Effects of spaceflight on human calf hemodynamics

Chronic microgravity may modify adaptations of the leg circulation to gravi tational pressures. We measured resting calf compliance and blood flow with Venous occlusion plethysmography, and arterial blood pressure with sphygmo manometry, in seven subjects before, during, and after spaceflight. Calf va scular resistance equaled mean arterial pressure divided by calf flow. Comp liance equaled the slope of the calf volume change and venous occlusion pre ssure relationship for thigh cuff pressures of 20, 40, 60, and 80 mmHg held for 1, 2, 3, and 4 min, respectively, with 1-min breaks between occlusions . Calf blood flow decreased 41% in microgravity ito 1.15 +/- 0.16 ml 100 ml (-1).min(-1)) relative to 1-G supine conditions (1.94 +/- 0.19 ml 100 ml(-1 ).min(-1), P = 0.01), and arterial pressure tended to increase (P = 0.05), such that calf vascular resistance doubled in microgravity (preflight: 43 /- 4 units; in-flight: 83 +/- 13 units; P < 0.001) yet returned to prefligh t levels after flight. Calf compliance remained unchanged in microgravity b ut tended to increase during the first week postflight (P > 0.2). Calf vaso constriction in microgravity qualitatively agrees with the "upright set-poi nt" hypothesis: the circulation seeks conditions approximating upright post ure on Earth. No calf hemodynamic result exhibited obvious mechanistic impl ications for postflight orthostatic intolerance.