Abstract
In normal subjects, the second of two successive partial expiratory flow-volume (PEFV 2) curves often had higher isovolume maximal expiratory flow rates (Vmax) than the first (PEFV 1) (mean increase 30.2 +/- 13%). The higher Vmax on PEFV 2 was present only when there was a greater lung elastic recoil pressure (Pst(L)). In eight subjects the Pst(L) derived from sequential partial quasi-static pressure-volume curves, from interruption of the flow-volume maneuvers and at the start of the PEFV curves showed that isovolume upstream resistance increased although Vmax also increased after going to residual volume (RV). In four subjects the RV volume history did not change the pressure flow relationship across the upstream airways. If airways dimensions were the sole determinant of Vmax, then Vmax on PEFV 2 would be the same or smaller than on PEFV 1. That the opposite was observed in our study indicates that the increase in Pst(L), which results from parenchymal hysteresis, offsets any dimensional decrease in upstream airways due to airways hysteresis.
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