Peripheral vascular changes after short-term simulated microgravity.

Cardiovascular deconditioning and orthostatic intolerance are the main symptoms experienced by astronauts after space flights. Alterations in the cardiovascular neural regulation have been implicated in the genesis of these disorders, but the mechanisms have not been clearly established. Alterations in the reflex control of circulation from the arterial and cardiopulmonary baroreceptive areas have been mainly hypothesized on the basis of long-term simulated microgravity studies. However, symptoms of orthostatic intolerance, and even fainting, are also observed after short-term exposure to simulated micro-G conditions. The possibility does exist that short-term exposure to micro-G conditions could affect the cardiovascular regulation differently from long-term exposure. Previous studies from this laboratory have shown that arterial baroreflex control of heart rate is not altered after 4 hours Head Down (HD) at -6 degrees, whereas it is significantly decreased after 28 days. Again, the role played by possible disturbances of the vasomotor regulation in the genesis of orthostatic intolerance after short term exposure to simulated microgravity has not yet been clarified. The aim of this study was therefore to evaluate peripheral blood flow changes following 2 and 4 h HD -6 degrees in healthy volunteers.