Contributory factors to orthostatic intolerance after simulated weightlessness

Various factors may contribute to orthostatic intolerance (OI) observed aft er space flights or simulated weightlessness such as bed rest experiments: individual physical and physiological factors (arterial blood pressure (BP) , height), physiological changes induced by real or simulated weightlessnes s (hypovolaemia, increase in venous distensibility,) and space flight or si mulation conditions (duration and counter-measure application). Our purpose was to test which of these factors were dominant in contributing to the OI . This was assessed in 47 healthy men participating in bed rest experiments of 4, 14, 28, 30 and 42 days, with or without counter-measures (medical st ockings, lower-body negative pressure (LBNP), LBNP + muscular exercise). Ni neteen subjects did not finish the orthostatic test (60 degrees head-up til t or stand test) after bed rest. The occurrence of OI was associated with g reater height, low resting BP, greater changes in resting lower-limb venous distensibility throughout the bed rest, and absence of counter-measures.