Influence of diaphragmatic contraction and expiratory flow on the pattern of lung emptying

Abstract

Transdiaphragmatic pressure (Pdi) and expiratory flow (V̇) were monitored during vital capacity single breath N 2 washouts in 7 seated subjects. Transient increases in V̇ were produced (1) actively, by subjects increasing mouth pressure while expiring through a constant resistance or (2) passively, by the operator transiently decreasing the resistance. Voluntary contraction of the diaphragm (increased Pdi) was achieved when abdominal muscles were tensed while maintaining V̇ constant. In 5 subjects a transient increase in Pdi of 25–150 cm H 20 consistently produced a transient increase in expired N 2 concentration of 1.80 ± 0.06 % (Mean ± 1 SE); in 1 subject N 2 concentration decreased by 0.8% to 2.7% N 2, and in one subject the alveolar plateau was uninfluenced by changes in Pdi. Passive increases in V̇ up to 21/sec had no effect on Fe N 2 in any of the subjects. Active increase in V̇ changed Fe N 2 only when associated with increases in Pdi. Qualitatively similar results were obtained during helium (He) bolus washouts. However, whereas diaphragmatic contraction, maintained throughout expiration, had no measurable influence on the N 2 washout, it changed the slope of the He alveolar plateau in 6 out of 7 subjects. We conclude that in normal subjects the alveolar N 2 plateau is relatively insensitive to flow variations up to 21/sec. The fluctuations in Fe N 2 observed when the expiratory flow is varied are due to concomittant changes in Pdi. We propose that diaphragmatic contraction changes the pattern of lung emptying by altering the vertical gradient of pleural pressure.