Passive expiration as a test of lung function

Abstract

Twenty-five normal subjects, 14 non-smokers and 11 smokers, passively expired into a spirometer after a maximal active inspiration, and after a passive inflation of the chest by a pressure cycled intermittent positive-pressure breathing (IPPB) machine. Acceptable passive expirations could be performed by all subjects after a passive inspiration but by only 12 after an active inspiration. Expired volume was found to change exponentially with time (r>0.98), and the time constant of passive expiration (T(p)) was obtained. There was no significant difference between the smokers and non-smokers in age, sex, forced vital capacity, FEV1, FEV1/FVC%, maximum mid-expiratory flow rate, maximum expiratory flow at 50% and 25% of the vital capacity, or the magnitude of the fall in the dynamic compliance with increasing frequency of breathing (Cdyn/f). T(p) in smokers (1.06 s ± 0.47 SD) was significantly longer than in the non-smokers (0.65 s ± 0.25 SD P < 0.02). T(p) had a significant correlation with Cdyn/f(T(p)) =0.6 + 161.81 Cdyn/f ± 0.38 SE, r=0.49, P<0.02). We conclude that satisfactory passive expiratory spirograms can be easily obtained after a mechanically assisted passive inspiration. T(p) thus obtained is determined by the intrinsic properties of the respiratory system (lung plus thorax), and is significantly prolonged in smokers compared with non-smokers when other studies of pulmonary function including frequency dependence of compliance are unchanged.