Pulmonary Function Test: Spirometry

Pulmonary Function Test: SpirometryREVIEW
Pulmonary Function Test: Spirometry Duu Wen Sewa, MRCP (UK), Thun How Ong, MRCP (UK)
Department of Respiratory and Critical Care Medicine, Singapore General Hospital, Singapore
ABSTRACT
Spirometry is a useful tool in assessing the physiological lung function of a patient, and can be helpful in differentiating the etiology of the patient’s symptoms. Indications for the test and the actual procedure are as described. Validity of a spirometry depends on patient co-operation and criteria for acceptability and repeatability must be met for useful interpretation of the results. Commonly measured parameters are described and a simple logarithm for interpretation of a spirometry result is given. Physicians must be mindful when interpreting the result in the context of extreme of ages, size or differing ethnicity as reference values for these groups of individuals are often extrapolated and not validated.
Keywords: central airway obstruction, obstructive pulmonary disease, spirometry
INTRODUCTION Pulmonary function testing comprises of mainly three components: spirometry, lung volumes and diffusing capacity. Spirometry, from the Latin spiro “to breathe” and the Greek metron “measure” is one of the oldest and most commonly ordered tests of pulmonary function. It is a physiological test that measures how an individual inhales or exhales volume of air as a function of time. It is a valuable tool for evaluating the respiratory system, representing an important adjunct to the patient history, various lung imaging studies and invasive testing.
INDICATIONS FOR SPIROMETRY The clinical indications for spirometry are varied and depend on the clinical settings and questions to be addressed. Generally accepted clinical indications are listed in Table 1.
Spirometry has very few absolute contra- indications, although several conditions may raise caution and others may affect the quality of results. It is recommended that test not be performed within one month of an acute coronary syndrome or myocardial infarction1, or in pregnancy. Similarly,
patients with pain, nausea or other discomfort, as well as altered mental state may not be able to co- operate fully, giving suboptimal results.
Testing Procedure Spirometry requires a voluntary maneuver in which a patient inhales maximally from tidal breathing at rest to total lung capacity (TLC) and then rapidly exhales to the fullest extent until no further volume is exhaled at residual volume (RV), followed by a maximum inspiration back to TLC (Fig. 1). A volume vs. time plot and a flow vs. volume plot for the same maneuver can be generated as shown in Fig. 2. The maximal flow-volume curve is helpful in quality assurance, in detecting mild airflow obstruction, and in detecting central airway obstruction. For best results, both inspiratory and expiratory loops are obtained.
Acceptability and Repeatability Criteria
A number of spirometry standards have been developed over the years1–3. The pulmonary function lab at Singapore General Hospital and most other testing centres in Singapore use the American Thoracic Society and European
Table 1: Indications for Spirometry
Diagnostic To evaluate symptoms and/ or signs pertaining to respiratory system e.g chronic cough, dyspnea,wheezing etc To measure the effect of disease on pulmonary function To screen individuals at risk of having pulmonary disease To assess pre-operative risk To assess prognosis of patients with respiratory disease To assess health status before beginning strenuous physical activity programme
Monitoring To assess therapeutic intervention To describe the course of disease that affect lung function To monitor people exposed to injurious agents To monitor for adverse reactions to drugs with known pulmonary toxicity
Disability/ impairment evaluations To assess patients as part of a rehabilitation programme To assess risks as part of insurance evaluation To assess individuals for legal reasons
Respiratory Society (ATS/ERS) Task force criteria for acceptability and repeatability (see Table 2).
Failure to achieve this criterion means that the spirometry must be interpreted with caution,
if at all.
Reference values The interpretation of various spirometric indices is based on comparisons of data measured in an
Fig. 1. Static lung volume measurements
Fig. 2. Volume vs time and flow vs. time plots
Time Volume
Vo lu
m e
Fl ow
Table 2: Acceptability criteria for Spirograms4
Good start (extrapolated volume <5% of FVC or 0.15L, whichever is greater)5 Absence of artefacts (refer Fig.3) such as
a) submaximal effort at any point b) obstructed mouthpiece c) coughing d) early termination or cutoff e) glottis closure influencing measurement f ) leak
Satisfactory exhalation Duration ≥ 6s in adults or plateau in volume-time curve or if subject cannot continue to exhale
Between-maneuver criteria: Once 3 acceptable spirograms are obtained, apply the following: Two largest value for FVC must be within 0.15 L of each other Two largest value for FEV1 must be within 0.15L of each other If both of the above are met, testing session may be terminated If not, continue testing until: Both criteria are met with additional maneuvers performed OR Total of eight tests have been performed OR The patient is no longer able to or no longer wishes to continue
individual subject with reference values derived from a representative population of health subjects2,7–8. Normal values of pulmonary function vary with age, height, gender and ethnicity. Moreover, the range of normal is considerably varied. These factors complicate the choice of the most appropriate reference range regression equations to use in the pulmonary laboratory9–12.
Additionally, because reference values are often derive from populations with little representation at the extremes of age and size, interpret these ranges with caution. The ATS/ERS recommends the use of the Third National Health and Nutrition Examination Survey in the United States as a spirometry reference standard in USA.
Fig. 3. Acceptable and unacceptable flow-volume loops
Normal Cough
Glottis closure
Suboptimal effort
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Currently, there is no specific set of equations that has been validated for Asian population. Specific values for “normalcy” are controversial but in general, there is a move towards reporting the normal range in terms of a lower limit of normal (LLN) and an upper limit of normal (ULN). 2005 ATS/ ERS recommended that the LLN be determined from the fifth percentile (i.e. 1.96 SD: two-tailed t test, p <0.05)13. In contrast, the Global Initiative for Chronic Obstructive Lung Disease14 suggests using a fixed cutoff of 0.70 for the post bronchodilator FEV1/FVC to define obstruction. Both systems have their limitations15–17.
Most important, spirometry values need to be interpreted in the context of the patient, their symptoms, and other testing for the highest yield in diagnosis and management.
Commonly Measured Spirometric indices Forced expiratory Volume in 1 sec (FEV1) The FEV1 is the most widely used parameter to measure the mechanical properties of the lungs, i.e. we are measuring the “strength” of the patient’s lungs. The absolute value of the FEV1 obtained by the patient is then compared to his or her peers according to age, gender, height, and ethnicity.
Spirometric values below the 5th percentile of the frequency distribution of values measured in the reference population are considered to be below the expected “normal range” or below “the lower limit of normal (LLN)”. The practice of using 80% predicted as a fixed value for the lower limit of normal may be acceptable in children, but can lead to important errors when interpreting lung function in adults and as such is not recommended by the ATS/ERS task force.13 FEV1 is decreased in both obstructive and restrictive lung diseases. Severity of most lung pathology (e.g. Chronic Obstructive Pulmonary Disease or COPD) is based on the percentage predicted FEV1 rather than the absolute value of FEV1
18–20. Although there are good evidence that FEV1 correlates well with severity of symptoms and prognosis in many diseases21–22, the correlates do not allow physicians to accurately predict symptoms and prognosis in individual patients.
Forced Vital Capacity (FVC) FVC is a measure of lung volume and is usually reduced in disease that causes the lungs to be
smaller e.g., restrictive lung disease, disorders of the bellows (kyphoscoliosis, neuromuscular weakness). The absolute FVC is compared with the patient’s peers to obtain a percentage predicted value. A reduction in FVC can also occur in situation whereby the lungs are hyperinflated due to severe obstruction and air trapping, and increased residual volume, a phenomenon referred to as pseudorestriction. Therefore, FVC is not a reliable indicator of total lung capacity or restriction, especially in the setting of airflow obstruction, and lung volume measurements should be done when a restrictive pattern is seen on simple spirometry.
FEV1 / FVC ratio In healthy adults, this should be approximately 75–80%. FEV1/FVC ratio is measured using the absolute values of FEV1/FVC rather than percentage predicted. In obstructive diseases, FEV1 is reduced due to airway resistance to expiratory flow; the FVC may also be diminished due to premature closure of airway in expiration, but not in the same proportion as FEV1. This leads to a reduced FEV1/ FVC ratio. In restrictive disorders, the FEV1, FVC and TLC are all reduced, and the FEV1 / FVC ratio is normal or even elevated.
A ratio of FEV1/FVC of < 70% is generally taken to indicate obstructive airways disease. Note should be made, however, the as one ages (male >40yrs and females >50yrs), the ratio of FEV1/FVC tend to drop, and for elderly patients the lower limit of normal (LLN) for FEV1/FVC may be a more useful indicator of obstruction than an absolute value of 70%. Hence the ATS/ERS taskforce recommends the use of the 5th percentile of the normal distribution as cutoff for the lower limit of the reference range (LLN)13 so as to avoid over-diagnosis of obstructive lung disease.
One of the most common problems encountered in spirometry is that patients are unable to exhale completely (i.e. expiration time is <6 s or fails to plateau; in this case the FVC is underestimated and the FEV1/FVC ratio is overestimated, leading the physician to wrongly conclude that the patient does not have obstructive airways disease (see algorithm).
Flow-Volume Loop The flow-volume loop is a plot of inspiratory and expiratory flow (y-axis) against volume (x-axis) during the performance of maximally forced
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inspiratory and expiratory maneuvers. The normal expiratory portion of the flow volume loop is characterized by a rapid rise to the peak flow rate, followed by a near linear fall in flow as the patient exhales toward residual volume. The Inspiratory curve, in contrast, is a relative symmetrical, saddle shaped curve.
The shape of the flow-volume loop can indicate the location of airflow limitation. In common obstructive airflow disorders e.g. asthma and emphysema, early evidence of airflow obstruction is qualitatively demonstrated by a concave “scooped- out” shape of the expiratory flow-volume curve.
Typical flow-volume plots observed with differing types of physiologic central airway obstruction are shown in Fig. 4.
Case Studies: Common patterns of Abnormality Obstructive Airways Disease (Examples: chronic obstructive lung disease, asthma, cystic fibrosis, and bronchiolitis obliterans).
A 67-year old smoker presented with cough and wheeze. His spirometry is as follows: (see Fig. 5)
In normal patients, up to 70% of the vital capacity of the lungs is emptied in the first second (hence FEV1/
Fig. 4. Flow-volume curves in physiologic central airway obstructions: a) fixed central airway obstruction. b) variable extrathoracic obstruction. c) variable intrathoracic obstruction.
Fig. 5. Example of a spirometry of a patient with obstructive lung disease. Note the reduced FEV1/FVC ratio with decreased FEV1. Flow-volume curve shows the typical scoop appearance in the expiratory loop.
Pre-Bronch Post-Bronch
––SPIROMETRY––
FEV1/FVC (%) 79 71 34 44 34 -2
1 2 3 4 5
10
8
6
4
2
0
-2
-4
-6
-8
-10
2
0 2 4 0 2 4 0 2 4
6
3
0
-3
-6
6
3
0
-3
-6
6
3
0
-3
-6
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Fig. 6. The spirometry of a patient with restrictive lung disease. The flow volume shape is maintained but there is a proportional decrease in both FEV1 and FVC (normal FEV1/FVC ratio).
Assess acceptability, repeatability criteria
Confirm with lung volume test
Fig. 7. A simplified algorithm in interpretation of spirometry results. LLN: lower limit of normal.
Pred LLN Actual % Pred
FEV1/FVC (%) 84 76 94 112
2
10 8 6 4 2 0 -2 -4 -6 -8
-10
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FVC ratio > 70%) With obstructive lung disease, it takes longer for the lung to empty. Smaller airways are narrower, resulting in lower flow with sharp fall in the flow volume loop (scooped out appearance). FEV1 and FEF (forced expiratory flow at 25% to 75% of forced vital capacity) are low. Typically, the patient will have a normal FVC, though an incomplete expiration may falsely lower this value, leading to an overestimation of the FEV1/ FVC ratio.
Note that spirometry only gives us the pattern of lung function abnormality – in this case, obstructive airways disease. It does not tell us if this patient has asthma or COPD. The diagnosis will depend on the patient’s clinical presentation and the spirometry merely provides collaborative evidence.
Restrictive airway disease (Examples: idiopathic pulmonary fibrosis, hypersensitivity pneumonitis, and obesity).
A 55-year old woman, having a history of rheumatoid arthritis, presenting with progressive dyspnea on exertion and clinical findings of fine inspiratory velcro-like crepitations at both lung bases. (See Fig. 6).
Restrictive airway disease means that the total lung volume is too low. Although an accurate diagnosis of total lung volume is not possible with spirometry (residual lung volume cannot be measured with a spirometer) spirometry results can be very suggestive for a restrictive lung disease.
Since the airways are normal, the flow volume loop will have a normal shape: the curve will descend in a straight line from the PEF to the x-axis. Total lung volume is low, which results in a low FVC. PEF (peak expiratory flow) can be normal or low. FEV1 is equally lowered than FVC, so the FEV1/FVC ratio will be normal or even raised.
A simple algorithm for interpretation of spirometry results (As seen in Fig. 7)
Related tests Bronchodilator test Reversibility test after administration of a bronchodilator is frequently undertaken. Baseline testing is done followed by administration of a short-acting bronchodilator (i.e. salbutamol
400ug or ipratiopium 160ug). An increase in either the FEV1 or the FVC of ≥12% AND ≥200ml is now considered a “positive” bronchodilator response13. Note that a positive bronchodilator test does not have adequate sensitivity and specificity in differentiating a patient with asthma from chronic obstructive lung disease.23-25 In addition, response well below the significant thresholds may still be associated with improvement in the patient symptoms and performance.26
Airway challenge test Airway hyperresponsiveness (AHR) is one of the characteristic features of asthma. Use of an exogenous stimuli e.g. methacholine to directly challenge the airway to elicit airway responsiveness is a commonly performed test in the diagnostic workup of a suspected asthmatic patient. Methacholine responsiveness is usually reported as the provocation concentration causing a 20% decrease in the FEV1 (i.e. PC20).27 The results are typically interpreted as followed: normal PC20>16mg/mL; borderline PC20=4 to 16mg/ mL; mild AHR PC20=1 to 4mg/mL; moderate AHR PC20=0.25 to 1mg/mL; and marked AHR PC20 <0.25mg/mL. From a diagnostic point of view, the test has a strong negative predictive value, and functions best when used to exclude a diagnosis of asthma in a currently symptomatic patient. False negative results can occur in the following situation:
• Airway responsiveness may have been suppressed by recent use of anti-inflammatory medication;
• The patient is asymptomatic due to passing of season for aeroallergen exposure28;
• Patients with occupational asthma may only response when challenged with the specific agent29.
On the other hand, bronchial hyperresponsiveness may be seen in a wide variety of other diseases, including smoking-induced chronic airway obstruction, congestive heart failure, cystic fibrosis, acute bronchitis, and allergic rhinitis30.
CONCLUSION Spirometry is a useful and easy to perform test to assess the physiological lung function of a patient. Care must be taken to ensure that it is
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done with the proper equipment and correct technique for maximal results. The clinician must have an appreciation of the common pitfalls in the performance, reporting and interpretation of results so that he can apply the results to any particular clinical scenario.
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