Assessment of sympathetic baroreflex control during orthostatic challenge before and after prolonged head-down bed rest

The study characterizes the sympathetic baroreflex control through the assessment of sympathetic baroreflex sensitivity (sBRS). sBRS was assessed as the slope of the regression line of the conditional probability of detecting a burst on the integrated muscle nerve sympathetic activity given an assigned diastolic arterial pressure (DAP) on DAP. sBRS was assessed during orthostatic challenge before and after 21 days -6 degrees head-down bed rest (HDBR) confinement. We identified an effect of the prolonged HDBR and an impairment of the sympathetic baroreflex just before pre-syncope symptoms induced by the orthostatic challenge, thus suggesting a possible role of the sympathetic baroreflex in the origin of neurally-mediated syncope.