Passive expiration as a test of lung function.

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 greater than 0.98), and the time constant of passive expiration (Tp) 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). Tp in smokers (1.06 +/- 0.47 SD) was significantly longer than in the non-smokers (0.65 +/- 0.25 SD P less than 0.02). Tp had a significant correlation with Cdyn/f(Tp = 0.6 + 161.81 Cdyn/f +/- 0.38 SE, r = 0.49, P less than 0.02). We conclude that satisfactory passive expiratory spirograms can be easily obtained after a mechanically assisted passive inspiration. Tp 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.