ORTHOSTATIC INTOLERANCE AFTER SPACEFLIGHT

Orthostatic intolerance occurs commonly after spaceflight, and importa nt aspects of the underlying mechanisms remain unclear. We studied 14 individuals supine and standing before and after three space shuttle m issions of 9-14 days. After spaceflight, 9 of the 14 (64%) crew member s could not complete a 10-min stand test that all completed preflight. Pre- and postflight supine hemodynamics were similar in both groups e xcept for slightly higher systolic and mean arterial pressures preflig ht in the finishers [15 +/- 3.7 and 8 +/- 1.2 (SE) mmHg, respectively; P < 0.05]. Postflight, finishers and nonfinishers had equally large p ostural reductions in stroke volume (-47 +/- 3.7 and -48 +/- 3.3 ml, r espectively) and increases in heart rate (35 +/- 6.6 and 51 +/- 5.2 be ats/min, respectively). Cardiac output during standing was also simila r (3.6 +/- 0.4 and 4.1 +/- 0.3 1/min, respectively). However, the fini shers had a greater postflight vasoconstrictor response with higher to tal peripheral resistance during standing (22.3 +/- 1.2 units prefligh t and 29. +/- 1.1 2.3 units postflight) than did the nonfinishers (20. 1 +/- 1.1 units preflight and 19.9 +/- 1.4 units postflight). We concl ude that 1) the primary systemic hemodynamic event, i.e., the postural decrease in stroke volume, was similar in finishers and nonfinishers and 2) the heart rate response and cardiac output during standing were not significantly different, but 3) the postural vasoconstrictor resp onse was significantly greater among the finishers (P < 0.01).