EFFECT OF SIMULATED MICROGRAVITY ON CARDIOPULMONARY BAROREFLEX CONTROL OF FOREARM VASCULAR-RESISTANCE

The stimulus-response characteristics of cardiopulmonary baroreflex co ntrol of forearm vascular resistance (FVR) were studied in 11 healthy men before and after 7 days of 6 degrees head-down bedrest to test the hypothesis that microgravity alters this reflex response. We assessed the relationship between stimulus [changes in central venous pressure (Delta CVP)] and reflex response (Delta FVR) during unloading of card iopulmonary baroreceptors with lower body negative pressure (LBNP; 0 t o -20 mmHg). Delta CVP during bedrest and LBNP was estimated from peri pheral vein pressures in the dependent right arm. Compared with prebed rest baseline, plasma volume and estimated CVP were decreased by 13 an d 33%, respectively, at 7 days of bedrest. Progressive reflex forearm vasoconstriction occurred in response to graded reductions in estimate d CVP during LBNP, and Delta FVR per unit Delta CVP was doubled after bedrest. The increase in sensitivity of the cardiopulmonary baroreflex control of FVR was related to reduced circulating blood volume, sugge sting that enhanced peripheral vasoconstriction in individuals adapted to microgravity can be attributed, in part, to hypovolemia. In additi on, microgravity appears to alter the stimulus for cardiopulmonary bar oreceptors to a lower operational range of CVP, suggesting the possibi lity of chronic resetting.