Configuration of the chest wall and occlusion pressures in awake humans.

During CO2 rebreathing in sitting position seven of nine conscious men showed a progressive fall in expiratory reserve volume, most of it due to a decrease in abdominal volume. Diaphragm length at end expiration was thus increased, and some elastic recoil pressure became available to drive inspiration. In four out of six subjects, when CO2 tension was greater than 55 Torr, there was a dip in abdominal pressure at the beginning of inspiration, and the change in transdiaphragmatic pressure during the first 100 ms of an occluded inspiration was smaller than the simultaneous change in mouth pressure (P0.1). In the subjects who showed the smallest diaphragmatic pressure in this 100 ms, electromyogram recordings showed that abdominal activity ceased before the onset of inspiration, and diaphragm activity did not appear until later than 100 ms into inspiration. We conclude that, in four our of our six subjects in the sitting posture, P0.1 can be generated in whole or in part by release of chest wall elastic recoil or in intercostal muscle contraction. In the supine posture, there was no change by end-expiratory chest wall configuration, and onset of diaphragm contraction coincided with beginning of inspiration in the two subjects in whom diaphragm electromyogram was recorded.