[Orthostatic hypotension after space flight].

Cardiovascular changes occur during spaceflight. Because of the loss of hydrostatic pressure there is, during weightlessness, a fluid shift of extra- and intra-vascular volumes to the upper part of the body. This provokes objective and subjective symptoms, especially in the first days of spaceflight. The fluid shift is perceived by baroreceptors as a relative central hypervolaemia and induces neuro-hormonal mechanisms which provoke a real hypovolaemia. Cardiovascular deconditioning exists during spaceflight with the following symptoms when astronauts return to earth: decreased orthostatic tolerance with possible tendency towards spontaneous syncope and decreased effort capacity. The mechanisms of orthostatic intolerance are not well known. Several factors are probably involved, including hypovolaemia, modifications of the venous system, baroreflex changes, etc. Various countermeasures have been tested in microgravity simulations on the ground, then in flight.