CHANGES IN SYMPATHOADRENAL RESPONSE TO STANDING IN HUMANS AFTER SPACEFLIGHT

Plasma catecholamine levels and cardiovascular responses to standing w ere determined in astronauts before and after several Space Shuttle mi ssions. Blood pressure, heart rate, and cardiac output were measured a nd blood samples for catecholamine analyses were drawn at the end of t he supine and standing periods. Supine plasma norepinephrine and epine phrine concentrations increased 34 and 65%, respectively, on landing d ay compared with before flight. Standing on landing day resulted in a 65 and 91% increase in plasma norepinephrine and epinephrine, respecti vely. Supine and standing norepinephrine levels remained elevated 3 da ys after landing while epinephrine levels returned to preflight levels . On landing day, supine heart rate and systolic blood pressure increa sed 18 and 8.9%, respectively, and standing heart rate and diastolic b lood 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 orthosta tic function after spaceflight results largely from the decreased stro ke volume. Possible mechanisms contributing to this condition are disc ussed.