Baroreflex dysfunction induced by microgravity: potential relevance to postflight orthostatic intolerance

Microgravity imposes adaptive changes in the human body. This review focuse s on the changes in baroreflex function produced by actual spaceflight, or by experimental models that simulate microgravity, e.g., bed rest. We will analyze separately studies involving baroreflexes arising from carotid sinu s and aortic arch afferents ("high-pressure baroreceptors"), and cardiopulm onary afferents ("low-pressure receptors"). Studies from unrelated laborato ries using different techniques have concluded that actual or simulated exp osure to microgravity reduces baroreflex function arising from carotid sinu s afferents ("carotic-cardiac baroreflex"). The techniques used to study th e carotid-cardiac baroreflex, using neck suction and compression to simulat e changes in blood pressure, have been extensively validated. In contrast, it is more difficult to selectively study aortic arch or cardiopulmonary ba roreceptors. Nonetheless, studies that have examined these baroreceptors su ggest that microgravity produces the opposite effect, ie, an increase in th e gain of aortic arch and cardiopulmonary baroreflexes. Furthermore, most s tudies have focus on instantaneous changes in heart rate, which almost excl usively examines the vagal limb of the baroreflex. In comparison, there is limited information about the effect of microgravity on sympathetic functio n. A substantial proportion of subjects exposed to microgravity develop tra nsient orthostatic intolerance. It has been proposed that alterations in ba roreflex function play a role in the orthostatic intolerance induced by mic rogravity. The evidence in favor and against this hypothesis is reviewed.