a. Uses. Pulmonary function tests can be used to:
i. Differentiate obstructive from restrictive lung disease.
ii. Assess the severity of lung disease.
iii. Evaluate response to therapy.
b. Types of information assessed. Pulmonary function tests allow assessment of three types of information:
i. Lung volumes
1. The total lung capacity (TLC) is the volume of air in the lungs after a maximal inspiratory effort.
2. The functional residual capacity (FRC) is the volume of air at the end of tidal expiration with the glottis open.
3. The residual volume (RV) is the volume of air remaining in the lungs after a maximal expiratory effort.
4. The vital capacity (VC) is the maximal volume of air that can be expelled from the lungs following a maximal inspiration.
ii. Expiratory flow rate. The forced expiratory volume in 1 second (FEV1) is the most commonly used screening test for airway disease. FEV1 represents the volume of air expired during the first second of expiration.
iii. Forced vital capacity. The forced vital capacity is the complete forced expiratory volume. If it is low (<80% predicted), restrictive disease is likely. However, total lung capacity by plethysmography is required to confirm. This test is useful in assessing respiratory muscle weakness.
iv. Diffusion capacity. Evaluation of the diffusing capacity for carbon monoxide (Dlco) indicates the adequacy of alveolar-capillary gas exchange. Common causes of a decreased Dlco are emphysema, interstitial lung disease, and pulmonary vascular disease.
c. Interpretation. An organized and systematic approach to interpretation is required. When interpreting the results of pulmonary function testing, it is important to note both the observed values and the percent predicted. Keep in mind that the range of normal for all values will vary slightly depending on the pulmonary function testing laboratory.
B. Obstructive Versus Restrictive Lung Disease
a. The relationship between flow rate, lung volume, resistance, and compliance is as follows: Flow rate = Lung volume/(Airway resistance)(Compliance). Therefore, the flow rate (as measured by FEV1) will decrease if the lung volume decreases (i.e., restrictive disorders), airway resistance increases (i.e., asthma or chronic bronchitis), or compliance increases (i.e., emphysema).
b. Obstructive disorders cannot be distinguished from restrictive disorders on the basis of FEV1alone. To distinguish between obstructive and restrictive lung disorders, the lung volume must be eliminated from the equation; dividing by the FVC will accomplish this goal (FEV1/FVC ratio). The FEV1/FVC ratio is thus the percent of the forced vital capacity expelled in 1 second.
i. If the FEV1/FVC ratio is decreased (< 70%), then the patient has airflow limitation, suggesting obstructive disease.
ii. If the FEV1/FVC ratio is normal or increased (>80%) in the setting of a low FEV1, then the patient may have restrictive disease.
C. Approach to the Interpretation of Pulmonary Function Tests
a. First, look at the volume-time loop. This is a form of quality control because it allows you to gauge the patient’s effort. For the reading to represent a valid effort, the patient should be able to exhale for at least 6 seconds and for the curve to flatten as the patient exhales.
b. Then, look at the flow-volume loops.
i. A narrowed loop suggests restriction.
ii. A loop that has a “coved” or “scalloped” appearance suggests obstruction. Think of the curve at the end of a slide (obstructive pattern) rather than a hill with an even grade although way down (normal).
c. Examine the FEV1.
i. If the patient has a normal FEV1, TLC, and Dlco, significant pulmonary disease is unlikely.
ii. If the FEV1 is low, look at the FEV1(%) to document airflow limitation (obstructive disease) or to suggest restrictive disease.
1. If the FEV1/FVC ratio is normal or high, the patient may have restrictive disease (which will be confirmed by a low TLC).
2. If the FEV1 is less than 80% predicted and the FEV1/FVC ratio is <70, the patient has airflow limitation, suggesting obstructive disease.
a. If the patient has airflow limitation, then determine whether it is reversible.
i. If the FEV1 improves by ≥12% and at least 200 mL after bronchodilator administration, reversible airflow limitation is present.
d. Next, look at the lung volumes.
i. Increased TLC, RV: this suggests hyperinflation and gas trapping, consistent with obstructive lung disease.
ii. Decreased TLC, RV: this suggests restriction and is diagnostic of restrictive lung disease.
e. Finally, look at the Dlco.
i. Dlco values should always be corrected for hemoglobin (Dlco-C).
ii. If the patient has obstructive disease, a low Dlco-C suggests the presence of emphysema.
iii. If the patient has normal spirometry and lung volumes with a low Dlco-C, pulmonary thromboembolic disease, pulmonary hypertension, or early interstitial lung disease should be suspected.
Suggested Further Readings
Crapo RO. Pulmonary-function testing. N Engl J Med 1994;331:25–30. (Classic Article.)Find this resource:
Gildea TR, McCarthy K. Pulmonary function testing. Cleveland clinic: current clinical medicine, 2010.Find this resource:
Johnson JD, Theurer WM. A stepwise approach to the interpretation of pulmonary function tests. Am Fam Physician 2014;89:359–66.Find this resource:
Pellegrino R, Viegi G, Brusasco V, et al. Interpretative strategies for lung function tests. Eur Respir J 2005;26:948–68.Find this resource: