To examine the effect of acute hyperbaric exposure on cardiovascular response to orthostasis, a passive 70 degrees head-up tilt (HUT) test was performed for 15 min in a simulated compressed-air hyperbaric environment of 3 atm abs at ambient temperature of 31 degrees C (thermoneutral) on 8 male subjects. Heart rate (HR), blood pressure, cardiac output (CO) by impedance cardiography, forearm blood flow (FBF) by the occlusion plethysmography, and laser-Doppler skin blood flow (BFLD) on the thigh were measured for 15 min before, during, and after HUT. Esophageal temperature and HR data were recorded continuously. An identical test was performed in a 29 degrees C (thermoneutral) normal atmospheric condition. None of the subjects showed signs of syncope during HUT in either environment. Baseline HR was significantly lower (P less than 0.05) at 3 atm abs, and the increase in HR (delta HR) during HUT was of the same magnitude (15 beats/min) at both atmospheric pressures. The reduction of systolic blood pressure (delta SBP) was identical in both environments. Thus, the chronotropic response to HUT (delta HR/delta SBP) was the same. A marked reduction in CO (P less than 0.05) was attributed to a reduction of stroke volume during HUT, and the reduction was greater (P less than 0.05) at 3 atm abs. There were no pressure-dependent changes during HUT in FBF, forearm vascular resistance, and BFLD except for a greater increase (P less than 0.05) in total peripheral resistance at 3 atm abs. These observations suggest that orthostatic tolerance was maintained in the presence of lower CO at 3 atm abs, probably by a greater vasoconstrictor response in the splanchnic areas. We conclude that the substantial bradycardia which occurred at 3 atm abs did not interfere with a normal response to orthostasis in humans because of a peripheral vasoconstriction caused by the elevated oxygen pressure and an enhanced increase in total peripheral resistance which occurred during HUT in 3 atm abs.

Sagawa, Miki, Tajima, Shiraki, , , , , (1992). Cardiovascular responses to upright tilt in man during acute exposure to 3 atm abs air. Undersea biomedical research, 1992 Mar;19(2):97-106. https://www.ncbi.nlm.nih.gov/pubmed/1561723