Human autonomic responses to actual and simulated weightlessness.

Orthostatic dysfunction occurs after exposure to microgravity, and is not completely understood. The authors developed a device for stimulating carotid baroreceptors to test the hypothesis that exposure to microgravity leads to impairment of arterial baroreflex mechanisms. Data obtained before and after two head-down bedrest studies and before and after brief Space Shuttle missions indicate that baroreceptor-cardiac reflex control is impaired by simulated or actual weightlessness. The authors speculate that arterial baroreflex derangements combine with blood volume reductions and increased venous compliance to provoke orthostatic hypotension after microgravity exposure. Altered baroreflex function after missions may result from autonomic neuronal plasticity that develops during missions secondary to changes of cardiopulmonary and arterial dimensions and consequent changes of autonomic sensory input profiles.