Effect of an acute increase in central blood volume on cerebral hemodynamics 1

61 Systemic blood distribution is an important factor involved in regulating cerebral blood 62 flow (CBF). However, the effect of an acute change in central blood volume (CBV) on 63 CBF regulation remains unclear. To address our question, we sought to examine the 64 CBF and systemic hemodynamic responses to microgravity during parabolic flight. 65 Twelve healthy subjects were seated upright and exposed to microgravity during 66 parabolic flight. During the brief periods of microgravity mean arterial pressure was 67 decreased (-26 ± 1%, P<0.001) despite an increase in cardiac output (+21 ± 6%, 68 P<0.001). During microgravity central arterial pulse pressure and estimated carotid 69 sinus pressure increased rapidly. In addition, this increase in central arterial pulse 70 pressure was associated with an arterial baroreflex-mediated decrease in heart rate 71 (r=-0.888, P<0.0001) and an increase in total vascular conductance (r=0.711, P<0.001). 72 The middle cerebral artery mean blood velocity (MCA Vmean) remained unchanged 73 throughout parabolic flight (P=0.30). During microgravity the contribution of cardiac 74 output to MCA Vmean was gradually reduced (P<0.05) and its contribution was negatively 75 correlated with an increase in total vascular conductance (r=-0.683, P<0.0001). These 76 findings suggest that the acute loading of the arterial and cardiopulmonary 77 baroreceptors by increases in CBV during microgravity results in acute and marked 78 systemic vasodilation. Furthermore, we conclude that this marked systemic vasodilation 79 decreases the contribution of cardiac output to CBF. These findings suggest that the 80 arterial and cardiopulmonary baroreflex mediated peripheral vasodilation along with 81 dynamic cerebral autoregulation counteracts a cerebral over-perfusion, that otherwise 82 would occur during acute increases in CBV. (247 words) 83 84