Alveolar gas exchange during simulated breath-hold diving to 20 m.

Alveolar gas exchange, as affected by changes in pulmonary blood flow, was studied in five subjects performing breath holds lasting 75 s at the surface and during compression to 20 m in a hyperbaric chamber. After reaching the maximal depth, VO2 started to increase, compared to control, reaching a maximum of 346 +/- 66 (SE) ml (STPD).min-1.m2 (body surface area) at 50 s, i.e., early in the ascent; it exceeded the 50-s surface breath-hold value by 214 +/- 9 ml.min-1.m2. During descent, CO2 was absorbed from the alveoli into the blood, initially at 140 +/- 24 ml.min-1.m2; during ascent CO2 was transferred back into the lungs. These changes reflected compression and expansion of lung air. The increase in VO2 during the dives, which are not steady states, may be explained by an increasing cardiac output at depth. An augmented cardiac output had earlier been observed under identical conditions and explained by a drop in transthoracic pressure, enhancing venous return. Upon surfacing, the PAO2 was about 20 mmHg lower than after surface breath holds, reflecting the effects of changes in cardiac output.