Mouth pressure curve on abrupt interruption of airflow during forced expiration.

The mouth pressure curve after abrupt interruption during forced expiratory maneuver was investigated to evaluate the collapsing state of the airway downstream to the choke point. Immediately after the airflow interruption at the mouth by means of the electromagnetic valve, the mouth pressure suddenly increased (1st phase), followed by a slower rise (2nd phase) within about 100 msec until the pressure reaches the alveolar pressure. The pleural and alveolar pressures remained constant during this process. It was evidenced that, from point of view of mean flow, the airflow flowed at a rate of Vmax through the choke point during the second phase. Thus, it is strongly suggested that the choke point remained at the same point during the 2nd phase. From these results, the 2nd phase of the mouth pressure is expected to represent the specific characteristics for the downstream airway.