Effects of breathing-gas pressure on pulmonary function and work capacity during immersion.

Upright immersion induces respiratory mechanical changes that may impair pulmonary function during hyperbaric exercise. To evaluate this, 10 divers performed an incremental cycling protocol while immersed upright at 1.02 and 6.05 atmospheres absolute (atm abs). Air was supplied at each of two hydrostatic pressures: mouth pressure (Pm: to stimulate a mouth-held demand regulator) and lung centroid pressure (PLC). Subjects perceived air delivery at PLC to be more comfortable at each level of exercise at both absolute pressures (P less than 0.05). At 6.05 atm abs subjects perceived narcosis to be greater for Pm than for PLC air delivery. Hypoventilation was encountered at 6.05 atm abs with PLC air delivery and was further exacerbated when air was delivered at Pm (P less than 0.05). It was concluded that hypoventilation and narcosis are reduced whereas respiratory comfort is increased when air is delivered at PLC. This change is possibly due to improved pulmonary mechanics accompanying PLC air supply pressure.