Mechanisms of vasovagal syncope: relevance to postflight orthostatic intolerance.

Orthostatic intolerance can present a very serious limitation to astronauts attempting any functions requiring the upright position. Vasovagal syncope is a common cause of fainting characterized by paradoxic bradycardia and vasodilation in the presence of decreasing arterial pressure. Several mechanisms have been proposed to mediate vasovagal syncope and it seems likely that redundant mechanisms exist both centrally and peripherally which can mediate these responses. Spaceflight alters reflex control of cardiovascular function in several ways which may influence susceptibility to vasovagal syncope provoked by the Bezold-Jarisch reflex.