Pulmonary Function Tests
Guidelines for Interpretation and Sample Problems
This appendix provides an outline of a simplified approach to interpreting pulmonary function tests and gives several examples of test results presented as unknown problems. Because details of the interpretation of these tests may vary among laboratories, the approach here focuses on the general concepts rather than the specific details, providing a step-by-step approach to analyzing pulmonary function tests. The concepts underlying this step-by-step approach are covered in the relevant section on pulmonary function tests in Chapter 3.
Analysis of Pulmonary Function Tests
1. Examination of lung volumes:
a. A decrease in total lung capacity (TLC) generally indicates the presence of a restrictive pattern. However, TLC measured by helium dilution may also be artificially depressed when there are poorly communicating or noncommunicating regions within the lung (e.g., in bullous lung disease).
b. Are lung volumes symmetrically reduced (i.e., are TLC, residual volume [RV], functional residual capacity [FRC], and vital capacity [VC] all decreased to approximately the same extent)? If so, this suggests interstitial lung disease as the cause of the restrictive pattern. A low diffusing capacity also supports the diagnosis of interstitial lung disease as the cause of the restrictive pattern.
c. A relatively preserved RV and a normal diffusing capacity suggest another cause of restrictive disease, such as neuromuscular or chest wall disease. Poor effort from the patient may also create this type of pattern.
2. Examination of the mechanics—that is, flow rates measured from the forced expiratory spirogram:
a. A decrease in the ratio of forced expiratory volume in 1 second to forced vital capacity (FEV1/FVC) indicates obstruction. In some cases of airflow obstruction, both FEV1 and FVC are reduced by approximately the same extent, and FEV1
