Pulmonary function hysteresis during compression to, and decompression from 31.3 ATA.

Tracheal gas density breathing heliox at 31.3 atmospheres absolute (O2 at 0.42 ATA) is 6.287 g l-1, or approximately 5.5 times greater than air at 1 ATA. This constitutes a significant respiratory load, previously shown to induce respiratory adaptation. During a saturation dive to 31.3 ATA, five divers were exposed to this load for 16 days. This project aimed at investigating possible hysteresis in pulmonary function during dive compression, adaptation and decompression phases. Pulmonary function tests were performed at the surface in air, and at four pressure stops during compression and decompression, with divers breathing the helium-oxygen gas mixture. Significant hysteresis patterns were observed for pooled maximal voluntary ventilation, forced expired volume at 1 s, peak expiratory flow, and maximum expiratory flows (P < 0.05), with post-adaptation flows consistently exceeding those observed during compression. Two mechanisms may explain these observations. Differences may be attributable to positive effort-dependence in the forced expiratory flow; or it is possible the subjects adapted to the respiratory load by modifying neural input to airway smooth muscle, thereby modifying airway resistance.