CARDIOVASCULAR-RESPONSES TO UPRIGHT TILT IN MAN DURING ACUTE EXPOSURETO 3 ATM ABS AIR

To examine the effect of acute hyperbaric exposure on cardiovascular r esponse to orthostasis, a passive 70-degrees head-up tilt (HUT) test w as performed for 15 min in a simulated compressed-air hyperbaric envir onment of 3 atm abs at ambient temperature of 31-degrees-C (thermoneut ral) on 8 male subjects. Heart rate (HR), blood pressure, cardiac outp ut (CO) by impedance cardiography, forearm blood flow (FBF) by the occ lusion plethysmography, and laser-Doppler skin blood flow (BF(LD)) on the thigh were measured for 15 min before, during, and after HUT. Esop hageal temperature and HR data were recorded continuously. An identica l test was performed in a 29-degrees-C (thermoneutral) normal atmosphe ric condition. None of the subjects showed signs of syncope during HUT in either environment. Baseline HR was significantly lower (P < 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 redu ction of systolic blood pressure (DELTA-SBP) was identical in both env ironments. Thus, the chronotropic response to HUT (DELTA-HR/DELTA-SBP) was the same. A marked reduction in CO (P < 0.05) was attributed to a reduction of stroke volume during HUT, and the reduction was greater (P < 0.05) at 3 atm abs. There were no pressure-dependent changes duri ng HUT in FBF, forearm vascular resistance, and BF(LD) except for a gr eater increase (P < 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 vasoco nstrictor response in the splanchnic areas. We conclude that the subst antial bradycardia which occurred at 3 atm abs did not interfere with a normal response to orthostasis in humans because of a peripheral vas oconstriction caused by the elevated oxygen pressure and an enhanced i ncrease in total peripheral resistance which occurred during HUT in 3 atm abs.