- Email: [email protected]
Pulmonary function tests
INVESTIGATIONS
Pulmonary function tests
What’s new?
Rachel Dancer C
David Thickett
C
Abstract Pulmonary function tests are frequently used to investigate patients with respiratory symptoms and signs such as dyspnoea, cough and wheeze. They are also used to monitor disease progression in patients with a known lung disease, such as chronic obstructive pulmonary disease or interstitial lung disease, and also to monitor for the development of a lung disease in patients with connective tissue disorders or in people who work with substances that may cause lung diseases. They can be helpful before cardiothoracic or abdominal surgery to help determine operative risk. This article will discuss the measurement and interpretation of commonly used pulmonary function tests.
C
outwards. This increases the difference in pressure between the alveoli and the external environment (the negative pressure), so air flows into the lungs. Expiration can occur passively e when the muscles stop contracting at the end of inspiration, the elastic recoil of the lungs leads to a reduction in the negative pressure and air flows out of the lung. However, when respiratory demands increase (e.g. in exercise), active contraction of the muscles of the abdominal wall and internal intercostals increases the outward flow of air.1
Keywords gas transfer; lung; lung volumes; obstruction; physiology; pulmonary function tests; spirometry
Introduction Pulmonary function tests (PFTs) are invaluable in the diagnosis and monitoring of respiratory diseases such as chronic obstructive pulmonary disease (COPD) and pulmonary fibrosis. Knowledge of lung physiology is required in order to fully interpret the tests.
Gas exchange In order for gas exchange to occur the alveoli must be ventilated and perfused. Ideally, the amount of ventilation (V) will match the amount of perfusion (Q), to maximize the gas exchange that will occur. However, due to the effects of gravity, even in normal, healthy lungs, ventilation will exceed perfusion in the lung apices and perfusion will exceed ventilation in the lung bases, so that V/Q matching will not be perfect. In a ventilated and perfused alveolus, gas will move from high concentration to low concentration, so oxygen will move from the alveoli to the blood and carbon dioxide will move in the opposite direction. There are several reasons why gas exchange may be limited in this situation1: increased membrane thickness (e.g. in fibrosis) loss of surface area (e.g. in emphysema) reduced numbers of red blood cells to take up diffused oxygen (i.e. anaemia) reduced cardiac output (results in reduced pulmonary capillary volumes).
Lung physiology The lungs are designed to facilitate the supply of oxygen to, and the removal of carbon dioxide from, the body. Ventilation is the movement of air from the external environment to the alveoli and gas exchange describes the movement of oxygen into the bloodstream and carbon dioxide into the alveoli. These functions need to be highly controlled and co-ordinated in order to work efficiently. In order to ventilate, the lungs must generate sufficient negative pressure to move air down the airways to the alveoli and be able to distribute the air throughout the lungs. The main static lung volumes are shown in Figure 1. Any factor, such as exercise, disease or an unfavourable environment, which alters these volumes will affect the ability of the lungs to ventilate. Ventilation Ventilation is an active process e the diaphragm is the primary muscle that contracts to enable inspiration. As the diaphragm contracts it shortens, moves downwards and moves the rib cage
Preparing for pulmonary function tests PFTs can be performed with the patient sitting or standing. The position used should be recorded on the report. Generally, the seated position is preferred in case the patient should have a syncopal episode, but if the standing position is used a chair should be placed behind the patient so that they can sit down if they feel light-headed. In obese patients, improved values may be seen if the patient is standing. False teeth should generally be kept in unless they are loose or prevent an adequate seal being formed around the mouthpiece.2
Rachel Dancer MBBS MRCP is a Clinical Lecturer in Respiratory Medicine at the University of Birmingham, UK. Her research focuses on acute lung injury. Conflicts of interest: none declared. David Thickett FRCP DM is a Reader in Respiratory Medicine at the University of Birmingham, UK with research interests in acute lung injury and interstitial lung disease. Conflicts of interest: none declared.
MEDICINE 40:4
The definition of severity of obstruction on COPD has changed to stress that FEV1 should be measured post-bronchodilator, with FEV1 >80% predicted classified as mild obstruction and FEV1 50e79% predicted classified as moderate obstruction. The lung clearance index has been shown to detect lung disease sensitively in young children with cystic fibrosis, who are unable to perform spirometry. The latest NICE guidelines on lung cancer have revised recommendations for lung function testing before lung resection surgery.
186
Ó 2012 Elsevier Ltd. All rights reserved.
INVESTIGATIONS
Static lung volumes
Inspiratory reserve volume
Total lung capacity
Vital capacity
Tidal volume Expiratory reserve volume Residual volume Time Figure 1
Contraindications to performing pulmonary function tests
The results from spirometry can either be displayed as a volumeetime graph or as a flowevolume loop (see Figure 2). This can then be used to calculate the forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), FEV1/FVC Ratio and peak expiratory flow rate (PEFR). The FVC is the volume of air that can be breathed out after full inspiration. It is the difference between the total lung capacity (TLC) and residual volume (RV) (see Figures 1 and 2a). The FEV1 is the volume of air that is breathed out in the first second of forced expiration (Figure 2a). In healthy individuals the FEV1 and FVC will depend mainly on height, sex and age. Predicted value calculators are available and predicted values will normally be quoted on the report. Reference should also be made to previous readings if available. The FEV1/FVC ratio is used to differentiate between obstructive and restrictive defects. The Global Initiative for Chronic Obstructive Lung Disease (GOLD) COPD guidelines5 define a ratio of less than 70% as consistent with an obstructive disorder. A ratio under 70% implies that a lower percentage of the total air breathed out is breathed out in the first second. This definition is most commonly used in practice. However, the European Respiratory Society (ERS) guidelines on lung function testing suggest that an obstructive disorder should be defined as ‘a reduced FEV1/VC ratio below the fifth percentile of the predicted value’.6 Controversy also exists over using arbitrary cut offs to diagnose airflow obstruction, particularly in the healthy elderly. The severity of obstruction is determined by the FEV1. Again, different guidelines define mild, moderate and severe obstruction differently but the GOLD COPD guideline definition is given in Table 1. A ratio above 70% is consistent with greater reductions in the vital capacity as seen in restrictive disorders. The morphology of the flowevolume loop can also be used to differentiate between obstructive and restrictive disorders. Examples are shown in Figure 3: The peak expiratory flow rate (PEFR) is the maximum speed of flow of air on expiration (see Figure 2b). It is useful in situations
PFTs require patient cooperation and effort, and should not be carried out on patients who are confused or unable to understand the instructions they are given by the respiratory physiologist. In addition, results are likely to be sub-optimal in patients who are in pain or acutely unwell, and in patients who suffer with stress incontinence.2 There are also specific contraindications to pulmonary function testing, including current pneumothorax, recent cardiothoracic, abdominal or ophthalmic surgery, and recent myocardial infarction. Cross-contamination as a result of pulmonary function testing has been reported3 and PFTs in patients with active respiratory infections such as tuberculosis should be deferred until the risk of cross-contamination can be minimized. In addition, extra precautions may be needed to ensure equipment is disinfected. Activities that may result in a sub-optimal reading and should be avoided are listed below: drinking alcohol within 4 hours of the test eating a large meal within 2 hours of the test smoking within 1 hour of the test vigorous exercise within 30 minutes of the test. In addition, patients should be advised not to wear tight, restrictive clothing as this may also result in a sub-optimal reading.2
Spirometry Spirometry measures the volume and flow of air that can be inhaled and exhaled. The patient will be asked to inhale maximally, then to breathe out into the spirometer as hard and as fast as they can for as long as they can. Normally, the patient will be expected to breathe out for at least 6 seconds and the procedure will be repeated at least three times to ensure the results are accurate.4 Nose clips can be used to prevent air escaping and not being measured. If forced inspiratory measurements are also needed (for example in patients in whom upper airways obstruction is suspected) this can be measured as the breath in is being taken.
MEDICINE 40:4
187
Ó 2012 Elsevier Ltd. All rights reserved.
INVESTIGATIONS
Figure 2
where a quick simple measure of airflow obstruction is required (e.g. in patients with asthma in whom measurements are required several times per day). However, it is less useful in patients with fixed airflow obstruction, such as patients with COPD, in whom FEV1 and FVC are more accurate reflections of disease severity.
Lung clearance index The lung clearance index is a measure of lung physiology derived from multiple breath washout tests. It has been shown to detect early lung disease sensitively in young children with cystic fibrosis and is easier to perform in this population than conventional lung function tests.7
GOLD definition of severity of obstruction5 Post-bronchodilator FEV1 (% predicted)
Severity of obstruction
>80 50e79 30e49 <30
Mild Moderate Severe Very severe Figure 3
Table 1
MEDICINE 40:4
188
Ó 2012 Elsevier Ltd. All rights reserved.
INVESTIGATIONS
Static lung volumes
tests as prognostic indicators in COPD, pulmonary fibrosis and pulmonary hypertension.
Static lung volumes are commonly measured by whole body plethysmography. They can also be measured by nitrogen or helium washout.8 Whole body plethysmography requires the patient to sit in an airtight box and breathe normally. To carry out the measurement a shutter is placed across the breathing tube and the patient continues to make respiratory efforts against the closed shutter. The plethysmograph measures changes in the pressure or volume inside the box and uses Boyle’s law to calculate static lung volumes. Boyle’s law states that volume of gas is inversely proportional to pressure and, therefore, increases in the volume of air inside the lung as a result of decreased intrathoracic pressure will reduce the volume of air outside the lung, which will result in an increase in pressure inside the box.8 Interpretation of static lung volumes will frequently focus on the total lung capacity (TLC) and the residual volume (RV). The total lung capacity is the total volume of air in the lungs at maximal inspiration. The residual volume is the amount of air left in the lungs after maximal expiration (see Figure 1). TLC and RV are increased in patients with obstructive defects such as asthma and emphysema, and decreased in patients with restrictive defects such as chest wall deformities or interstitial lung disease.8
Use of pulmonary function tests in preoperative assessment Pulmonary function tests are frequently used to assess whether patients are suitable for lung resection surgery. The NICE lung cancer guidelines recommend that spirometry should be measured in all patients being considered for lung resection, and that DLCO should be measured in any patient with dyspnoea that is out of proportion to their known disease or with other lung pathology such as fibrosis. Predicted post-operative lung function that is lower than 30% predicted carries a high risk of post-operative complications. Patients with a moderateto-high risk of post-operative dyspnoea can be assessed with a shuttle walk test, with a distance of 400 m being considered good function, or with a cardiopulmonary exercise test to establish VO2max.11
Conclusion Pulmonary function tests can provide vital information about patients’ respiratory physiology. They are useful in patients with either known or suspected respiratory disease. Interpretation of the tests requires knowledge about respiratory physiology and should be made in the context of the patient’s history and examination findings. A
Transfer factor The transfer factor is the measurement of diffusion of gas across the alveolar blood membrane. It should normally be corrected for haemoglobin. The patient is asked to rapidly breath in gas containing 10% helium and 0.3% carbon monoxide, hold their breath for 10 seconds, then breathe out. The remaining carbon monoxide in the expired breath is measured and the fractions of CO in the inspired and expired gas compared to calculate diffusing capacity of the lung for carbon monoxide (DLCO). The KCO is the DLCO corrected for alveolar volume e for example a patient who has undergone a pneumonectomy may have a low DLCO due to reduced alveolar volume, but a normal KCO when this is corrected for volume.9
REFERENCES 1 Maskell N, Miller A, eds. Oxford desk reference: respiratory medicine. OUP, 2009. 2 Miller MR, Crapo R, Hankinson J, et al. General considerations for lung function testing. Eur Respir J 2005; 26: 153e61. 3 Hazaleus RE, Cole J, Berdischewsky M. Tuberculin skin test conversion from exposure to contaminated pulmonary function testing apparatus. Respir Care 1981; 26: 53e5. 4 Miller MR, Hankinson J, Brusasco V, et al. Standardisation of spirometry. Eur Respir J 2005; 26: 319e38. 5 Global Strategy for the Diagnosis, Management and Prevention of COPD. Global Initiative for Chronic Obstructive Lung Disease (GOLD). Available from: http://www.goldcopd.org; 2010. 6 Pellegrino R, Viegi G, Brusasco V, et al. Interpretative strategies for lung function tests. Eur Respir J 2005; 26: 948e68. 7 Horsley A. Lung Clearance Index in the assessment of airways disease. Respir Med 2009 Jun; 103: 793e9. 8 Wanger J, Clausen JL, Coates A, et al. Standardisation of the measurement of lung volumes. Eur Respir J 2005; 26: 511e22. 9 Macintyre N, Wanger J, Viegi G, et al. Standardisation of the singlebreath determination of carbon monoxide uptake in the lung. Eur Respir J 2005 Oct; 26: 720e35. 10 ATS Committee on Proficiency Standards for Clinical Pulmonary Function Laboratories. ATS statement: guidelines for the six-minute walk test. Am J Respir Crit Care Med 2002; 166: 111e7. 11 National Institute for Health and Clinical Excellence. The diagnosis and treatment of lung cancer. CG121. London: National Institute for Health and Clinical Excellence, 2011.
Exercise testing The six-minute walk test (SMWT) is commonly used for the assessment of functional status in patients with respiratory disease. It can be used in conjunction with measurement of patient’s oxygen saturation to determine whether a patient is suitable for consideration of ambulatory oxygen. The SMWT measures how far a patient can walk on a flat, hard surface in 6 minutes. It should be measured in a long flat corridor (at least 30 m long) where interruptions are unlikely. The patient should walk with any aids they would normally use (e.g. a walking stick) and the technician should not walk alongside them. The report will normally state the pre- and post-test Borg dyspnoea score and the distance the patient walked. If the patient was unable to walk for the full 6 minutes, the report will indicate for how long they walked. If a portable pulse oximeter is used the report will also record the pre- and post-test oxygen saturations and heart rate.10 The shuttle walk test is an alternative method of exercise testing in which the patient walks between two cones in time with a beep that gradually increases in speed as the test progresses. There remains debate about the relative merits of these
MEDICINE 40:4
189
Ó 2012 Elsevier Ltd. All rights reserved.
Pulmonary function tests
What’s new?
Rachel Dancer C
David Thickett
C
Abstract Pulmonary function tests are frequently used to investigate patients with respiratory symptoms and signs such as dyspnoea, cough and wheeze. They are also used to monitor disease progression in patients with a known lung disease, such as chronic obstructive pulmonary disease or interstitial lung disease, and also to monitor for the development of a lung disease in patients with connective tissue disorders or in people who work with substances that may cause lung diseases. They can be helpful before cardiothoracic or abdominal surgery to help determine operative risk. This article will discuss the measurement and interpretation of commonly used pulmonary function tests.
C
outwards. This increases the difference in pressure between the alveoli and the external environment (the negative pressure), so air flows into the lungs. Expiration can occur passively e when the muscles stop contracting at the end of inspiration, the elastic recoil of the lungs leads to a reduction in the negative pressure and air flows out of the lung. However, when respiratory demands increase (e.g. in exercise), active contraction of the muscles of the abdominal wall and internal intercostals increases the outward flow of air.1
Keywords gas transfer; lung; lung volumes; obstruction; physiology; pulmonary function tests; spirometry
Introduction Pulmonary function tests (PFTs) are invaluable in the diagnosis and monitoring of respiratory diseases such as chronic obstructive pulmonary disease (COPD) and pulmonary fibrosis. Knowledge of lung physiology is required in order to fully interpret the tests.
Gas exchange In order for gas exchange to occur the alveoli must be ventilated and perfused. Ideally, the amount of ventilation (V) will match the amount of perfusion (Q), to maximize the gas exchange that will occur. However, due to the effects of gravity, even in normal, healthy lungs, ventilation will exceed perfusion in the lung apices and perfusion will exceed ventilation in the lung bases, so that V/Q matching will not be perfect. In a ventilated and perfused alveolus, gas will move from high concentration to low concentration, so oxygen will move from the alveoli to the blood and carbon dioxide will move in the opposite direction. There are several reasons why gas exchange may be limited in this situation1: increased membrane thickness (e.g. in fibrosis) loss of surface area (e.g. in emphysema) reduced numbers of red blood cells to take up diffused oxygen (i.e. anaemia) reduced cardiac output (results in reduced pulmonary capillary volumes).
Lung physiology The lungs are designed to facilitate the supply of oxygen to, and the removal of carbon dioxide from, the body. Ventilation is the movement of air from the external environment to the alveoli and gas exchange describes the movement of oxygen into the bloodstream and carbon dioxide into the alveoli. These functions need to be highly controlled and co-ordinated in order to work efficiently. In order to ventilate, the lungs must generate sufficient negative pressure to move air down the airways to the alveoli and be able to distribute the air throughout the lungs. The main static lung volumes are shown in Figure 1. Any factor, such as exercise, disease or an unfavourable environment, which alters these volumes will affect the ability of the lungs to ventilate. Ventilation Ventilation is an active process e the diaphragm is the primary muscle that contracts to enable inspiration. As the diaphragm contracts it shortens, moves downwards and moves the rib cage
Preparing for pulmonary function tests PFTs can be performed with the patient sitting or standing. The position used should be recorded on the report. Generally, the seated position is preferred in case the patient should have a syncopal episode, but if the standing position is used a chair should be placed behind the patient so that they can sit down if they feel light-headed. In obese patients, improved values may be seen if the patient is standing. False teeth should generally be kept in unless they are loose or prevent an adequate seal being formed around the mouthpiece.2
Rachel Dancer MBBS MRCP is a Clinical Lecturer in Respiratory Medicine at the University of Birmingham, UK. Her research focuses on acute lung injury. Conflicts of interest: none declared. David Thickett FRCP DM is a Reader in Respiratory Medicine at the University of Birmingham, UK with research interests in acute lung injury and interstitial lung disease. Conflicts of interest: none declared.
MEDICINE 40:4
The definition of severity of obstruction on COPD has changed to stress that FEV1 should be measured post-bronchodilator, with FEV1 >80% predicted classified as mild obstruction and FEV1 50e79% predicted classified as moderate obstruction. The lung clearance index has been shown to detect lung disease sensitively in young children with cystic fibrosis, who are unable to perform spirometry. The latest NICE guidelines on lung cancer have revised recommendations for lung function testing before lung resection surgery.
186
Ó 2012 Elsevier Ltd. All rights reserved.
INVESTIGATIONS
Static lung volumes
Inspiratory reserve volume
Total lung capacity
Vital capacity
Tidal volume Expiratory reserve volume Residual volume Time Figure 1
Contraindications to performing pulmonary function tests
The results from spirometry can either be displayed as a volumeetime graph or as a flowevolume loop (see Figure 2). This can then be used to calculate the forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), FEV1/FVC Ratio and peak expiratory flow rate (PEFR). The FVC is the volume of air that can be breathed out after full inspiration. It is the difference between the total lung capacity (TLC) and residual volume (RV) (see Figures 1 and 2a). The FEV1 is the volume of air that is breathed out in the first second of forced expiration (Figure 2a). In healthy individuals the FEV1 and FVC will depend mainly on height, sex and age. Predicted value calculators are available and predicted values will normally be quoted on the report. Reference should also be made to previous readings if available. The FEV1/FVC ratio is used to differentiate between obstructive and restrictive defects. The Global Initiative for Chronic Obstructive Lung Disease (GOLD) COPD guidelines5 define a ratio of less than 70% as consistent with an obstructive disorder. A ratio under 70% implies that a lower percentage of the total air breathed out is breathed out in the first second. This definition is most commonly used in practice. However, the European Respiratory Society (ERS) guidelines on lung function testing suggest that an obstructive disorder should be defined as ‘a reduced FEV1/VC ratio below the fifth percentile of the predicted value’.6 Controversy also exists over using arbitrary cut offs to diagnose airflow obstruction, particularly in the healthy elderly. The severity of obstruction is determined by the FEV1. Again, different guidelines define mild, moderate and severe obstruction differently but the GOLD COPD guideline definition is given in Table 1. A ratio above 70% is consistent with greater reductions in the vital capacity as seen in restrictive disorders. The morphology of the flowevolume loop can also be used to differentiate between obstructive and restrictive disorders. Examples are shown in Figure 3: The peak expiratory flow rate (PEFR) is the maximum speed of flow of air on expiration (see Figure 2b). It is useful in situations
PFTs require patient cooperation and effort, and should not be carried out on patients who are confused or unable to understand the instructions they are given by the respiratory physiologist. In addition, results are likely to be sub-optimal in patients who are in pain or acutely unwell, and in patients who suffer with stress incontinence.2 There are also specific contraindications to pulmonary function testing, including current pneumothorax, recent cardiothoracic, abdominal or ophthalmic surgery, and recent myocardial infarction. Cross-contamination as a result of pulmonary function testing has been reported3 and PFTs in patients with active respiratory infections such as tuberculosis should be deferred until the risk of cross-contamination can be minimized. In addition, extra precautions may be needed to ensure equipment is disinfected. Activities that may result in a sub-optimal reading and should be avoided are listed below: drinking alcohol within 4 hours of the test eating a large meal within 2 hours of the test smoking within 1 hour of the test vigorous exercise within 30 minutes of the test. In addition, patients should be advised not to wear tight, restrictive clothing as this may also result in a sub-optimal reading.2
Spirometry Spirometry measures the volume and flow of air that can be inhaled and exhaled. The patient will be asked to inhale maximally, then to breathe out into the spirometer as hard and as fast as they can for as long as they can. Normally, the patient will be expected to breathe out for at least 6 seconds and the procedure will be repeated at least three times to ensure the results are accurate.4 Nose clips can be used to prevent air escaping and not being measured. If forced inspiratory measurements are also needed (for example in patients in whom upper airways obstruction is suspected) this can be measured as the breath in is being taken.
MEDICINE 40:4
187
Ó 2012 Elsevier Ltd. All rights reserved.
INVESTIGATIONS
Figure 2
where a quick simple measure of airflow obstruction is required (e.g. in patients with asthma in whom measurements are required several times per day). However, it is less useful in patients with fixed airflow obstruction, such as patients with COPD, in whom FEV1 and FVC are more accurate reflections of disease severity.
Lung clearance index The lung clearance index is a measure of lung physiology derived from multiple breath washout tests. It has been shown to detect early lung disease sensitively in young children with cystic fibrosis and is easier to perform in this population than conventional lung function tests.7
GOLD definition of severity of obstruction5 Post-bronchodilator FEV1 (% predicted)
Severity of obstruction
>80 50e79 30e49 <30
Mild Moderate Severe Very severe Figure 3
Table 1
MEDICINE 40:4
188
Ó 2012 Elsevier Ltd. All rights reserved.
INVESTIGATIONS
Static lung volumes
tests as prognostic indicators in COPD, pulmonary fibrosis and pulmonary hypertension.
Static lung volumes are commonly measured by whole body plethysmography. They can also be measured by nitrogen or helium washout.8 Whole body plethysmography requires the patient to sit in an airtight box and breathe normally. To carry out the measurement a shutter is placed across the breathing tube and the patient continues to make respiratory efforts against the closed shutter. The plethysmograph measures changes in the pressure or volume inside the box and uses Boyle’s law to calculate static lung volumes. Boyle’s law states that volume of gas is inversely proportional to pressure and, therefore, increases in the volume of air inside the lung as a result of decreased intrathoracic pressure will reduce the volume of air outside the lung, which will result in an increase in pressure inside the box.8 Interpretation of static lung volumes will frequently focus on the total lung capacity (TLC) and the residual volume (RV). The total lung capacity is the total volume of air in the lungs at maximal inspiration. The residual volume is the amount of air left in the lungs after maximal expiration (see Figure 1). TLC and RV are increased in patients with obstructive defects such as asthma and emphysema, and decreased in patients with restrictive defects such as chest wall deformities or interstitial lung disease.8
Use of pulmonary function tests in preoperative assessment Pulmonary function tests are frequently used to assess whether patients are suitable for lung resection surgery. The NICE lung cancer guidelines recommend that spirometry should be measured in all patients being considered for lung resection, and that DLCO should be measured in any patient with dyspnoea that is out of proportion to their known disease or with other lung pathology such as fibrosis. Predicted post-operative lung function that is lower than 30% predicted carries a high risk of post-operative complications. Patients with a moderateto-high risk of post-operative dyspnoea can be assessed with a shuttle walk test, with a distance of 400 m being considered good function, or with a cardiopulmonary exercise test to establish VO2max.11
Conclusion Pulmonary function tests can provide vital information about patients’ respiratory physiology. They are useful in patients with either known or suspected respiratory disease. Interpretation of the tests requires knowledge about respiratory physiology and should be made in the context of the patient’s history and examination findings. A
Transfer factor The transfer factor is the measurement of diffusion of gas across the alveolar blood membrane. It should normally be corrected for haemoglobin. The patient is asked to rapidly breath in gas containing 10% helium and 0.3% carbon monoxide, hold their breath for 10 seconds, then breathe out. The remaining carbon monoxide in the expired breath is measured and the fractions of CO in the inspired and expired gas compared to calculate diffusing capacity of the lung for carbon monoxide (DLCO). The KCO is the DLCO corrected for alveolar volume e for example a patient who has undergone a pneumonectomy may have a low DLCO due to reduced alveolar volume, but a normal KCO when this is corrected for volume.9
REFERENCES 1 Maskell N, Miller A, eds. Oxford desk reference: respiratory medicine. OUP, 2009. 2 Miller MR, Crapo R, Hankinson J, et al. General considerations for lung function testing. Eur Respir J 2005; 26: 153e61. 3 Hazaleus RE, Cole J, Berdischewsky M. Tuberculin skin test conversion from exposure to contaminated pulmonary function testing apparatus. Respir Care 1981; 26: 53e5. 4 Miller MR, Hankinson J, Brusasco V, et al. Standardisation of spirometry. Eur Respir J 2005; 26: 319e38. 5 Global Strategy for the Diagnosis, Management and Prevention of COPD. Global Initiative for Chronic Obstructive Lung Disease (GOLD). Available from: http://www.goldcopd.org; 2010. 6 Pellegrino R, Viegi G, Brusasco V, et al. Interpretative strategies for lung function tests. Eur Respir J 2005; 26: 948e68. 7 Horsley A. Lung Clearance Index in the assessment of airways disease. Respir Med 2009 Jun; 103: 793e9. 8 Wanger J, Clausen JL, Coates A, et al. Standardisation of the measurement of lung volumes. Eur Respir J 2005; 26: 511e22. 9 Macintyre N, Wanger J, Viegi G, et al. Standardisation of the singlebreath determination of carbon monoxide uptake in the lung. Eur Respir J 2005 Oct; 26: 720e35. 10 ATS Committee on Proficiency Standards for Clinical Pulmonary Function Laboratories. ATS statement: guidelines for the six-minute walk test. Am J Respir Crit Care Med 2002; 166: 111e7. 11 National Institute for Health and Clinical Excellence. The diagnosis and treatment of lung cancer. CG121. London: National Institute for Health and Clinical Excellence, 2011.
Exercise testing The six-minute walk test (SMWT) is commonly used for the assessment of functional status in patients with respiratory disease. It can be used in conjunction with measurement of patient’s oxygen saturation to determine whether a patient is suitable for consideration of ambulatory oxygen. The SMWT measures how far a patient can walk on a flat, hard surface in 6 minutes. It should be measured in a long flat corridor (at least 30 m long) where interruptions are unlikely. The patient should walk with any aids they would normally use (e.g. a walking stick) and the technician should not walk alongside them. The report will normally state the pre- and post-test Borg dyspnoea score and the distance the patient walked. If the patient was unable to walk for the full 6 minutes, the report will indicate for how long they walked. If a portable pulse oximeter is used the report will also record the pre- and post-test oxygen saturations and heart rate.10 The shuttle walk test is an alternative method of exercise testing in which the patient walks between two cones in time with a beep that gradually increases in speed as the test progresses. There remains debate about the relative merits of these
MEDICINE 40:4
189
Ó 2012 Elsevier Ltd. All rights reserved.