The Effects of Leaks in Spirometers on Measurements of Pulmonary Function

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Abstract
Although the effects of spirometric leaks on forced expiratory tracings have been noted in retrospect in epidemiologic studies, the present study was designed to examine the effects of leaks of known size on the tracings of subjects with varying degrees of airway obstruction. Such information is needed to assess the potential impact of spirometric leaks on epidemiologic studies using existing spirograms collected in the absence of spirometer leak tests. Spirometric leaks of 5-9, 22-30, 34-46, 69-97 and 150-190 ml/s were produced by drilling holes of 5 sizes into spirometer mouthpieces. Eight subjects were studied, of whom 5 had FEV1/FVC [forced expiratory volume in 1s/forced vital capacity] 0.70 (normal), and 3 had FEV1/FVC < 0.70 (obstructed). Although leaks .ltoreq. 34-46 ml/s were invisible in the forced expiratory tracings, they caused statistically significant (P < 0.05) FVC decrements in all 8 subjects, with a mean decrement of -0.35 l in the normal subjects and -0.79 l in the obstructed subjects. The 34-46 ml/s leaks caused no significant FEV1 decrement in 7 of 8 subjects. Since marked FVC decrements occurred even with the smaller leaks, the FVC should not be analyzed in epidemiologic studies in the absence of frequent, documented leak tests. The FEV1 may be robust enough for analysis if recorded in the presence of small spirometric leaks.