[Collapsibility of the downstream segment].

With an abrupt interruption of airflow at the mouth during the V-V curve maneuver, the mouth pressure suddenly increased (1st phase), followed by a slower rise (2nd phase). The pleural pressure and alveolar pressure remained constant during this process. The first phase represented the pressure induced by the instantaneous interruption of the airflow itself. Analysis of Vsupramax which occurred just after resumption of the airflow suggested that the choke point was maintained constant during the 2nd phase. Based on the above results, a method was introduced for the determination of the airway compliance (Cb) of the downstream segment. Cb was below 1.5 ml/cmH2O in healthy subjects, and above 1.5 ml/cmH2O in COPD. Cb should be useful for the pathophysiological evaluation of the V-V curve and large airway in various diseases.