, Boersma L, Kirchhof C, et al. Double-wave reentry as a mechanism of

ventricular tachycardia acceleration. Circulation 1991;84:1633-1643. 4. Brugada J, Brugada P, Boersma L, et al. On the mechanisms of ventricular tachycardia acceleration during programmed electrical stimulation. Circulation 1991;82: 1621-1629. 5. Brugada J, Boersma L, Abdollah H, et al. Echo-wave termination of ventricular tachycardia. A common mechanism of termination of reentrant arrhythmias by various pharmacological interventions. Circulation 1992;85:1879-1887. 6. Brugada J, Boersma L, Kirchhof C, et al. Proarrhythmic effects of flecainide. Experimental evidence for increased susceptibility to reentrant arrhythmias. Circulation 1991;84:1808-1818. 7. Brugada J, Brugada P, Boersma L, et al. Value of an anisotropic model of ventricular tachycardia in the screening of antiarrhythmic drugs. Pacing Clin Electrophysiol 1989;12:102. 8. Brugada J, Boersma L, Kirchhof C, et al. Induction of reentry is related to the refractory period at the site of stimulation. Pacing Clin Electrophysiol 1990; 13: 512. 9. Brugada J, Boersma L, Escande D, et al. Mechanism of action of Class III antiarrhythmic drugs. J Am Coll Cardiol 1991; 17: 42A. 10. Boersma L, Brugada J, Kirchhof C, et al. Drug-induced acceleration of reentrant ventricular tachycardia by double-wave reentry. Importance of the wavelength. Eur Heart J 1991;12:146. 11. Allessie M, Hoeks APG, Schmitz GML, et al. On-line mapping system for the visualization of the electrical activation of the heart. Int J Card Imaging 1987;2:59. 12. Spach MS, Dolber PC, Heidlage JF. Influence of the passive anisotropic properties on directional differences in propagation following modification of the sodium conductance in human atrial muscle. Circ Res 1988:62:811.

Chapter 15

Microscopic Discontinuities as a Basis for Reentrant Arrhythmias Madison S. Spach, MD

level allows excitation wavefronts to be represented by isochrones and to be ana5 During the past 2 decades the con- lyzed for wavefront curvatures and for 6 7 cept of discontinuous conduction has spiral waves. - Second, normal uniform gained acceptance as a paradigm for anisotropic cardiac bundles have extenstructural biophysical mechanisms of sive electrical coupling between cells and normal and abnormal propagation events groups of cells, which provides a protecthat lead to cardiac reentrant arrhyth- tive effect against the initiation of reen4 mias.1'2 It has also become clear that dis- trant arrhythmias. On the other hand, continuous anisotropic propagation has side-to-side electrical connections between a dual nature dependent upon the size cells are lost when there is deposition of scale at which the propagating events are collagenous septa in interstitial space, evaluated.3 Examples of this dual char- which occurs following myocardial infarc8 9 acter of discontinuous anisotropic con- tion and with aging. At that point the duction include the following: First, at a electrical loading effects become distribmicroscopic level the events of conduc- uted over many cells and the structure 10 tion appear stochastic (nonuniform and becomes proarrhythmic. Based on the foregoing, it now appears irregular) due to electrical loading effects produced by the complex microscopic that an integral part of a complete theory structure of cardiac muscle.4 However, of normal and abnormal cardiac conducthese effects at a microscopic level become tion will have to include electrical loading averaged at the larger macroscopic size mechanisms produced by the complex scale where conduction appears to be uni- architecture of cardiac muscle. These bioform and regular as occurs in a continu- physical mechanisms involve several comous medium. This feature at a macroscopic ponents of cardiac microstructure that Introduction

This work was supported by U.S. Public Health Service Grant HL 50537 and The North Carolina Supercomputing Center. From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; e2003.

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produce variable loading of the sarcolemmal membrane within individual cells and groups of cells.4,11 Consequently, an important goal for the future is to develop yet to be achieved high-resolution mapping of propagation events in naturally occurring cardiac bundles to demonstrate the manner by which the stochastic phenomena of discontinuous propagation at a microscopic level are translated into the smooth appearing macroscopic excitation waves like those that occur in a continuous medium. The purpose of this chapter, therefore, is to identify several features of the dual nature of discontinuous anisotropic conduction and to describe how propagating action potentials are affected by naturally occurring electrical remodeling of cardiac muscle secondary to changes in

the distribution of the gap junctions and by variations in the composition of interstitial space. Interpretation of Excitation Spread from Isochrones and Extracellular Waveforms in Uniform Anisotropic Normal Bundles The multidimensional spread of excitation at a macroscopic level in normal anisotropic ventricular muscle is depicted by isochrones in Figure 1A for propagation initiated at a single site.12 The pattern and shapes of the isochrones indicate fast propagation along the longitudinal axis of the fibers (LP) and slower propagation along an axis transverse to the orientation

Figure 1. A. Multidimensional excitation spread at a macroscopic level as represented by an isochrone map when propagation was initiated at a point stimulus (*) in a uniform anisotropic left ventricular preparation from an adult dog. The isochrones were drawn at 1-ms intervals. The first isochrone begins 2 ms from the onset of the stimulus. At the bottom of the isochrone map, the solid circles indicate the location of a microarray containing 11 unipolar extracellular electrodes with 100-(im center-to-center spacing. B. Original unipolar extracellular waveforms (Oe) and the first and second derivative extracellular waveforms (dOe/dt and d2Oe/dt2, respectively) recorded with the microarray. Reproduced from Spach MS, Heidlage JF, Darken ER, et al. Cellular Vmax reflects both membrane properties and the load presented by adjoining cells. Am J Physiol 1992;263:H1855-H1863, with permission of the American Physiological Society.

MICROSCOPIC DISCONTINUITIES AS A BASIS FOR REENTRANT ARRHYTHMIAS of the fibers (TP). The effective velocity along the longitudinal axis of the fibers was 0.51 m/s and in the transverse direction it was 0.17 m/s. These directionally different velocities produced an LP-to-TP velocity ratio of 3. The isochrones produced a picture at the macroscopic level that is consistent with the spread of excitation in a continuous anisotropic medium. A microarray of unipolar electrodes was located as shown at the bottom of Figure 1A to record the extracellular waveforms (<E>e) during LP and during the change from LP to TP at varying angles to the long axis of the fibers. The original Oe waveforms during LP (waveform #1) and TP (waveform #11) appear smooth in contour (Figure IB), which also is expected in a continuous anisotropic medium.13 Although the original Oe waveforms appeared smooth in contour, the first and second derivatives of the original Oe waveforms revealed small notches (dOe/dt and d2Oe/dt2, respectively, in Figure IB).12 Also, the notches increased in number as the amplitude of Oe decreased when propagation changed from along the longitudinal axis of the fibers to a direction across the fibers. Notches in the derivative waveforms indicate asynchronous excitation of small groups of cells at a microscopic level.3,9 Thus, the irregular shapes of the first and second derivatives of the original extracellular waveforms indicate that propagation was discontinuous in nature at a small size scale in this normal uniform anisotropic ventricular preparation. Electrical Loading in Normal Mature Myocardium at a Microscopic Level When one attempts to study the mechanisms of discontinuous conduction at a microscopic level in naturally occurring normal or diseased cardiac muscle, significant difficulties become evident.

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A major problem is that electrical loading, which greatly affects the safety factor of conduction,1,14 is strongly dependent on the nonuniform and irregular distribution of the electrical connections between cells. Consequently, electrical loading in a multicellular network is like the effective coupling coefficient between cells15 in that it cannot be measured directly. Further, this problem cannot be resolved by measurements in isolated cell pairs because their isolation removes the nonuniform loading produced by the stochastic distribution of their interconnections to other contiguous cells present in the intact cellular network. These difficulties likely account for the paucity of experimental information about the most fundamental features of cardiac excitation waves—the events associated with excitation spread within individual cells and the transfer of the excitatory impulse to neighboring cells in normal and abnormal cardiac muscle. The high-resolution optical mapping techniques used by Fast and Kleber16,17 and by Rohr18 provide an important recent experimental advance for the measurement of excitation spread at a microscopic level in synthetic neonatal cellular monolayers. However, at present it is not clear how to achieve the increased spatial resolution needed to evaluate electrical loading within individual myocytes, as well as the transfer of the excitation impulse from cell to cell, in naturally occurring cardiac muscle. Possibly combined optical and microelectrode measurements, along with high-resolution extracellular potential measurements like those in Figure 1, or the greater spatial resolution of Oe measurements achieved by Hofer et al.19 with thin-film sensors, will be a next experimental step. Increases in electrical load produce a decrease inV max , and decreases in electrical load are associated with increases in Vmax.4,20 Therefore, one experimental approach to evaluate spatial differences

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in electrical loading, as well as differences in load at a given site for different directions of conduction, is to measure Vmax at multiple sites and at each site change the direction of conduction from along the longitudinal axis of the fibers to a direction across the fibers. This method changes the complex geometric relationships between the impalement site, the

boundaries of the impaled and juxtaposed cells, and the associated contiguous gap junctions.12 Figure 2A shows the arrangement of 2 pairs of electrodes used to produce bidirectional longitudinal propagation (LP1 and LP2) and bidirectional transverse propagation (TP1 and TP2) at each of 17 centrally located microelectrode impalement sites in an uniform

Figure 2. A. Arrangement of 4 pairs of unipolar stimulus electrodes to produce 4-way plane wave conduction along the longitudinal and transverse axis of the fibers. The solid circles represent 17 microelectrode impalement sites at which 4-way conduction was produced. B. Histograms illustrate values of the maximum rate of rise of the transmembrane potential (Vmax) for one direction of longitudinal propagation (LP) and one direction of transverse propagation (TP) at each impalement site. C. Bar graph shows values of Vmax during one direction of LP and TP at each of the 17 microelectrode impalement sites. Reproduced from reference 12, with permission of the American Physiological Society.

MICROSCOPIC DISCONTINUITIES AS A BASIS FOR REENTRANT ARRHYTHMIAS anisotropic adult canine ventricular preparation. Figure 2B shows the histograms of the Vmax values obtained at each site for one direction of LP and for one direction of TP. The mean value of TP Vmax was greater than the mean value of LP Vmax (P < 0.001), as expected1; however, there was considerable variation in the values of Vmax in both directions. The histograms for each group (Figure 2B) show that a few of the lowest values of TP Vmax were in the same range of some of the LP Vmax values. The paired values of LP and TP Vmax for each of the 17 impalement sites are shown in the bar graph of Figure 2C. Note that at 2 sites LP Vmax exceeded TP Vmax (arrows), and some of the lowest values of LP Vmax occurred at the same sites that had the highest TP Vmax values. The major point illustrated by Figure 2 is that in mature uniform anisotropic bundles the distribution of electrical load on the sarcolemmal membrane is quite nonhomogeneous. Also, changes in electrical load are quite sensitive to the direction of propagation relative to the impalement site, as reflected in the differences of Vmax at the same site for different directions of conduction and at different sites for the same direction of conduction. It is interesting that these variations in electrical load produce undulating values of Vmax in any given direction of propagation, and it is the average Vmax value that is larger during TP than LP.12 Electrical Description of Myocardial Architecture and Its Application to Conduction Only recently have analyses of cardiac conduction begun to include the details of the arrangement of cardiac myocytes, their irregular shapes, and the associated nonuniform distribution of

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their gap junctions. Also, to this point, experimental mapping has not provided sufficient detailed data to produce a clear picture of the cellular events that occur with anisotropic conduction in naturally occurring preparations. Thus, electrical representation (models) of the microarchitecture of normal and abnormal cardiac muscle should become an integral part of the task to gain insight to propagation events at a cellular level. As an example, the effects of normal cellular structure on conduction have been shown in a recent 2-dimensional cellular model based on the nonuniform shapes of disaggregated adult ventricular canine myocytes. Details of the model are presented in prior papers.4'21 Variations in cell shape and in the topology of the gap junctions of any one model, however, represent only one of an infinite number of possible configurations. Thus, rather than emphasize a specific cardiac structure, the following results illustrate how the variable sizes and shapes of cardiac myocytes, along with the nonuniform arrangement of the gap junctions, have a major effect on propagation events at a microscopic level. Figure 3 shows diagrams of the formation of a 33-cell basic unit of the 2-dimensional cellular model.4 Intracellular space is represented by the cytoplasm of each myocyte surrounded by discrete cellular boundaries, and the myocytes are coupled together by gap junctions located in the intercalated disks and in areas juxtaposed to the disks as described by Hoyt et al.,22 i.e., plicate and interplicate gap junctions, respectively. This model, like several well-known 1-dimensional23,24 and 2-dimensional16,25 models, is a "monodomain" model in which the active membrane (sarcolemma) separates intracellular space from a large volume conductor of low resistance. Thus, the cellular model of Figure 3 does not include an electrical representation of the

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Figure 3. Diagrams showing the formation of a 33-myocyte basic unit to produce a 2-dimensional cellular model. A. Outlines of 5 myocytes and the manner they were fitted together to form a multicellular network. The stippled areas next to the intercalated disks represent interplicate gap junctions.22 The grids show 10-^im x 10-fim segments that represent the sarcolemmal membrane patches and the interior of each cell. B. Arrangement of 3 types of gap junctions (symbols) that electrically interconnect the 5 myocytes (a through e). C. Arrangement of 33 myocytes and the distribution of their intercellular connections (symbols). The 33-cell unit could be replicated longitudinally and vertically by fitting the ends and sides together to form cellular arrays of different sizes and shapes. The group of 5 myocytes (a through e) is highlighted within the 33-cell unit by marking each segment within these myocytes. Reproduced from Spach MS, Heidlage JF. The stochastic nature of cardiac propagation at a microscopic level. An electrical description of myocardial architecture and its application to conduction. Circ Res 1995;76:366-380, with permission.

interstitium that separates myocytes cellular boundaries was best depicted by beneath the surface of a cardiac bundle.11 perspective plots, which provided a view of the multidimensional spatial distribution of the activation times. A represenThe Nature of Excitation Spread tative result is presented in Figure 4 for Between Myocytes in the the 5 myocytes highlighted within the Multicellular Network 33-cell unit shown in Figure 3C. To examine cell-to-cell excitation spread during LP and TP, the times of Vmax were identified in each of the segments of 16 interconnected myocytes at the center of an array of 700 cells.4 The sensitivity of excitation spread to the

Longitudinal Propagation During LP, step increases in activation time (discontinuities) occurred at the end-to-end connections between myocytes

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Figure 4. Perspective plots of activation spread through a network of 5 myocytes within the 2-dimensional cellular model. The activation sequence was determined from the time of Vmax at each of the segments (8 rows of longitudinal marks) with the 5 myocytes that formed a group 80 urn wide and 300 |im long as shown at the top of the figure. A. Longitudinal propagation in a left-to-right direction. B. Transverse propagation in a bottom-to-top direction. In each perspective plot, the activation times along each of the 8 rows of segments are plotted as a function of distance along the long axis of the cells. The hatched steps denoted delays across the plicate gap junctions at the intercalated disks. In both panels the letters a through e are included to identify each myocyte with the activation times along the corresponding rows. Reproduced from Spach MS, Heidlage JF. The stochastic nature of cardiac propagation at a microscopic level. An electrical description of myocardial architecture and its application to conduction. Circ Res 1995;76:366-380, with permission.

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(Figure 4A). The major increases in activation time, however, occurred along the sarcolemmal membrane of each cell. The overall process resulted in a predominantly smooth pattern of activation spread. A major feature of LP was that the locations of the discontinuities of propagation along the longitudinal axis corresponded to the irregular distribution of the plicate gap junctions at the intercalated disks. These irregular delays at the end-to-end connections of the cells produced asynchrony of excitation in different portions of myocytes located side by side. Consequently, superimposed on the overall smooth process of LP, the nonuniformly distributed discontinuities of activation spread reflected the irregular shapes of the cells (Figure 4A).

Excitation Spread Within Individual Myocytes Longitudinal Propagation

Representative intracellular excitation sequences during LP are shown in Figure 5A for the 5 interconnected myocytes shown in Figure 4. The isochrones maintained a vertical orientation throughout each myocyte, except there was slight bending near the intercalated disks at the ends of the irregularly shaped cells. Within each cell, however, the isochrones shifted further apart as excitation moved from the subcellular area where the action potential entered the myocyte to the area where the action potential exited the myocyte. As a result, the major subcellular feature of LP was that conduction was slower in the proximal part and Transverse Propagation faster in the distal part of each cell.4 These events produced an alternating During TP, there were large lateral sequence of slower and faster conduction '^jumps'' of activation time between myocytes, along the pathway of longitudinal conwhile within each myocyte the entire sar- duction. Reversing the direction of longicolemma activated very rapidly (Figure 4B). tudinal conduction showed that the Further, there were a few prominent step subcellular differences in the speed of increases in activation time in the region conduction were not created by variations of the end-to-end plicate gap junctions in the cross-sectional area within each (Figure 4B, steps connecting cells c and e). myocyte. As shown in Figure 5A, when Therefore, the cell-to-cell pattern of trans- the direction of longitudinal conduction verse activation spread was quite differ- was reversed, the proximal part of each ent from that which occurred during LP. myocyte with respect to the direction of LP TP occurred as large jumps in activation remained the region of slowest conduction time between the lateral borders of jux- and the distal part of each cell remained taposed cells, while within each myocyte the region of fastest conduction. there was almost simultaneous activation of the entire sarcolemmal membrane. A general conclusion to be drawn Transverse Propagation from the results of Figure 4 is that for The major subcellular feature of actiany given direction of conduction, plane waves of excitation do not occur at a vation spread during TP was the rapidity microscopic level because of the disrup- with which excitation of each myocyte tion of the excitation wave by the irregu- occurred (Figure 5B). Unlike LP, during larly distributed cell boundaries and the TP the pattern of excitation spread was associated irregular locations of the gap different in each myocyte. Within the same cell, the isochrones were oriented junctions.4

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Figure 5. Intracellular activation sequences during 4-way propagation along the longitudinal and transverse axes of the myocytes. A. Isochrones within each of the myocytes (a through e) during propagation in both directions (arrows) along the longitudinal axis of the myocytes. The isochrones are separated by 4 (is. B. Intracellular isochrones during propagation in both directions (arrows) along the transverse axis of the cells. The isochrones are separated by 3 jus. Cells a through e are the same 5 myocytes shown in Figure 4. Reproduced from Spach MS, Heidlage JF. The stochastic nature of cardiac propagation at a microscopic level. An electrical description of myocardial architecture and its application to conduction. Circ Res 1995;76:366-380, with permission.

in different directions, and there were collisions (asterisks) inside a few cells when TP occurred in a top-to-bottom direction (cells a and c in Figure 5B, top). The pattern of excitation spread within

each cell changed markedly when the direction of conduction was reversed along the transverse axis of the myocytes (Figure 5B), and the collisions shifted to other cells; e.g., a collision occurred only

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in cell b when TP occurred in a bottom-to- (Figure 6, LP). When comparing the 2top direction. dimensional cellular model results to the experimentally observed variation in Vmax at differnt locations along the axis of conVmax Variations Within Adult duction (Figure 2C), it is reasonable to Ventricular Myocytes presume that in the experimental measurements the tip of the microelectrode Longitudinal Propagation varied randomly in its intracellular location relative to the ends of the impaled cell. During LP, Vmax was lowest in the prox- The cellular model results show that at difimal area of each cell (Figure 6, LP), where ferent locations within each cell the values intracellular conduction was slowest. Vmax of Vmax would be different due to the flucincreased to a maximum value between the tuation of Vmax within the individual cells. middle and distal fourth of each myocyte. Consequently, the cellular model results In the distal part of each myocyte, Vmax indicate that variations in electrical load decreased although subcellular conduction within individual cells can account for the was fastest in this area. During longitu- experimental variety of Vmax values observed dinal conduction, the Vmax minimum in at different impalements sites during LP. the output area of each cell had a higher That is, the variable values of Vmax within value than did the minimum at input area each myocyte resulted in an undulating

Figure 6. Subcellular distribution of Vmax within 3 myocytes during longitudinal and transverse propagation. In each panel the values of Vmax are plotted as a function of distance (|o.m) along a line of consecutive segments extending between the ends of the each cell. The outline of each myocyte with the accompanying intracellular activation sequences for transverse conduction (isochrones separated by 3 |is) is presented above each panel. A similar outline of the myocyte with the accompanying intracellular activation sequence for longitudinal conduction (isochrones separated by 4 |is) is presented below each panel. Myocytes a, c, and e are those of the previous 2 figures. Reproduced from Spach MS, Heidlage JF. The stochastic nature of cardiac propagation at a microscopic level. An electrical description of myocardial architecture and its application to conduction. Circ Res 1995;76:366-380, with permission.

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region of the highest value of Vmax. Consequently, in normal adult myocardium the greatest electrical load within each cell is concentrated at the intercalated disks near the ends of the cells where curTransverse Propagation rents are received and transferred to The same general subcellular pat- adjoining cells via the associated gap tern of Vmax variability occurred during junctions. TP as during LP; i.e., in each cell the maximum Vmax value occurred near the center Microfibrosis with Loss of of the myocyte and lower Vmax values Side-to-Side Cellular occurred at the ends of each cell. The Interconnections Alters the mean Vmax value within almost every cell was greater during TP than during LP. Curvature of Wavefronts However, the TP Vmax minima near the ends of each cell were often lower in value In the foregoing sections, adult carthan the LP Vmax maximum located near diac myocytes have been considered to form the center of each cell (cells c and e of networks in which each myocyte is conFigure 6). This feature resulted in con- nected to multiple surrounding cells.22'26'27 siderable overlap of TP and LP Vmax val- This normal arrangement produces tight ues when comparing Vmax values from electrical coupling in all directions relative different subcellular areas of the same or to the orientation of the fibers. However, different cells. The different TP Vmax values aging is associated with the deposition within each myocyte thereby were consis- of fine, longitudinally oriented collagetent with the experimental results, which nous septa (microfibrosis) that surround demonstrated considerable variation in the small groups of cells in atrial bundles TP Vmax values, and some of the TP Vmax (Figure 7), and coarse collagenous septa values overlapped those that occurred develop in healed myocardial infarcts.8 during longitudinal conduction. When the Because sarcolemmal membrane appopaired values of Vmax were compared for sition does not occur between cells sepeach membrane segment (Figure 6), TP arated by collagen, the deposition of Vmax was greater than LP Vmax through- collagenous septa in interstitial space out most myocytes. However, near the marks areas where there has been loss of ends of a few cells there was a reversal of side-to-side electrical connections over the usual TP > LP Vmax relationship, e.g., variable distances. The "normal" deposicell e in Figure 6. These areas near the ends tion of collagenous septa occurs in atrial of myocytes likely provide a subcellular bundles at 2 widely separated time peribasis for the experimental result that at ods—during the first months of life 28 and a few microelectrode impalement sites, during the aging process.9 This structural TP Vmax was found to be less than LP Vmax. arrangement produces nonuniform A major conclusion to be drawn from anisotropic electrical properties of carFigure 6 is that for all directions of prop- diac bundles. agation the greatest load within each cell The deposition of collagen also reoccurs in the sarcolemmal membrane sults in the loss of tight packing of the located adjacent to the intercalated disks myocytes; i.e., there is a relative widenat the ends of cells in the regions of the ing of interstitial space, which should Vmax minima, and the least load occurs reduce the interstitial resistance to curtoward the center of each cell in the rent flow.11 A reduction in interstitial sequence of lower and higher Vmax values along the longitudinal axis of the network of cells.

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Figure 7. Collagenous septa in an atrial bundle with nonuniform anisotropic electrical properties from a 64-year-old patient. The collagenous septa (gray to black) are thick and long and result in isolation of adjacent cells and groups of cells (white). Bar = 50 |im. Reproduced from Spach MS, Dolber PC, Heidlage JF. Influence of the passive anisotropic properties on directional differences in propagation following modification of the sodium conductance in human atrial muscle. A model of reentry based on anisotropic discontinuous propagation. Circ Res 1988;62:811-832, with permission.

resistance secondary to an increase in interstitial volume at sites of collagen deposition is consistent with the demonstration by Fallert et al.29 that the impedance of the dense scar of healed infarcts is 50% lower than the impedance of normal myocardium. Further, recent evidence indicates that there are unexpected (and unexplored) mechanisms of extracellular loading of the sarcolemmal membrane due to resistive discontinuities produced by the spatial variations in the volume of interstitial space associated with collagenous septa.11 When analyzing the curvature of wavefronts, puzzling questions arise as to whether the conduction events are due to 1 of the 2 following phenomena: (1) At the larger macroscopic size scale (>1 to 2 mm), variations in conduction velocity are considered to occur in relation to the degree of

curvature of wavefronts that form spiral waves in a continuous medium,5-7 i.e., a greater curvature is associated with a greater electrical load, which generates a decrease in conduction velocity. (2) Conduction events are related to variations in loading produced by differences in cellular connectivity that occur in anisotropic cardiac bundles.2,3 To illustrate that differences in the curvature of cardiac wavefronts are highly determined by the anisotropic distribution of the cellular interconnections, Figure 8 shows activation sequences in 2 types of anisotropic cardiac muscle. In the well-coupled uniform anisotropic preparation (Figure 8, left), the effective longitudinal conduction velocity was 0.51 m/s and the effective transverse velocity was 0.17 m/s, which resulted in an LP/TP velocity ratio of 3.0. In the nonuniform anisotropic preparation with

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Figure 8. Effect of uniform versus nonuniform anisotropy on the spread of activation. The sequence for a uniform anisotropic preparation of canine ventricular epimyocardium is shown on the left, along with unipolar extracellular waveforms (bottom) recorded at the sites noted on the isochrone map. The sequence for a nonuniform anisotropic canine atrial preparation (crista terminalis) is shown on the right. Each activation sequence was associated with normal action potentials. The isochrones represent 1-ms intervals. Reproduced from Spach MS. The stochastic nature of cardiac propagation due to the discrete cellular structure of the myocardium. Int J Bifurcation Chaos 1996;6:16371656, with permission.

decreased side-to-side coupling between fibers (Figure 8, right), the effective LP conduction velocity was 1.0 m/s and the effective TP conduction velocity was 0.1 m/s, which produced an LP/TP velocity ratio of 10. Myocardial bundles with uniform anisotropic properties usually have an LP/TP ratio between 2 and 4, and bundles with nonuniform anisotropic properties produce a much higher LP/TP velocity ratio ranging from 7 to 15.2 As can be seen in Figure 8, in the shift from LP to TP the curvature of the wavefronts was much greater in the preparation with -I side-toside coupling between the fibers than in the well-coupled anisotropic preparation.30 Figure 8 also shows that a major effect of the loss of side-to-side electrical

coupling between fibers is a prominent decrease in the effective conduction velocity in the transverse direction. The decrease in the effective transverse conduction velocity largely accounts for the high LP/TP velocity ratio exhibited by nonuniform anisotropic bundles with microfibrosis. Further, in nonuniform anisotropic atrial bundles from humans older than 60 years of age, the effective transverse conduction velocities are as low as 0.04 m/s in the presence of rapid upstroke action potentials.10 This type of very slow anisotropic conduction should be of increasing importance in the analysis of the low conduction velocities that enhance the initiation of atrial fibrillation in the geriatric age group, as well as in the initiation of

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have different effective refractory periods in the absence of repolarization inhomogeneities.10 A representative wellcoupled uniform anisotropic atrial bundle Proarrhythmic Effects of from a child is illustrated in Figure 9A. Microfibrosis with Loss of This shows the pattern of activation spread and associated extracellular waveSide-to-Side Gap Junctions forms of TP (waveform 1) and LP (waveA widely used electrophysiological forms 2 and 3) of a normal beat at an parameter is the effective refractory period, interstimulus interval of 800 ms (left which is measured as the shortest pre- side) and the earliest propagated premamature interval at which a propagated ture beat that occurred at an interval of action potential occurs. Moe et al.31 initially 345 ms (right side). As the premature demonstrated that an abrupt increase (dis- interval was reduced to the earliest stimcontinuity) in a conduction time curve, ulus that would produce a propagated which is produced by initiating progres- response, all of the extracellular wavesively earlier premature stimuli, indi- forms remained smooth in contour and cates that there are 2 or more functionally maintained the same general shape as different pathways in the total conduc- they decreased in amplitude. Earlier pretion circuit. Such discontinuities in a con- mature stimuli failed to produce conducduction time curve occur when the tion in any direction. Figure 9B shows the corollary results effective refractory period of one pathway is longer than that of another. In this sit- in a nonuniform anisotropic atrial bundle uation, when a premature beat occurs at from a 64-year-old patient. Normal beats an interval that is shorter than that of the produced TP extracellular waveforms that longer refractory period of one pathway, the had irregular deflections superimposed on propagation of premature impulses can the overall waveforms (waveforms 1 and 2); continue in another pathway that has a these multiphasic or fractionated waveforms shorter effective refractory period. This were associated with the underlying microphenomenon produces unidirectional fibrosis shown in this patient's atrial bundle block, which is a requirement for the in Figure 7. When the premature interval initiation of a reentrant arrhythmia. was decreased to 327 ms (Figure 9B, right), Mechanistically, the importance of uni- stable but very slow TP continued in assodirectional block is that it must occur in ciation with a considerable increase in the order for the remaining activation wave time intervals between the multiple small to circulate back and reexcite the initial deflections in the multiphasic TP wavestimulus area. Spatial differences in forms. In the longitudinal direction of the action potential duration (inhomogeneities fibers, however, decremental conduction to of repolarization) are usually consid- block occurred. The same events occurred ered the underlying cause of spatial dif- when the stimulus site was moved to differences in the effective refractory period ferent locations throughout the bundle, and unidirectional block. Figure 9, how- which again demonstrated that the direcever, demonstrates that changes in cel- tion of unidirectional block of the premature lular connectivity associated with the impulses was related to the orientation of microstructural remodeling of uniform the fibers. Although not shown, further anisotropic properties to nonuniform aniso- slight shortening of the premature interval tropic properties create the general property resulted in anisotropic reentry within the of functionally different pathways that muscle bundle.10 ventricular tachycardia in patients with healed infarcts.8

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Figure 9. Propagation responses to premature action potentials in atrial bundles with (A) uniform and (B) nonuniform anisotropic properties. Each panel shows the normal activation sequence and a few of the extracellular waveforms on the left, and those of an early premature beat are shown on the right. The interstimulus intervals (ms) are listed in the boxes above each set of extracellular waveforms. A. Isochrones are separated by 1 ms. The atrial bundle is from a 12-year-old patient. B. Activation sequences in the transverse direction were so complex that it was not possible to construct isochrone maps. In the drawing of the normal activation sequence (B, left), the elongated open arrow represents the narrow region of fast longitudinal conduction. On the right side, the elongated triangle represents decremental conduction to block along the longitudinal axis of the fibers. Reproduced from Spach MS, Dolber PC, Heidlage JF. Influence of the passive anisotropic properties on directional differences in propagation following modification of the sodium conductance in human atrial muscle. A model of reentry based on anisotropic discontinuous propagation. Circ Res 1988;62:811-832, with permission.

activation waves at a macroscopic level, the events of propagating excitation at the The results presented in this chapter microscopic level are constantly changing show that the microstructure of the and disorderly due to intracellular loading myocardium creates inhomogeneities of effects and conduction delays between electrical load that cause cardiac propaga- cells.4 An important feature of the stotion to be discontinuous and stochastic in chastic nature of discontinuous propaganature at a microscopic level. Thus, under- tion at a microscopic level is that small lying the movement of smooth appearing input changes produce major changes in Significance

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the events of propagation. For example, a simple change in the direction of conduction of the excitation wave produces considerable change in the excitatory events during the upstroke of the action potential and in the delays between cells. Although the events of discontinuous conduction are stochastic in nature at a microscopic level, these stochastic events become averaged and appear consistent with a continuous medium at the larger macroscopic size scale, as depicted experimentally.32 It may seem bewildering that excitation spread in normal anisotropic muscle can be viewed as continuous and discontinuous. Although these phenomena may appear contrasting at first sight, they present a biological analogy to the theory of Bohr and Heisenberg in quantum physics in the context that there are 2 truths rather than 1. At a large size scale in normal ventricular muscle, curved wavefronts appear continuous, but when analyzed at a microscopic size scale they are discontinuous and stochastic in nature.4 This is only one example of a general principle that when biological phenomena (e.g., bursting of pancreatic (3-cells,33 ion channel events, etc.) are examined at a small enough size scale, they are found to occur in steps with abrupt changes (discontinuous), and normally their stochastic properties become averaged to produce order at a larger spatial or time scale. This relationship is a feature of the central limit theorem,34 which provides a course from discontinuous events at a microscopic level to smoothed (averaged) events at a macroscopic level. A fundamental property of the stochastic nature of normal propagation is that it provides a major protective effect against arrhythmias by reestablishing the general direction of wavefront movement when small variations in excitation events occur. However, a loss of side-toside electrical connections between small groups of fibers results in a decrease in

the diversity of conduction events at a very small size scale. This produces relatively isolated groups of cells throughout cardiac bundles in which larger fluctuations of electrical load can develop and be distributed over more cells than occurs normally. Now, due to the relative isolation of small groups of cells, the discontinuous events at a microscopic size scale no longer can be averaged to produce the smooth curved wavefronts that occur at a macroscopic level in normal cardiac muscle. As illustrated in Figure 9B, at this point premature impulses can induce abnormal conduction events that produce breaks in the overall wavefront,35 resulting in the initiation of a reentrant circuit. This view implies an important synthesis for future combined structuralelectrical mapping studies to establish a new relationship between discontinuous propagation at a microscopic level and spiral waves at a macroscopic size scale.3 Such a synthesis will be necessary to understand how the occurrence of wavefront breaks35 and spiral waves5-7 (reentry) are enhanced by the loss of side-to-side cellular interconnections in association with collagen accumulation in interstitial space. Thus, a variety of improved mapping techniques will be needed to obtain needed information about microscopic conduction events in normal and diseased cardiac structures. Such highresolution mapping studies should be analogous to the fact that quantum physics takes advantage of the "microscopic" variations of particles to predict new phenomena not possible from the measurements of classic physics at a larger size scale. There are some events related to discontinuous anisotropic propagation at a microscopic level that are not accounted for by the theory of the curvature of wavefronts at a macroscopic level. One example is the relationship between conduction velocity and Vmax. According to the theory of spiral

MICROSCOPIC DISCONTINUITIES AS A BASIS FOR REENTRANT ARRHYTHMIAS waves in a continuous medium, when the local curvature of a wavefront becomes sufficiently great, there is a reduction in the local conduction velocity.36 These decreases in velocity related to wavefront curvature should be associated with a reduction in the rate of rise of the action potential37; i.e., changes in wavefront curvature produce a monotonic relationship between the local conduction velocity and Vmax. However, in anisotropic cardiac bundles, there is a general reciprocal relationship between the local conduction velocity and Vmax. Fast longitudinal conduction is associated with a relatively low Vmax compared to the increase in Vmax that occurs when the local conduction velocity decreases as the wavefront propagates at progressively greater angles with respect to the longitudinal axis of the fibers. Here, the highest values of Vmax are associated with the lowest conduction velocities of the macroscopic activation wavefront (see Figures 4 and 5 in reference 1). Of considerable long-term interest is the elucidation of the specific proarrhythmic mechanisms associated with the accumulation of collagen in interstitial space. The results presented here indicate there is a close relationship between the deposition of collagen and the loss of lateral interconnections between small groups of fibers. This relationship provides a major challenge to develop interventions that alter collagen deposition as an antiarrhythmic therapeutic measure. References 1. Spach MS, Miller WT III, Geselowitz DB, et al. The discontinuous nature of propagation in normal canine cardiac muscle: Evidence for recurrent discontinuities of intracellular resistance that affect the membrane currents. CircRes 1981;48:39-45. 2. Spach MS. Discontinuous cardiac conduction: Its origin in cellular connectivity with long-term adaptive changes that

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cause arrhythmias. In Spooner P, Joyner RW, Jalife J (eds): Discontinuous Conduction in the Heart. Armonk, NY: Futura Publishing Co.; 1997:5-51. 3. Spach MS, Heidlage JF, Dolber PC. The dual nature of anisotropic discontinuous conduction in the heart. In: Zipes DP, Jalife J (eds): Cardiac Electrophysiology: From Cell to Bedside. 3rd ed. Philadelphia: W.B. Saunders; 2000:213-222. 4. Spach MS, Heidlage JF. The stochastic nature of cardiac propagation at a microscopic level. An electrical description of myocardial architecture and its application to conduction. CircRes 1995;76:366380. 5. Cabo C, Pertsov AM, Baxter WT, et al. Wave-front curvature as a cause of slow conduction and block in isolated cardiac muscle. CircRes 1994;75:1014-1028. 6. Davidenko JM, Kent PF, Chialvo DR, et al. Sustained vortex-like waves in normal isolated ventricular muscle. Proc NatlAcad Sci USA 1990;87:8785-8789. 7. Pertsov AM, Jalife J. Three-dimensional vortex-like reentry. In: Zipes DP, Jalife J (eds): Cardiac Electrophysiology: From Cell to Bedside. 2nd ed. Philadelphia: W.B. Saunders; 1995:403-410. 8. Dillon SM, Allessie MA, Ursell PC, et al. Influences of anisotropic tissue structure on reentrant circuits in the EBZ of subacute canine infarcts. CircRes 1988;63:182-206. 9. Spach MS, Dolber PC. Relating extracellular potentials and their derivatives to anisotropic propagation at a microscopic level in human cardiac muscle: Evidence for electrical uncoupling of side-to-side fiber connections with increasing age. Circ Res 1986;58:356-371. 10. Spach MS, Dolber PC, Heidlage JF. Influence of the passive anisotropic properties on directional differences in propagation following modification of the sodium conductance in human atrial muscle. A model of reentry based on anisotropic discontinuous propagation. CircRes 1988;62:811-832. 11. Spach MS, Heidlage JF, Dolber PC, et al. Extracellular discontinuities in cardiac muscle: Evidence for capillary effects on the action potential foot. Circ Res 1998;83: 1144-1164. 12. Spach MS,.Heidlage JF, Darken ER, et al. Cellular Vmax reflects both membrane properties and the load presented by adjoining cells. Am J Physiol 1992;263: H1855-H1863.

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13. Spach MS, Miller WT III, Miller-Jones E, et al. Extracellular potentials related to intracellular action potentials during impulse conduction in anisotropic canine cardiac muscle. Circ Res 1979;45:188— 204. 14. Rushton WA. Initiation of the propagated disturbance. Proc R Soc Lond (Biol) 1937; 124:124-210. 15. Socolar SJ. The coupling coefficient as an index of junctional conductance. J Membr Biol 1977;34:29-37. 16. Fast VG, Kleber AG. Microscopic conduction in cultured strands of neonatal rat heart cells measured with voltage-sensitive dyes. Circ Res 1993;73:591-595. 17. Fast VG, Kleber AG. Anisotropic conduction in monolayers of neonatal rat heart cells cultured on collagen substrate. Circ Res 1994;75:591-595. 18. Rohr S. Determination of impulse conduction characteristics at a microscopic scale in patterned growth heart cell cultures using multiple site optical recording of transmembrane voltage. J Cardiovasc Electrophysiol 1995;6:551-568. 19. Hofer E, Urban G, Spach MS, et al. Measuring activation patterns of the heart at a microscopic size scale with thin-film sensors. Am JPhysiol 1994;35:H2136-H2145. 20. Joyner RW, Westerfield M, Moore JW. Effect of cellular geometry on current flow during a propagated action potential. Biophys J I980;3l:183-194. 21. Spach MS, Heidlage JF. A multidimensional model of cellular effects on the spread of electrotonic currents and on propagating action potentials. Crit Rev Biomed Eng 1992;20:141-169. 22. Hoyt RH, Cohen MI, Saffitz JE. Distribution and three-dimensional structure of intercellular junctions in canine myocardium. Circ Res 1989;64:563-574. 23. Joyner RW. Effects of the discrete pattern of electrical coupling on propagation through an electrical syncytium. Circ Res 1982;50:192-200. 24. Rudy Y, Quan W. A model study of the effects of the discrete cellular structure on electrical propagation in cardiac tissue. Circ Res 1987;61:815-823.

25. Leon LJ, Roberge FA. Directional characteristics of action potential propagation in cardiac muscle. A model study. Circ Res 1991;69:378-395. 26. Gourdie RG, Green CR, Severs NJ. Gap junction distribution in adult mammalian myocardium revealed by anti-peptide antibody and laser scanning confocal microscopy. J Cell Sci 1991;99:41-55. 27. Dolber PC, Beyer EC, Junker JL, et al. Distribution of gap junctions in dog and rat ventricle studied with a double-label technique. JMol Cell Cardiol 1992;24:1443-1457. 28. Spach MS, Miller WT III, Dolber PC, et al. The functional role of structural complexities in the propagation of depolarization in the atrium of the dog. Cardiac conduction disturbances due to discontinuities of effective axial resistivity. Circ Res 1982;50:175-192. 29. Fallert MA, Mirotznik MS, Downing EB, et al. Myocardial electrical impedance mapping of ischemic sheep hearts and healing aneurysms. Circulation 1993;87:199-207. 30. Spach MS. The stochastic nature of cardiac propagation due to the discrete cellular structure of the myocardium. Int J Bifurcation Chaos 1996;6:1637-1656. 31. Moe GK, Preston JM, Burlington H. Physiologic evidence for a dual A-V transmission system. Circ Res 1956;4:357-375. 32. Clerc L. Directional differences of impulse spread in trabecular muscle from mammalian heart. J Physiol (Lond) 1976;255: 335-346. 33. Sherman A, Rinzel J. Model for synchronization of pancreatic (3-cells by gap junction coupling. Biophys J 1991;59:547-559. 34. Kleinbaum DG, Kupper LL, Muller KE. Applied Regression Analysis and Other Multiuariable Methods. Boston: PWSKent Publishing Co.; 1988:17. 35. Agladze K, Keener JP, Muller SC, et al. Rotating spiral waves created by geometry. Science 1994;264:1746-1748. 36. Tyson JJ, Keener JP. Singular perturbation theory of traveling waves in excitable media. Physica D 1988;32:327-361. 37. Zykov VS. Spiral waves in two-dimensional excitable media. Ann NY Acad Sci 1990;591:75-88.

Chapter 16

Mapping in Explanted Hearts Jacques M. T. de Bakker, PhD and MichielJ. Janse, MD

Introduction

Electrophysiological changes in cardiac muscle and mechanisms of arrhythmias caused by heart diseases cannot easily be studied in patients, and therefore experimental models are frequently used. Animal models are usually the first choice for investigations on electrophysiological mechanisms of arrhythmias. In the development of animal models, attempts must be made to fulfill the criteria that would make the model a realistic representation of the clinical events. A large number of laboratory models that use experimental animals have been developed in an attempt to achieve this goal. In these models, different approaches are applied. Different procedures of occlusion have been used to create infarction, resulting in different anatomical characteristics of the infarcts, which makes comparison between the models and the clinical setting difficult. x~5 In addition, various species are being used, which introduces differences in electrophysiological properties of the heart and differences in cardiac and coronary anatomy. In an effort to introduce some sort

of standardization, a set of guidelines for the study of arrhythmias caused by ischemia, infarction, and reperfusion, known as the Lambeth Convention, was published by Walker et al.6 in 1988. This convention addresses statistical problems, uniformity of animals, classification and detection of arrhythmias, and definition and detection of ischemia and infarction. To delineate the electrophysiological abnormalities in dilated cardiomyopathy, various experimental models have been developed as well.7"10 While arrhythmias frequently occur in patients with dilated cardiomyopathy, monomorphic, sustained ventricular tachycardias (VTs) are rare,11'12 hampering their mechanistic delineation. The animal models have also mainly been used to study cellular electrophysiological abnormalities. Although animal models have all fallen short in one respect or another, each has provided important information that has improved our understanding of the pathophysiology of arrhythmias in the diseased human heart. The use of Langendorff-perfused human hearts from patients who underwent

From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; e2003.

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heart transplantation because of extensive infarction or cardiomyopathy presents an alternative method to study electrophysiology and arrhythmias caused by these diseases. Although the pathology is a realistic representation of the clinical events, this model also lacks some clinical characteristics. Explantation results in denervation of the heart, which may affect arrhythmogenicity. It has been shown in animal models that transmural infarcts that extend to the epicardial surface may damage efferent sympathetic fibers in the subepicardium and produce heterogeneous sympathetic denervation of normal myocardium apical to the infarct.13'14 In addition, only studies in the healed phase of myocardial infarction are possible, because infarctions in these patients already exist long before transplantation. Although sustained VTs are usually not present spontaneously in the explanted infarcted hearts, they often can be induced by programmed stimulation. This is compatible with clinical observations that sustained VTs can be induced in approximately 50% of patients with healed myocardial infarction but without documented arrhythmias.15^17 This suggests that in at least half of the patients who survive myocardial infarction, the substrate for sustained VT is present but, in most cases, the trigger for starting the tachycardia never occurs because only a small percent of patients who survive myocardial infarction develop sustained VT late after the onset of infarction. In contrast to infarcted hearts, we were unable to induce sustained monomorphic VTs in explanted hearts from patients with dilated cardiomyopathy. Although innervation in explanted hearts is corrupted by the resection procedure, study of these hearts is particularly helpful to correlate electrophysiology with anatomy of the arrhythmogenic area.

LangendorfF Perfusion of the Isolated Human Heart Langendorff perfusion of explanted human hearts does not differ essentially from that procedure applied in animal hearts except for the cannulation procedure. In animal hearts, a cannula is inserted into the aorta and fixed with a suture around the aortic root. In the explanted human heart, the aorta is often too short for the cannula and the left main artery and right coronary artery must be cannulated separately. To avoid blood clotting in the arteries and to protect the heart directly after explantation, hearts were perfused with cardioplegia just before explantation. After removal, hearts were submerged in Tyrode's solution containing (in mmol/L): sodium (Na+) 156.5; potassium (K+) 4.7; calcium (Ca2+) 1.5; phosphate (HaPO^) 0.5; chlorine (Clr) 137; bicarbonate (HCOg) 28; and glucose 20. The left and right coronary arteries were cannulated via the ostia with the heart submerged in the cold Tyrode's solution. Thereafter, the heart was attached to the Langendorff perfusion system. Details of this system are given elsewhere.18 The perfusion fluid consisted of a mixture of 50% human blood and 50% of the Tyrode's solution (total volume 2 L). After the heart was connected, coronary flow was stabilized to approximately 200 mL/min. The ventricles were drained by rubber tubes in the apex, one into the left and the other into the right cavity. Hearts started to beat spontaneously or to fibrillate within minutes of perfusion. If fibrillation occurred, the heart was defibrillated after approximately 5 minutes. Perfusion was maintained at a temperature of 37±0.5°C until either the measurements were completed or contraction noticeably decreased (after 4 to 5 hours).

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Recording Electrical Activity

Histology of the Hearts

Electrograms recorded with bipolar hook electrodes attached to the base of the left and right ventricles served as a time reference and were used to distinguish the configuration of induced tachycardias. Epicardial as well as endocardial mapping of the electrical activity was performed during tachycardia. Tachycardia was induced by premature stimulation via a bipolar hook electrode attached to the left ventricular wall. Endocardial electrical activity of the left ventricle (and of the right ventricle in a number of cases) was recorded with a balloon electrode covered with 64 electrode terminals. The interelectrode distance was approximately 1.2 cm. The balloon was introduced into the left ventricular cavity through the mitral (or tricuspid) valve orifice. For recording of epicardial signals, flexible grid electrodes with 64 electrode terminals were used. High-resolution mapping of electrical activity using plaque electrodes (105 to 208 electrode terminals; interelectrode distance 0.8 and 0.5 mm, respectively) was carried out to determine the role of the architecture of interstitial fibrosis for activation delay.

Figure 1 shows typical examples of the histology of the explanted hearts. Panel A, a section of the posteroseptal wall of one of the hearts, illustrates that the endocardium (endo) is thickened by fibroelastosis (dark rim). This is a frequent finding in these hearts, where the endocardium can be as thick as 1 mm.19 A major part of the posterior wall is fibrotic (F), but a thin rim of surviving myocardial tissue is found subendocardially along the entire posterior wall (arrow) and contacts remaining healthy myocardium (M) of the septal wall. A surviving subendocardial rim is a common finding in infarcted myocardium and is supposed to be caused by the presence of a subendocardial plexus.20'21 White areas along the epicardium (epi) point to fatty tissue. Panel B, a section from the septal wall of another heart, shows that myocardial bundles (dark areas) may also survive deep in the infarcted zone. Note that these surviving myocardial bundles are often divided into smaller bundles by strands of fibrous tissue, yielding a complex structure of isolating and conducting tissue. Such areas of intermingled muscle bundles and fibrous septae may cause profound conduction delay (discussed later in this chapter). A similar architecture has been found in the infarcted canine heart.22 Panel C is a section from the apical part of the septum in the same heart. The left wall (top) of the septum is entirely fibrotic (light area marked F) in contrast to the right wall (bottom). A myocardial bundle, entirely encaged by fibrosis, seems to have survived in the left septal wall (arrow). It is conceivable that such bundles, when they traverse the infarcted zone, may constitute the return path of a reentry circuit.

Characteristics of the Infarcted Human Heart Studies were carried out at 9 hearts from patients who underwent heart transplantation because of congestive heart failure caused by myocardial infarction. The location of the infarct was in the anterior wall including the septum in 6 patients, inferoposterior in 2, and diffuse in 1. Ejection fraction before transplantation ranged from 13% to 27%. A history of sustained VT was documented in one patient only.

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Figure 1. A. Histologic section of the infarcted area in the posteroseptal region of an explanted human heart. The posterior wall consists mainly of fibrous tissue (F); in contrast, the septal region is not affected and consists of healthy myocardial tissue (M). The endocardium is thickened by fibroelastosis, which is present along the entire posterior wall (dark rim). A small rim of surviving myocardium is present between the endocardium and the infarcted area (arrow). The white zone along the epicardium indicates the presence of fatty tissue. B. Histologic section from the infarcted mid septal region of another explanted heart. In contrast to A, this region consists of numerous surviving myocardial bundles (dark areas) that survive within the fibrotic regions (F). Note that the myocardial bundles are often divided into smaller bundles by fibrotic strands. Areas in which numerous myocardial bundles are embedded in fibrous tissue are a common finding in infarcted myocardium.C. Histologic section from the same heart as in B. This section is taken from the septal wall as well, but at a more apical position. The infarction is almost transmural at this level, but a large strand of myocardium was able to survive in the middle of the septum (arrow). F = fibrous tissue.

MAPPING IN EXPLANTED HEARTS Spread of Activation During Tachycardia In the 9 isolated hearts, 15 monomorphic sustained VTs were induced. The cycle length (CL) ranged from 260 to 560 ms. In 7 of 10 tachycardias, earliest epicardial activation appeared more than 20 ms after earliest endocardial activation, whereas in 3 tachycardias, earliest epicardial and endocardial activation was almost simultaneous. In all cases, the endocardial activation pattern showed a focal area of earliest activity from which activation spread more or less centrifugally with activation block toward the infarcted zone. The site of earliest activation was always located within 2 cm of the border of the infarct. These characteristics were similar to those of the tachycardias recorded in patients during arrhythmic surgery.23"27 Although the tachycardias induced in healed myocardial infarction usually reveal a focal activation pattern, there was convincing evidence from clinical and experimental studies28"33 that reentry is the underlying mechanism of these arrhythmias. It was supposed that activation returned from the latest activated site of one cycle to the earliest activated site of the next cycle via surviving myocardial fibers in the infarcted zone. Remaining healthy myocardium constituted the other part of the circuit. The exact path within the infarcted area was, however, unknown. Because electrophysiology and histology can be well correlated in explanted hearts, detection of the reentry circuit within the compromised area was expected to be possible. In 2 tachycardias recorded in 2 different hearts, activation delay between earliest and latest endocardial activation was approximately 30 ms. The distance between earliest and latest activated sites was approximately 1.2 cm. Assuming that activation returns from the latest activated

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site to the earliest activated site via surviving myocardial bundles yields a mean conduction velocity of 0.4 m/s in the return path. Thus, in these cases, which are rather exceptional, the conduction velocity in the return tract was close to normal. In contrast, the majority of activation maps showed delays of more than 120 ms between latest and earliest activation over a similar distance, resulting in conduction velocities of less than 0.1 m/s. Endocardial activation patterns of 2 tachycardias induced in the same heart are shown in Figure 2. VT1 shows a tachycardia where conduction in the return path was close to normal; in contrast, the activation map of VT2 shows that the conduction velocity in the presumed return path is low. Hatching indicates that an area of damaged tissue extended from base to apex in the posterior wall. Activation of VT1 started at the right border of the infarct, and main activation (arrows) spread out toward the base and the lateral wall. Activation toward the septum was blocked after 120 ms; 240 ms after the onset of endocardial activation, activity reached the septal side of the infarct (site e). Delay between activation at this site and the onset of the next cycle (at site a) was only 30 ms, suggesting reexcitation at the "origin" by means of activation through surviving myocardial tissue between sites e and a. The electrogram recorded at site f, located between a and e, had 3 deflections; the first and third deflection were remote and caused by activation removing from the right side of infarcted zone and the wavefront approaching the left side of the infarcted zone, respectively. The second deflection did not coincide with documented activity anywhere else in the tissue, which indicated that it was generated by viable tissue underneath the recording site. The time delay of 30 ms between activation at the sites e and a (distance approximately 1.2 cm) suggested that activation was carried back toward

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Figure 2. Endocardial activation patterns of 2 monomorphic sustained ventricular tachycardias that were induced in a Langendorff-perfused human heart with extensive posterior infarction. Isochrones are in ms and timed with respect to the onset of endocardial activation. Arrows indicate main spread of activation. The activation map of VT1 shows that there is a time gap of 30 ms between latest activation of one cycle (recorded at site e) and earliest activation of the next cycle (recorded at site a). Reentry probably occurred through surviving pathways in the infarct zone between sites e and a (distance, 1.2 cm). The conduction velocity in the return tract, is approximately 0.4 m/s, which is close to normal. In contrast, delay in the return tract of VT2 is 140 ms, while the distance between the earliest and latest activated sites is 1.2 cm as well. This implies that the conduction velocity in the return tract of VT2 is low, approximately 0.07 m/s.

MAPPING IN EXPLANTED HEARTS the "origin" with close to normal conduction velocity (0.4 m/s). VT2 shows the activation pattern of another tachycardia induced in the same heart. Earliest endocardial activation arose at the septal side of the infarct zone. At the base of the septum, spread of activation toward the posterior wall was blocked. However, the wavefront reached the posterior wall by way of the anterolateral wall and apex. The wavefronts merged at a mid posterior level (160 ms isochrone) to arrive 240 ms after the onset at the base of the posterior septal border (site a). Because the CL of the tachycardia is 400 ms, a time gap of 160 ms had to be bridged to close the reentrant circuit that reactivated the "origin." The distance between latest and earliest activated sites was about 1.2 cm, yielding a mean conduction velocity in the presumed return tract of approximately 0.07 m/s.

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Characteristics of the Return Path For both tachycardias shown in Figure 2 the location of the return path of the presumed reentry circuit could be determined from the activation maps; however, only for the tachycardia of VT1 were we able to trace the exact route. After fixation in formalin, the area comprising the earliest and latest activated site was removed and sectioned. Sections of 10-u,m thickness were produced. A total of 500 sections were analyzed. Figure 3 shows a stacking of transparencies made of 10 sections 100 |im apart. In the upper and lower sections, the infarction was transmural. The stacking, however, suggests that a continuous tract (arrow) was present traversing the infarcted area, connecting the septum with the posterior wall. Thus, it could

Figure 3. Schematic drawings of sections of the area harboring the return tract for reentry of VT1 in Figure 2. Black areas are myocardial tissue; white areas are fibrotic. Transparencies of 10 sections have been stacked. The infarction is transmural at the level of sections 1 and 10. The stacking, however, suggests that there is a bridge of surviving myocardial tissue (white arrow) connecting surviving myocardial tissue at either side of the infarcted zone. Note that this tract is located endocardially, almost intramurally. In the same area an epicardially located tract could be traced in the area indicated by the open arrow.

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Figure 4. Three-dimensional reconstruction of a tract of surviving myocardial tissue traversing the infarcted zone as suggested by Figure 3. Surviving myocardium of the sections is indicated in black and that of the tract in gray. Eleven consecutive sections, 100 jim apart, were used for the reconstruction. The tract is connected with remaining healthy tissue of the posterior wall at level d and with the septal wall at level a. The length of the tract is approximately 13 mm. The width varies along the tract, with bottlenecks appearing in both the descending and the ascending parts of the tract. Reproduced from reference 33, with permission.

well be that a surviving bundle of myocardial tissue was present, connecting viable tissue at either side of the infarct zone and possibly forming a return path for reentry.

To prove that the supposed tract was indeed continuous, a 3-dimensional reconstruction of the tract had to be made (Figure 4). The schematic drawings of 10 successive sections (a to k), each 100 um

MAPPING IN EXPLANTED HEARTS apart, show that the tract (gray area) fuses with remaining healthy tissue of the posterior wall at level d. From here, the tract runs down toward level k, and remains horizontal over a distance of about 6 mm in level j before it ascends and finally merges with remaining healthy tissue of the septal wall at level a. Although the tract appears to be continuous, there are a number of narrow passages, especially in the descending and ascending parts of the tract. The smallest width of these bottlenecks was approximately 250 um. One might speculate that unidirectional activation block, necessary for initiating the tachycardia, could preferentially occur at such sites.34-36 The length of the endocardial tract within the infarct zone was approximately 13 mm. The short length of the tract may account for the small delay of activation in this area. It occurred to us that the subendocardial tract did not constitute the only possibility for reentry. The epicardial activation map suggested the presence of a surviving tract in the subepicardial part of the infarct, and an anatomical substantiation for such a tract could be demonstrated by applying the same procedure as described before.37 There was only one other heart in which delay between latest activation of one cycle and earliest activation of the next cycle was less than 30 ms, whereas the distance between the sites was only 1.2 cm. A tract of surviving myocardium, located 1.5 mm from the epicardial surface of the anterior wall, was traced. Its smallest width was approximately 350 |im. Two morphologically different tachycardias could be induced in this heart. In both tachycardias, the return path of the reentry circuit was the same. Because we could trace only one surviving tract in this area, it was likely that this return path was used for activation in both

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tachycardias. The tachycardias revolved in opposite directions, indicating that unidirectional block in this area could occur in either direction. These observations show that surviving tracts may be present throughout the entire infarcted ventricular wall and that return tracts may be used in either direction. The latter gives rise to VTs with different configuration. Direction of Surviving Fibers The area of the return path in VT2 in Figure 2, and in 3 other hearts where the apparent conduction velocity between the site of latest activation of one cycle and earliest activation of the next was low, consisted of a collection of surviving bundles with various diameters, separated from each other by fibrous or fatty tissue. Surviving bundles coursed separately over a few hundred micrometers and then merged into a single bundle (Figure 5). The fiber direction of the bundles was perpendicular to the line connecting the sites of earliest and latest activation, indicating that it was necessary for activation to proceed perpendicular to the fibers. The clusters of surviving muscle bundles separated by fibrous tissue and the repeated fusion and bifurcation of these bundles resembles inhomogeneous anisotropy, which could well account for the delayed conduction.38,39 Because of the complicated architecture of the branching bundles in the infarct, it was impossible to reconstruct a possible return path in either of these hearts. In the remaining hearts, the surviving fibers had no fixed direction, but varied throughout the area of the presumed return path. In the infarcted dog heart too, disorganization of surviving myocardial cells due to the ingrowth of fibrous tissue is a common finding.40

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Figure 5. Photomicrographs of an area revealing slow conduction during tachycardia. The left panel shows that the area consists of a number of surviving myocardial bundles (bright areas), separated from each other by fibrous tissue (dark areas). The photomicrograph at the right shows histological features in a section 100 jj,m beneath the one at the left. Here, the separated bundles have merged into a single bundle. Fiber direction in the area shown was perpendicular to the line connecting earliest and latest activated sites, indicating that activation had to proceed perpendicular to the fiber direction. Modified from reference 37.

slow conduction in healed infarcts. Impaired coupling between cells presents Unraveling the mechanism of con- another option for impaired conduction. duction delays of greater than 100 ms over Several investigators have shown that in distances less than 1 cm, which were compromised myocardium the connecfound in the majority of the tachycardias, tions of cells in side-to-side apposition are presented a challenge. Microelectrode reduced, whereas end-to-end connections recordings of myocardial tissue surviving are virtually unaffected.43,44 This is comin the infarcted zone have shown that patible with our finding that conduction action potentials were close to normal in delay preferentially occurred perpendichealed myocardial infarction.33,40-42 There- ular to the fiber direction. fore, abnormal membrane characteristics Disruption of side-to-side connection were unlikely candidates as the cause of may increase path length. Histology showed Superfused Preparations

MAPPING IN EXPLANTED HEARTS proliferation of fibrous tissue resulting in longitudinally oriented shells of connective tissue, insulating adjacent groups of myocardial fibers. Side-to-side electrical coupling among these fibers could be absent over distances of several millimeters, but interconnections farther away transferred activation to neighboring bundles. Diameter of the surviving bundles ranged from a few millimeters to the diameter of single cells. The small diameter of the isolated bundles together with the large delays over short distances suggested that very high resolution measurements were required to unravel the mechanism of activation delay. Measurements in the isolated hearts were not very suitable for such high-resolution mapping and therefore we studied conduction in and histology of papillary muscles resected from the explanted hearts.45 Muscles were superfused in a tissue bath; this has the advantage that only a rim of subendocardial myocardium 300 to 600 um thick survives. Thus, these preparations were more or less 2 dimensional. Papillary muscles were chosen because bundles of viable myocardial cells that survive in the subendocardial rim remain parallel in orientation.31 In infarcted myocardium, the parallel orientation of the surviving fibers often is not preserved and the surviving fibers appear to course in different directions.38,40 During basic stimulation, spread of activation was determined from recordings made at distances of 200 um from up to 416 sites. Electrograms showed multiple deflection in virtually all preparations (fractionated electrograms). Such electrograms are a common finding in diseased myocardium and point to asynchronous conduction. Electrical separation of myocardial fibers by anatomical barriers such as fibrous or fatty tissue give rise to separated conducting paths, which may result in asynchronous conduction. Separation of

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conducting paths is not exclusively the result of anatomic barriers. Reduced coupling between cells caused by a decrease in the number of gap junctions or reduction of the conduction properties may result in fractionation of electrograms as well. Analysis of activation times revealed that activation in the infarcted papillary muscle spread in myocardial tracts parallel to the fiber direction. Conduction velocity in the tracts was high (0.79 m/s). Tracts were separated by collagenous septa over distances up to 3 mm, but often connected with each other at one or more sites, forming a complex network of merging and diverging tracts. For propagation perpendicular to the fiber direction, delays of up to 36.5 ms over a distance of 1.2 mm were observed. Conduction delay in the direction perpendicular to the fibers was caused mainly by the increase of the route activation had to travel through the branching and merging bundles. Figure 6 shows an example of the complex architecture of merging and diverging bundles in one of the papillary muscles. The preparation was stimulated in the lower left corner (near site A); numbers within the tracts indicate activation times with respect to the stimulus. To reach site B, at a distance of 1.2 mm from site A, activation had to follow the indicated zigzag route. At site B, activation arrived after a delay of 36.5 ms, indicating that the apparent conduction velocity perpendicular to the fiber direction was 0.04 m/s. Note that in a number of tracts activation proceeded from the right to the left, that is toward the site of stimulation. The tracings in Figure 7 are 8 electrograms recorded along a line perpendicular to the fiber direction. The interelectrode distance of the recording electrode was 0.2 mm. All electrograms are highly fractionated because of the asynchronous conduction within the different bundles. The architecture

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Figure 6. Tracts along which activation proceeded in an infarcted papillary muscle from an explanted human heart. The tracts, which were reconstructed using electrograms recorded with a resolution of 200 jim, run parallel to the fiber direction and are separated over distances of several millimeters before they connect to neighboring tracts. The preparation was stimulated at the lower left corner (close to site A). Numbers in the tracts indicate activation times measured with respect to the stimulus. Activation delay between sites A and B along a line perpendicular to the fiber direction is 36.5 ms. Sites A and B are 1.2 mm apart, resulting in an apparent conduction velocity between A and B of 0.04 m/s. The continuous line indicates the tortuous route activation had to travel from A to reach B. Activation delay between A and B is caused mainly by the increase of the route activation must travel. Conduction velocity in the tracts is close to normal (0.78 m/s).

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Figure 7. Eight of 240 unipolar recordings made to determine the tracts in Figure 6. Electrograms are highly fractionated indicating asynchronous activation in the various tracts. Modified from reference 45.

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Figure 8. Photomicrograph of a section of the papillary muscle in Figure 6. A surviving subendocardial rim (bright area) surrounds a core of dense connective tissue (dark area). The surviving subendocardial rim is divided into several bundles of myocytes that are sheathed by septae of fibrous tissue. Reproduced from reference 45, with permission.

of isolated myocardial bundles in the surviving subendocardial rim is shown in Figure 8. A narrow surviving subendocardial rim (bright area) surrounds a core of dense connective tissue (dark area). The surviving subendocardial rim is divided into several bundles of myocytes that are sheathed by septae of fibrous tissue. The infarcted papillary muscle presents the simplest model for studying conduction abnormalities imposed by healed myocardial infarction. As illustrated, very high resolution mapping is required to gain insight into the mechanism of slow conduction. Such recordings are difficult to perform in the isolated Langendorffperfused heart. In addition, superfusion

virtually reduces the 3-dimensional structure to 2 dimensions and fiber direction in the papillary muscles remains almost parallel.

Conduction in Dilated Cardiomyopathy Electrophysiological abnormalities have been observed in various experimental models of dilated cardiomyopathy.7"10 Anderson et al.46 have shown that the severity of abnormal propagation correlates with the amount of myocardial fibrosis in patients with dilated cardiomyopathy who underwent heart transplantation.

MAPPING IN EXPLANTED HEARTS To unravel the mechanism of abnormal conduction in dilated cardiomyopathy, we carried out similar measurements as described before in papillary muscles derived from explanted hearts of patients who underwent heart transplantation while in the end stage of heart failure due to dilated cardiomyopathy.47 Although preparations revealed fractionated electrograms and conduction delay as observed in the infarcted preparations, architecture of the cardiomyopathic papillary muscles differed from that of the infarcted ones. Impaired conduction in dilated cardiomyopathy was indeed associated with collagen infiltration, but the matrix of intermingling myocardial bundles and fibrosis differed. Discernible lines of block consisting of broad strands of fibrosis were present but most often electrical barriers consisted of short stretches of fibrous tissue. Delayed conduction was caused by curvature of activation around the distinct lines of block and by a wavy course of activation between the short barriers. The latter reflects extreme nonuniform anisotropy. Architecture of Interstitial Fibrosis and Conduction Delay The previous observations suggest that the architecture of interstitial fibrosis is an important parameter in determining the amount of conduction delay. To investigate the role of the architecture of fibrosis on conduction, we performed high-resolution mapping in another 8 Langendorff-perfused human hearts.48 Three patients had coronary artery disease (with myocardial infarction), 1 suffered from hypertrophic cardiomyopathy, and 4 had dilated cardiomyopathy. Multiterminal plaque electrodes harbored 105 or 208 terminals consisting of 70-jimdiameter silver wires, isolated except at

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the tip. Terminals were arranged in a 9 x 12 or 16 x 13 matrix at interelectrode distances of 0.8 and 0.5 mm, respectively. Plaque electrodes were positioned over nonfatty epicardial areas. Electrical stimulation was applied with bipolar hook electrodes positioned adjacent to any one of the 4 sides of the multielectrode. Pacing was at twice diastolic current threshold with an 8-pulse drive train (CL 600 ms) and one premature stimulus. Coupling intervals of premature stimuli were from 500 ms down to the refractory period in steps of 10 ms. Tissue at 16 of the 26 positions of the multielectrode was subjected to histological investigation. Mean density of fibrosis in the recording areas ranged from 7% to 43% (mean 18 ± 10%). Three types of fibrosis with regard to architecture were distinguished: • Patchy: patchy fibrosis with long, compact groups of strands • Diffuse: more or less diffusely distributed fibrosis with short strands • Stringy: homogeneously distributed fibrosis with long "single" strands; sometimes local, compact areas of fibrosis were present in this type. Density of fibrosis did not correlate with conduction delay, but the architecture of fibrosis played a major role. Figure 9 shows 2 types of fibrosis, patchy (left panel) and diffuse (right panel) with similar mean density of fibrosis (22% and 33%, respectively). Conduction velocity during basic CL was similar for both types of fibrosis when propagation was parallel to the fiber direction. However, for propagation perpendicular to the fiber direction, conduction velocity, which was 0.24 ± 0.04 m/s during basic CL, reduced to 0.17 ± 0.06 m/s after premature stimulation with a coupling interval 10 ms longer than the refractory period. In contrast, conduction velocity reduced from

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Figure 9. Histological sections showing patchy fibrosis with long strands (left panel) and diffuse fibrosis (right panel). See text for discussion. See color appendix. Adapted from reference 48.

0.28 ± 0.07 m/s to 0.05 ± 0.01 m/s for patchy fibrosis. This is illustrated in Figure 10. Stimulation at site A (panel A) resulted in propagation of activation nearly perpendicular to the fiber direction. Tracings at the left in panel b show a progressive increase in conduction delay during premature stimulation at site A. Activation maps (panel C) during baseline stimulation at site A revealed widely separated isochronal lines, compatible with continuous conduction. Following premature stimulation, however, conduction became irregular with functional lines of conduction block at decreasing coupling intervals. Delay of activation within the recording area increased from 11 ms to 68 ms. Activation patterns resulting from stimulation at site B (lower maps) showed no irregularities after premature stimuli, and only a marginal increase of conduction delay was observed (from 7 to 17 ms). Thus, this study showed that, in chronically diseased human myocardium,

progressive increase of conduction delay during premature stimulation arises in areas with patchy fibrosis having long, compact, groups of strands. Delay strongly depends on the direction of wavefront propagation with respect to fiber direction, the effect during propagation perpendicular to the fiber direction being large as compared to that found upon parallel propagation. Diffusely distributed fibrosis with short strands only marginally affected conduction delay, even at high densities of fibrosis. In summary, mapping of the electrical activity in Langendorff-perfused human hearts allowed detection of the reentry circuit by correlating electrophysiology with anatomy in only a minority of the cases. In these cases, surviving myocardial bundles within the infarct zone constituted a continuous tract that traversed the infarct; activation delay in these tracts was close to normal. In the majority of the cases, activation delay in the area supposed to harbor the return

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Figure 10. a. Histological section of the recording area (rectangle) in a heart showing patchy fibrosis with long strands (red areas). Yellow areas show myocardial tissue. A and B are pacing sites. b. Extracellular electrograms recorded at the center of the recording electrode (asterisks). Numbers are coupling intervals of the premature stimuli, c. Activation maps during stimulation at site A (upper maps) and site B (lower maps) during basic cycle length of 600 ms and premature stimuli at coupling intervals from 500 to 320 ms. Numbers beneath the maps indicate the time in which the recording area was activated. See color appendix. Adapted from reference 48.

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Figure 11. Schematics of delayed conduction in infarcted myocardium. Fibrous tissue separates surviving myocardial bundles within the infarcted zone, although bundles may merge at a number of sites to connect with neighboring tracts. Activation starting at site A must follow the tortuous route indicated by the black line to reach site B. Activation delay between sites A and B is caused mainly by the increase of the route activation must travel.

tract was, however, large and the architecture of the area was too complex to correlate electrophysiology with histology. To reveal the mechanism of conduction delay in the return tract in these cases, refined measurements in papillary muscles superfused in a tissue bath were more appropriate. These measurements showed that slow conduction perpendicular to the fiber direction in infarcted myocardial tissue is caused by zigzag conduction at normal speed due to lengthening of the pathway by branching and merging of surviving myocardial bundles ensheathed by collagenous septae (Figure 11). Impaired conduction in dilated cardiomyopathy is caused, at least in part,

by the development of fibrous tissue as well. In these hearts, delayed conduction was caused by curvature of activation around distinct barriers (Figure 12A) and by the wavy course of activation between short barriers of fibrous tissue (Figure 12B). Finally, architecture of fibrosis plays a major role in determining progressive increase of conduction delay at incremental shortening of the coupling interval of premature stimuli. The effect of long fibrotic strands on conduction was much greater than that of diffuse fibrosis with short strands. The architecture of fibrosis is more important than its density for generating conduction disturbances.

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Figure 12. Schematic representations of activation in myocardium impaired by cardiomyopathy. Conduction delay arises because of curvature of activation around distinct barriers (A) or by the wavy course of activation between short barriers (B). Numbers indicate activation times.

References 1. Harris AS. Delayed development of ventricular ectopic rhythms following experimental coronary occlusion. Circulation 1950;1:1318-1328. 2. Uemura N, Knight DR, Shen J, et al. Increased myocardial infarct size because of reduced coronary collateral blood flow in beagles. Am JPhysiol 1989;257(6 Pt 2): H1798-H1803. 3. El-Sherif N, Scherlag BJ, Lazarra R, et al. Reentrant ventricular arrhythmias in the late myocardial infarction period. 1. Conduction characteristics in the infarcted zone. Circulation 1977;55:686-702. 4. Wit AL, Allessie MA, Bonke FIM, et al. Electrophysiologic mapping to determine the mechanism of experimental ventricular tachycardia initiated by premature impulses. Experimental approach and initial results demonstrating reentrant excitation. Am J Cardiol 1982;49:166-185.

5. Kramer JB, Saffitz JE, Witkowski FX, et al. Intramural reentry as a mechanism of ventricular tachycardia during evolving canine myocardial infarction. Circ Res 1985; 56:736-754. 6. Walker MJA, Cirtis MJ, Hearse DJ, et al. The Lambeth Conventions: Guidelines for the study of arrhythmias in ischemia, infarction and reperfusion. Cardiouasc Res 1988;22:447-455. 7. Hano O, Mitsuoka T, Matsumoto Y, et al. Arrhythmogenic properties of the ventricular myocardium in cardiomyopathic Syrian hamster, BIO 14.6 strain. Cardiovasc Res 1991;25:49-57. 8. Weber KT, Pick R, Silver MA, et al. Fibrillar collagen and remodeling of dilated canine left ventricle. Circulation 1990;82: 1387-1401. 9. Einzig S, Detloff BLS, Borgwardt BK, et al. Cellular electrophysiological changes in "round heart disease" of turkeys: A potential basis for dysrhythmias in myopathic

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ventricles. Cardiouasc Res 1981;15:643— 21. Estes HE Jr, Entman ML, Dixon HB, 651. et al. The vascular supply of the left ven10. Fein FS, Capasso JM, Aronson RS, et al. tricular wall: Anatomic observations, plus a hypothesis regarding acute events in Combined renovascular hypertension in diabetes in rats: A new preparation for coronary artery disease. Am Heart J 1966;71:58-67. congestive cardiomyopathy. Circulation 1984;70:318-330. 22. Gardner PI, Ursell PC, Due Pham T, 11. Poll DS, Marchlinski FE, Buxton AE, et al. Experimental chronic ventricular et al. Sustained ventricular tachycardia in tachycardia: Anatomic and electrophysiopatients with idiopathic dilated cardiomylogic substrates. In Josephson ME, Wellens HJJ (eds): Tachycardias: Mechaopathy: Electrophysiologic testing and lack of response to antiarrhythmic drug therapy. nisms, Diagnosis, Treatment. PhiladelCirculation 1984;70:451-456. phia: Lea and Febiger; 1984:29. 12. Milner PG, DiMarco JP, Lerman BB. Elec- 23. de Bakker JMT, van Capelle FJL, Janse trophysiologic evaluation of sustained MJ, et al. Macroreentry in the infarcted human heart: The mechanism of ventricventricular tachyarrhythmias in idiopathic dilated cardiomyopathy. Pacing ular tachycardias with a "focal" activation pattern. J Am Coll Cardiol 1991; 128: Clin Electrophysiol 1988;ll:562-568. 1005-1015. 13. Barber MJ, Mueller TM, Henry DP, et al. Transmural myocardial infarction in the 24. Josephson ME, Horowitz LN, Farshidi A, dog produces sympathectomy in noninet al. Recurrent sustained ventricular farcted myocardium. Circulation 1983; tachycardia. I. Mechanisms. Circulation 1978;57:431-439. 67:787-796. 14. Zipes DP. Influence of myocardial 25. Horowitz LN, Josephson ME, Harken AM. ischemia and infarction on autonomic Epicardial and endocardial activation innervation of heart. Circulation 1990; during sustained ventricular tachycardia 82:1095-1105. in man. Circulation 1980;61:1227-1238. 15. Brugada P, Waldecker B, Kersschot Y, 26. Mickleborough LL, Harris L, Downar E, et et al. Ventricular arrhythmias initiated al. A new intraoperative approach for by programmed stimulation in four groups endocardial mapping of ventricular tachyof patients with healed myocardial infarccardia. JThorac Cardiouasc Surg 1988;95: 271-280. tion. JAm Coll Cardiol 1986;8:1035-1040. 16. Roy DE, Marchand E, Theroux P, et al. 27. Mason JW, Stinson EB, Oter PE, et al. The mechanism of ventricular tachycardia Programmed ventricular stimulation in survivors of an acute myocardial infarcin humans determined by intraoperative recording of the electrical activation tion. Circulation 1985;72:487-494. 17. Kuck KH, Costard A, Schlulter M, et al. sequence. Int J Cardiol 1985;8:163-172. Significance of timing programmed elec- 28. Josephson ME, Buxton AE, Marchlinski FE, et al. Sustained ventricular tachytrical stimulation after acute myocardial cardia in coronary artery disease— infarction. J Am Coll Cardiol 1986;8: evidence for reentrant mechanism. In: 1279-1288. 18. Downar E, Janse MJ, Durrer D. The effect Zipes DP, Jalife J (eds): Cardiac Electroof acute coronary artery occlusion on physiology and Arrhythmias. Orlando: subepicardial transmembrane potentials Grune & Stratton; 1985:409-418. in the intact porcine heart. Circulation 29. El Sherif N, Smith RA, Evans K. Canine 1977;56:217-224. ventricular arrhythmias in the late 19. Fenoglio JJ, Due Pham T, Harken AM, et myocardial infarction period: Epicardial mapping of reentrant circuits. Circ Res al. Recurrent sustained ventricular tachy1981;49:255-271. cardia: Structure and ultrastructure of subendocardial regions in which tachy- 30. Karagueuzian HS, Fenoglio JJ, Weiss MB, et al. Protracted ventricular tachycardia originates. Circulation 1983; cardia induced by premature stimulation 68:518-533. in the canine heart after coronary artery 20. Fulton WFM. The dynamic factor in occlusion and reperfusion. Circ Res 1979; enlargement of coronary arterial anasto44:833-846. moses, and paradoxical change in the subendocardial plexus. Br Heart J 1964; 31. Mehra R, Zeiler R, Cough WB, et al. Reentrant ventricular arrhythmias in the 26:39-50.

MAPPING IN EXPLANTED HEARTS late myocardial infarction period. 9. Electrophysiologic-anatomic correlation of reentrant circuits. Circulation 1983;67: 11-24. 32. Gessman LJ, Agarwal JB, Endo T, et al. Localization and mechanism of ventricular tachycardia by ice mapping 1 week after the onset of myocardial infarction in dogs. Circulation 1983;68:657-666. 33. de Bakker JMT, van Capelle FJL, Janse MJ, et al. Reentry as a cause of ventricular tachycardia in patients with chronic ischemic heart disease: Electrophysiologic and anatomic correlation. Circulation 1988;77:589-606. 34. De la Fuente D, Sasyniuk B, Moe GK. Conduction through a narrow isthmus in isolated canine atrial tissue: A model of the WPW syndrome. Circulation 1971; 44:803-809. 35. Fast VG, Kleber AG. Cardiac tissue geometry as a determinant of unidirectional conduction block: Assessment of microscopic excitation spread by optical mapping in patterned cell cultures and in a computer model. Cardiovasc Res 1995; 29:697-707. 36. Maglaveras N, de Bakker JMT, van Capelle FJL, et al. Activation delay in healed myocardial infarction: A comparison between model and experiment. Am J Physiol 1995;269(4 Pt 2):H1441-H1449. 37. de Bakker JMT, Coronel R, Tasseron S, et al. Ventricular tachycardia in the infarcted Langendorff-perfused human heart: Role of the arrangement of surviving cardiac fibers. J Am Coll Cardiol 1990;15:1594-1607. 38. Dillon S, Allessie MC, Ursell PC, Wit AL. Influence of anisotropic tissue structure on reentrant circuits in the epicardial border zone of subacute canine infarcts. CircRes 1988;63:182-206. 39. Spach MS, Miller WT, Dolber PC, et al. The functional role of structural complexities in the propagation of depolar-

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ization in the atrium of the dog. Cardiac conduction disturbances due to discontinuities of effective axial resistivity. Circ Res 1982;50:175-191. 40. Ursell PC, Gardner PI, Albala A, et al. Structural and electrophysiological changes in the epicardial border zone of canine myocardial infarcts during infarct healing. CircRes 1985;56:436-451. 41. Meyerburg RJ, Gelband H, Nilsson K, et al. Long-term electrophysiologic abnormalities resulting from experimental myocardial infarction in cats. Circ Res 1977;41: 73-84. 42. Spear JF, Horowitz LN, Hodess AB, et al. Cellular electrophysiology of human myocardial infarction, I: Abnormalities of cellular activation. Circulation 1979;59: 247-256. 43. Luke RA, Saffitz JE. Remodeling of ventricular conduction pathways in healed canine infarct border zones. J Clin Invest 1990;87:1594-1602. 44. Hoyt RH, Cohen ML, Saffitz JE. Distribution and three-dimensional structure of intercellular junctions in canine myocardium. Circ Res 1988;64:563-574. 45. de Bakker JMT, van Capelle FJL, Janse MJ, et al. Slow conduction in the infarcted human heart. 'Zigzag' course of activation. Circulation 1993;88:915-926. 46. Anderson KP, Walker R, Urie P, et al. Myocardial electrical propagation in patients with idiopathic dilated cardiomyopathy. J Clin Invest 1993;92:122-140. 47. de Bakker JMT, van Capelle FJL, Janse MJ, et al. Fractionated electrograms in dilated cardiomyopathy: Origin and relation to abnormal conduction. J Am Coll Cardiol 1996;27:1071-1078. 48. Kawara T, Derksen R, de Groot JR, et al. Activation delay after premature stimulation in chronically diseased myocardium relates to the architecture of interstitial fibrosis. Circulation 2001;104: 3069-3075.

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Chapter 17 Efferent Autonomic Innervation of the Atrium: Assessment by Isointegral Mapping Pierre L. Page, MD and Rene Cardinal PhD

Introduction Autonomic nerves affect heart rate, atrioventricular (AV) conduction, and contractile force through their efferent projections to the sinus node, AV node, and atrial musculature. These parameters have therefore been used as markers for the study of atrial innervation in the canine heart.1"3 From these previous reports, we have learned that parasympathetic postganglionic efferent neurons were localized in the right atrial (RA) ganglionated plexus, a collection of neural elements contained in a triangular fat pad on the ventral aspect of the RA free wall (pulmonary vein fat pad), as well as in a ganglionated plexus located in fatty tissues overlying the junction of the inferior vena cava and inferior left atrium (LA).4"7 Selective stimulation4'8 or surgical removal9"12 of these structures were used to demonstrate the specificity of parasympathetic efferent innervation of the sinus node and AV node that may

occur through the RA and inferior LA ganglionated plexi, respectively. However, little information was available on functional pathways to regions of the atria other than the sinus and AV nodes. In addition, a precise knowledge of the anatomical distribution of autonomic neural elements may have important implications for clinical procedures such as those used in cardiac surgery or during invasive electrophysiological ablation sessions. In this chapter, new information on the autonomic innervation of the atrium resulting from experimental and clinical work based on repolarization mapping is discussed. Atrial Integral Distribution Mapping The repolarization phase of the cardiac action potential is very sensitive to parasympathetic and sympathetic nerve stimulation.13'14 Therefore, refractory

From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; e2003. 363

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period determinations have been used to study regional atrial effects in response to stimulation of intrathoracic autonomic nerves. However, the extrastimulus technique commonly used for refractory period measurements is limited by the fact that only one site can be sampled at a time and that each determination requires several seconds to complete. Other parameters—QRST area, activationrecovery interval, QT interval, and T wave amplitude—have been used to determine ventricular recovery properties.15'16 In conjunction with a multichannel recording system and multiple electrode arrays, the QRST area calculated from unipolar electrograms was used to assess ventricular patterns of sympathetic innervation.17 This concept was then applied to atrial unipolar electrograms to assess repolarization properties on a beatto-beat basis, thereby analyzing the spatial distribution of electrical responses induced by stimulation of specific efferent autonomic neural elements with a high degree of spatial and temporal resolution.18 In the studies summarized herein, the integral of the atrial unipolar electrogram waveform was calculated with reference to the isoelectric diastolic baseline. The distribution of integral values measured from multiple simultaneous recordings (Figure 1) was used to detect regional changes in atrial electrical events induced by stimulation of individual intrathoracic and intracardiac neural elements. To allow the analysis of atrial unipolar electrograms without interference made by ventricular QRS complexes usually occurring before the end of the atrial T wave, a complete AV block was induced by the injection of formaldehyde into the His bundle region in all of our canine studies. Unipolar electrograms were simultaneously recorded from 192 sites by means of flexible electrode tern-

Figure 1. A. Unipolar atrial electrogram recorded during control conditions (sinus rhythm, atrioventricular block, no ventricular pacing, no nerve stimulation). B. Atrial unipolar electrogram recorded during stimulation of the right vagosympathetic complex. C. Superimposition of electrograms shown in A and B. Neural stimulation affected primarily the T wave (arrows) of unipolar electrograms. The hatched area represents the integral of changes in the T wave morphology. This value was plotted on the atrial grid for each recording site and used to generate an integral distribution map.

plates sutured to the epicardial surfaces of both atria. The signals were recorded with reference to the Wilson central terminal, and amplified, filtered (0.05 to 200 Hz), multiplexed, digitized at I kHz, and stored on hard disk using a data acquisition system (Institut de Genie Biomedical, Universite de Montreal).18 Data obtained

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Figure 2. Integral distribution maps showing effects of neural stimulation in selected preparations (A through C). The grids represent a posterior view of the atria. The left atrium is displayed on the left-hand side of the map. The two upper corners of the maps correspond to the atrial appendages. Isoarea lines delimit color-coded zones including points where integral measurements were within a given 60 mV/ms range of values. Increases in T wave amplitude are shown as positive values (green to blue), whereas reductions of T wave amplitude or inversion are represented as negative values (orange to red). The major effects common to the right (A) and left (B) vagosympathetic stimulation and right atrial ganglionated plexus (C) consisted of a blue shift in the right atrial free wall. The map in panel D was obtained during nerve stimulation after an inverted Y-shaped incision was performed in the right atrium. Compared to map A, the effect of RVSC stimulation was suppressed. RVSC = right vagosympathetic complex; LVCS = left vagosympathetic complex; RAGP =right atrial ganglionated plexus (fat pad). RVSC-Y = RVSC stimulation after a Y-shaped incision in the right atrium. See color appendix.

during each period of neural stimulation were compared to control data obtained prior to stimulation. The net area (integral) under the intrinsic deflection and T wave of each atrial electrogram was computed by an integration process using a modified Simpson's technique and custom software that added the values of sample sectors multiplied by the duration of each sampling period (Institut de Genie Biomedical, Universite de Montreal).17'18 The difference between prestimulation integral and that measured

during stimulation was calculated for each recording site (Figure 1C). This value was plotted on an atrial grid using color-coded zones including points displaying values within a given range (Figure 2). The incidences, among preparations, of specific regional effects were assessed by counting the number of preparations that displayed integral changes beyond a threshold of ±60 mV/ms corresponding to 2 standard deviations of changes obtained when measurements were repeated under basal conditions.

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14

Figure 3. Histogram representing the incidence, in 12 preparations, of electrogram integral changes beyond ±60 mV/ms in each atrial region in response to nerve stimulation. Nerves: LVSC = left vagosympathetic complex; RAGP = right atrial ganglionated plexus; RVSC = right vagosympathetic complex. IAB = interatrial band; LAFW = left atrial free wall; LatRA = lateral right atrial free wall, adjacent to the AV groove; LowRA = lower half of right atrial free wall; SAN = sinus node area.

Stimulation of Autonomic Neural Elements Figure 3 shows the incidence of significant changes (>60 mV/ms) in each region of the RA and LA in a series of 12 preparations. Stimulation of the right vagosympathetic complex induced positive integral changes in the RA free wall in all preparations (Figure 2A). Interanimal variability of responses occurred, as illustrated by the fact that right vagosympathetic complex stimulation induced similar global patterns of effects, albeit with a variable distribution in the RA. The interatrial band and the LA free wall were both affected in half of the preparations. On the other hand, negative integral changes were induced in the LA in approximately half of the animals (Figure 3).

Stimulation of the left vagosympathetic complex also induced positive integral changes in the sinus node area in most preparations (Figure 2B). However, positive changes were also induced in the LA free wall in all animals. Stimulation of loci in the RA ganglionated plexus produced positive changes in the sinus node area and the RA free wall (Figure 2C). These effects were more restricted than those of the right vagosympathetic complex, encompassing smaller regions of the RA (31 ± 13 cm2 and 43 ± 10 cm2, respectively, P< 0.01). Negative integral changes of a lesser amplitude were induced in small regions of the LA free wall in less than 25% of the preparations. Efferent sympathetic neural elements in the RA ganglionated plexus were also identified following atropine administration

EFFERENT AUTONOMIC INNERVATION OF THE ATRIUM in 11 preparations, since the induction of positive integral changes in the RA was inhibited, whereas changes similar to those generated in response to right or left stellate ganglion stimulation were induced. These data indicate that the RA ganglionated plexus may contain neural elements originating from neural structures other than the right vagosympathetic complex, possibly from the right stellate ganglion.

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Surgical procedures designed to prevent or cure atrial fibrillation must comply with a wide range of electrophysiological conditions. Therefore, the major rationale of the development of the Maze operation led to a global, biatrial approach.22 This operation consists of an arrangement of numerous incisions performed in both the RA and the LA that are meant, first, to eliminate most opportunities for reentrant circuits, and second, to reduce the likeliEffects of Atrial Incisions hood of fibrillation in any remaining single fragment of tissue because of the Incisions in the RA free wall are myocardial mass reduction. As shown in often used in experimental models of the previous section, all parts of the reentry19'20 or during human open-heart atrium are richly innervated by efferent surgery aimed at the correction of intra- parasympathetic elements. Experiments atrial abnormalities. The effect of a Y-- were conducted in canines to determine shaped incision in the RA free wall was whether extensive surgical modification investigated through integral mapping of atrial anatomy lead to parasympathetic during vagal stimulation in 7 anes- denervation.23"26 thetized dogs. Right and left vagal stimIn the experiments with the Maze III ulation and that of the RA ganglionated procedure, the sinus bradycardia induced plexus decreased heart rate by 87 ± 25, by vagal stimulation was suppressed in all 76 ± 17, and 42 ± 15 beats per minute 9 preparations. Isointegral maps (Figure 4) (bpm), respectively, before the incision, indicated that neurally induced repolarand by 25 ± 21, 14 ± 9, and 8 ± 9 bpm, ization changes were suppressed by the respectively, after the incision (P< 0.01).21 Maze procedure in most atrial regions in Figure 2D shows an example of suppres- 7 of 9 preparations. Neural effects induced sion of integral changes induced by right by left vagal stimulation persisted in the vagal stimulation. The incidence of sup- Bachmann's bundle region (4 of 9) and in pression of significant neural effects after the superior RA (3 of 9), and in the supethe incision suggested the following con- rior RA (3 of 9) during right vagal stimuclusions: (1) The denervated area included lation. We concluded that the Maze III the rostral RA encompassing the anterior procedure induces parasympathetic denpart of the tricuspid ring. (2) The supe- ervation of the sinus node in canine rior LA and, to a lesser extent, the infe- preparations. The procedure also interrior LA are innervated directly by the feres with parasympathetic innervation right and/or the left vagal nerve. (3) Post- of atrial regions remote from the sinus ganglionic axons of the RA ganglionated node, mainly the inferior RA and LA. The plexus may innervate the LA by coursing extent to which this effect contributes to through the RA free wall. Thus, incision the antiarrhythmic mechanism of the in the RA induces parasympathetic procedure and whether this effect will denervation of a significant portion of persist over the long term is not yet the RA. established.

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Figure 4. Integral maps during stimulation of the right vagosympathetic complex in a dog experiment before and 90 minutes after a Maze procedure. In this example, neurally induced repolarization changes persisted in the interatrial band but were completely suppressed on the right atrial free wall. See color appendix.

Stimulation of Intrinsic Cardiac Ganglia in Humans The implication of the autonomic nervous system in the occurrence and maintenance of postoperative atrial arrhythmias motivated us to investigate the regional distribution of neural terminations throughout the atrium in human subjects during routine open-heart surgery. In 7 patients scheduled for elective coronary bypass procedures, atrial mapping was performed during normothermic cardiopulmonary bypass.27 Multielectrode templates containing 128 unipolar contacts were sutured onto the epicardial surface of the RA and the LA. In order to allow the analysis of whole atrial activation, a programmed stimulation protocol independently applied to the RA was used to increase the PR interval. This protocol was performed during the control acquisition and during the stimulation of the RA ganglionated plexus (i.e., the pulmonary vein fat pad). The stimulation of the fat pad

decreased the sinus rate from a mean of 73 ± 12 bpm to 52 ± 9 bpm, indicating the recruitment of parasympathetic neural elements within the RA ganglionated plexus. The map shown in Figure 5 shows cumulative data obtained in all 7 patients. The results show that the changes occurred only in the RA and in the RA appendage. The area encompassed by significant changes has been calculated with planimetry of the maps. Neural stimulation induced changes in 32% of the surface of the RA and in 8.5% of the surface of the LA; however, although mathematically significant, changes noted on the LA were always measured on a single, isolated electrode, thus not reflecting an actual local neural effect. These data suggest that the electrophysiological effects of stimulation of the RA ganglionated plexus in humans are parasympathetic, as also demonstrated by others,28 and appear to be more specific to the sinus node region than in the dog. The reasons for these observations are not yet well understood.

EFFERENT AUTONOMIC INNERVATION OF THE ATRIUM

369

Figure 5. Cumulative integral mapping data obtained in 7 patients during electrical stimulation of the right atrial ganglionated plexus. Effects are restricted in the right atrial free wall and the sinus node area. See color appendix.

Summary and Conclusion

The spatial distribution of repolarization changes induced by neural stimulation can be determined using integral distribution maps obtained from multiple atrial recordings.18 The data so obtained indicate that the autonomic nervous system displays a heterogeneity of atrial electrical responses when its various extrinsic and intrinsic neural elements are stimulated electrically. In our experiments, consistent atrial electrogram integral changes were induced in the RA free wall during stimulation of the right and left vagosympathetic com-

plexes as well as the RA ganglionated plexus. Such integral changes were also induced by vagal stimulation in other atrial regions (Figures 2 and 3). Our data also indicate that stimulation of loci in the RA ganglionated plexus induced changes in regions of the RA remote from the sinus node region and, in about a third of the animal preparations, also in the LA, indicating that neural elements in the RA ganglionated plexus project to many more atrial sites than suggested previously.6'9"12'29'30 Other work performed in laboratory used intrapericardial dissections to delineate the contribution of specific neural structure to atrial innervation.31 Interestingly,

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the data from these studies have sugReferences gested that (1) parasympathetic dener1. Randall WC, Priola DV, Ulmer RHG. A vation induced by the ablation of the RA functional study of the distribution of fat pad largely exceeds the region of the cardiac sympathetic nerves. Am JPhysiol sinus node, and (2) the RA fat pad con1963'205' 1227—1231 tains parasympathetic axons from the 2. GeisWP, Kaye MP, Randall WC. Major right vagal nerve that terminate in both autonomic pathways to the atria and S-A and A-V nodes of the canine heart. Am J atria and also contains elements from the Physiol 1973;224:202-208. left vagal nerve that distribute only to 3. Stuesse SL, Wallick DW, Levy MN. Autothe RA, as the LA receives parasympanomic control of right atrial contractile thetic axons directly from the left vagus. strength in the dog. Am JPhysiol 1979;236: In another group of studies, the interaH860-H865. 4. Lazzara R, Scherlag BJ, Robinson MJ, trial septum was mapped using an inflatSamet P. Selective in situ parasympaable balloon electrode array.32'33 These thetic control of the canine sinoatrial and studies demonstrated that the atrial atrioventricular nodes. Circ Res 1973;32: septum is richly innervated by parasym393-401. pathetic efferent pathways coursing near 5. Randall WC, Ardell JL, Calderwood D, et al. Parasympathetic ganglia innervating the the superior vena cava and via the RA canine atrioventricular nodal region. J ganglionated plexus. Tissues near the Autonom Nerv Syst 1986;16:311-323. pulmonary artery or inferior vena cava 6. Randall WC, Ardell JL, Wurster RD, do not appear to play a significant role in Miloslavljevic M. Vagal postganglionic septal innervation. innervation of the canine sinoatrial node. JAuton Nerv Syst 1987;20:13-23. Integral changes identified in the pre7. Gagliardi M, Randall WC, Bieger D, et al. sent study are consistent with heterogeActivity of in vivo canine cardiac plexus neous modifications of repolarization. The neurons. Am JPhysiol 1988;255(4 Pt 2): nonuniform distribution of atrial refracH789-H800. tory period shortening is a well-known 8. Butler CK, Smith FM, Cardinal R, et al. Cardiac responses to electrical stimulaeffect of vagal stimulation.13'14 Using tion of discrete loci in canine atrial and refractory period determinations by the ventricular ganglionated plexi. Am J extrastimulus technique, Zipes et al.14 Physiol 1990;259(5 Pt 2):H1365-H1373. have shown that right vagal stimulation 9. Ardell JL, Randall WC. Selective vagal elicits greater effects in the RA than the innervation of sinoatrial and atrioventricular nodes in canine hearts. Am J LA and that shortening of atrial refracPhysiol 1986;251(4 Pt 2):H764-H773. tory periods is more pronounced during 10. Randall WC, Ardell JL. Selective parasymright than left vagal stimulation. The prepathectomy of automatic and conductile sent study not only supported these findtissues of the canine heart. Am J Physiol ings but also provided information depict1985;248(2 Pt 2):H61-H68. ing the complexity of atrial parasympa- 11. Randall WC, Wurster RD, Duff M, et al. Surgical interruption of postganglionic thetic innervation. Furthermore, the coninnervation of the sinoatrial nodal region. cept of integral distribution mapping JThorac Cardiovasc Surg 1991;101:66-74. based on multiple-site simultaneous record- 12. Mick JD, Wurster RD, Duff M, et al. ings appears to be feasible in humans Epicardial sites for vagal mediation of sinoatrial function. Am JPhysiol 1992;262 during cardiac surgical procedures. This (5 Pt 2):H1401-H1406. new concept opens the way to further 13. Alessi R, Nusynowitz M, Abildskov JA, investigate the functional anatomy of the Moe GK. Nonuniform distribution of vagal cardiac autonomic nervous system in effects on the atrial refractory period. Am humans. JPhysiol 1958;194:406-410.

EFFERENT AUTONOMIC INNERVATION OF THE ATRIUM 14. Zipes DP, Mihalik MJ, Robbins GT. Effects of selective vagal and stellate ganglion stimulation on atrial refractoriness. Cardiovasc Res 1974;8:647-655. 15. Abildskov JA, Evans AK, Lux RL, Burgess MJ. Ventricular recovery properties and the QRST deflection area in cardiac electrograms. Am JPhysiol 1980;239:H227-H231. 16. Millar CK, Kralios FA, Lux RL. Correlation between refractory periods and activationrecovery intervals from electrograms: Effects of rate and adrenergic interventions. Circulation 1985;72:1372-1379. 17. Savard P, Cardinal R, Nadeau RA, Armour JA. Epicardial distribution of ST segment and T wave changes produced by stimulation of intrathoracic ganglia or cardiopulmonary nerves in dogs. JAuton Nerv Syst 1991;34:47-58. 18. Page PL, Dandan N, Savard P, et al. Regional distribution of atrial electrical changes induced by stimulation of extracardiac and intracardiac neural elements. J Thorac Cardiovasc Surg 1995;109:377-388. 19. Frame LH, Page RL, Hoffman BF. Atrial reentry around an anatomic barrier with a partially refractory excitable gap. Circ Res 1986;58:495-511. 20. Derakhchan K, Page P, Lambert C, Kus T. Effects of procainamide and propafenone on the composition of the excitable gap in canine atrial reentry tachycardia. J Pharmacol Exp Ther 1994;270:47-54. 21. Dandan N, Do Q-B, Page P, Cardinal R. The right atrial Y-shaped incision model of atrial flutter affects parasympathetic innervation of the reentry pathway: Assessment by integral distribution mapping. Pacing Clin Electrophysiol 1996; 19 (Pt II): 705. 22. Cox JL, Schuessler RB, DAgostino HJ Jr, et al. The surgical treatment of atrial fibrillation. III. Development of a definitive surgical procedure. J Thorac Cardiovasc Surg 1991;101:569-583. 23. Do Q-B, Page P, Dandan N, Cardinal R. Maze procedure against atrial fibrillation abolishes atrial parasympathetic efferents. Cardiostim '96. Nice, France: June

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19-22, 1996. Eur J Card Pacing Electrophysiol 1996;6(Suppl 5):21. 24. Do Q-B, Page P, Dandan N, Cardinal R. The Maze III procedure for surgical treatment of atrial fibrillation abolishes parasympathetic influences on the atrium. Pacing Clin Electrophysiol 1996;9(Pt II): 628. 25. Do QB, Page P, Dandan N, Cardinal R. Effects of Maze III procedure on atrial parasympathetic innervation. Can J Cardiol 1996;12(Suppl E):145E. 26. Do QB, Page PL, Dandan N, Cardinal R. Influence of the Maze III procedure on atrial autonomic innervation: Assessment by repolarization mapping. Circulation 1996;94(Suppl I):I493-I494. 27. Page P, Corriveau MM, Cardinal R. Atrial parasympathetic innervation in the human: Assessment by intraoperative epicardial isointegral mapping. Can J Cardiol 1998;14(Suppl F):101F. 28. Carlson MD, Geha A, Hsu J, et al. Selective stimulation of parasympathetic nerve fibers to the human sinoatrial node. Circulation 1992;85:1311-1317. 29. Furakawa Y, Narita M, Takei M, et al. Differential intracardiac sympathetic and parasympathetic innervation to the SA and AV nodes in anesthetized dog hearts. Jpn JPharmacol 1991;55:381-390. 30. Wallick DW, Martin PJ. Separate parasympathetic control of the heart rate and atrioventricular conduction of dogs. Am J Physiol 1990;259:H536-H542. 31. Do QB, Page P, Dandan N, Cardinal R. Effect of intrapericardial dissections on atrial innervation: Assessment by integral distribution mapping. Can J Cardiol 1995;ll(Suppl E):95E. 32. Do QB, Page P, Dandan N, Cardinal R. Autonomic innervation of the atrial septum: Assessment by integral mapping. Can J Cardiol 1996;12(Suppl E):112E. 33. Do QB, Dandan N, Cardinal R, Page P. Etude de 1'innervation autonomique du septum interauriculaire par cartographic isointegrale chez le chien. Ann Chir 1996; 50:659-666.

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Chapter 18

Mapping of Atrial Flutter Wolfgang Schoels, MD and Nabil El-Sherif, MD

It is now very well accepted that typical atrial flutter results from single-loop reentry around a large combined functional-anatomical obstacle consisting of the circumference of both caval veins and an arc of functional conduction block in between.1'2 The rim of atrial tissue bounded by the inferior vena cava and the adjacent atrioventricular (AV) ring constitutes a crucial part of the reentrant circuit, since any obstacle within this isthmus linking both boundaries will inevitably stop the circulating wavefront. Accordingly, "isthmus ablation" has now become the treatment of choice for recurrent typical atrial flutter.3"5 It might therefore appear that there is no need for further mapping studies of this particular arrhythmia. There are still, however, several unsettled issues regarding the pathophysiology of typical and atypical atrial flutter that are of relevance for the development of future preventive therapeutic strategies and the selection of current treatment options. This relates, for example, to the nature of functional conduction block in general, to the electrophysiological properties of the intercaval region as a preferential site for extensive

conduction block, to the question on hemodynamic-electrophysiological interactions, and to the value of surface EGG characteristics for the prediction of epicardial activation patterns during atrial flutter. Standard electrophysiological mapping techniques were used to analyze activation and refractory patterns in dogs with right atrial (RA) enlargement,6 to specifically determine conduction properties and gradients of refractoriness within the intercaval region in normal canine hearts, to assess the effects of acute RA pressure load on atrial repolarization, and to correlate surface EGG characteristics and epicardial activation maps in dogs with sterile pericarditis and inducible sustained atrial flutter.7 We used a custom-designed electrode array for simultaneous recording of 127 bipolar epicardial electrograms (interpolar distance 1 to 2 mm, interelectrode distance 3 to 8 mm) from the in situ canine heart. A high-density patch-electrode (10 x 10 bipoles, interelectrode distance 1.5 mm) allowed for more detailed analysis of activation and refractory patterns in the intercaval region. Data were stored and

From Shenasa M, Borggrefe M, Breithardt G (eds): Cardiac Mapping, Second Edition. Elmsford, NY: Blackwell Publishing, Inc./Futura Division; e2003. 373

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analyzed on a 256-channel computerized multiplexer mapping system. Details of the model preparation, the recording techniques, and the methods for constructing isochronal maps have been described elsewhere.1 Activation and Refractory Patterns in the RA Enlargement Model As in the sterile pericarditis model, atrial flutter in the RA enlargement model generally results from single-loop reentry around combined functional-anatomical obstacles.8 In response to a premature stimulus, an arc of functional conduction block occurs. With an isolated short arc of block, the stimulated wavefront proceeds around both of its ends. The 2 wavelets then collide on the distal side of the arc. For either wavelet, the pathway includes the distance from the site of stimulation to the respective end of the arc and then half of its length distally. Based on the length of this pathway and the conduction velocity within atrial tissue, one might calculate the conduction time required to reach the site of collision. For reentry to occur, this conduction time must exceed the refractory period of the tissue on the proximal side of the arc. Obviously, conduction and repolarization characteristics of the atria imply rather long pathways and, thus, rather long arcs of functional conduction block. Two aspects of the normal atrial anatomy seem to facilitate reentry. The atrial vessels and, especially, the caval veins introduce large discontinuities, which might combine with arcs of block to form large functionalanatomical obstacles. Furthermore, the atrial surface area is relatively small, being electrically isolated from the ventricles by the AV ring. If one end of an arc of block occurring in response to a premature stimulus reaches the AV ring,

bidirectional activation of the tissue on the distal side of this arc is no longer possible. Instead, only one stimulated wavefront might proceed around the free end of the arc, reaching the distal AV ring connection as the last site to be activated. This sudden increase in the length of the effective pathway seems to facilitate reentry, possibly explaining why most reentrant circuits underlying typical and atypical atrial flutter reveal some spatial relationship to the AV ring. Although there is some temporal dispersion of refractoriness even in normal hearts, the transition of areas with relatively long refractory periods, typically located below the superior vena cava and in the lateral RA wall, and areas with shorter refractoriness, predominantly found in the left atrium (LA), is gradual for most of the epicardial surface. Only a few small areas exhibit local gradients of refractoriness reaching 20 ms or more. In response to premature atrial stimulation, short arcs of functional conduction block tend to coincide with respective areas (Figure 1). In enlarged atria, the temporal dispersion of refractoriness is not much different. However, the refractory pattern appears to be much more inhomogeneous, with areas of short and long refractoriness being commonly located next to each other. This spatiotemporal dispersion of refractoriness seems to underlie the occurrence of regional conduction block, since there is generally a compelling correlation between the location of these arcs of block and the isochrones of refractoriness (Figure 2). At individual sites, a lack of correlation might indicate that anisotropic conduction properties are also of some relevance. Thus, our interpretation would be that the long arcs of functional conduction block seen during atrial flutter are primarily based on local inhomogeneities in repolarization with anisotropic conduction properties serving as a modifying factor.

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Figure 1. Epicardial refractory map (left panel) and endocardia! activation map during premature atrial stimulation (right panel) in a normal dog. The site of stimulation is indicated by E. Here and in all subsequent figures in this chapter, the epicardial atrial surface is displayed as a planar projection of a posterior view. The atria are separated from the ventricles along the atrioventricular ring, the atrial appendages are incised inferiorly and unfolded. The refractory map shows a gradual transition from areas of short refractoriness to areas with longer refractoriness. Only at a few sites, local gradients in refractoriness of 20 ms occur. During S2 stimulation, short arcs of functional conduction block (the heavy solid lines) occur at those very sites.

Figure 2. Epicardial refractory map (l