Cardiac output changes during hyperbaric hyperoxia

Objectives: Increased ambient pressure and oxygen partial pressure (pO(2)) influence cardiovascular regulation during diving and caisson work. We meas ured the cardiac output (< (Q)over dot >) in subjects who practiced moderat e work at a usual diving depth of 30 m. Methods: In 23 healthy male Navy di vers who performed steady state bicycle exercises (100 W workload) in a hyp erbaric chamber < (Q)over dot > was measured by a CO2-rebreathing technique at normal pressure(100 kPa) and at raised ambient pressure (400 kPa), in a random order. During the rebreathing maneuver the subjects were exposed to pO(2) values which theoretically may have reached a maximum value of 87 kP a (normobaric) and 388 kPa (hyperbaric). During the experiments the ambient temperature ranged between 22 and 25 degreesC. Results: There was a signif icant decrease of the directly measured < (Q)over dot >, heart rate (HR) an d the calculated stroke volume at depth when compared with normoxic and nor mobaric exercise. The decrease of < (Q)over dot > amounted to 64% of the no rmobaric value (8.9 l min(-1) versus 13.9 l min(-1)). The mean HR decreased from 104.7 min(-1) (100 kPa) to 94.0 min(-1) (400 kPa), The calculated mea n stroke volume decreased from 133 mi (100 kPa) to 96 ml (400 kPa). Conclus ions: During hyperoxic hyperbaria the peripheral vascular tonus increases d ue to the consecutively increased arterial oxygen content. The cardiac outp ut may correlate to the peripheral vasoconstriction and is therefore indire ctly influenced by elevation of inspiratory pO(2) i.e. during the rebreathi ng maneuver.