WAVE SPEED DURING MAXIMAL EXPIRATORY FLOW AND PHASE-VELOCITY FROM FORCED-OSCILLATIONS

To test the hypothesis that pressure waves in the airways propagate at the speed obtained from maximal expiratory now we compared wave speed s (WS) associated with flow limitation and phase velocities (PV) of os cillatory pressure waves in four excised calf tracheae for transmural pressures (P-tm) between 0 and - 10 kPa. WS was calculated from static area-P-tm curves using the acoustic reflection technique. PV was dete rmined by the forced oscillation method between 16 and 1024 Hz. WS ran ged from 80 to 120 m/sec slightly increasing with decreasing P-tm. PV was relatively constant between 60 and 160 Hz with values between 170 and 310 m/sec. With decreasing P-tm, PV also increased, however, at 10 0 Hz it was 1.5-2.5 times higher than WS at all P-tm. In one additiona l trachea we found that PV decreased from similar to 200 m/sec at 7 Hz to similar to 130 m/sec at 0.23 Hz approaching WS. We suggest that VP is larger than WS because of the differences in airway wall mechanics during small-amplitude oscillations and large amplitude unidirectiona l wall motion such as a forced expiration. These results may provide a n additional explanation why expiratory flow during rapid breathing or expiratory transients can exceed the maximum expiratory flow-volume e nvelope.