Vital Lung Function (Vital Guides)

Vital

LUNG FUNCTION Your essential reference for the management and assessment of lung function Rachel Booker RGN DN (Cert) HV Independent Specialist Respiratory Nurse

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LUNG FUNCTION

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Vital

LUNG FUNCTION Your essential reference on lung function testing Rachel Booker RGN DN (Cert) HV Independent Specialist Respiratory Nurse

CLASS HEALTH • LONDON

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Text © Rachel Booker 2008 © Class Publishing (London) Ltd 2008 All rights reserved. Without limiting the rights under copyright reserved above, no part of this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording or otherwise), without the prior written permission of the above publisher of this book. The author asserts her rights as set out in Sections 77 and 78 of the Copyright Designs and Patents Act 1988 to be identified as the author of this work wherever it is published commercially and whenever any adaptation of this work is published or produced including any sound recordings or films made of or based upon this work. NOTICE The information presented in this book is accurate and current to the best of the author’s knowledge. The author and publisher, however, make no guarantee as to, and assume no responsibility for, the correctness, sufficiency or completeness of such information or recommendation. The reader is advised to consult a doctor regarding all aspects of individual health care. Printing history First published 2008 The author and publishers welcome feedback from the users of this book. Please contact the publishers. Class Publishing, Barb House, Barb Mews, London W6 7PA, UK Telephone: 020 7371 2119 Fax: 020 7371 2878 [International +4420] Email: [email protected] A CIP catalogue for this book is available from the British Library ISBN 13: 978 1 85959 161 1 ISBN 10: 1 85959 161 2 10 9

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Edited by Caroline Taylor Designed and typeset by Martin Bristow Diagrams by David Woodroffe Printed and bound in Slovenia by Delo Tiskarna by arrangement with Korotan, Ljubljana

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Contents Introduction

9

Acknowledgements

10

Foreword

11

1 Peak expiratory flow measurement Definition Uses Limitations Peak expiratory flow meters The Wright meter The Mini Wright meter Other meters Reference values Peak expiratory flow scales Care and maintenance Recording peak expiratory flow Using PEF to diagnose asthma Reversibility testing Exercise testing Serial PEF recording Patient and carer information

13 13 13 14 15 15 15 15 16 17 17 18 20 20 20 21 23

2 Spirometry equipment Volumetric spirometers Bellows spirometer Volume displacement spirometers Flow-measuring spirometers Differential pressure pneumotachograph Turbine or rotary vane Ultrasound flow sensor Choosing a spirometer Essential features Desirable characteristics Spirometers to avoid

25 25 26 26 27 27 27 28 29 29 30 30 C ONTENTS | 5

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3 Spirometry measurements and reference values Vital capacity Relaxed vital capacity Forced vital capacity Forced expiratory volume in 1 second Measurements of airflow Peak expiratory flow FEV1/FVC ratio (FEV1% or FER) FEV1/VC ratio Mid-expiratory flow rates (MEF25, MEF50, MEF75) Presentation of results Volume/time trace Flow/volume trace Reference values

32 32 33 34 34 34 35 35 36 36 37 37 38 39

4 Using a spirometer Calibration Calibrations checks Calibration syringes Verification using a biological control Infection control Mouthpieces, nose-clips and filters Cleaning procedures Contraindications to spirometry Patient preparation Performing the test VC FVC/FEV1 Maximal flow/volume curve Technical acceptability and reproducibility Common errors: recognition and correction Sub-maximal effort Coughing Failure to expire to FVC Slow start to the forced expiratory manoeuvre Air leak Patient and carer information

41 41 41 42 43 43 44 44 45 45 47 47 48 49 50 51 51 52 52 53 53 55

5 Spirometry interpretation Normal spirometry Lung volumes – FVC and FEV1 FEV1/FVC (FEV1%) and FEV1/VC

57 57 57 58

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The volume/time trace The flow/volume trace Obstructive spirometry Lung volumes – FEV1 and FVC FEV1/FVC (FEV1%) and FEV1/VC The volume/time trace The flow/volume trace Restrictive spirometry Lung volumes – FEV1 and FVC FEV1/FVC (FEV1%) and FEV1/VC The volume/time trace The flow/volume trace Severely obstructed spirometry Lung volumes – FEV1 and FVC FEV1/FVC (FEV1%) and FEV1/VC The volume/time trace The flow/volume trace Maximal flow/volume traces Fixed intra-thoracic and extra-thoracic obstruction Variable intra-thoracic obstruction Variable extra-thoracic obstruction Standard deviation scores

59 59 59 60 61 62 62 63 64 64 65 66 66 67 67 67 67 69 70 70 71 72

6 Static lung volumes and gas transfer Static lung volumes Residual volume (RV) Total lung capacity (TLC) Functional residual capacity (FRC) Residual volume/total lung capacity ratio (RV/TLC) Measuring static lung volumes Nitrogen washout Multiple breath helium dilution Body plethysmography Diffusing capacity and gas transfer Conditions that reduce the efficiency of oxygen transfer Diffusing capacity test Single breath DLco Krogh constant (Kco) Patient and carer information

75 75 75 76 77 77 78 78 79 81 82 82 83 83 83 85

7 Blood gases and pulse oximetry Arterial blood gases (ABG)

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PaCO2 pH Acid–base balance HCO3– PaO2 Respiratory failure Type I respiratory failure Type II respiratory failure Compensated respiratory failure Collecting arterial blood gas samples SpO2 Haemoglobin–oxygen dissociation curve Pulse oximetry Using a pulse oximeter Limitations of pulse oximetry Patient and carer information

89 90 91 92 92 92 92 93 94 94 96 97 97 98 99 100

Glossary

103

Useful addresses and contacts

107

Feedback form

109

Priority order form

112

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Introduction

Dear Colleagues Welcome to Vital Lung Function This practical book is designed for you to use during your work in general practice, the community or the hospital. It will enable you to record simple tests of lung function accurately and to interpret the results. The book gives you the vital information you need to understand the commonly used tests of lung function. The contents list allows you to pinpoint specific topics easily. The text is divided into seven distinct chapters with topics clearly presented. At the end of each topic you will find one or more vital points that will give you the essential information in a few key words. At the end of some of chapters you will find an associated section called patient and carer information. These pages can be enlarged and photocopied for your patients. You will find useful addresses and contact numbers at the end of the book, as well as references and further reading, and details of training courses. There is also a feedback form on page 109, which I hope you will use. I would welcome comments and suggestions for improvements. I hope that you will find this book useful, time-saving and vital to your everyday clinical practice.

Rachel Booker

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Acknowledgements

With thanks to my colleagues at Education for Health for their unfailing support and encouragement, my numerous clinical colleagues and friends for their words of wisdom and enlightenment, and my family for their support, tolerance and forbearance of my periods of distraction and the time I spend away from them.

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Foreword

This reference book is easy to use and will be a cost-effective guide for respiratory nurses, primary care practitioners and any healthcare professional who needs a working understanding of lung function testing. It is an excellent guide to the essential diagnostic tests for lung disease spirometry and blood gases, and offers an introduction to other lung function tests, such as static lung volume measurement. This will be a very popular book and ideally complements Vital COPD by the same author.

Dr Brendan G Cooper Consultant Clinical Scientist Association for Respiratory Technology and Physiology

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Peak expiratory flow measurement

■ Peak expiratory flow (PEF) is probably the most widely used and familiar lung function test ■ There are a variety of small, cheap peak flow meters available, making measurement of PEF practical for all healthcare settings, and in patients’ homes or workplaces ■ PEF meters are available on prescription in the UK

DE F I NI TI O N ‘Peak expiratory flow (PEF) is the highest flow achieved from a maximal forced expiratory manoeuvre started without hesitation from a position of maximal lung inflation’ (Miller et al 2005) ■ PEF occurs very early in a forced expiration – within the first tenth of a second ■ The major contribution to the PEF is airflow from the larger generations of airways ■ PEF can be measured with a flow-measuring spirometer, but is most commonly measured with a PEF meter

USE S ■ PEF measurement is particularly useful for the diagnosis and monitoring of asthma ■ It is also useful as part of a personalised ‘asthma action plan’

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V ITAL POINT ✱ The range of the normal reference (predicted) value for PEF in any individual is wide. Values within 100 litres/minute (L/min) of the reference value for men and 85 L/min for women are considered to be within the normal range. You should compare a PEF reading with that patient’s personal ‘best ever’ PEF, if this is known, rather than the reference value

■ PEF is essential for objective assessment of the severity of an asthma attack and to measure the patient’s response to emergency treatment ◆ PEF > 75% of patient’s best or reference value = mild attack ◆ PEF 50–75% of patient’s best or reference value = moderate attack ◆ PEF 33–50% of patient’s best or reference value = severe attack ◆ PEF < 33% of patient’s best or reference value = life-threatening attack

V ITAL POINT ✱ Measuring the PEF is essential for assessing the severity of an attack of asthma and its response to therapy

LI M I TA TI O NS ■ PEF measures airflow from large, conducting airways. Chronic obstructive pulmonary disease (COPD) is characterised by small airway obstruction ■ PEF is unreliable for detecting airflow obstruction in early COPD and can seriously underestimate the degree of obstruction in more advanced COPD ■ PEF is unaffected by diseases that reduce lung volumes without causing airflow obstruction, eg fibrosing alveolitis ■ PEF measurement is an effort-dependent test and is therefore prone to inaccuracy and variability 1 4 | V I T AL LU NG F UNC T I O N

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VITAL POINTS ✱ If PEF measurements are to be done by patients at home, they must be adequately instructed in the technique and the need for absolutely maximum effort must be stressed ✱ PEF measurement is useful for the diagnosis and monitoring of asthma, but is of limited use in other respiratory diseases ✱ A single, normal or abnormal PEF reading cannot be used to disprove or confirm asthma. Further tests such as serial PEF readings or reversibility tests are necessary

P E F M E TE RS The Wright meter ■ The original Wright peak flow meter was developed in the 1950s by Wright and McKerrow ■ It consists of a spring-loaded, pivoted vane inside a metal drum ■ When the patient blows into the meter the vane rotates in proportion to the maximum flow generated

The Mini Wright meter ■ The Wright meter is too expensive and bulky for everyday use and has been largely superseded by the Mini Wright meter (Figure 1.1) ■ When the patient blows into the meter the plastic baffle moves down inside the body of the meter, pushing the pointer down the scale on the outside of the meter

Other meters ■ There are a large number of other cheap, portable meters available ■ Electronic PEF meters can store PEF recordings for downloading into a computer record. They record both the PEF and the time at which the measurement was taken

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■ There may be variations between the readings obtained with different makes of meter and between different meters of the same make

Scale Pointer Spring

Air exhaust Metal rod

Plastic baffle Mouthpiece

Figure 1.1 Mini Wright peak flow meter

V ITAL POINT ✱ Repeat or serial measurements of PEF should be done using the same meter, and if possible, the patient’s own meter

RE F E RE NC E V A LUES ■ The reference value for PEF in an adult is dependent on: ◆ Age ◆ Gender ◆ Height ■ In children the reference value for PEF is dependent on height alone ■ The most widely accepted reference values were determined from large population surveys conducted in the 1980s (Nunn & Gregg 1989) 1 6 | V I T AL LU NG F UNC T I O N

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P E F SC A LE S ■ The original Wright–McKerrow scale for PEF meters was determined using a biological method. Subsequent research, using accurate, computerised, flow-generating equipment, demonstrated that the Wright–McKerrow scale does not accurately reflect flow rates, particularly at the lower and higher ends of the range ■ A new scale, overcoming the problems of the Wright–McKerrow scale, has been introduced across Europe. All meters sold within the EU now use the new scale, complying with EU standard EN13826

VITAL POINTS ✱ All PEF meters sold in the EU use the new EU standard scale. The reference values for PEF determined by Nunn and Gregg using the Wright–McKerrow scale need to be adapted when using the new EU standard meters. Adapted reference values and an online conversion facility are available at www.peakflow.com ✱ It is important to use the correct reference value for the scale on the meter – Wright–McKerrow or EU scale. They are not interchangeable

■ To calculate a patient’s PEF as a percentage of the reference value: Patient’s reading Reference value

× 100

■ Values over 80% of the reference value are considered to be within normal limits

C A R E A ND M A I NT EN AN CE ■ PEF meters labelled for single patient use should not be used between patients ■ Meters for multiple patient use are available

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■ There is no evidence that patients have come to harm from using PEF meters, but to avoid any possible risk, both single patient use and multiple patient use meters must be regularly cleaned and disinfected according to the manufacturers’ instructions ■ The Mini Wright meter mouthpiece incorporates a one-way valve, but one-way disposable mouthpieces that prevent a patient from inhaling through the meter must be used with multiple patient use meters ■ Meters should be replaced annually. With regular use the spring becomes slack and the meter becomes inaccurate ■ Use a ‘biological control’ to check the meter in the surgery. Use a member of staff without respiratory disease, and whose PEF is known, to verify the reading on the meter

V ITAL POINTS ✱ PEF meters must be cleaned and disinfected regularly. In healthcare settings where meters are used between patients, a log should be kept of cleaning procedures and cleaning should be a regular routine ✱ One-way disposable mouthpieces reduce the possibility of cross-infection and are a minimum requirement in healthcare settings

RE C O RDI NG P E F ■ Set the pointer on the meter to zero ■ Ask the patient to stand, or sit upright. To ensure reproducibility of subsequent tests they should always be performed in the same position – standing or sitting

V ITAL POINT ✱ It is good practice to stipulate in your own local guideline or protocol whether the patient should stand or sit. Any deviation can be noted so that future tests are done in the same position

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■ Ask the patient to gently hold the meter horizontally, avoiding: ◆ Unnecessary pressure on the body of the meter ◆ Occluding the air exhaust holes at the end of the meter ◆ Obstructing the movement of the pointer ■ Ask the patient to take a maximal unforced but steady breath in. It is important to ensure absolutely maximal inhalation ■ The patient should immediately place the mouthpiece of the meter between their teeth so that the tongue does not occlude the mouthpiece, and should blow hard down the meter, without delay and with an open glottis – a short, sharp ‘huff’ ■ Record the reading on the meter and return the pointer to zero ■ Allow the patient time to recover and repeat the manoeuvre a minimum of a further two times ■ The three efforts should be within 20 L/min of each other. All three readings should be recorded to indicate any variation and highlight the best ■ If there is unacceptable variation between efforts further blows, up to a maximum of eight, can be recorded ■ Record the highest of the readings ■ Errors in technique include: ◆ Failing to take a maximal inhalation ◆ Holding the breath at maximal inhalation and delaying blowing into the meter ◆ Blocking the mouthpiece with the tongue or teeth ◆ A ‘coughing’ or ‘spitting’ exhalation technique ◆ Failure to make a maximum effort ◆ Leaks around the mouthpiece, due to blowing the cheeks out, loose-fitting false teeth or facial palsy

VITAL POINTS ✱ Demonstrate the technique to the patient. Demonstration combined with explanation is generally more effective than verbal explanation alone! ✱ Children under the age of 5–6 years are usually unable to produce reliable and reproducible PEF recordings

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USI NG P E F TO DI A GN OSE AST H MA Reversibility testing ■ Measure the PEF ■ Give a dose of short-acting β2 agonist, eg 200–400 μg salbutamol, via a dry powder inhaler or holding chamber, or 2.5–5 mg via a nebuliser, or the equivalent dose of terbutaline. Wait 15 minutes ■ Record the PEF again ■ Calculate the percentage reversibility: Post-bronchodilator PEF − Pre-bronchodilator PEF Pre-bronchodilator PEF

× 100

■ An improvement in the PEF of 20% or more and a PEF of at least 60 L/min is highly suggestive of asthma ■ In adults reversibility can also be assessed by recording the postbronchodilator PEF before and immediately at the end of a course of oral prednisolone – 30 mg/day for 14 days

V ITAL POINT ✱ Asthma is a variable condition. A negative reversibility test does not always exclude asthma. If the PEF is near that individual’s best when the test is performed then reversibility testing may be inconclusive.

Exercise testing ■ Exercise testing exploits the fact that exercise is a potent trigger of bronchoconstriction in many asthma patients ■ Measure the PEF and then get the patient to exercise for 6 minutes ■ Supervise the patient during the exercise ■ Take another reading at the end of the 6 minutes and every 10 minutes for 30 minutes

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■ Calculate the percentage change in PEF pre- and post-exercise: Pre-exercise PEF − Lowest post-exercise PEF Pre-exercise PEF

× 100

■ A drop in PEF of 20% or more and a PEF of at least 60 L/min is suggestive of asthma

VITAL POINTS ✱ Exercise testing can occasionally provoke a significant asthma attack. Short-acting ß2 agonists must be immediately available to treat this, should it occur ✱ Exercise testing in a primary care setting is only suitable for young patients or for otherwise fit adults who are accustomed to exercise and have no serious medical problems. Suitable facilities that allow the patient to exercise in safety are also essential

Serial PEF recording ■ Variability in the PEF is characteristic of asthma. The PEF is usually lower in the morning than the evening and this ‘diurnal variability’ can be used to diagnose asthma ■ Ask the patient to record the highest of three PEF readings on a chart: ◆ First thing in the morning (before using any inhalers) and ◆ In the early evening ◆ Every day ◆ For at least 2 weeks ■ Calculate the variability between the morning and evening readings: Highest PEF reading − Lowest PEF reading Highest PEF reading

× 100

■ A variability of more than 20% on at least 3 days per week over at least a 2-week period is highly suggestive of asthma PEA K EXPI RA TORY FLOW M EA SUREM ENT | 2 1

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■ Electronic storage PEF meters are now available. They can record the time of the test and will also record the quality of the effort that the patient has made

V ITAL POINTS ✱ Diurnal variability is specific to asthma, but serial PEF is a relatively insensitive test and may not be conclusive. Some people with asthma will demonstrate more than the normal 5% diurnal variability, but less than the diagnostic 20% variability ✱ Encourage patients to complete the chart as they do the readings. If they forget to take a reading they must leave the chart blank and not try to guess what the reading was! ✱ Requests to record a serial PEF for a prolonged period may not be complied with. You should be wary of a ‘pristine’ peak flow chart. It may have been filled in retrospectively and is unlikely to be accurate! The more ‘dog-eared’ the chart the more likely it is to be genuine and reliable

REFERENCES Miller MR, Hankinson J, Brusasco V et al (2005) ATS/ERS Task Force: Standardisation of lung function testing. European Respiratory Journal 26: 319–38 Nunn AJ, Gregg I (1989) New regression equations for predicting peak expiratory flow in adults. British Medical Journal 298: 1068–70

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PA T IENT A ND CARER INFORMATION: PEA K EXPIRATORY FL OW ■ A peak flow meter measures the speed, or flow rate, at which you can blow air out of your lungs. Asthma, and some other lung problems, cause the air passages in your lungs to tighten and narrow. This reduces the speed at which you can blow air out, so measuring your peak flow is a quick and easy way to assess how tight, or narrowed, your air passages are ■ To record your peak expiratory flow, or peak flow, you should: ❏ Set the pointer on the meter to zero by pushing it down with your finger all the way to the bottom of the scale ❏ Stand up straight, or sit upright. Always do your tests in the same position – either standing or sitting. It does not matter which as long as it is consistent ❏ Hold the meter on its side. Do not to squeeze it tightly and keep your hands clear of the air holes at the bottom and the pointer on the side of the meter ❏ Breathe in steadily until you cannot take in any more air. It is important to absolutely fill your lungs ❏ Once your lungs are full you should put the meter into your mouth, with the mouthpiece between your teeth so that your tongue and teeth are out of the way. Make a tight seal with your lips around of the mouthpiece ❏ Immediately after putting the meter into you mouth you need to blow as hard and as fast as you can in a short, sharp ‘huff’. Put as much effort as you can into it but try not to cough down the meter, or blow it like a trumpet with your cheeks bulging! ❏ Make a note of the reading on the meter and return the pointer to zero ❏ Take a few moments’ rest ❏ Repeat the test again at least twice more so that you have at least three recordings

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❏ Make a note of the highest of the readings, or record it on your chart, if you have been given one ❏ Your doctor or nurse will tell you how often and when to record your peak flow ■ If you have been prescribed a peak flow meter to keep at home there will be an instruction leaflet in the box. This will tell you how and when to clean your meter ■ There is a spring inside the meter and, with time and repeated use, this spring can become loose and slack. This may make your readings inaccurate. If you use the meter regularly it will need replacing every year. You should ask your doctor or nurse for a repeat prescription

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Spirometry equipment

■ The first spirometer was developed by John Hutchinson, a London surgeon, in the mid 19th century ■ A spirometer is used to measure accessible lung volumes – the volume of air that can be inhaled and exhaled (see p. 32) ■ Until recently spirometers were rarely available in primary care settings and spirometry was usually performed in secondary care ■ National COPD management guidelines highlighting the need for widespread access to spirometry (British Thoracic Society 1997, National Institute for Clinical Excellence 2004), together with payment for performing spirometry under the Quality Outcome Framework of the General Medical Services Contract (NHS Confederation and British Medical Association 2003) have led to an increase in the use of spirometers in primary care ■ There are two types of spirometer: ◆ Those that measure volume directly – volumetric spirometers ◆ Those that measure flow rates and calculate volume from flow – flow-measuring spirometers

V O LUM E TRI C SPIROMET ERS ■ Volumetric spirometers are considered the ‘gold standard’ ■ They are simple and accurate but are larger and less portable than the flow-measuring devices ■ They are commonly used in secondary care settings and lung function laboratories, but are less commonly used in primary care ■ They are of two basic types: the ‘bellows’ spirometer and the rolling-seal spirometer

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Bellows spirometer ■ This is probably the most familiar volumetric spirometer ■ Expired air is collected inside the bellows, causing it to inflate ■ A stylus is attached to the bellows and as it inflates the stylus moves downwards. The stylus presses on the pressure sensitive, recording paper attached to the front of the spirometer to make a trace ■ The bellows are calibrated so that the stylus moves the correct distance to accurately record the volume of air in the bellows onto the graduated recording paper ■ As the bellows inflate, a motor also moves the recording paper horizontally, taking 6 or 12 seconds (depending on the model of spirometer) to move the recording paper its full distance of travel ■ The stylus moving in the vertical plane and the paper moving horizontally produces a graph of volume against time on the chart paper Recording stylus

Bellows (expanded)

Chart paper

Paper motor device

Bellows (collapsed)

Expired air from subject

Figure 2.1 Bellows spirometer

Volume displacement spirometers ■ There are two types: the water-sealed and the dry rolling-seal spirometer ■ These spirometers calculate the volume of expired air by measuring the volume of water or air that the expired air displaces ■ They are simple and accurate, but are not portable and are expensive. They are used mainly in lung function laboratories 2 6 | V I T AL LU NG F UNC T I O N

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F LO W -M E A SUR I N G SPIROMET ERS ■ The currently available types of flow-measuring spirometer work on different principles. They are: ◆ The differential pressure pneumotachograph ◆ The turbine or rotary vane ◆ The ultrasound flow sensor

Differential pressure pneumotachograph ■ When air is blown down a partially obstructed tube there is a drop in air pressure beyond the obstruction ■ The extent of the pressure drop is dependent on the rate of airflow ■ In a differential pressure pneumotachograph air pressure is measured instantaneously before and after the obstruction and airflow and volume calculated from the pressure drop ■ The two most commonly used are the Lilly and the Fleisch (Figure 2.2) Lilly

Fleisch

Mesh

Capillary tubes

Differential pressure transducer

Differential pressure transducer

Figure 2.2 Pneumotachographs

Turbine or rotary vane ■ Air blown down a tube spins a low-inertia vane ■ Light from two light-emitting diodes is interrupted by each rotation of the vane ■ Each of these interruptions produces a digital pulse ■ The volume of air is calculated from the number of pulses and the flow rate from the frequency of the pulses (Figure 2.3) SPI ROM ETRY EQUI PM ENT | 2 7

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Light source

Mouthpiece

Optical sensor Swirl plate

Moving vane

Figure 2.3 Turbine spirometer

Ultrasound flow sensor ■ Ultrasound, generated by a piezoelectric crystal, is used to measure airflow in two ways: ◆ Air blown into the spirometer is broken into waves by partial obstructions inside the air tube. Each wave passing through the ultrasound beam produces a pulse proportional to its volume. The pulses are counted to calculate the volume (Figure 2.4) Struts to partially obstruct airflow

Gas flow

Piezoelectric crystal transducer

Figure 2.4 Ultrasound spirometer ◆

The second method operates on the principle that the speed of an ultrasound signal is reduced or increased depending on the speed of the airflow. Two piezoelectric crystal transducers within the airflow through the spirometer send ultrasonic pulses to each other. The time taken for a pulse to reach the other crystal is proportional to the flow rate

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C HO O SI NG A SP IROMET ER ■ If possible ‘test drive’ the spirometer for a trial period before purchasing it. You need to ensure that it is easy for you to use and is suitable for your needs! ■ Consider who will be using it and where it is going to be used. For example, you will need a robust and lightweight spirometer if it is going to be moved from room to room or site to site, or used in patients’ homes ■ Consider the running costs. All spirometers need servicing and maintenance and there will be ongoing costs, such as mouthpieces and recording paper ■ There are some essential features that you should look for, some desirable characteristics you might like to consider and some things that you should avoid

Essential features ■ The spirometer produces a hard copy of the results, including graphs of volume/time and flow/volume of sufficient size to check the results manually ■ A real-time graphic display of the patient’s effort will allow you to check and correct their technique

VITAL POINT ✱ Poorly performed spirometry is meaningless and misleading. It is impossible to check that the patient’s technique is good and that the test is reproducible and valid if your spirometer does not produce a graphical display or a hard copy of the graph ■ It should also be possible to save all the patient’s efforts so that you can select the best – from different efforts if necessary ■ The spirometer must comply with the American Thoracic Society and European Respiratory Society standards for accuracy and reliability (Miller et al 2005) SPI ROM ETRY EQUI PM ENT | 2 9

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Desirable characteristics ■ Check whether training in the use of the spirometer is provided ■ Easy-to-use software that will allow patient’s results to be downloaded into a patient database for easy storage and retrieval is a very useful feature ■ Check whether technical support services and a free ‘helpline’ are available ■ A calibration syringe is essential. Check whether this is included in the purchase price

Spirometers to avoid ■ Imported equipment with no technical support or servicing facility ■ Spirometers requiring expensive consumables, such as single patient use airflow tubes or expensive recording paper ■ Small, cheap, hand-held spirometers with no hard copy or real-time graphic facility

V ITAL POINT ✱ The purchase of a spirometer involves capital outlay and ongoing costs. It is well worth spending time examining and testing a variety of equipment, to ensure it will meet your short- and long-term needs, before deciding which to purchase

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REFERENCES Association for Respiratory Technology & Physiology (2006) The ARTP spirometry handbook. ARTP, Birmingham British Thoracic Society (1997) BTS guidelines for the management of chronic obstructive pulmonary disease. Thorax 52 (Suppl 5): S1–28 Miller MR, Hankinson J, Brusasco V et al (2005) ATS/ERS Task Force: Standardisation of lung function testing. European Respiratory Journal 26: 319–38 National Institute for Clinical Excellence (2004) Chronic obstructive pulmonary disease: National Clinical Guideline for management of chronic obstructive pulmonary disease in adults in primary and secondary care. www.nice.org.uk/page.aspx?o=cg012&c=respiratory (accessed 20 May 2006) NHS Confederation and British Medical Association (2003) Investing in general practice: The new General Medical Services Contract. February. NHS Confederation and BMA

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Spirometry measurements and reference values

■ A spirometer primarily measures lung volumes ■ Lung volumes are measured in two ways: ◆ As dynamic lung volumes during forced inspiration or expiration ◆ As static lung volumes during relaxed breathing ■ Dynamic lung volumes are most commonly measured with a spirometer – the two basic measurements are: ◆ The forced vital capacity (FVC) ◆ The forced expiratory volume in 1 second (FEV1) ■ The static lung volume measured with a spirometer is the relaxed vital capacity (RVC or VC). Other static lung volumes such as total lung capacity (TLC) or residual volume (RV) require more complicated techniques such as body plethysmography (box) or gas dilution tests (see p. 78) ■ Flow-measuring spirometers calculate lung volumes from airflow rates and therefore measure airflow directly. When volumetric spirometers are used flow rates are calculated from the lung volumes ■ The basic measurement of airflow measured during spirometry is: ◆ The ratio of FEV1 to FVC (FEV1/FVC or FEV1%). This is sometimes also referred to as the forced expiratory ratio (FER)

V I TA L C A P A C I TY ■ VC is defined as: ‘The volume, measured at the mouth, between the positions of full inspiration and full expiration’ ■ It is the total amount of air that an individual can breathe in and out of their lungs in a single maximum breath 3 2 | V I T AL LU NG F UNC T I O N

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■ The term ‘vital capacity’ was coined by John Hutchinson, the inventor of the first spirometer ■ Hutchinson believed that this volume was essential for life – hence the term ‘vital’ capacity. And he was right! VC is a good predictor of mortality from both respiratory and other causes ■ VC can be measured as both a dynamic and static measurement with a conventional spirometer ■ It can also be measured as an inspired or expired volume

Relaxed vital capacity ■ This is most often measured as the expired relaxed vital capacity (EVC or RVC) ■ The expired relaxed vital capacity is defined as: ‘The maximum volume of air that can be expired from the lungs during a relaxed, but complete expiration from a position of full inspiration’ ■ The inspiratory vital capacity is defined as: ‘The maximum volume of air that can be inspired into the lungs during a relaxed but complete inspiration from a position of full expiration’ ■ Airways are normally compressed a little during expiration, so the inspiratory vital capacity is often greater than the expiratory vital capacity – particularly in obstructive airways disease

VITAL POINT ✱ When the term VC is used it conventionally refers to the expired relaxed vital capacity

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Forced vital capacity ■ The FVC is defined as: ‘The maximum volume of air that can be expired from the lungs during a forced and complete expiration from a position of full inspiration’ ■ FVC is conventionally deemed to have been reached after a maximum of 15 seconds’ forced expiration, or when the expiratory flow rate has fallen below 0.05 L/second (L/sec)

F O R C E D E XP I RA TORY VOLU ME I N 1 SE C O ND ■ FEV1 is defined as: ‘The maximum volume of air that can be expelled from the lungs in the first second of a forced expiration from a position of full inspiration’ ■ FEV1 is the most widely used of all the lung function measurements ■ It is a simple to do, reproducible and has clearly defined reference values

V ITAL POINT ✱ FEV1 is affected in all patterns of lung disease and is a good predictor of all-cause mortality. Measurement of FEV1 is therefore one of the most useful tests for evaluating a patient with respiratory symptoms and for routine health screening

M E A SUR E M E NTS O F AIRFLOW ■ The speed at which air can be expired is reduced when airways are obstructed

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VITAL POINT ✱ Obstructive airways diseases, such as asthma and chronic obstructive pulmonary disease (COPD) are common and accurate measurement of airflow obstruction is essential for diagnosis

Peak expiratory flow ■ PEF (see p. 13) is usually measured with a peak flow meter, but can also be recorded with a flow-measuring spirometer during a forced expiratory manoeuvre ■ PEF cannot be recorded with a volumetric spirometer ■ PEF can be used from the spirometry manoeuvre for FEV1 and should be the same as the reading on an EU scale PEF meter if the technique is good

VITAL POINT ✱ The Nunn and Gregg (1989) reference values do not apply to PEF readings obtained with a spirometer. The patient’s recording must be compared with the spirometric reference values for PEF (Quanjer et al 1993)

FEV1/FVC ratio (FEV1% or FER) ■ FEV1/FVC ratio, FEV1% and FER all refer to the same measurement ■ FEV1/FVC is defined as: ‘The amount of air blown out in the first second of a forced expiration from a position of maximal inspiration expressed as a percentage of the total amount expired (regardless of time) during that forced manoeuvre’

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FEV1/VC ratio ■ The FEV1/VC ratio is defined as: ‘The amount of air expired during the first second of a forced expiration from a position of maximal inspiration expressed as a percentage of the total amount expired during a relaxed vital capacity manoeuvre’

V ITAL POINT ✱ In patients with airflow obstruction air trapping may occur during a forced expiratory manoeuvre, causing the VC to be greater than the FVC. In this case FEV1/VC, rather than the FEV1/FVC, should always be calculated as this will be a more reliable indicator of airflow obstruction

Mid-expiratory flow rates (MEF25, MEF50, MEF75) ■ Mid-expiratory flow rates are expressed in litres per second (L/sec) ■ The MEF25 is defined as: ‘The maximum flow achievable when 75% of the FVC has been expired’ ■ In other words it is the maximum flow rate achievable when 25% of the FVC remains in the lungs ■ Similarly, the MEF75 refers to the maximum flow achievable when 75% of the FVC remains in the lungs and the MEF50 is the maximum flow rate achievable when the lungs are half-empty ■ These are less commonly used measures of airflow and have less robust reference values. Recordings of more than 50% of the reference value are considered to be within normal limits

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VITAL POINTS ✱ Reduced mid-expiratory flow rates are sometimes cited as a sign of very early airflow obstruction, but this is not universally accepted and its validity is controversial ✱ Some spirometers use the North American equivalent of MEF: the forced expiratory flow (FEF25, FEF50 and FEF75). The FEF25 is equivalent to the MEF75, and so on. FEF25–75 refers to the flow rate generated between the middle part of a forced expiratory manoeuvre between 25% and 75% of the FVC

P R E SE NTA TI O N OF RESU LT S ■ The FEV1, FVC and VC are reported as both the measured volume and as a percentage of the reference value for someone of that age, height, gender and ethnicity ■ To calculate the FEV1, FVC and VC as a percentage of their respective reference value: Measured volume Reference value

× 100

■ The FEV1/FVC (and/or FEV1/VC if the VC is greater than the FVC) is calculated as: Measured FEV1



Measured FVC or VC

× 100

■ The FEV1/FVC and or FEV1/VC are sometimes also presented as a percentage of the reference value on the spirometer printout ■ The results of the forced expiratory manoeuvre are also presented graphically as the volume/time and flow/volume trace

Volume/time trace ■ This is a graph of the volume exhaled during a forced expiratory manoeuvre against the time taken to exhale fully ■ Volume in litres is plotted on the vertical axis and time in seconds on the horizontal (Figure 3.1) S P I ROM ETRY M EA SUREM ENTS A ND REFERENC E VA LUES | 3 7

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FVC

Volume (litres)

FEV1

1

2

3

4

5

6

Time (seconds)

Figure 3.1 Normal volume/time trace

Flow/volume trace ■ This is a graph of the expiratory flow rates during a forced manoeuvre against the volume expired (Figure 3.2)

Peak expiratory flow

Flow (litres/second)

FEF25 (MEF75) FEF50 (MEF50)

FEF75 (MEF25) FVC

Volume (litres)

Figure 3.2 Normal flow/volume trace 3 8 | V I T AL LU NG F UNC T I O N

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■ Flow rate (expressed in L/sec or L/min depending on the spirometer) is plotted on the vertical axis and volume in litres on the horizontal axis

VITAL POINT ✱ The flow/volume trace is an alternative or additional way of looking at a graphical representation of a forced expiratory manoeuvre. It can be useful for detecting early airflow obstruction

RE F E RE NC E V A LU ES ■ Lung volumes are dependent on gender, age, height and ethnic group: ◆ Males have larger lung volumes than females ◆ Lung volumes decline with age after 21 years old ◆ Tall individuals have larger lung volumes than shorter people ◆ The anthropometric characteristics of different ethnic groups have an affect on lung volumes. For example Afro-Caribbean people have a smaller thorax in relation to their overall height than white people and consequently have smaller lung volumes

VITAL POINTS ✱ Correction factors can be applied to reference values for European populations that adjust for ethnicity. European reference values can be multiplied by a factor of 0.9 for people of Japanese, Polynesian, Indian, Pakistani and African descent. However, if a correction factor is applied this must be reported and must also be consistently applied in subsequent tests ✱ It may be difficult to determine whether to apply a correction factor if the individual is of mixed ethnic background and their exact ethnic group cannot be determined

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■ In order to determine whether an individual’s spirometry is normal or abnormal their results will need to be compared with the ‘normal’ reference value ■ Large population surveys in different populations have been conducted to determine the normal reference values for spirometry. Tables of the results, showing the mean reference value for individuals within a range of age and height, and either gender, are available ■ Regression equations have been developed from these population surveys. The exact reference range for any individual can be calculated. The regression equations for use in European subjects are available at www.spirxpert.com/epidemiol7.htm

V ITAL POINTS ✱ Actual data from population surveys are limited for adolescents and the elderly. Robust data are only available for the 18–70-year-old age range. These data are extrapolated to cover adolescents and the over-70-year-old age group and are therefore less robust. This needs to be borne in mind when interpreting spirometry from these individuals ✱ A large number of reference values for different populations have been produced. The standard reference equations for use in Europe are those produced for the European Community for Coal and Steel (Quanjer et al 1993)

REFERENCES Nunn AJ, Gregg I (1989) New regression equations for predicting peak expiratory flow in adults. British Medical Journal 298: 1068–70 Quanjer PH, Tammeling GJ, Cotes JE et al (1993) Lung volumes and forced ventilatory flows: Report working party standardization of lung function tests, European Community for Steel and Coal. Official statement of the European Respiratory Society. European Respiratory Journal 6 (Suppl 16): 5–40

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Using a spirometer

■ The accuracy of spirometry equipment must be checked regularly ■ Spirometry equipment is used between patients, so appropriate cleaning, disinfection and sterilisation procedures must be undertaken ■ Personnel conducting spirometry tests must be trained to: ◆ Understand contraindications ◆ Get a good technique from a patient ◆ Recognise poor technique ◆ Know how to correct it

C A LI BR A TI O N Calibration checks ■ Spirometers are calibrated to record the true volume of air exhaled into or through them ■ The accuracy of this calibration must be regularly checked with a calibration syringe to ensure that the spirometer continues to record volumes accurately ■ The spirometer must record within 3% of the syringe volume to be considered accurate. If the recording is outside this range the calibration will need adjusting ■ The calibration of some spirometers, such as the ultrasonic or turbine flow-measuring devices, and the bellows volumetric spirometers, will require servicing by an engineer in order to adjust calibration. Others, such as the Fleisch pneumotachograph, can be updated on a daily basis or a sessional basis if necessary ■ If there is a significant change in temperature during a spirometry session calibration should be rechecked

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V ITAL POINTS ✱ Calibration checks must be carried out on all types of spirometer as a daily routine. Whilst modern spirometers are generally robust and reliable, things can still occasionally go wrong. Without regular calibration checks these errors will go undetected! ✱ A log of calibration checks and their results must be kept for medico-legal purposes ✱ Some spirometers use disposable, single patient use flow sensors. Calibration checks will need to be performed on each new batch of disposable flow sensors, in addition to the routine daily checks ✱ A spirometer that is transported from one location to another and exposed to changes in temperature or ‘jarring’ movement (eg a spirometer that is transported in a car to a patient’s home) will need to be left to come up to room temperature and will need to have the calibration checked before use

Calibration syringes ■ A calibration syringe injects an exact, known volume of air (usually 3 litres, although 1 litre syringes are also available) through the spirometer ■ The syringe must be accurate to within 0.5%: 15 ml for a 3 litre syringe and 5 ml for a 1 litre syringe ■ The syringe should be kept next to the spirometer so that it is at the same temperature and humidity ■ Calibration syringes are delicate, precision instruments that require servicing according to the manufacturers’ recommendations. A syringe that has been dropped should be considered inaccurate until it has been serviced ■ The syringe should be checked for leaks on a monthly basis by attempting to expel air from it with the outlet blocked

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V E RI F I C A TI O N USIN G A BIOLOG ICAL CON T ROL ■ The accuracy of a spirometer can also be verified using a biological control – ie a member of staff with known lung function values and no respiratory disease ■ To determine the normal physiological range for a biological control you will need to: ◆ Record spirometry daily, at the same time of day for 2 weeks (at least 10 recordings) ◆ Calculate the mean (average) value for each lung function parameter ◆ Calculate the normal range, which is plus or minus 5% of the mean of each parameter ■ Once the normal range for that person has been established, a spirometry recording from them can be used to verify the accuracy of the spirometer. If the spirometer measures a value outside the normal range for that person then it may need to be recalibrated

I NF E C TI O N C O NTROL ■ Sensible hygiene precautions and simple measures to prevent cross-infection must be taken, even though cross-infection from spirometry equipment is rare

VITAL POINT ✱ One of the simplest and most effective methods of preventing cross-infection is hand-washing. Make sure that you wash your hands before and after handling spirometry equipment and between patients ■ Patients with known, active respiratory infection should not have spirometry measurements taken, except when they are urgently needed for sound medical reasons. Such situations are rare, but in this case do the test at the end of the day on equipment that can then be dismantled and sterilised after use

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■ Immunocompromised patients (such as those with HIV, AIDS, posttransplant patients, etc) should be tested at the beginning of each day on newly sterilised equipment

Mouthpieces, nose-clips and filters ■ Mouthpieces should be disposable or autoclavable, and nose-clips, if they are used, should be disposable ■ In primary care settings inspiratory manoeuvres are rarely necessary and disposable, one-way mouthpieces that prevent accidental inhalation through the spirometer can reduce cross-infection risk ■ Careful hand-washing and the use of disposable gloves for handling used mouthpieces will also reduce risk ■ Disposable viral and bacterial filters can be used to prevent contamination of equipment used for inspiratory manoeuvres

Cleaning procedures ■ Any part of a spirometer that comes into contact with mucous membranes can be contaminated with saliva or mucus. These secretions will need to be removed by washing in hot, soapy water prior to disinfection or sterilisation

V ITAL POINTS ✱ All spirometers must be cleaned and disinfected or sterilised according to the manufacturers’ instructions. Inappropriate disinfection or sterilisation methods can destroy the equipment ✱ With the rising prevalence of tuberculosis and multi-drug-resistant bacterial infection, infection control is increasingly important. It is essential that you are aware of and follow infection control policies for your place of work ✱ Keep a log of cleaning procedures and the date, time and details of patients tested on the equipment. In the event of an infectious patient being inadvertently tested you will then be able to assess risk and trace any vulnerable patients

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C O NTRA I NDI C A TION S T O SPIROMET RY ■ Forced expiratory manoeuvres raise intra-cranial, intra-thoracic and intra-abdominal pressures ■ There are no absolute contraindications to performing spirometry. It is generally a safe procedure and is rarely needed as an emergency. The following conditions, however, could pose some risk and you should seek additional expert advice or consider delaying the test ◆ Recent eye, thoracic or abdominal surgery ◆ Recent myocardial infarction, uncontrolled hypertension or pulmonary embolism ◆ Recent cerebrovascular haemorrhage or known cerebral or abdominal aneurysm ◆ Pneumothorax ◆ Haemoptysis of unknown origin

P A TI E NT P RE P A RAT ION ■ Spirometry can be recorded ‘opportunistically’ but it is helpful to give the patients some instruction so that they are properly prepared ■ If possible ask patients: ◆ To avoid eating a large meal within 2 hours of the start of the test ◆ Not to smoke within 1 hour of the start of the test ◆ Not to consume alcohol within 4 hours of the start of the test ◆ To wear loose and comfortable clothing that does not restrict their breathing ◆ To avoid vigorous exercise within 30 minutes of the start of the test ◆ To arrive for their appointment in plenty of time so that they are relaxed, and have time to visit the toilet ■ If diagnostic bronchodilator reversibility testing is needed patients should also be advised to withhold their usual bronchodilators prior to the test: ◆ Short-acting inhaled β2 agonists for 2–4 hours ◆ Short-acting inhaled anticholinergics for 4–6 hours USI NG A SPI ROM ETER | 4 5

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Long-acting inhaled and oral sustained release β2 agonists for 12–24 hours Long-acting inhaled anticholinergics for 24–36 hours Theophyllines for 12 hours and sustained release theophyllines for 24 hours

V ITAL POINTS ✱ Bronchodilators should not be withheld when spirometry is performed as a routine monitoring procedure in established conditions such as COPD or asthma. To do so is likely to cause the patient unnecessary distress ✱ Post-bronchodilator FEV1 is used to categorise disease severity and to monitor disease progression ✱ Giving a simple instruction and information leaflet to the patient when they make their appointment can help ensure that they are properly prepared for the test. This will save you time

■ Explain the procedure and do your best to put the patient at ease ■ Before performing the test check that any instructions, such as withholding bronchodilators, not smoking, etc, have been complied with. Record any deviations from the ideal so that subsequent tests can be carried out under the same conditions ■ Ensure the patient is comfortable and does not have a full bladder ■ Briefly check the medical history to ensure that there are no major contraindications to testing ■ Accurately measure height, standing, without shoes ■ If patients are unable to stand, or have a severe spinal deformity such as a scoliosis, height can be estimated by measuring arm span – middle finger tip to middle finger tip with the arms outstretched at 90º to the thorax ■ Weigh the patient and calculate the body mass index ■ Check all the patient’s details (age, gender, name, hospital number, etc) and, if you are using an electronic spirometer, enter these details, together with the height and weight, into the spirometer 4 6 | V I T AL LU NG F UNC T I O N

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■ Sit the patient in a chair, preferably with arms, making sure that they are sitting upright, are well supported and have both feet on the floor. The chair should not have wheels and if the patient is in a wheelchair ensure that the wheels are locked

VITAL POINTS ✱ Performing a forced expiratory manoeuvre can make patients feel light-headed or cause syncope. For safety reasons, spirometry should be performed in the sitting position ✱ False teeth, unless they are very ill-fitting and loose, should be left in

P E R F O R M I NG THE T EST ■ Spirometry manoeuvres can be both inspiratory and expiratory ■ A combination of forced expiratory and inspiratory manoeuvres is needed for the production of maximal flow/volume curves – the flow/volume ‘loop’

VITAL POINT ✱ If possible, demonstrate the manoeuvre you require the patient to perform. A demonstration is generally more effective than a verbal explanation

VC ■ This is most commonly performed as a relaxed expiratory manoeuvre: the RVC, or VC ■ It should be undertaken before any forced manoeuvres ■ Explain the manoeuvre and if possible demonstrate it ■ Place a nose-clip on the patient’s nose and ask them to take a maximum breath in USI NG A SPI ROM ETER | 4 7

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■ When they have completely filled their lungs they should place the mouthpiece of the spirometer between their teeth, so that their tongue does not occlude the mouthpiece, and make a tight seal around the mouthpiece with their lips ■ Without delay they should exhale steadily and gently until they are unable to exhale any further ■ Wait a minute or so before attempting another recording

V ITAL POINTS ✱ The exhalation technique for VC must be steady, but unforced. It can be compared to a ‘big sigh’ ✱ Verbally encourage the patient to keep exhaling steadily until they have completely emptied their lungs and are unable to exhale further. Some electronic spirometers detect when airflow through the spirometer has ceased and give an audible or visual signal. This informs you when the patient has reached VC and can stop blowing ✱ Observe the patient throughout the manoeuvre. Encourage them to remain sitting upright and to keep their shoulders relaxed. Make sure that they maintain a tight seal around the mouthpiece with their lips

■ Inspiratory vital capacity can be recorded. This may be significantly higher than expiratory vital capacity in patients with airflow obstruction

FVC/FEV1

■ Nose-clips are not essential for forced manoeuvres, but can be tried if there are difficulties obtaining reproducible results ■ Demonstrate or explain the manoeuvre to the patient. Stress that they will need to make an absolutely maximum effort and should try to empty their lungs as completely as possible, as fast as possible ■ Ask them to take a rapid, but unforced, maximum breath in

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■ When they have completely filled their lungs they should place the mouthpiece of the spirometer between their teeth, so that their tongue does not occlude it, and make a tight seal around it with their lips ■ They should then immediately, using maximum effort, exhale as hard and as fast as possible until they are unable to exhale any further ■ Wait at least 1 minute before attempting another recording

VITAL POINTS ✱ Spirometry is hard work! Verbal encouragement to make a rapid, maximum effort at the start of the blow and to keep blowing as hard as possible is vital. Failure to do this will usually result in an inadequate effort from the patient and an abrupt end to the test ✱ Observe the patient throughout the manoeuvre. Any tendency to bend over at the waist should be discouraged. Make sure that they maintain a tight seal around the mouthpiece

Maximal flow/volume curve ■ Nose-clips should be used ■ Place the nose-clip on the patient’s nose and ask them to breathe in through their mouth to maximum inspiration ■ They should place the mouthpiece in their mouth, taking care not to occlude it with their tongue, and make a tight seal around it with their lips ■ Then, using the same manoeuvre described for FVC and FEV1, they should immediately exhale as hard and fast as possible to a position of maximum expiration ■ When they have exhaled completely they should immediately inspire, quickly and completely, to a position of maximum inspiration

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TE C HNI C A L A C C E P TABILIT Y A ND R E P R O DUC I BILIT Y ■ A technically acceptable effort is where the individual: ◆ Has used maximum effort for the forced manoeuvre ◆ Has exhaled immediately from the position of maximal inspiration ◆ Has not made a slow start to the forced manoeuvre ◆ Has not coughed ◆ Has exhaled completely to a position of maximum expiration ■ Traces need to be smooth and free of irregularity ■ The volume/time trace should plateau for at least 1 second and there should not be an ‘S’ shape to the beginning of the trace ■ The flow/volume trace should rise almost vertically to a peak and the trace should merge smoothly with the horizontal axis at the end of the blow ■ To ensure reproducibility you need a minimum of three RVC measurements, with less than 100 ml difference between the two best efforts ■ If reproducibility cannot be achieved with three efforts a further manoeuvre (up to a maximum of four) can be attempted ■ The highest reading is reported ■ You also need a minimum of three forced manoeuvres with less than 5% difference between the best two technically acceptable FVC and FEV1 readings ■ If reproducibility cannot be achieved with three efforts further manoeuvres (up to a maximum of eight) can be attempted ■ The highest reading of FVC and FEV1 is reported and these can be taken from different efforts if necessary

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VITAL POINTS ✱ Some patients, particularly the frail and elderly, will find it exhausting to perform up to eight forced expiratory manoeuvres. You will need to use common sense and if your patient is becoming distressed and exhausted you will need to abandon the test and try again on another occasion ✱ It is absolutely essential that personnel conducting spirometry are adequately trained, and ideally certified as competent, in obtaining valid, technically acceptable and reproducible tests

C O M M O N E R RO RS: RE C O GNI TI O N AN D CORRECT ION

VITAL POINT ✱ The expiratory techniques needed for spirometry have to be learnt. Some patients will grasp the technique quickly and others will need several attempts before they get it right. The operator’s role as coach and support is essential in helping patients to get the technique right and ensuring that results are meaningful

Sub-maximal effort ■ Failure to make a maximum effort will result in non-reproducible results: greater than 5% variation between efforts ■ Explain the procedure again and give vigorous, verbal encouragement

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Coughing ■ Forced expiratory manoeuvres will make some patients cough, particularly those with uncontrolled asthma or bronchial hyperreactivity ■ The volume/time and flow/volume traces will be irregular and the patient will have been seen to cough during the test ■ Allow the patient time to recover between efforts, but if coughing is a persistent problem testing may need to be abandoned ■ If an RVC has been recorded and the patient has blown to FEV1 without coughing an FEV1/VC can be calculated, although an FVC recording will not be possible

Failure to expire to FVC ■ The volume/time trace will fail to plateau ■ The flow/volume trace will not merge with the horizontal axis and will ‘drop off’ (Figure 4.1) ■ Verbally encourage the patient to ‘blow . . . go on . . . keep going . . .’

Volume (litres)

Flow (litres/second)

Trace fails to plateau

Time (seconds)

Figure 4.1 Abrupt end to effort

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Trace ‘drops off’

Volume (litres)

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Slow start to the forced expiratory manoeuvre ■ Failure to make an immediate, maximal effort from a position of maximal inspiration will give an ‘S’ shape to the start of the volume/time trace (Figure 4.2)

Flow rate (litres/second)

Volume (litres)

4 3 2 1

FEV1

0

1

2 3 4 Time (seconds)

5

6 Volume (litres)

Figure 4.2 Slow start to effort

■ The flow/volume trace will have a sloping, rather than vertical start (Figure 4.2) ■ Explain and/or demonstrate the technique again, stressing the need to try and ‘empty your lungs as fast as you can ... in the first few seconds if possible’ ■ Give a loud verbal encouragement and make a forceful gesture, such as stamping your foot, as the patient begins to blow to encourage maximum effort

Air leak ■ If the patient has not made a tight seal around the mouthpiece air will leak as they exhale ■ The volume/time trace will ‘dip’ downwards, rather than rise steadily to a plateau (Figure 4.3) ■ Any leaks in a volumetric spirometer system will cause similar problems ■ Patients with no teeth or facial palsy may have particular difficulties. As a last resort hold the patient’s lips around the mouthpiece

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4

Volume (litres)

3

2

1

FEV1

0

1

2

3

4

5

6

Time (seconds)

Figure 4.3 Air leak

■ Check a volumetric spirometer for leaks

V ITAL POINTS ✱ The only way to ensure that the patient’s technique is good is to observe them carefully while they are doing the test and to check the volume/time and flow/volume traces ✱ A real-time graphic display allows you to check the patient’s technique as they are performing the test

REFERENCES British Thoracic Society and Association for Respiratory Technology and Physiology (1994) Guidelines for the measurement of respiratory function. Respiratory Medicine 88: 165–94

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PA T IENT A ND CARER INFORMATION: S PIROMETRY ■ Spirometry is a test used to accurately measure the amount of air you can breathe in and out of your lungs and how fast you can breathe out ■ This test is carried out in hospital, at your doctor’s surgery and, in some circumstances, in your workplace as part of an occupational health check ■ Spirometry is painless, but it can be hard work. It is essential that you try as hard as you can. If you don’t, the tests will not be helpful and you will have wasted your time ■ Although some tests require you to blow as hard as you can, you will be able to rest between tests and get your breath back ■ Being properly prepared for your appointment will help to make the tests as easy and useful as possible ■ When you have an appointment for a spirometry test the following simple ‘dos and don’ts’ will help you prepare and will help get the most accurate information from the test results: ❏ DO wear loose and comfortable clothing that does not restrict or interfere with your breathing ❏ DO arrive in plenty of time for your appointment so that you are relaxed and have time to visit the toilet before your test ❏ DON’T eat a big meal within 2 hours of the start of your appointment. A full stomach will restrict your breathing and may make you feel uncomfortable ❏ DON’T smoke within at least 1 hour before the start of the test. Smoking affects the test result ❏ DON’T drink alcohol within 4 hours of the start of your appointment. Again, this can affect the accuracy of the test results ❏ DON’T take any vigorous exercise within 30 minutes of the start of the test

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❏ Certain medicines can also affect the test. If you are taking medicine for a chest complaint you may be asked not to take some of them before the spirometry test. You will be given instructions about which medicines you can and can’t take, and for how long before the test – but if you are in any doubt, ask! ❏ Written information may be given to you about your test. DO read it and follow any instructions you are given ■ You will be asked to: ❏ Breathe in until you have filled your lungs with air and cannot take any more air in ❏ Put the mouthpiece into your mouth – far enough to avoid blocking it with your tongue or teeth ❏ Make a good seal with your lips around the mouthpiece so that air does not escape around the edges ❏ You may also have to wear a clip on your nose to prevent air escaping down your nose during some of the tests. These clips are not uncomfortable ❏ Then you will have to breathe out into the spirometer, completely emptying your lungs and squeezing out every last drop of air ❏ You will be asked to do this several times. You will be asked to breathe out slowly and completely in a relaxed way and will also be asked to blow out as hard and as fast as you possibly can ❏ When you are asked to blow hard it is essential that you put as much effort into it as you possibly can ❏ Some tests will also require you to breathe in through the spirometer ❏ Whatever tests you are asked to do you will be instructed and given encouragement to help you with the test

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Spirometry interpretation

■ Spirometry is used to identify whether the pattern of ventilation is normal or whether there is an abnormal ventilatory defect

VITAL POINT ✱ Spirometry is an essential part of the diagnostic process for respiratory disease, but is insufficient on its own to reach a diagnosis. The cause of any abnormality in the spirometry can only be determined in the light of a full clinical history and other diagnostic tests

NO RM A L SP I RO MET RY Lung volumes – FVC and FEV1 ■ In adults, lung volumes are dependent on age, gender, height and ethnicity (see p. 39) ■ Large studies have been conducted, in a variety of populations, to determine the mean or average lung volumes for individuals of each age, gender and height within that population ■ Regression equations, derived from these population studies, allow calculation of the mean reference value (sometimes also referred to as the predicted value), the standard deviation from the mean and the reference value for any individual. Tables that give a reference value for a group of individuals within small ranges of height and age are also available ■ The regression equations for use with European Community for Coal and Steel (ECCS) reference values are given in the section on standard deviation scores ■ Lung volumes are expressed as absolute volumes (in litres) and as a percentage of the reference value SPI ROM ETRY I NTERPRETA TI ON | 5 7

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■ The calculation of the lung volume as a percentage of the reference value is described on p. 32 ■ Not all individuals of the same age, gender, height and ethnicity will have identical lung volumes – in just the same way that not all individuals of the same height will have the same size feet or be the same weight! ■ There is a normal ‘range’ for lung volume, just as there is a range of shoe size and weight ■ Lung volumes are generally considered normal if they are within 20% of the mean, or reference value. Therefore a normal range for lung volume is between 80% and 120% of the reference value

FEV1/FVC (FEV1%) and FEV1/VC ■ FEV1/FVC is the FEV1 expressed as a percentage (or ratio) of the FVC ■ The FEV1/VC is the FEV1 as a percentage (or ratio) of the RVC. It is interpreted in the same way as the FEV1/FVC ■ A healthy individual can exhale about three-quarters (75%, or a ratio of 0.75) of their VC in the first second of a forced expiration ■ A normal FEV1/FVC is therefore about 75% (or 0.75) ■ The FEV1/FVC can also be expressed as a percentage of the reference value for FEV1/FVC for a person of that age, height, gender and ethnicity ■ The FEV1/FVC is considered normal if it is over 80% of the reference value

V ITAL POINT ✱ Spirometry parameters are considered to be within the normal range if: ◆ The FEV1, FVC and VC are between 80% and 120% of the reference value for someone of that age, gender, height and ethnic group ◆ The FEV1/FVC is about 75% (0.75) or over 80% of the reference value for someone of that age, gender, height and ethnic group

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FVC

Volume (litres)

FEV1

1

2

3

4

5

6

Time (seconds)

Figure 5.1 Normal volume/time trace

The volume/time trace ■ A normal volume/time trace has a typical shape (Figure 5.1) ■ There is a rapid rise to the trace as three-quarters of the air is expired in the first second ■ The trace plateaus between 4 and 6 seconds

The flow/volume trace ■ A normal flow/volume trace also has a typical shape (Figure 5.2) ■ The trace rises almost vertically to PEF ■ As air is cleared from small airways the flow rate decreases steadily ■ The trace merges smoothly with the horizontal axis of the graph at FVC

O BSTR UC TI V E SPIROMET RY ■ Diseases that cause obstructive ventilatory defects are common. They include asthma and COPD SPI ROM ETRY I NTERPRETA TI ON | 5 9

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Flow (litres/second)

Peak expiratory flow

FVC

Volume (litres)

Figure 5.2 Normal flow/volume trace

■ Obstructive airways diseases cause a reduction in the rate at which air can be expired from the lungs, but do not always affect the maximum volume of air that can be expired

Lung volumes – FEV1 and FVC ■ Obstruction of the airways slows the rate at which air can be exhaled. This reduces the volume of air forcibly expired in the first second of a forced expiratory manoeuvre ■ Thus the volume of FEV1 will be reduced: usually to less than 80% of the reference value ■ In mild and moderate airflow obstruction the FVC is usually normal (over 80% of its reference value) ■ In severe airway obstruction the FVC may also be reduced, as a result of dynamic airway collapse and air trapping. However, this is not usually reduced to the same extent as the FEV1

V ITAL POINT ✱ The VC as well as the FVC should be routinely recorded. If the VC is greater than the FVC the FEV1/VC should be calculated and used to determine airflow obstruction

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FEV1/FVC (FEV1%) and FEV1/VC ■ Airway obstruction reduces the FEV1, but the FVC is relatively well preserved ■ The FEV1 as a ratio or percentage of the FVC or VC is therefore reduced in obstructive airways disease ■ A ratio of less than 70% (0.7) is usually considered to be diagnostic of airflow obstruction ■ An FEV1/FVC or FEV1/VC of less than 80% of the reference value is also indicative of airflow obstruction ■ Airflow obstruction results in prolonged expiratory time. A healthy person can usually expire to FVC within 4–6 seconds. A person with airflow obstruction may be able to expire completely, but this will usually take longer

VITAL POINTS ✱ Spirometry parameters compatible with airflow obstruction are: A reduced FEV1/FVC, or FEV1/VC. Values of less than 70% (0.7) and/or less than 80% of the reference value are considered to be diagnostic of airflow obstruction

◆

◆

An FEV1 of less than 80% of the reference value

✱ Lungs lose elasticity with age and an FEV1/FVC ratio of less than 70% (0.7) may be normal in an elderly subject. Calculation of the FEV1/FVC ratio as a percentage of the reference value, and/or calculation of the standard deviation score (see below), may be helpful in determining whether the lung function is abnormal or not ✱ It is vital to interpret the spirometry alongside the clinical history in all cases. An FEV1/FVC ratio of greater than 70% may still be abnormal, particularly in a young patient with symptoms of airflow obstruction

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Volume (litres)

Predicted

Measured

1 Time (seconds)

Figure 5.3 Obstructive volume/time trace

The volume/time trace ■ The slower rate at which air can be expired produces a flatter volume/time trace (Figure 5.3) ■ It will take longer for the trace to plateau. In severe obstruction this may take up to 15 seconds

V ITAL POINT ✱ The motor in a bellows spirometer moves the recording paper for a set time: 6 or 12 seconds. A patient with airflow obstruction may not have blown to FVC within this time and must be encouraged to continue to blow until the stylus ceases to move down the side of the paper. FVC is recorded at the point where the stylus stops

The flow/volume trace ■ The trace must rise almost vertically to PEF. Although PEF may be reduced it must still be reached in the first 10 milliseconds of the blow 6 2 | V I T AL LU NG F UNC T I O N

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■ As air is cleared from obstructed airways the flow rate decreases more rapidly than normal, producing a concave, ‘scooped’ shape to the curve (Figure 5.4) ■ The trace should still merge smoothly with the horizontal axis of the graph at FVC

Flow (litres/second)

Peak expiratory flow

Volume (litres)

Figure 5.4 Obstructive flow/volume trace

VITAL POINTS ✱ The flow/volume trace is useful for the detection of early or mild airflow obstruction. It may show ‘scooping’ suggestive of obstruction when the spirometry parameters are within normal limits ✱ Loss of lung elasticity with increasing age can produce slight ‘scooping’ towards FVC in normal, healthy individuals

RE STRI C TI V E SPIROMET RY ■ Restrictive spirometry can arise as a result of problems within and outside the lungs SPI ROM ETRY I NTERPRETA TI ON | 6 3

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■ Intra-pulmonary causes of restrictive spirometry include diseases that cause fibrosis of lung tissue, such as fibrosing alveolitis or sarcoidosis ■ Pulmonary oedema, as a result of cardiac failure, will produce a restrictive ventilatory defect ■ Any condition that prevents full expansion of the thoracic cavity can cause restrictive spirometry, such as: ◆ Deformity of the thoracic spine (scoliosis or kyphoscoliosis) ◆ Neuromuscular problems (muscular dystrophy, motor neurone disease, Guillain–Barré syndrome, paralysis of the diaphragm, etc) ◆ Obesity ◆ Tight clothing, eg corsets

V ITAL POINT ✱ Respiratory causes of restrictive ventilatory defects are comparatively rare. A common cause of apparent restrictive spirometry is poor technique: failure to make a maximum effort and expire to FVC

Lung volumes – FEV1 and FVC ■ Lung volumes are reduced, but airflow is unaffected in restrictive ventilatory defects ■ The volume of FEV1 and FVC are both reduced in proportion to each other ■ Both the FEV1 and FVC will be less than 80% of their reference values

FEV1/FVC (FEV1%) and FEV1/VC ■ Since airways are not obstructed the ratio of FEV1/FVC will be normal: 75% (0.75) or more ■ When the lung volumes are significantly reduced it may be possible to expire more than 75% of the total volume in the first second, simply because the volume is so small. In this case the FEV1/FVC will be abnormally high ■ An FEV1/FVC of greater than 85% in an adult is suggestive of a restrictive defect 6 4 | V I T AL LU NG F UNC T I O N

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■ As lung volumes are small and airflow is normal the expiratory time may be very short: 2–4 seconds or less

VITAL POINT ✱ Spirometry parameters compatible with a restrictive ventilatory defect are: ◆ An FEV1, FVC and VC reduced in proportion to each other to less than 80% of their reference value ◆ A normal or high FEV1/FVC, or FEV1/VC (about 75% or a ratio of 0.75). The FEV1/FVC will be over 80% of the reference value

The volume/time trace ■ The volume/time trace will be a normal shape, but will be small and will plateau early (Figure 5.5)

Volume (litres)

Predicted

Measured

1 Time (seconds)

Figure 5.5 Restricted volume/time trace

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The flow/volume trace ■ Restrictive defects do not affect airflow, so the trace must rise almost vertically to a normal or near-normal PEF ■ Airflow from the airways is normal so the trace will not show the ‘scooping’ typical of obstruction ■ The FVC is reduced so the trace will reach the horizontal axis of the graph quickly, giving a narrow appearance to the trace (Figure 5.6)

Flow (litres/second)

Predicted normal curve

Volume (litres)

Figure 5.6 Restricted flow/volume trace

SE V E R E LY O BSTRU CT ED SPIROMET RY ■ During a forced expiratory manoeuvre the pressure on the outside of the airway (extra-mural pressure) is increased, causing it to narrow, even in healthy people ■ In severe obstructive airways disease, where the airways are already significantly narrowed, the increased extra-mural pressure during a forced expiration may cause them to collapse, trapping air in the lungs and reducing the FVC ■ This is sometimes referred to as ‘dynamic airway collapse’ ■ Where there is dynamic airway collapse the VC is often significantly greater than the FVC 6 6 | V I T AL LU NG F UNC T I O N

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VITAL POINT ✱ When the relaxed vital capacity is higher than the FVC the FEV1/VC should be calculated

Lung volumes – FEV1 and FVC ■ The FEV1 will always be significantly reduced. Values of less than 30% of the reference value are compatible with a severe obstructive defect ■ In severe obstructive defects the FVC is also reduced because of dynamic airway collapse during the forced manoeuvre ■ The reduction in FVC can give a superimposed restrictive pattern due to the spirometry. Inspection of the flow/volume trace will show concavity in the trace and reveal the obstruction ■ The reduction in FVC is usually less than the reduction in FEV1; they are not usually reduced in proportion to each other

FEV1/FVC (FEV1%) and FEV1/VC ■ Dynamic airway collapse may produce a superimposed restrictive pattern and the FEV1/FVC may be within normal limits ■ Calculation of FEV1/VC will reveal the obstruction ■ In severe airflow obstruction expiratory time may be 15 seconds or more

The volume/time trace ■ The volume/time trace will be flat and small (Figure 5.7) ■ It may take up to 15 seconds for the patient to blow to FVC

The flow/volume trace ■ The trace must still rise almost vertically to PEF but the PEF will be reduced ■ Dynamic airway collapse will cause a rapid reduction in airflow through small generations of airways and a marked concavity to the trace ■ The reduction in FVC will produce a narrower trace than normal, but the major feature is the dramatically ‘scooped’ shape of the trace (Figure 5.8) SPI ROM ETRY I NTERPRETA TI ON | 6 7

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Volume (litres)

Predicted

Measured

1 Time (seconds)

Figure 5.7 Severely obstructed volume/time trace

Flow (litres/second)

Predicted normal curve

Volume (litres)

Figure 5.8 Severely obstructed flow/volume trace

Table 5.1 summarises the effect of different ventilatory defects on lung function parameters. 6 8 | V I T AL LU NG F UNC T I O N

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Table 5.1 Summary of spirometry parameters affected by different ventilatory defects Normal

Obstruction

Restriction

Severe obstruction

FVC

More than 80% of reference value

More than 80% of reference value

Less than 80% of reference value. Reduced in proportion to FEV₁

Often less than 80% of reference value but less reduced than FEV₁

VC

Same as FVC

May be higher than FVC

Same as FVC

Greater than FVC

FEV₁

More than 80% of reference value

Less than 80% of reference value

Less than 80% of reference value

Less than 30% of reference value

FEV₁ /FVC

About 75% (0.75) and more than 80% of reference value

Less than 70% (0.7) and less than 80% of reference value

In excess of 75% (0.75) and more than 80% of reference value

Usually less than 70% (0.7) and 80% of reference value – but may be higher if there is significant air trapping and loss of volume at FVC

FEV₁/VC

About 75% (0.75) and more than 80% of reference value

Less than 70% (0.7) and less than 80% of reference value

In excess of 75% (0.75) and more than 80% of reference value

Less than 70% (0.7) and less than 80% of reference value

M A XI M A L F LO W/ VOLU ME T RACES ■ Maximal flow/volume traces are sometimes known as flow/volume ‘loops’ ■ The forced expiratory manoeuvre is immediately followed by a maximal inspiratory manoeuvre ■ The inspiratory trace should join the start of the expiratory trace at the point of maximal inspiration, forming a ‘loop’ ■ Failure to form a ‘loop’ can be due to: ◆ Failure to start at maximal inspiration ◆ Sub-maximal inspiratory effort ◆ Air leak ■ Maximal flow/volume traces are particularly useful for helping to determine the site of obstruction in large airways, eg localised, intra-thoracic obstruction, such as tracheal stenosis, or extra-thoracic airway obstruction, eg laryngeal obstruction due to ‘goitre’ SPI ROM ETRY I NTERPRETA TI ON | 6 9

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■ The technique requires particular attention to infection control. Patients have to inhale through the spirometer so the use of viral and bacterial filters is advisable ■ Maximal flow/volume traces are not essential in primary care settings, but can be useful in more difficult respiratory patients where the diagnosis is in doubt or where they are failing to respond to therapy as expected

Fixed intra-thoracic and extra-thoracic obstruction ■ There is a flattening of both the expiratory and inspiratory curves ■ Expiration and inspiration are both limited to the same extent so that the expiratory and inspiratory traces are a mirror image of each other (Figure 5.9)

Variable intra-thoracic obstruction ■ Increased extra-mural pressure on the airway within the thorax during forced expiration causes increased narrowing of the airway during expiration (see p. 66)

Expiratory flow

FEF50%

Inspiratory flow

Volume

FIF50%

Figure 5.9 Fixed intra- and extra-thoracic obstruction 7 0 | V I T AL LU NG F UNC T I O N

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Expiratory flow

FEF50%

Inspiratory flow

Volume

FIF50%

Figure 5.10 Variable intra-thoracic obstruction

■ There is flattening of the expiratory curve but less flattening of the inspiratory curve (Figure 5.10)

Variable extra-thoracic obstruction ■ During forced expiration the upper airway is ‘blown’ open, but during inspiration it is narrowed ■ The inspiratory curve is flattened, but the expiratory curve is relatively well preserved (Figure 5.11)

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Expiratory flow

FEF50%

Inspiratory flow

Volume

FIF50%

Figure 5.11 Variable extra-thoracic obstruction (FIF50% is less than FEF50%)

STA NDA RD DE V I A TION SCORES ■ The presentation of spirometry values as a percentage of the reference value can be misleading. It can result in a large number of false positive and false negative results, particularly in elderly and adolescent populations ■ Some laboratories now present spirometry results as standard deviation from normal ■ The standard deviation score is calculated as: Measured volume − Reference volume Residual standard deviation ■ The residual standard deviation is derived from the ‘scatter’ of results around the mean and is included in the regression equation table below (Table 5.2) 7 2 | V I T AL LU NG F UNC T I O N

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Table 5.2 Regression equations for ECCS reference values Regression equation

Units

Residual standard deviation (RSD)

Adult men FEV1 FVC FEV1/VC PEF

(4.30 × height) – (0.029 × age) – 2.49 (5.76 × height ) – (0.026 × age) – 4.34 (–0.18 × age ) + 87.21 (6.14 × height) – (0.043 × age) + 0.15

L L % L/sec

0.51 0.61 7.71 1.21

L L % L/sec

0.38 0.43 6.51 0.90

L L % L/sec

0.52 0.54 6.72 1.65

L

0.42

% L/sec

7.66 1.34

Adult women FEV1 FVC FEV1/VC PEF

(3.95 × height ) – (0.025 × age) – 2.60 (4.43 × height ) – (0.026 × age) – 2.89 (–0.19 × age ) + 89.10 (5.5 × height) – (0.030 × age) – 1.11 Male children

FEV1 FVC FEV1/VC PEF

(4.40 × height ) – (0.045 × age) – 4.81 (5.00 × height ) – (0.078 × age) – 5.51 (–8.7 × height) – (0.14 × age ) + 103.6 (78 × height) – (0.166 × age) – 8.06 Female children

FEV1 FVC FEV1/VC PEF

(2.70 × height ) – (0.085 × age) – 2.70 (3.30 × height ) – (0.092 × age) – 3.47L0.50 (–11.1 × height) – (0.11 × age ) + 107.4 (78 × height) – (0.166 × age) – 8.06

For ECCS (European Community for Coal and Steel) reference values, see Quanjer et al 1993 Height (metres); Age (years)

VITAL POINTS ✱ The normal standard deviation, which will include 90% of a population, is 1.645. Standard deviation scores outside a range of –1.645 to +1.645 are likely to be abnormal ✱ Standard deviation scores are independent of age, height and gender bias and have the same scale and value for all lung function parameters

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REFERENCES Pellegrino R, Viegi G, Brusasco V et al (2005) Interpretive strategies for lung function tests. European Respiratory Journal 26: 948–68 Quanjer PH, Tammeling GJ, Cotes JE et al (1993) Lung volume and forced ventilatory flows: Report working party standardization of lung function tests, European Community for Steel and Coal. Official statement of the European Respiratory Society. European Respiratory Journal 6 (Suppl 16): 5–40 Quanjer PH et al. Expressing test results. Available at www.spirxpert.com (accessed 16 July 2006)

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Static lung volumes and gas transfer

■ Spirometry is used to measure VC and the dynamic lung volumes that go to make up the VC ◆ FVC ◆ FEV1 ■ The static lung volume measured with a spirometer is the RVC (see p. 32) ■ Different techniques are required to measure the other static lung volumes: ◆ RV ◆ TLC ◆ Functional residual capacity (FRC) ■ These static volume measurement techniques are carried out by trained staff in a lung function laboratory ■ The primary function of the lungs is to exchange gas between the atmosphere and the bloodstream ■ An estimate of the efficiency of the lungs in exchanging gas is measured during a gas transfer test

STA TI C LUNG V OLU MES Residual volume (RV) ■ RV is the volume of air left in the thorax at the end of a full expiration ■ The volume of RV is affected by: ◆ Chest wall mechanics ◆ The efficiency and strength of the expiratory muscles ◆ The elastic recoil of the lungs ◆ Dynamic airway collapse STA TI C LUNG VOLUM ES A ND G A S TRA NSFER | 7 5

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V ITAL POINTS ✱ RV is increased in airways obstruction due to air trapping as a result of closure of the airways during expiration ✱ RV is increased in respiratory muscle disease ✱ RV is reduced when the lungs are ‘stiffened’ by conditions such as pulmonary fibrosis and pulmonary oedema ✱ RV is reduced in obesity

Total lung capacity (TLC) ■ TLC is the volume of air in the thorax at full inspiration and is the maximum volume of air that the lungs can contain. It includes the VC (the ‘accessible’ volume of air that can be measured with a spirometer) and the RV (the ‘inaccessible’ volume) ■ TLC is limited by: ◆ Chest wall mechanics ◆ Inspiratory muscle strength ◆ The size of the heart (eg cardiomegaly) ◆ Lung compliance – how ‘stretchy’ the lung tissue is

V ITAL POINTS ✱ A reduced TLC is the cardinal feature of restriction ✱ Neuromuscular diseases (such as muscular dystrophy) and chest wall deformity (such as kyphoscoliosis) reduce TLC. In these conditions the lung tissue and airways are unaffected and the RV remains within normal limits. This leads to a high ratio of RV to TLC ✱ Obstructive lung disease leads to air trapping and hyperinflation. Air trapping leads to an increase in RV and hyperinflation leads to an increase in TLC. These values are often both increased, although the RV is usually increased to a greater extent than the TLC. Occasionally a raised RV is found without a rise in TLC

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Functional residual capacity (FRC) ■ FRC is the volume of air left in the lungs after a normal, unforced expiration ■ FRC is reached when the outward ‘springiness’ of the chest wall is matched by the inward elastic recoil of the lungs

Residual volume/total lung capacity ratio (RV/TLC) ■ In healthy individuals the RV to TLC ratio is between 0.2 and 0.35 (ie 20–35% of the TLC is composed of the RV) ■ The ratio of RV to TLC is affected in disease (Figure 6.1) TLC

TLC

TLC

TLC FRC

FRC

FRC RV

RV

RV Normal

FRC RV

Air trapping

Hyperinflation

Restrictive

Obstructive airways disease

Figure 6.1 Static lung volumes in respiratory disease

■ The relationship between dynamic and static lung volumes is shown in Figure 6.2 ■ Measurement of static lung volumes is particularly useful when spirometry is borderline or equivocal ■ These measurements are helpful in developing a full picture of the nature of any ventilatory defect and can be a useful part of the diagnostic process STA TI C LUNG VOLUM ES A ND G A S TRA NSFER | 7 7

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7 Total lung capacity 6

Volume (litres)

5

Inspiratory reserve volume

Inspired vital capacity

Inspiratory capacity

4 Tidal volume 3

2

1

Expiratory reserve volume

Functional residual capacity

Residual volume

0

Figure 6.2 Lung volumes

■ As with dynamic lung volumes, static lung volume reference values are related to age, gender, height and ethnicity ■ Static lung volumes are reported as a percentage of the reference value and/or as standard deviation from it

M E A SUR I NG STA TI C LU N G VOLU MES ■ Static lung volumes are measured by estimating the volume of gas inside the thorax ■ This can be done in a variety of ways: ◆ Open circuit: nitrogen wash-out ◆ Closed circuit: multiple breath helium dilution ◆ Body plethysmography (the ‘body box’)

Nitrogen washout ■ This test utilises the fact that breathing 100% oxygen (O2) ‘washes’ nitrogen (N2) out of the lungs 7 8 | V I T AL LU NG F UNC T I O N

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■ The volume of N2 ‘washed out’ is used to measure FRC ■ The patient breathes through an open circuit containing a flow spirometer, an O2 source and a rapid N2 analyser ■ 100% O2 is introduced into the system during inspiration ■ Each expiration is measured and the N2 content analysed ■ The test is continued until the exhaled N2 is stable, for about 7 minutes, at 1% of the expired breath ■ A healthy person will ‘wash out’ all the N2 from their lungs in about 3– 4 minutes ■ The volume of N2 expired and the change in N2 concentration from the beginning to the end of the test is used to calculate the volume of air in the thorax

Multiple breath helium dilution ■ This method uses a closed breathing circuit and uses the dilution of the concentration of an inert gas (helium in this case) in an O2 mixture to calculate lung volumes (Figure 6.3) ■ A known volume of about 10% helium and O2 mix is put into a volumetric spirometer in a closed circuit ■ The patient is ‘switched into’ the helium dilution circuit at FRC ■ The patient re-breathes this gas mixture and the carbon dioxide (CO2) in the expired air is removed with a CO2 absorber ■ O2 is introduced into the system to replace the CO2 and to maintain the volume of gas in the system ■ As the patient inhales the helium into their lungs the helium concentration in the circuit drops ■ The test is continued until the concentration of helium in the system is stable. At this point the concentration of helium in the lungs will be the same as the concentration within the circuit – equilibrium ■ In healthy people equilibrium is reached in about 3 minutes when a 6–8 litre closed circuit is used ■ The dilution of the helium is used to calculate the volume of air in the thorax. The reduction in helium concentration is proportional to the size of the lungs at the point of ‘switch in’. The greater the dilution, the larger the lungs STA TI C LUNG VOLUM ES A ND G A S TRA NSFER | 7 9

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Start of test Circuit closed

Helium molecules Lungs Spirometer containing helium mixture Patient rebreathing Circuit open

Equilibrium

Figure 6.3 Helium dilution test

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Body plethysmography ■ Body plethysmography (Figure 6.4) is sometimes considered the most accurate method of measuring static lung volumes, but relies on adequate ventilation of all the lung tissue ■ The test is complex and some patients are either unable to tolerate it or are unable to co-operate with instructions ■ The patient sits inside an airtight box (the plethysmograph or ‘body box’) ■ The pressure inside the box is monitored ■ The patient breathes through a pressure transducer connected to the atmosphere outside the box. The transducer measures the pressure in the lungs ■ The patient ‘pants’ through the mouthpiece, which is then occluded for a few moments. Breathing against this occlusion alternately compresses and decompresses the air in the thorax causing slight changes to the pressure inside the box

Air flow sensor

Mouth pressure sensor

Box pressure sensor

Figure 6.4 Body plethysmography STA TI C LUNG VOLUM ES A ND G A S TRA NSFER | 8 1

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■ Changes in lung volume are detected as changes in the pressure inside the box ■ The volume fluctuation and the pressure changes are used to calculate the volume of gas in the thorax

DI F F USI NG C A P A C IT Y AN D G AS T RAN SFER ■ The efficiency of O2 transfer depends on: ◆ The ventilation of the alveolar surface area ◆ The thickness of the alveolar membrane ◆ The amount of haemoglobin (Hb) available for the O2 to attach to

Conditions that reduce the efficiency of gas transfer ■ Problems with ventilation of the alveoli such as: ◆ Pneumonia ◆ Obstructive airways disease, such as emphysema ◆ Acute respiratory distress syndrome (ARDS) ◆ Chest wall disease (muscle weakness) ◆ Kyphoscoliosis ■ Conditions that thicken the alveolar membrane, such as: ◆ Pulmonary fibrosis ■ Problems with pulmonary circulation, such as: ◆ Pulmonary embolism ◆ Vasculitis (eg scleroderma, systemic lupus erythematosus) ■ Cardiac diseases, such as: ◆ Pulmonary oedema ◆ Right-to-left shunt ■ Conditions that reduce the availability of haemoglobin, such as: ◆ Anaemia ◆ Recent smoking or environmental exposure to carbon monoxide

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Diffusing capacity test ■ How efficient the lungs are at exchanging gas is measured in a diffusing capacity test, DLco – also known as a gas transfer test, TLCO ■ Diffusing capacity and gas transfer tests utilise the fact that carbon monoxide (CO) is very similar to oxygen in its ability to defuse across the alveolar surface. It also binds strongly with haemoglobin and is not exhaled immediately ■ The transfer test measures the rate of disappearance of CO from the inhaled sample ■ There are a variety of methods used. They all involve the inhalation of a small, measured amount of CO ■ The most commonly used method is the single breath technique

VITAL POINT ✱ Diffusion of oxygen across the alveolar–capillary membrane is normally very efficient and there has to be considerable damage and loss of surface area before the DLco is affected

Single breath DLco ■ The patient inhales, from RV to TLC, a mixture of gases of known concentration: CO, O2 and an inert gas, such as helium ■ The patient then holds their breath for 10 seconds before exhaling ■ The first part of the exhaled breath is discarded, to allow for anatomical dead space, and the concentration of gases in the middle part of the expiration is analysed to calculate the amount of CO taken up by the lungs ■ Some laboratories will correct the DLco to allow for the haemoglobin concentration (if this is known)

Krogh constant (Kco ) ■ A small person will have a smaller alveolar surface area and will not be able to take up as much CO as a larger individual, and will have a lower DLco

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■ When lung has been resected the DLco will be reduced because of loss of alveoli, but Kco will be normal if the remaining lung is healthy ■ Kco is particularly useful to distinguish between intrinsic lung disease and loss of lung tissue ■ Results are presented as unadjusted for haemoglobin, adjusted and as a percentage of the reference value ■ Using the TLco and Kco together is helpful in differentiating between intra- and extra-pulmonary abnormality

REFERENCES MacIntyre N, Crapo RO, Viegi G et al (2005). Standardisation of the single-breath determination of carbon monoxide uptake in the lung. European Respiratory Journal 26: 720–35 Pellegrino R, Viegi G, Brusasco V et al (2005). Interpretive strategies for lung function tests. European Respiratory Journal 27: 948–68 Wanger J, Clausen JL, Coates A et al (2005). Standardisation of the measurement of lung volumes. European Respiratory Journal 26: 511–22

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PA T IENT A ND CARER INFORMATION: ST A T IC LU NG VOL UMES AND GAS TRANS FER Measuring your residual volume ■ It is possible to measure the amount of air you can breathe in and out quite easily using a piece of equipment called a spirometer ■ A spirometer is very useful but is not always enough to tell your doctor or nurse exactly how your lungs are working ■ There is always some air left behind in your lungs, even when you have tried to empty your lungs completely. This air is called the ‘residual volume’ ■ Knowing how big the residual volume is can be helpful to your doctor in deciding what, if anything, is wrong ■ The tests needed to measure residual volume are done at the hospital in a lung function laboratory ■ They are safe and painless and you will not need to stay in the hospital overnight ■ Before the test you should not smoke for at least an hour, and you should not drink alcohol within 4 hours of the start of the test as this can affect the test results ■ There are several different ways of measuring residual volume ❏ Two of the techniques used involve wearing a clip on your nose while breathing normally in and out through a mouthpiece for several minutes ❏ The air that you breathe in during these tests will have oxygen added to it, so there is no danger of you suffocating ❏ For one test you may need to breathe a mixture of oxygen and another harmless, inactive gas called helium ❏ The other commonly used method involves you sitting inside a big, air tight, plastic box, sometimes called a ‘body box’

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❏ This may sound alarming, but the test takes only a few moments, you will be able to see out of the box, there will be someone with you all the time and the box will only be closed for a very short time ❏ When the box is closed you will have a clip on your nose and will need to breathe through a tube connected to the air outside the box ❏ You will be asked to put your hands on your cheeks to stop you blowing your cheeks outwards as you breathe ❏ First you will be asked to breathe normally for a few breaths ❏ Then the mouthpiece will be closed with a shutter and you will be asked to gently ‘pant’ against the shutter for a few breaths ❏ Again, this does sound rather alarming, but this part of the test only lasts for a few seconds and there is no danger of you becoming short of oxygen or coming to any harm ❏ As the shutter is opened you will be asked to either to breathe out completely, immediately followed by a maximum breath in, or to breathe in completely, followed by a maximum breath out ❏ The door of the box will be opened and you will be given time to rest ■ Whichever type of test you are asked to do there will be a qualified and competent specialist in lung function testing with you at all times ■ He or she will be able to tell you what to do and will make sure that you are safe and as comfortable as possible

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Measuring how well your lungs are getting oxygen into your blood ■ Oxygen is essential for life and the main function of your lungs is to get oxygen from the air you breathe into your bloodstream so that it can be carried all around your body ■ How effective your lungs are at getting oxygen into your blood is measured with a gas transfer test ■ This test is done in the hospital lung function laboratory ■ Before this test it is very important not to smoke for at least 1 hour as this can affect the test results ■ You will need to wear a nose-clip and will need to breathe through the equipment ■ You will usually be asked to breathe normally through the equipment for a few moments ■ You will then be asked to breathe out as completely as you can ■ You will then be asked to breath in rapidly and to fill your lungs ■ This breath in will contain a very small quantity of carbon monoxide, mixed with oxygen and another, harmless, inactive gas ■ When you have filled your lungs you will be asked to hold your breath for a few seconds before breathing out again completely ■ You may need to repeat this test a few times, but will be given time to relax between tests

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Blood gases and pulse oximetry

■ Measuring arterial blood gases (ABG) is an important, basic test that indicates the overall capacity of the lungs to exchange CO2 and O2 ■ Measurement of ABG also allows the acid–base balance to be assessed ■ ABG are essential for the diagnosis of respiratory failure and for differentiating between type I and type II failure ■ It is critical for the monitoring and management of patients in acute respiratory failure, eg to determine a safe level of O2 to administer and to assess the need for artificial ventilation ■ It is an essential part of the assessment of patients’ suitability for long-term oxygen therapy ■ Oxygen saturation is estimated as part of an ABG report, but can also be measured directly with a pulse oximeter ■ Pulse oximetry measures the percentage of haemoglobin saturated with O2 (SpO2) ■ Pulse oximetry is a simple, non-invasive test that should be routinely available in all healthcare settings

V ITAL POINT ✱ Pulse oximetry is useful, but has limitations. It does not give any indication of acid–base balance or CO2 level and should be interpreted with caution, particularly in patients with chronic lung disease. It is insufficient on its own to guide the management of acutely ill patients

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A RTE R I A L BLO OD G ASES ( ABG ) ■ Samples of arterial blood are analysed to determine: ◆ The pressure, or ‘tension’ of CO2 (PaCO2) ◆ The hydrogen ion concentration, or ‘acidity’ of the blood (pH) – acid– base balance ◆ An estimate of bicarbonate ion (HCO3−) concentration ◆ The pressure, or ‘tension’ of O2 (PaO2) ■ PaCO2 and PaO2 pressures are measured in kilopascals (kPa) and bicarbonate ion concentration in millimoles per litre (mmol/L)

VITAL POINTS ✱ Normal values for ABG are: PaO₂ (kPa)

PaCO₂ (kPa)

pH

HCO₃– (mmol/L)

11–13

4.7–5.9

7.36–7.44

21–28

✱ ABG must be interpreted in the context of the clinical history and presentation. In particular, the use of supplemental O2 at the time of sampling will significantly alter the ABG results and affect how they are interpreted. It is vital that you record whether the patient is breathing air or is receiving supplemental O2 and the concentration of O2 given

PaCO2 ■ CO2 readily diffuses from the blood into the alveoli ■ The rate at which CO2 is excreted through the lungs, and therefore the PaCO2, is dependent on the rate of alveolar ventilation: ◆ The higher the alveolar ventilation the lower the PaCO2 ◆ The lower the alveolar ventilation the higher the PaCO2 ■ During hyperventilation an excess of CO2 is ‘blown off or ‘washed out’ of the alveoli, and the PaCO2 falls ■ In hypoventilation CO2 is retained and the PaCO2 rises

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V ITAL POINT ✱ Alveolar hypoventilation is due to failure of the respiratory ‘pump’ (the lungs and diaphragm) to work efficiently. The main reasons for failure of the ‘pump’ are: ◆ Severe airflow obstruction, leading to overload of the pump ◆

◆

Severe restrictive lung disease, also leading to pump overload

Neuromuscular disease or skeletal deformity, such as kyphoscoliosis, leading to weakness and ineffective working of the pump

◆ Central respiratory failure – failure of the central nervous system to drive the pump

pH ■ pH is a measure of the concentration of hydrogen ions and is used to gauge the acidity or alkalinity (basicity) of the blood. This is sometimes termed the acid–base balance ■ A neutral pH is 7. Values of less than 7 are acid and over 7 are alkaline ■ CO2 combines with water in the blood to produce HCO3− and hydrogen (H+) ions. Dissolved CO2 acts as an acid: ◆ An increase in PaCO2 will cause an increase in the ‘acidity’ of the blood and a fall in the pH: acidosis ◆ A fall in PaCO2 produces a drop in the acidity of the blood and a rise in the pH: alkalosis

V ITAL POINT ✱ The normal pH of arterial blood is 7.4, which is slightly alkaline. Maintenance of normal pH is critical for a wide variety of physiological functions

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Metabolic alkalosis

Normal pH Normal

Low pH Acidosis

Metabolic acidosis

Low PaCO2

Respiratory acidosis

Normal PaCO2

High PaCO2

Figure 7.1 Respiratory and metabolic alkalosis and acidosis

Acid–base balance ■ Acidosis and alkalosis can be due to either respiratory or metabolic causes ■ Acidosis ◆ Respiratory acidosis can be caused by a variety of diseases and is due to alveolar hypoventilation. The pH is low (acid) and the PaCO2 is high ◆ In metabolic acidosis (eg diabetic ketoacidosis) the pH will be low (acid), but because the body responds by increasing ventilation to ‘blow off’ CO2 and raise the pH, the PaCO2 will also be low ■ Alkalosis ◆ Hyperventilation will produce respiratory alkalosis. An excess of CO2 is ‘blown off’, the PaCO2 falls and the pH rises (alkaline) BLOOD G A SES A ND PULSE OXI M ETRY | 9 1

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Metabolic alkalosis occurs as a result of a loss of body acid, eg due to prolonged vomiting. The rise in pH as the body becomes more alkaline reduces central respiratory drive. This results in hypoventilation which increases the PaCO2 and lowers the pH, thus correcting the alkalosis

■ Figure 7.1 is a useful way of remembering the concept of acid–base balance, and respiratory and metabolic alkalosis and acidosis

HCO3– ■ The body will respond rapidly to acidosis by increasing ventilation, but if the acid–base disturbance is prolonged renal compensatory mechanisms come into play. This will occur after a few days ■ Renal compensatory mechanisms result in retention of HCO3− ions and a large increase in HCO3− concentration ■ Estimation of HCO3− is therefore useful in determining whether the disturbance in acid–base balance is acute or chronic

PaO2 ■ O2 is much less soluble in water than CO2 and most of the O2 in the arterial system is carried bound to haemoglobin in the red blood cells ■ O2 diffuses across the alveolar membrane less efficiently than CO2 ■ PaO2 is sensitive to changes in both ventilation and alveolar disease. Almost any respiratory disease can lead to low levels of arterial O2 (hypoxia) if it is sufficiently severe

RE SP I RA TO RY F A I LU RE Type I respiratory failure ■ A low PaO2 with a normal PaCO2 indicates type 1 failure ◆ It is an isolated disorder of oxygenation where the PaO2 is reduced to less than 8 kPa

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VITAL POINT ✱ Causes of a low PaO2 with a normal (or low) PaCO2 include: ◆ ◆

Interstitial lung disease (eg fibrosing alveolitis) ◆ ◆

◆

Acute asthma

Severe pneumonia

Pulmonary embolism

Right-to-left cardiac shunt (eg ventricular septal defect, cyanotic heart disease)

Type II respiratory failure ■ In type II failure the PaCO2 is raised to above 6.7 kPa ■ The PaO2 will also be low (< 7.3–8 kPa), unless it has been corrected with supplemental oxygen

VITAL POINTS ✱ An easy way of remembering the difference between type I and type II respiratory failure is: ◆

In type I failure one arterial gas level is abnormal – O2

In type II failure there are two abnormal gas levels – O2 and CO2 ✱ A common cause of type II respiratory failure is chronic obstructive pulmonary disease (COPD). The respiratory drive of COPD patients with chronic type II failure may be dependent on a degree of hypoxia – a hypoxic drive. Administration of too high a percentage of supplementary O2 will suppress respiratory drive, worsen respiratory failure and may lead to respiratory arrest. Supplementary O2 must be given with caution, in a controlled fashion and a safe percentage of O2 determined by ABG monitoring ◆

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Compensated respiratory failure ■ Respiratory compensation occurs rapidly. There is an increase in ventilation and a fall in PaCO2 ■ Renal compensation occurs after a few days and produces a rise in HCO3− level

V ITAL POINT ✱ Compensation for respiratory failure is relative. The body is rarely able to fully compensate and it can never over compensate

The ABG values compatible with the types of respiratory failure are summarised in Table 7.1 (abnormal values are highlighted in blue) Table 7.1 ABG values and types of respiratory failure PaO2

PaCO2

pH

HCO3−

Type I respiratory failure

<8 kPa

<5.9 kPa

7.36–7.44

21–28

Type II respiratory failure

<8 kPa

>6.7 kPa

<7.36

21–28

Compensated respiratory failure

<8 kPa

>6.7 kPa

<7.36

>28

C O LLE C TI NG A R TE RIAL BLOOD G AS SAMPLES ■ Patients in intensive therapy units may have an arterial line in place. ABG samples can be taken from the arterial line, avoiding the need for repeated arterial stabs ■ In the absence of an arterial line samples are usually obtained from a radial or brachial artery. The femoral or dorsalis pedis can also be used, but the radial artery is the preferred site ■ Before a radial puncture is attempted the adequacy of the collateral circulation to the hand must be checked (Allen’s test): 9 4 | V I T AL LU NG F UNC T I O N

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◆ ◆

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Occlude both the radial and ulnar artery by pressing down over the wrist The hand will become pale and bloodless Release the pressure over the ulnar artery while maintaining the pressure over the radial artery If the ulnar supply is adequate the hand will become rapidly perfused again within 5–10 seconds of the ulnar artery being released

VITAL POINTS ✱ If the ulnar blood supply is inadequate an alternative site for sampling must be used ◆

2–4 ml of arterial blood is collected in a heparinised syringe, from which all the air has been expelled, using aseptic technique Firm pressure must be applied over the sampling site for at least 2 minutes. Longer pressure will be needed for patients who are anticoagulated

◆

◆

Air contamination of samples can seriously alter blood gas values

✱ Samples must be analysed immediately. If there is going to be a delay of more than a few minutes the sample must be packed in ice and kept at 0ºC. The sample must be analysed within 1 hour ✱ Arterial blood sampling is not without risk. Complications include: ◆ Haematoma ◆ Air embolus ◆ Infection ◆ Trauma and subsequent occlusion of the artery ◆ Arterial spasm

■ An alternative method of collecting samples for ABG is to use ‘arteriolised’ capillary samples, from the heel of an infant or ear lobe of an adult: ◆ The area is heated with a warm compress or a topical vasodilator is applied to dilate the capillaries and encourage blood flow BLOOD G A SES A ND PULSE OXI M ETRY | 9 5

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◆

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The area is lanced and blood allowed to flow naturally. ‘Squeezing’ the area is avoided as this will produce a ‘venous’ sample The sample is collected in a heparinised capillary tube that is then sealed to prevent air entry This is analysed immediately or packed in ice

V ITAL POINT ✱ There is no significant difference in the PaO2 measured by capillary gas or arterial sample when the PaO2 is below 8.0 kPa

Sp O 2 ■ SpO2 is expressed as a percentage and refers to the percentage of available haemoglobin that is saturated with O2 ■ Approximately 97% of the O2 that passes across the alveolar membrane into the circulation is carried around the body attached to haemoglobin as oxyhaemoglobin ■ Haemoglobin is ‘blue’ and absorbs more light than ‘red’ oxyhaemoglobin ■ Pulse oximetry and the measurement of SpO2 depend on the differing light absorption characteristics of the two compounds, haemoglobin and oxyhaemoglobin ■ A normal SpO2 is approximately 97% ■ The relationship between SpO2 and PaO2 is not direct and is described by the ‘haemoglobin–oxygen dissociation curve’ (Figure 7.2)

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SpO2 (%)

100 97 90 85

7

8

11

13

PaO2 (kPa)

Figure 7.2 Haemoglobin–oxygen dissociation curve

Haemoglobin–oxygen dissociation curve ■ At normal levels of PaO2 (11–13 kPa) the SpO2 will be above 95%, on the ‘level’ part of the curve ■ Once the PaO2 falls to 8 kPa and below, SpO2 falls steeply

VITAL POINT ✱ Oximetry, when properly performed, is accurate to within ±3% at higher levels of saturation, but is less accurate at values below 80%

P ULSE O XI M E TRY Measurement of SpO2 requires a pulsating blood flow. The volume of oxygenated blood passing through the blood vessels increases at the point of pulsation with each contraction of the heart. SpO2 is measured at maximum volume as the heart contracts. BLOOD G A SES A ND PULSE OXI M ETRY | 9 7

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V ITAL POINT ✱ When blood flow is sluggish, because of vasoconstriction or hypovolaemia (‘shock’) the oximeter may not be able to function, or will give inaccurate results

■ A pulse oximeter consists of a probe, that emits two wavelengths of light, and a sensor, attached to computerised unit ■ The probe is attached to a patient’s finger or ear lobe ■ Light is passed through the capillary bed from the probe to the sensor ■ The sensor detects the amount of light passing through ■ Other areas of the body where the light source can be shone through a capillary bed can also be used, eg the heel in infants ■ The SpO2 is calculated from the amount of light that passes through. The higher the percentage of oxyhaemoglobin the ‘redder’ the blood and the greater the amount of light that passes through

USI NG A P ULSE O X IMET ER ■ Ensure that the fingertip or ear lobe is warm and well perfused with blood ■ Remove the patient’s nail varnish if you are using a fingertip ■ Position the probe and sensor securely, but not so tightly as to occlude pulsating blood flow ■ Check that the oximeter is recording a good pulse waveform. If not, reposition the probe ■ Some drugs can affect the performance of oximeters and produce errors in SpO2 ■ The probe will emit heat as well as light and can cause local burns. If the SpO2 is being continuously monitored over a prolonged period the probe must be repositioned regularly

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VITAL POINT ✱ The SpO2 must always be interpreted in the context of the patient’s clinical state. Pulse oximetry is not a replacement for close observation of the patient and alarm systems on the pulse oximeter must never be ignored

Limitations of pulse oximetry ■ Shivering or tremor will interrupt the light signal and may give inaccurate results ■ Sluggish circulation, eg peripheral vascular disease or cardiac failure, will also give inaccurate readings ■ Cardiac arrhythmias, eg atrial fibrillation, will affect pulsating blood flow ■ Strong ambient lighting may interrupt the light signal from the probe ■ Nail varnish, particularly those with blue pigments, will cause falsely low readings

VITAL POINT ✱ Carboxyhaemoglobin (due to carbon monoxide poisoning) has similar light absorption characteristics to oxyhaemoglobin. Pulse oximetry in such cases is unreliable and misleading. ABG are essential

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PA T I ENT A ND CARER INFORMATION Your doctor may want to find out how well your lungs are getting oxygen into your blood. There are two main ways of finding this out: either by taking a blood sample or by attaching a small machine called a pulse oximeter to your finger or ear lobe. Both types of test may be necessary to give the doctor all the information he needs.

Blood samples ■ Blood samples are usually done in the hospital because the sample will need to be analysed straight away ■ Blood samples are taken in two ways and it is worth asking which method your doctor or local hospital is going to use

Arterial blood gases (ABG) ■ To do this test your doctor will need to take a small amount of blood from one of your arteries ■ This is usually taken from the artery in your wrist, but may also be taken from an artery in your foot, groin or shoulder ■ This should be no more painful than an ordinary blood test

Ear lobe samples ■ Some hospitals can find out how much oxygen is in your blood by taking a special blood sample from your ear lobe ■ Before taking the sample the doctor will need to get blood flowing freely through your ear lobe. You will either be given a warm pad to put on your ear, or a cream will be put on your ear lobe to open up the blood vessels. You ear will feel hot and will be red, but this passes off quite quickly ■ Your ear lobe will be pricked with a small needle and the blood collected in a small tube ■ This is not unduly painful, but your ear may be a little bruised for a few days after the test

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Pulse oximetry ■ This test is completely painless ■ A small clip is placed on your fingertip or ear ■ This shines a light through your finger or ear and the oximeter detects how much light is coming through ■ When you have a lot of oxygen in your blood more light gets through than when there is not enough oxygen ■ This test takes just a few moments, but if you are in hospital the clip may be left on for a longer period to allow staff to monitor you over a period of time

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Glossary

Airflow The speed at which air can be expired. Reduced when airways are obstructed. Anticholinergic bronchodilator A drug which dilates the airways by blocking the action of acetylcholine on the parasympathetic nerve endings in the lungs. Arterial blood gases (ABG) Analysis of samples of arterial blood to determine the pressure, or ‘tension’, of carbon dioxide (PaCO2), the hydrogen ion concentration, or ‘acidity’, of the blood (pH) – acid–base balance, the concentration of bicarbonate ions (HCO3–), and the pressure, or ‘tension’, of oxygen (PaO2). Asthma Chronic inflammatory condition of the airways, leading to widespread, variable airway obstruction that is reversible spontaneously or with treatment. Beta2 (ß2) agonist bronchodilator A drug which that stimulates β2 receptors of the sympathetic nervous system in the lungs, causing them to dilate. Biological control A normal individual with known lung function values and no respiratory disease used to verify the accuracy of the spirometer. Body plethysmography An accurate method of measuring static lung volumes: residual volume, total lung capacity and functional residual capacity. Calibration check Spirometers are calibrated to record the true volume of air exhaled into or through them and the accuracy of this calibration must be regularly checked with a calibration syringe to ensure that the spirometer continues to record volumes accurately. Calibration syringe A precisely engineered device to inject an exact, known volume of air into a spirometer to check the accuracy of its recording. Chronic obstructive pulmonary disease (COPD) A disease characterised by airflow obstruction that is usually progressive. It is not fully reversible and symptoms that do not vary markedly. It is predominantly caused by smoking. Correction factor A calculation applied to reference values for European populations to adjust for other ethnic groups. G LOSSA RY | 1 0 3

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Differential pressure pneumotachograph Measures the drop in pressure in a flow of air either side of a partial obstruction. Used in some flow-measuring spirometers. Diffusing capacity and gas transfer Tests used to assess the efficiency of the lungs in exchanging oxygen and carbon dioxide. Dynamic airway collapse In severe obstructive airways disease, where the airways are already significantly narrowed, the increased pressure on the outside of the airways during a forced expiration may cause them to collapse, trapping air in the lungs and reducing the forced vital capacity. Dynamic lung volumes Lung volumes measured during forced expiration and/or inhalation. EU peak flow meter scale New scale, complying with EU standard EN 13826, introduced across Europe to replace the Wright–McKerrow scale. Exercise testing A method of diagnosing asthma that exploits the fact that exercise is a potent trigger of bronchoconstriction in many asthma patients. Expired relaxed vital capacity The maximum volume of air that can be expired from the lungs during a relaxed but complete expiration from a position of full inspiration. Fibrosing alveolitis A condition causing progressive fibrosis of lung tissue, loss of lung volume and hypoxaemia. Flow/volume loop A graph of forced, maximal expiration and maximal inhalation produced by a spirometer. Flow rate (in litres per second or minute) is plotted on the vertical axis and volume (in litres) on the horizontal axis. The expiratory graph is above the horizontal axis and the inspiratory under the horizontal axis. Flow/volume trace A graph of forced expiration produced by a spirometer where flow rate (in litres per second or minute) is plotted on the vertical axis and volume (in litres) on the horizontal axis. Forced expiratory flow rates (FEF25, FEF50 and FEF75) The FEF25 is the maximum flow achievable when 75% of the forced vital capacity remains in the lungs. FEF25–75 refers to the flow rate generated between the middle part of a forced expiratory manoeuvre between 25% and 75% of the forced vital capacity. Forced expiratory volume in 1 second (FEV1) The maximum volume of air that can be expelled from the lungs in the first second of a forced expiration from a position of full inspiration. Forced vital capacity (FVC) The maximum volume of air that can be expired from the lungs during a forced and complete expiration from a position of full inspiration. Functional residual capacity (FRC) The volume of air left in the lungs after a normal, unforced expiration. 1 04 | V I T AL LUNG F UNC T I O N

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Haemoptysis Coughing blood. A contraindication to spirometry. Helium dilution A method of measuring static lung volumes: residual volume, total lung capacity and functional residual capacity. Inspiratory vital capacity The maximum volume of air that can be inspired into the lungs during a relaxed but complete inspiration from a position of full expiration. Maximal flow/volume curve See flow/volume loop. Mid expiratory flow rates (MEF25, MEF550, MEF75) The MEF25 is the maximum flow achievable when 25% of the forced vital capacity remains in the lungs. Similarly, the MEF75 is the maximum flow achievable when 75% of the forced vital capacity remains in the lungs and the MEF50 is the maximum flow rate achievable when the lungs are half-empty. Nitrogen washout A method of measuring static lung volumes: residual volume, total lung capacity and functional residual capacity. Obstructive airways diseases Diseases such as asthma and chronic obstructive pulmonary disease, where airways are narrowed and airflow reduced. Obstructive spirometry Arises as a result of obstructive airways disease and produces a reduced FEV1 and reduction in flow rate (FEV1/FVC or FEV1/VC). Oxygen saturation (SpO2) The percentage of available haemoglobin that is saturated with oxygen. Peak expiratory flow (PEF) The highest flow achieved from a maximal forced expiratory manoeuvre started without hesitation from a position of maximal lung inflation. Piezoelectric crystal A crystal that generates ultrasonic vibrations when an electric current is passed through it. Used to generate ultrasound in ultrasonic spirometers. Pneumothorax Condition where air is present between the pleural membranes. A contraindication to spirometry. Pulse oximetry A non-invasive method of measuring oxygen saturation. Ratio of forced expiratory volume in 1 second to forced vital capacity (FEV1/FVC) The amount of air blown out in the first second of a forced expiration from a position of maximal inspiration expressed as a percentage of the total amount expired (regardless of time) during that forced manoeuvre. Ratio of forced expiratory volume in 1 second to relaxed vital capacity (FEV1/VC) The amount of air expired during the first second of a forced expiration from a position of maximal inspiration expressed as a percentage of the total amount expired during a relaxed vital capacity manoeuvre. Reference value The normal, predicted or expected lung volume for an G LOSSA RY | 1 0 5

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individual of a particular gender, age, height and ethnic group. Regression equation Mathematical formula used to determine an individual’s reference (normal) spirometry values. Derived from population studies. Residual volume (RV) The volume of air left in the thorax at the end of a full expiration. Restrictive spirometry Arises as a result of problems within and outside the lungs causing a reduction in lung volumes with normal airflow. Reversibility testing A test to determine the response to bronchodilators or steroids. Used to differentiate between asthma and chronic obstructive pulmonary disease. Serial peak flow A method of diagnosing asthma by detecting the diurnal variability in peak flow that is characteristic of asthma. Small airway obstruction Pathological changes to airways 2–5 mm in diameter. Often present in COPD. Standard deviation Derived from the ‘scatter’ of results around the mean lung volumes measured in population studies to determine reference values. The normal standard deviation that will include 90% of a population is 1.645. Standard deviation scores outside a range of –1.645 to +1.645 are likely to be abnormal. Standardardised residuals The most statistically sound method to detect how abnormal a patient’s value is compared with predicted values. Theophyllines Oral bronchodilator drugs. Not now commonly used now. Total lung capacity (TLC) The maximum volume of air the lungs can contain at full inspiration. It includes the vital capacity (the ‘accessible’ volume of air that can be measured with a spirometer) and the residual volume (the ‘inaccessible’ volume). Type I respiratory failure Low PaO2 with a normal PaCO2. An isolated disorder of oxygenation where the PaO2 is reduced to less than 8 kPa. Type II respiratory failure PaCO2 raised to above 6.7 kPa. The PaO2 is also low, unless it has been corrected with supplemental oxygen. Vital capacity The volume, measured at the mouth, between the positions of full inspiration and full expiration. Volume/time graph A graph produced by a spirometer where volume exhaled forcibly is plotted against the time taken to exhale completely. Volume (in litres) is plotted against the vertical axis of the graph and time (in seconds) on the horizontal axis. Wright–McKerrow scale Original scale for PEF meters determined using a biological method, now superseded by the EU scale, which reflects flow rates more accurately at the lower and higher ends of the range.

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Useful addresses and contacts TRAINING

PROFESSIONAL

ORGANISATIONS

ORGANISATIONS

Education for Health The Athenaeum 10 Church St Warwick CV34 4AB Tel: 01926 493313 Fax: 01926 493224 Website: www.educationforhealth.org.uk An accredited institute of the Open University offering 1-day short courses, diploma level and degree level modular courses in a wide range of respiratory disorders, cardiovascular disease and diabetes, and an MSc in respiratory care. Also offers an accredited diploma level module in spirometry, recognised by the Association for Respiratory Technology and Physiology Respiratory Education UK Tel: 0151 529 2598/6258 Website: www.respiratoryeduk.com Offers a range of 1-day training courses and diploma and degree level modules in respiratory disease

The British Thoracic Society 17 Doughty Street London WC1N 2PL Tel: 020 7831 8778 Fax: 020 7831 8766 Website: www.brit-thoracic.org.uk Seeks to preserve and protect public health by the provision of information in matters concerning respiratory and associated disorders and how they might be prevented Association for Respiratory Technology and Physiology Suite 4, Sovereign House Gate Lane, Boldmere Sutton Coldfield Birmingham B73 5TT Tel: 0845 226 3062 or 0121 354 8200 Fax: 0121 355 2420 Website: www.artp.org.uk A professional organisation for practitioners working in clinical respiratory physiology and technology. Offers national, professionally recognised and validated qualifications in all aspects of respiratory function testing including spirometry, blood gases and exercise testing USEFUL A DDRESSES A ND C ONTA C TS | 1 0 7

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PATIENT INFORMATION AND SUPPORT British Lung Foundation 73–75 Goswell Road London EC1V 7ER Tel: 020 7688 5555 Fax: 020 7831 5556 Website: www.lunguk.org Helpline: 0845 8505020 open Mon–Fri 10 a.m.–6 p.m. A charity working to support people with respiratory disease by raising money for research, campaigning for better health service provision for respiratory disease, providing useful information for patients in a range of topics and facilitating and supporting the development of Breathe-Easy patient self-help groups. A telephone hotline for patients is also available

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Feedback Form I, the author, would welcome your comments on this book. Would you like more on some subjects and less on others? Are there additional topics which you would like to see in future editions? Please help me by marking your comments on this page, and sending it to the publisher, post-free, at Class Publishing, FREEPOST, London, W6 7BR Current topics

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1 Peak expiratory flow measurement

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3 Spirometry measurements and reference values

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4 Using a spirometer

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5 Spirometry interpretation

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6 Static lung volumes and gas transfer

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7 Arterial blood gases and pulse oximetry

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Have you found Vital Lung Function useful and practical? If so, you may be interested in these other books on from Class Publishing. Chronic Obstructive Pulmonary Disease in Primary Care THIRD EDITION £29.99 Dr David Bellamy and Rachel Booker This clear and helpful resource manual addresses the management requirements of GPs and practice nurses. In this book, you will find guidance, protocols, plans and tests – all appropriate to the primary care situation – that will streamline your diagnosis and management of COPD. ‘I am sure it will become a classic in the history of COPD Care.’ Duncan Geddes, Professor of Respiratory Medicine and Consultant Physician, Royal Brompton Hospital Vital Asthma

£14.99

Sue Cross and Dave Burns This book contains the essential information you need if you are part of the community asthma care team, whether you are a practice nurse, specialist nurse, GP, community pharmacist or physiotherapist. ‘. . . will be welcomed as a concise learning resource by those currently in asthma training, and as an update and a source of reference information by those in practice.’ Dr Mike Thomas, FRCP, GPIAG, Research Fellow, Department of General Practice, University of Aberdeen

1 10 | V I T AL LUNG F UNC T I O N

Vital COPD

£14.99

Rachel Booker The definitive quick-reference manual for all health professionals caring for people with COPD. This handbook will enable you to treat patients with speed and confidence. It will give you all the information you need to run a high-quality, efficient COPD management service. ‘. . . provides sound and authoritative clinical advice in an easily accessible format. This is likely to be one of the reference books that is used in practice, rather than gathering dust in a corner.’ Dr Steve Holmes, Chair of the General Practice Airways Group (GPIAG) Vital Diabetes FOURTH EDITION

£14.99

Dr Charles Fox and Mary MacKinnon This handy reference guide gives you all the back up you need for best practice in diabetes care, and includes all the vital facts and figures about diabetes for your information and regular use, as well as providing patient and carer information sheets that you can photocopy for patients to take away with them. ‘Full of the kind of essential and up-todate information you need to deliver the best practice in diabetes care.’ M. Carpenter, Diabetes Grapevine

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VI T AL BOOK S F O R YO U R PR A CTI CE TE A M The definitive quick reference manuals for all health professionals

Armed with these straightforward handbooks, you can treat patients with speed and confidence. Ideal for GPs, practice nurses, specialist nurses, community pharmacists and students.

VIT AL AST H MA Sue Cross and Dave Burns V I T AL N EPH ROLOG Y Dr Andy Stein, Janet Wild and Dr Paul Cook VIT AL COPD Rachel Booker V I TAL LU N G FU N CT ION Rachel Booker This series provides you with: ■ Clear and concise information at a glance ■ All the essential medical background you need for daily

practice ■ Patient information sections to help you explain complex

subjects ■ ‘Vital Points’ highlighted throughout the text for easy

reference ■ Up-to-date information to help you structure treatment

in primary care

VI TA L BOOKS FOR Y OUR PRA C TI C E TEA M | 1 1 1

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