POST-SPACEFLIGHT ORTHOSTATIC HYPOTENSION

Cardiovascular changes occur during spaceflight. Because of the loss o f hydrostatic pressure there is, during weightlessness, a fluid shift of extra- and intra-vascular volumes to the upper part of the body. Th is 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 mechanism s which provoke a real hypovolaemia. Cardiovascular deconditioning exi sts during spaceflight with the following symptoms when astronauts ret urn to earth: decreased orthostatic tolerance with possible tendency t owards spontaneous syncope and decreased effort capacity. The mechanis ms 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 te sted in microgravity simulations on the ground, then in flight.