Closing volume after inspiratory resistive loading to fatigue.

Closing volume was measured by the single breath nitrogen washout test in six normal subjects both before and after inspiratory resistive loading to fatigue. Subjects breathed through an inspiratory resistance until they could no longer maintain the required mouth pressure throughout inspiration. There was electromyographic evidence of diaphragmatic fatigue in all experiments. Closing volume (expressed as a percentage of vital capacity) after resistive loading to fatigue (10.1 +/- 1.9%) was not significantly different from that before resistive loading (10.5 +/- 1.7%). Because pulmonary edema increases closing volume, this study suggests that the very negative intrathoracic pressures generated during resistive loading do not cause pulmonary edema. Therefore, the rapid shallow breathing following inspiratory resistive loading to fatigue is not due to pulmonary edema but is probably a direct consequence of fatigue.