Enhanced flow-dependent vasodilatation after bed rest, a possible mechanism for orthostatic intolerance in humans

We investigated the alteration in flow-dependent-dilatation in the orthosta tic intolerance occurring after bed-rest deconditioning. Eight men [aged me an (SEM) 32 (2) years] underwent two consecutive periods of 7 days of head- down-tilt (HDT, -6 degrees) during bed rest. A control age and sex matched group [n = 8, 30 (2) years], maintained its usual physical activity. Blood flow velocity (BFV) and diameter (Doppler and echotracking systems) were me asured in the brachial artery, under basal conditions and during the post i schaemic hyperaemia following occlusion. The increase in BFV post-ischaemia did not change before, during and after HDT but the relative increase in t he diameter was greater on the 7th day of the HDT period than before HDT [8.8(1.6)% compared to +3.7(1.0)%, P <0.001]. After HDT. 11 of 16 standing t ests (comprising eight subjects in the two HDT periods) had to be stopped b ecause of orthostatic intolerance. The flow-dependent-dilatation measured a t the end of HDT was negatively correlated with the post-bed-rest duration of orthostatic tolerance (r=0.78. P <0.01). After the sublingual administra tion of glyceryl trinitrate, there was no change in the increase in diamete r. No significant changes were observed in the control group. Bed-rest deco nditioning enhances the flow-dependent vasodilatation of large arteries and might contribute to the orthostatic intolerance observed following bed-res t.