Effects of spaceflight on human calf hemodynamics

Departments of Physiology and Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas 75235 Chronic microgravity may modify adaptations of the leg circulation to gravitational pressures. We measured resting calf compliance and blood flow with venous occlusion plethysmography, and arterial blood pressure with sphygmomanometry, in seven subjects before, during, and after spaceflight. Calf vascular resistance equaled mean arterial pressure divided by calf flow. Compliance equaled the slope of the calf volume change and venous occlusion pressure 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 (to 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 0.2). Calf vasoconstriction in microgravity qualitatively agrees with the "upright set-point" hypothesis: the circulation seeks conditions approximating upright posture on Earth. No calf hemodynamic result exhibited obvious mechanistic implications for postflight orthostatic intolerance. weightlessness; gravity; leg; vascular resistance; blood flow; venous compliance