- Email: [email protected]
Assessment of pulmonary function
INVESTIGATIONS
Assessment of pulmonary function
Key points C
Rachel Dancer David Thickett C
Abstract Assessment of pulmonary function plays a vital role in the investigation and monitoring of patients with and at risk of respiratory disease. The interpretation of pulmonary function tests requires knowledge of respiratory physiology and should always be made within the context of the patient’s history and examination findings. This article discusses the measurement and interpretation of commonly used pulmonary function tests including spirometry, lung volume and gas transfer measurements and exercise testing.
C
C
Keywords Gas transfer; lung; lung volumes; obstruction; physiology; pulmonary function tests; spirometry
There are a number of contraindications to pulmonary function tests. In particular, testing should be avoided in patients with current pulmonary infections as there is a risk of crosscontamination When interpreting spirometry results, the forced expiratory volume in 1 second (FEV1) to forced vital capacity (FVC) ratio (FEV1/FVC) ratio is used to determine whether there is an obstructive or restrictive defect. With an obstructive pattern, the severity of obstruction can be determined by comparing the FEV1 with predicted values Measurement of lung volumes using the plethysmography (body box) and dilution methods can be useful in patients with bullous lung disease: the dilution method measures lung volumes only in ventilated lung, whereas plethysmography includes the volume of non-communicating areas such as bullae Pulmonary function tests should be measured in all patients under consideration for lung resection. Patients considered to be at high risk of postoperative complications can be further assessed using exercise testing
Introduction alveoli and pleural space (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 negative pressure, and air flows out of the lung. However, when respiratory demands increase (e.g. exercise), active contraction of the abdominal wall muscles and internal intercostals increases the outward flow of air.1
Pulmonary function tests (PFTs) are invaluable for diagnosis and monitoring of respiratory diseases such as chronic obstructive pulmonary disease (COPD) and pulmonary fibrosis. Knowledge of lung physiology is needed for interpretation.
Lung physiology The lungs are designed to aid the supply of oxygen to, and 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 are coordinated to work efficiently. For ventilation, the lungs must generate sufficient negative pressure to move air down the airways to the alveoli and distribute it through the lungs. The main static lung volumes are shown in Figure 1. Any factor e exercise, disease, an unfavourable environment e that alters these volumes will affect the ability of the lungs to ventilate.
Gas exchange Gas exchange requires the alveoli to be ventilated and perfused. Ventilation and perfusion increase in a cranio-caudal direction, so the basal alveoli are better perfused than the apical ones. Ideally, the amount of ventilation (V) will match the amount of perfusion (Q), to maximize gas exchange. However, because of gravitational and non-gravitational factors, ventilation, even in normal, healthy lungs, exceeds perfusion in the lung apices and vice versa in the lung bases (the V/Q ratio is highest at the apex and lowest at the lung bases), so V/Q matching is never perfect. In a ventilated, perfused alveolus, gas moves from high to low concentration, so oxygen will move from the alveoli to the blood, and carbon dioxide will move in the opposite direction. Gas exchange may be limited in this situation because of:1 increased membrane thickness (e.g. fibrosis) loss of surface area (e.g. emphysema) reduced numbers of red blood cells to take up diffused oxygen (i.e. anaemia) reduced cardiac output (resulting in reduced pulmonary capillary volumes, including pulmonary emboli).
Ventilation Ventilation is an active process e the primary muscle that contracts to enable inspiration is the diaphragm. As the diaphragm contracts, it shortens, moves downwards and moves the rib cage outwards. This increases the difference in pressure between the
Rachel Dancer MBBS MRCP is a Clinical Lecturer in Respiratory Medicine at the University of Birmingham, UK. Her research focuses on acute lung injury and perioperative inflammation. Competing interests: none declared.
Preparing for pulmonary function tests PFTs can be performed with the patient sitting or standing; the position should be recorded on the report. The seated position is preferred for safety. If standing is used, place a chair behind the patient in case of light-headedness. In obese patients, improved
David Thickett FRCP DM is Professor of Respiratory Medicine at the University of Birmingham, UK, with research interests in acute lung injury and interstitial lung disease. Competing interests: none declared.
MEDICINE --:-
1
Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dancer R, Thickett D, Assessment of pulmonary function, Medicine (2016), http://dx.doi.org/10.1016/ j.mpmed.2016.02.007
INVESTIGATIONS
Static lung volumes
Inspiratory reserve volume
Total lung capacity
Vital capacity
Tidal volume Expiratory reserve volume Residual volume Time Figure 1
total lung capacity (TLC) and residual volume (RV) (Figures 1 and 2a). FEV1 is the volume of air that is breathed out in the first second of forced expiration (Figure 2a). In healthy individuals, 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 any previous readings. The FEV1/FVC ratio is used to differentiate between obstructive and restrictive defects. The Global Initiative for Chronic
values may be seen with standing. False teeth should be kept in unless loose or preventing an adequate seal around the mouthpiece.
Contraindications to performing pulmonary function tests PFTs require patient cooperation and effort. Confusion, inability to understand the instructions, pain, acute ill-health and stress incontinence may produce suboptimal tests. Specific contraindications to PFTs include current pneumothorax, recent cardiothoracic, abdominal or ophthalmic surgery, and recent myocardial infarction. Cross-contamination following PFTs has been reported so PFTs in patients with active respiratory infections such as tuberculosis should be deferred until the risk is minimized. In addition, extra precautions to disinfect equipment are needed. Activities that may result in a suboptimal reading and should be avoided are: drinking alcohol up to 4 hours before the test eating a large meal up to 2 hours before the test smoking up to 1 hour before the test vigorous exercise up to 30 minutes before the test. Patients should also be advised not to wear tight, restrictive clothing as this may result in suboptimal readings.
Volume (litres)
a. A volume–time graph
FEV1
FVC
Time (seconds)
Spirometry
b. A flow–volume loop
Spirometry measures the volume and flow of air that can be inhaled and exhaled. The patient is asked to inhale maximally and then breathe out into the spirometer as hard and fast as possible for as long as possible. A minimum exhalation time of 6 seconds is usually needed, and the procedure is repeated at least three times to ensure consistent results.2 Nose clips can be used to prevent air escaping and not being measured. If forced inspiratory measurements are needed (e.g. patients in whom upper airways obstruction is suspected), they can be recorded during inspiration. Results can be displayed as a volumeetime graph or a flowevolume loop (Figure 2). This is used to calculate the forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), FEV1/FVC ratio and peak expiratory flow rate (PEFR). FVC is the volume of air that can be maximally forcefully breathed out after full inspiration. It is the difference between
MEDICINE --:-
Peak expiratory flow
Flow
Expiratory
Volume
Inspiratory
FEV1, forced expiratory volume in 1 second; FVC, forced vital capacity. Figure 2
2
Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dancer R, Thickett D, Assessment of pulmonary function, Medicine (2016), http://dx.doi.org/10.1016/ j.mpmed.2016.02.007
INVESTIGATIONS
Obstructive Lung Disease (GOLD) COPD guidelines3 define a ratio less than 70% as consistent with an obstructive disorder as it implies that a lower percentage of the total exhaled air is breathed out in the first second. This definition is most commonly used in practice. However, the European Respiratory Society 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’. Controversy also exists over using arbitrary cut-offs to diagnose airflow obstruction, particularly in healthy elderly individuals. The severity of obstruction is determined by the FEV1. Again, different guidelines have different definitions of mild, moderate and severe obstruction; the commonly used GOLD COPD guideline definition is given in Table 1. A ratio greater than 70% may suggest greater reductions in vital capacity, as seen in restrictive disorders. However, a ratio greater than 80% has a higher positive predictive value for restrictive lung pathology. The morphology of the flowevolume loop can also be used to differentiate obstructive and restrictive disorders. Examples are shown in Figure 3. The PEFR is the maximum airflow speed on expiration (Figure 2b). It is useful when a quick simple measure of airflow obstruction is required (e.g. patients with asthma needing several recordings a day). However, it is less useful in patients with fixed airflow obstruction, such as patients with COPD; FEV1 and FVC are more accurate reflections of disease severity here.
Flow–volume loops The blue lines represent the patient’s efforts. Inspiration is represented by the line below the x-axis and expiration by the line above a. Severe airflow obstruction. Immediately after the peak expiratory flow, the flow drops to low values until the end of the forced expiration. This pattern is compatible with severe airflow obstruction.
Flow (litres/minute)
1
2
3
4
5
6
7
8
b. Restrictive spirometry in a patient with early pulmonary fibrosis. This is a characteristic pattern of a low FVC and comparatively high expiratory flow, giving the flow–volume curve a pointed cap appearance. Both FEV1 and FVC are well below predicted levels, but the FEV1 /FVC ratio is normal or even high.
FVC FEV1 FEV1/VC% PEF MEF75 MEF50 MEF25
10
Flow (litres/minute)
Expiration 5
0
1
2
3
4
5
6
7
2.67 2.41 7.02
74% 77% 86% 100% 99% 77% 61%
8
Volume (litres) Inspiration
–5
c. Upper airway obstruction: The inspiratory and expiratory flow–volume curves exhibit a flow plateau at low flows. Both the expiratory and inspiratory patterns are abnormal. The overall pattern is compatible with fixed obstruction in large extra- or intrathoracic airways (e.g. carcinoma of the larynx, or obstruction due to goitre).
Static lung volumes Static lung volumes are commonly measured by whole-body plethysmography and less often by inert gas dilution or nitrogen washout. Whole-body plethysmography is performed with the patient breathing normally in an airtight box. 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.
FVC 4.11 2.09 FEV1 FEV1/VC% MEF75 MEF50 MEF25 Empey index
Flow (litres/minute)
10
GOLD definition of severity of obstruction3 Severity of obstruction
Expiration 5
0
1
2
3
4
5
6
7
109% 68% 51% 31% 36% 68% 7%
8
Volume (litres)
Mild Moderate Severe Very severe
Inspiration
MEF, mid-expiratory flow; PEF, peak expiratory flow; VC, vital capacity.
Figure 3
Table 1
MEDICINE --:-
0
2.39
82% 25% 25% 26% 7% 5% 7%
Inspiration
The lung clearance index (LCI) is a measure of lung physiology derived from multiple breath washout tests. It represents the number of times the volume of gas present in the lung at the start of the washout (the FRC) must be turned over in order to wash out the inert tracer to the predefined endpoint. LCI increases with increasing disease severity. It has been shown to detect early lung disease sensitively in children with cystic fibrosis and is easier to perform in this population than conventional lung function tests.
>80 50e79 30e49 <30
Expiration 5
3.83 0.95
Volume (litres)
Lung clearance index
Post-bronchodilator FEV1 (% predicted)
FVC FEV1 FEV1/VC% PEF MEF75 MEF50 MEF25
10
3
Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dancer R, Thickett D, Assessment of pulmonary function, Medicine (2016), http://dx.doi.org/10.1016/ j.mpmed.2016.02.007
INVESTIGATIONS
Boyle’s law states that volume of gas is inversely proportional to pressure, so 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; this, in turn, results in an increase in pressure inside the box. The gas dilution technique requires a closed-circuit system and a known quantity of an inert gas such as helium, which is not significantly absorbed across the alveolarecapillary barrier. Therefore, once the patient has breathed in the helium and sufficient time has been given for it to distribute equally, the dilution of helium in expired air enables a calculation of lung volumes. Nitrogen dilution is a similar technique except the patient breaths 100% oxygen and dilution of nitrogen in expired air is measured. Interpretation of static lung volumes frequently focuses on TLC and RV. TLC is the total volume of air in the lungs at maximal inspiration. RV is the amount of air left in the lungs after maximal expiration (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. In patients with lung blebs or bullae, lung volumes should be measured using both techniques; the volume of non-communicating areas of the lung will be measured by plethysmography but not by dilution techniques.
Exercise testing The 6-minute walk test is commonly used for the assessment of functional status in patients with respiratory disease. It can be used in conjunction with measurement of oxygen saturation to determine whether a patient is suitable for ambulatory oxygen. The test measures how far a patient can walk on a flat, hard surface in 6 minutes. It should be carried out in a long, flat corridor (30 metre) where interruptions are unlikely. The patient should walk with any normal aids (e.g. 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 6 minutes, the report will indicate for how long they walked. If a portable pulse oximeter is used, the pre- and post-test oxygen saturation and heart rate will also be recorded. The shuttle walk test is an alternative test in which the patient walks between two cones in time with a beep that gradually speeds up as the test progresses. There is debate about the relative merits of these tests as prognostic indicators in COPD, pulmonary fibrosis and pulmonary hypertension.
Use of pulmonary function tests in preoperative assessment PFTs are frequently used to assess patients before surgery, particularly lung resection. NICE lung cancer guidelines recommend that spirometry should be undertaken in all patients considered for lung resection, and DLCO should be measured in any patient with dyspnoea out of proportion to their known disease or with other lung pathology such as fibrosis. Segment counting can be used to help predict lung function post-lung resection as the preoperative lung function and the impact of removal of part of the lung need to be considered. Predicted postoperative lung function less than 30% predicted carries a high risk of postoperative complications. Patients with a moderate-to-high risk of postoperative dyspnoea can be assessed with a shuttle walk test, a distance of 400 metre being considered good function, or with a cardiopulmonary exercise test to establish VO2max (a measure of patient’s individual aerobic capacity).5 A
Transfer factor This is a measurement of the diffusion of gas across the alveolarblood membrane. It should normally be corrected for haemoglobin concentration. The patient is asked to rapidly inspire gas containing 10% helium and 0.3% carbon monoxide, hold their breath for 10 seconds and then breathe out. The carbon monoxide in the expired breath is measured, and the fractions of carbon monoxide in the inspired and expired gas compared to calculate the diffusing capacity of the lung for carbon monoxide (DLCO). The KCO is the DLCO corrected for alveolar volume (VA). For example, a patient who has undergone a pneumonectomy may have a low DLCO due to reduced VA, but a normal KCO when this is corrected for functioning volume.
Fractional exhaled nitric oxide (FeNO) Eosinophilic airway inflammation is thought to increase levels of nitric oxide in the exhaled breath, so measurement of the FeNO concentration can be useful in patients with asthma. Effective treatment with inhaled corticosteroids reduces airway inflammation so serial FeNO measurements can be used to monitor response to treatment. In the test, patients need to breathe steadily into a mouthpiece for up to 12 seconds. National Institute for Health and Care Excellence (NICE) guidance recommends use of FeNO in adults and children over 5 years old in whom the diagnosis of asthma is unclear and in patients with asthma who have ongoing symptoms despite inhaled corticosteroid use. Although a negative FeNO result does not exclude asthma and exhaled NO levels can be raised for reasons other than asthma, a positive FeNO result can support the diagnosis in the context of other typical signs and symptoms. The normal range for FeNO depends on the device used and is lower for children than adults. Use of FeNO is also limited in smokers as cigarette smoke suppresses NO production in the lungs.4
MEDICINE --:-
KEY REFERENCES 1 Maskell N, Miller A, eds. Oxford desk reference: respiratory medicine. OUP, 2009. 2 Miller MR, Hankinson J, Brusasco V, et al. Standardisation of spirometry. Eur Respir J 2005; 26: 319e38. 3 Global Strategy for the Diagnosis. Management and prevention of COPD. Global initiative for chronic obstructive lung disease (GOLD). 2010. Available from: http://www.goldcopd. org. 4 National Institute for Health and Care Excellence. Measuring fractional exhaled nitric oxide concentration in asthma: NIOX MINO, NIOX VERO and NObreath. DG12. London: National Institute for Health and Care Excellence, April 2014. 5 National Institute for Health and Care Excellence. The diagnosis and treatment of lung cancer. CG121. London: National Institute for Health and Care Excellence, 2011.
4
Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dancer R, Thickett D, Assessment of pulmonary function, Medicine (2016), http://dx.doi.org/10.1016/ j.mpmed.2016.02.007
Assessment of pulmonary function
Key points C
Rachel Dancer David Thickett C
Abstract Assessment of pulmonary function plays a vital role in the investigation and monitoring of patients with and at risk of respiratory disease. The interpretation of pulmonary function tests requires knowledge of respiratory physiology and should always be made within the context of the patient’s history and examination findings. This article discusses the measurement and interpretation of commonly used pulmonary function tests including spirometry, lung volume and gas transfer measurements and exercise testing.
C
C
Keywords Gas transfer; lung; lung volumes; obstruction; physiology; pulmonary function tests; spirometry
There are a number of contraindications to pulmonary function tests. In particular, testing should be avoided in patients with current pulmonary infections as there is a risk of crosscontamination When interpreting spirometry results, the forced expiratory volume in 1 second (FEV1) to forced vital capacity (FVC) ratio (FEV1/FVC) ratio is used to determine whether there is an obstructive or restrictive defect. With an obstructive pattern, the severity of obstruction can be determined by comparing the FEV1 with predicted values Measurement of lung volumes using the plethysmography (body box) and dilution methods can be useful in patients with bullous lung disease: the dilution method measures lung volumes only in ventilated lung, whereas plethysmography includes the volume of non-communicating areas such as bullae Pulmonary function tests should be measured in all patients under consideration for lung resection. Patients considered to be at high risk of postoperative complications can be further assessed using exercise testing
Introduction alveoli and pleural space (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 negative pressure, and air flows out of the lung. However, when respiratory demands increase (e.g. exercise), active contraction of the abdominal wall muscles and internal intercostals increases the outward flow of air.1
Pulmonary function tests (PFTs) are invaluable for diagnosis and monitoring of respiratory diseases such as chronic obstructive pulmonary disease (COPD) and pulmonary fibrosis. Knowledge of lung physiology is needed for interpretation.
Lung physiology The lungs are designed to aid the supply of oxygen to, and 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 are coordinated to work efficiently. For ventilation, the lungs must generate sufficient negative pressure to move air down the airways to the alveoli and distribute it through the lungs. The main static lung volumes are shown in Figure 1. Any factor e exercise, disease, an unfavourable environment e that alters these volumes will affect the ability of the lungs to ventilate.
Gas exchange Gas exchange requires the alveoli to be ventilated and perfused. Ventilation and perfusion increase in a cranio-caudal direction, so the basal alveoli are better perfused than the apical ones. Ideally, the amount of ventilation (V) will match the amount of perfusion (Q), to maximize gas exchange. However, because of gravitational and non-gravitational factors, ventilation, even in normal, healthy lungs, exceeds perfusion in the lung apices and vice versa in the lung bases (the V/Q ratio is highest at the apex and lowest at the lung bases), so V/Q matching is never perfect. In a ventilated, perfused alveolus, gas moves from high to low concentration, so oxygen will move from the alveoli to the blood, and carbon dioxide will move in the opposite direction. Gas exchange may be limited in this situation because of:1 increased membrane thickness (e.g. fibrosis) loss of surface area (e.g. emphysema) reduced numbers of red blood cells to take up diffused oxygen (i.e. anaemia) reduced cardiac output (resulting in reduced pulmonary capillary volumes, including pulmonary emboli).
Ventilation Ventilation is an active process e the primary muscle that contracts to enable inspiration is the diaphragm. As the diaphragm contracts, it shortens, moves downwards and moves the rib cage outwards. This increases the difference in pressure between the
Rachel Dancer MBBS MRCP is a Clinical Lecturer in Respiratory Medicine at the University of Birmingham, UK. Her research focuses on acute lung injury and perioperative inflammation. Competing interests: none declared.
Preparing for pulmonary function tests PFTs can be performed with the patient sitting or standing; the position should be recorded on the report. The seated position is preferred for safety. If standing is used, place a chair behind the patient in case of light-headedness. In obese patients, improved
David Thickett FRCP DM is Professor of Respiratory Medicine at the University of Birmingham, UK, with research interests in acute lung injury and interstitial lung disease. Competing interests: none declared.
MEDICINE --:-
1
Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dancer R, Thickett D, Assessment of pulmonary function, Medicine (2016), http://dx.doi.org/10.1016/ j.mpmed.2016.02.007
INVESTIGATIONS
Static lung volumes
Inspiratory reserve volume
Total lung capacity
Vital capacity
Tidal volume Expiratory reserve volume Residual volume Time Figure 1
total lung capacity (TLC) and residual volume (RV) (Figures 1 and 2a). FEV1 is the volume of air that is breathed out in the first second of forced expiration (Figure 2a). In healthy individuals, 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 any previous readings. The FEV1/FVC ratio is used to differentiate between obstructive and restrictive defects. The Global Initiative for Chronic
values may be seen with standing. False teeth should be kept in unless loose or preventing an adequate seal around the mouthpiece.
Contraindications to performing pulmonary function tests PFTs require patient cooperation and effort. Confusion, inability to understand the instructions, pain, acute ill-health and stress incontinence may produce suboptimal tests. Specific contraindications to PFTs include current pneumothorax, recent cardiothoracic, abdominal or ophthalmic surgery, and recent myocardial infarction. Cross-contamination following PFTs has been reported so PFTs in patients with active respiratory infections such as tuberculosis should be deferred until the risk is minimized. In addition, extra precautions to disinfect equipment are needed. Activities that may result in a suboptimal reading and should be avoided are: drinking alcohol up to 4 hours before the test eating a large meal up to 2 hours before the test smoking up to 1 hour before the test vigorous exercise up to 30 minutes before the test. Patients should also be advised not to wear tight, restrictive clothing as this may result in suboptimal readings.
Volume (litres)
a. A volume–time graph
FEV1
FVC
Time (seconds)
Spirometry
b. A flow–volume loop
Spirometry measures the volume and flow of air that can be inhaled and exhaled. The patient is asked to inhale maximally and then breathe out into the spirometer as hard and fast as possible for as long as possible. A minimum exhalation time of 6 seconds is usually needed, and the procedure is repeated at least three times to ensure consistent results.2 Nose clips can be used to prevent air escaping and not being measured. If forced inspiratory measurements are needed (e.g. patients in whom upper airways obstruction is suspected), they can be recorded during inspiration. Results can be displayed as a volumeetime graph or a flowevolume loop (Figure 2). This is used to calculate the forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), FEV1/FVC ratio and peak expiratory flow rate (PEFR). FVC is the volume of air that can be maximally forcefully breathed out after full inspiration. It is the difference between
MEDICINE --:-
Peak expiratory flow
Flow
Expiratory
Volume
Inspiratory
FEV1, forced expiratory volume in 1 second; FVC, forced vital capacity. Figure 2
2
Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dancer R, Thickett D, Assessment of pulmonary function, Medicine (2016), http://dx.doi.org/10.1016/ j.mpmed.2016.02.007
INVESTIGATIONS
Obstructive Lung Disease (GOLD) COPD guidelines3 define a ratio less than 70% as consistent with an obstructive disorder as it implies that a lower percentage of the total exhaled air is breathed out in the first second. This definition is most commonly used in practice. However, the European Respiratory Society 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’. Controversy also exists over using arbitrary cut-offs to diagnose airflow obstruction, particularly in healthy elderly individuals. The severity of obstruction is determined by the FEV1. Again, different guidelines have different definitions of mild, moderate and severe obstruction; the commonly used GOLD COPD guideline definition is given in Table 1. A ratio greater than 70% may suggest greater reductions in vital capacity, as seen in restrictive disorders. However, a ratio greater than 80% has a higher positive predictive value for restrictive lung pathology. The morphology of the flowevolume loop can also be used to differentiate obstructive and restrictive disorders. Examples are shown in Figure 3. The PEFR is the maximum airflow speed on expiration (Figure 2b). It is useful when a quick simple measure of airflow obstruction is required (e.g. patients with asthma needing several recordings a day). However, it is less useful in patients with fixed airflow obstruction, such as patients with COPD; FEV1 and FVC are more accurate reflections of disease severity here.
Flow–volume loops The blue lines represent the patient’s efforts. Inspiration is represented by the line below the x-axis and expiration by the line above a. Severe airflow obstruction. Immediately after the peak expiratory flow, the flow drops to low values until the end of the forced expiration. This pattern is compatible with severe airflow obstruction.
Flow (litres/minute)
1
2
3
4
5
6
7
8
b. Restrictive spirometry in a patient with early pulmonary fibrosis. This is a characteristic pattern of a low FVC and comparatively high expiratory flow, giving the flow–volume curve a pointed cap appearance. Both FEV1 and FVC are well below predicted levels, but the FEV1 /FVC ratio is normal or even high.
FVC FEV1 FEV1/VC% PEF MEF75 MEF50 MEF25
10
Flow (litres/minute)
Expiration 5
0
1
2
3
4
5
6
7
2.67 2.41 7.02
74% 77% 86% 100% 99% 77% 61%
8
Volume (litres) Inspiration
–5
c. Upper airway obstruction: The inspiratory and expiratory flow–volume curves exhibit a flow plateau at low flows. Both the expiratory and inspiratory patterns are abnormal. The overall pattern is compatible with fixed obstruction in large extra- or intrathoracic airways (e.g. carcinoma of the larynx, or obstruction due to goitre).
Static lung volumes Static lung volumes are commonly measured by whole-body plethysmography and less often by inert gas dilution or nitrogen washout. Whole-body plethysmography is performed with the patient breathing normally in an airtight box. 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.
FVC 4.11 2.09 FEV1 FEV1/VC% MEF75 MEF50 MEF25 Empey index
Flow (litres/minute)
10
GOLD definition of severity of obstruction3 Severity of obstruction
Expiration 5
0
1
2
3
4
5
6
7
109% 68% 51% 31% 36% 68% 7%
8
Volume (litres)
Mild Moderate Severe Very severe
Inspiration
MEF, mid-expiratory flow; PEF, peak expiratory flow; VC, vital capacity.
Figure 3
Table 1
MEDICINE --:-
0
2.39
82% 25% 25% 26% 7% 5% 7%
Inspiration
The lung clearance index (LCI) is a measure of lung physiology derived from multiple breath washout tests. It represents the number of times the volume of gas present in the lung at the start of the washout (the FRC) must be turned over in order to wash out the inert tracer to the predefined endpoint. LCI increases with increasing disease severity. It has been shown to detect early lung disease sensitively in children with cystic fibrosis and is easier to perform in this population than conventional lung function tests.
>80 50e79 30e49 <30
Expiration 5
3.83 0.95
Volume (litres)
Lung clearance index
Post-bronchodilator FEV1 (% predicted)
FVC FEV1 FEV1/VC% PEF MEF75 MEF50 MEF25
10
3
Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dancer R, Thickett D, Assessment of pulmonary function, Medicine (2016), http://dx.doi.org/10.1016/ j.mpmed.2016.02.007
INVESTIGATIONS
Boyle’s law states that volume of gas is inversely proportional to pressure, so 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; this, in turn, results in an increase in pressure inside the box. The gas dilution technique requires a closed-circuit system and a known quantity of an inert gas such as helium, which is not significantly absorbed across the alveolarecapillary barrier. Therefore, once the patient has breathed in the helium and sufficient time has been given for it to distribute equally, the dilution of helium in expired air enables a calculation of lung volumes. Nitrogen dilution is a similar technique except the patient breaths 100% oxygen and dilution of nitrogen in expired air is measured. Interpretation of static lung volumes frequently focuses on TLC and RV. TLC is the total volume of air in the lungs at maximal inspiration. RV is the amount of air left in the lungs after maximal expiration (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. In patients with lung blebs or bullae, lung volumes should be measured using both techniques; the volume of non-communicating areas of the lung will be measured by plethysmography but not by dilution techniques.
Exercise testing The 6-minute walk test is commonly used for the assessment of functional status in patients with respiratory disease. It can be used in conjunction with measurement of oxygen saturation to determine whether a patient is suitable for ambulatory oxygen. The test measures how far a patient can walk on a flat, hard surface in 6 minutes. It should be carried out in a long, flat corridor (30 metre) where interruptions are unlikely. The patient should walk with any normal aids (e.g. 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 6 minutes, the report will indicate for how long they walked. If a portable pulse oximeter is used, the pre- and post-test oxygen saturation and heart rate will also be recorded. The shuttle walk test is an alternative test in which the patient walks between two cones in time with a beep that gradually speeds up as the test progresses. There is debate about the relative merits of these tests as prognostic indicators in COPD, pulmonary fibrosis and pulmonary hypertension.
Use of pulmonary function tests in preoperative assessment PFTs are frequently used to assess patients before surgery, particularly lung resection. NICE lung cancer guidelines recommend that spirometry should be undertaken in all patients considered for lung resection, and DLCO should be measured in any patient with dyspnoea out of proportion to their known disease or with other lung pathology such as fibrosis. Segment counting can be used to help predict lung function post-lung resection as the preoperative lung function and the impact of removal of part of the lung need to be considered. Predicted postoperative lung function less than 30% predicted carries a high risk of postoperative complications. Patients with a moderate-to-high risk of postoperative dyspnoea can be assessed with a shuttle walk test, a distance of 400 metre being considered good function, or with a cardiopulmonary exercise test to establish VO2max (a measure of patient’s individual aerobic capacity).5 A
Transfer factor This is a measurement of the diffusion of gas across the alveolarblood membrane. It should normally be corrected for haemoglobin concentration. The patient is asked to rapidly inspire gas containing 10% helium and 0.3% carbon monoxide, hold their breath for 10 seconds and then breathe out. The carbon monoxide in the expired breath is measured, and the fractions of carbon monoxide in the inspired and expired gas compared to calculate the diffusing capacity of the lung for carbon monoxide (DLCO). The KCO is the DLCO corrected for alveolar volume (VA). For example, a patient who has undergone a pneumonectomy may have a low DLCO due to reduced VA, but a normal KCO when this is corrected for functioning volume.
Fractional exhaled nitric oxide (FeNO) Eosinophilic airway inflammation is thought to increase levels of nitric oxide in the exhaled breath, so measurement of the FeNO concentration can be useful in patients with asthma. Effective treatment with inhaled corticosteroids reduces airway inflammation so serial FeNO measurements can be used to monitor response to treatment. In the test, patients need to breathe steadily into a mouthpiece for up to 12 seconds. National Institute for Health and Care Excellence (NICE) guidance recommends use of FeNO in adults and children over 5 years old in whom the diagnosis of asthma is unclear and in patients with asthma who have ongoing symptoms despite inhaled corticosteroid use. Although a negative FeNO result does not exclude asthma and exhaled NO levels can be raised for reasons other than asthma, a positive FeNO result can support the diagnosis in the context of other typical signs and symptoms. The normal range for FeNO depends on the device used and is lower for children than adults. Use of FeNO is also limited in smokers as cigarette smoke suppresses NO production in the lungs.4
MEDICINE --:-
KEY REFERENCES 1 Maskell N, Miller A, eds. Oxford desk reference: respiratory medicine. OUP, 2009. 2 Miller MR, Hankinson J, Brusasco V, et al. Standardisation of spirometry. Eur Respir J 2005; 26: 319e38. 3 Global Strategy for the Diagnosis. Management and prevention of COPD. Global initiative for chronic obstructive lung disease (GOLD). 2010. Available from: http://www.goldcopd. org. 4 National Institute for Health and Care Excellence. Measuring fractional exhaled nitric oxide concentration in asthma: NIOX MINO, NIOX VERO and NObreath. DG12. London: National Institute for Health and Care Excellence, April 2014. 5 National Institute for Health and Care Excellence. The diagnosis and treatment of lung cancer. CG121. London: National Institute for Health and Care Excellence, 2011.
4
Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dancer R, Thickett D, Assessment of pulmonary function, Medicine (2016), http://dx.doi.org/10.1016/ j.mpmed.2016.02.007