Volume dependence of instantaneous time constants derived from the maximal expiratory flow-volume curve. A new approach to the analysis of forced expiration.

A simple expansion of the information in the maximal expiratory flow-volume curve was used to measure the convexity to the volume axis of maximal expiratory flow-volume curves produced by 11 asymptomatic smokers and 11 asymptomatic nonsmokers. The ratio of remaining volume to maximal flow was equal to the product of upstream resistance and chord compliance. This "instantaneous time constant" increased more near residual volume in the smokers. Using this method, plethysmographically obtained maximal expiratory flow-volume curves were more sensitive than spirometrically obtained maximal expiratory flow-volume curves. This method compared favorably with moment analysis and helium response of maximal expiratory flow as a separator of asymptomatic smokers and nonsmokers.