Changes in sympathoadrenal response to standing in humans after spaceflight.

Plasma catecholamine levels and cardiovascular responses to standing were determined in astronauts before and after several Space Shuttle missions. Blood pressure, heart rate, and cardiac output were measured and blood samples for catecholamine analyses were drawn at the end of the supine and standing periods. Supine plasma norepinephrine and epinephrine concentrations increased 34 and 65%, respectively, on landing day compared with before flight. Standing on landing day resulted in a 65 and 91% increase in plasma norepinephrine and epinephrine, respectively. Supine and standing norepinephrine levels remained elevated 3 days after landing while epinephrine levels returned to preflight levels. On landing day, supine heart rate and systolic blood pressure increased 18 and 8.9%, respectively, and standing heart rate and diastolic blood pressure were elevated by 38 and 19%, respectively. On standing, stroke volume was decreased by 26% on landing day compared with before flight. Collectively, these data indicate that the decreased orthostatic function after spaceflight results largely from the decreased stroke volume. Possible mechanisms contributing to this condition are discussed.