The Biomedical Effects of the Hyperbaric Environment.

Hyperbaric bradycardia was observed in 3 of 4 subjects at rest and during exercise at 5.5 Ata while breathing normoxic helium with a gas density equivalent to air at 1 Ata. The bradycardia was evident within 1 hour after reaching the simulated depth but was not present after 24 hours of exposure. Cardiac output at rest and during exercise after 24-48 hours at 5.5Ata was not significantly different from the control values at 1 Ata; the bradycardia was not present at these times. Airway pressures during the first 100 msec of an occluded breath were greater at 5.5 Ata than at 1 Ata. The data indicate an increased respiratory center motor output at increased hydrostatic pressure. The response may be due either to the increased pressure or to inhalation of a gas with an increased density just prior to the occlusion. Hyperbaric bradycardia was not observed in five subjects after rapid compression to 5.5 Ata at which depth the subjects were breathing normoxic helium. (Author)