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1Departments of Physiology and 2Internal Medicine, Academic Medical Center, Amsterdam, The Netherlands
Submitted 29 October 2004 ; accepted in final form 7 January 2005
Reduction in plasma volume is a major contributor to orthostatic tachycardia and hypotension after spaceflight. We set out to determine time- and frequency-domain baroreflex (BRS) function during preflight baseline and venous occlusion and postflight orthostatic stress, testing the hypothesis that a reduction in central blood volume could mimic the postflight orthostatic response. In five cosmonauts, we measured finger arterial pressure noninvasively in supine and upright positions. Preflight measurements were repeated using venous occlusion thigh cuffs to impede venous return and "trap" an increased blood volume in the lower extremities; postflight sessions were between 1 and 3 days after return from 10- to 11-day spaceflight. BRS was determined by spectral analysis and by PRVXBRS, a time-domain BRS computation method. Although all completed the stand tests, two of five cosmonauts had drastically reduced pulse pressures and an increase in heart rate of 
30 beats/min or more during standing after spaceflight. Averaged for all five subjects in standing position, high-frequency interbeat interval spectral power or transfer gain did not decrease postflight. Low-frequency gain decreased from 8.1 (SD 4.0) preflight baseline to 6.8 (SD 3.4) postflight (P = 0.033); preflight with thigh cuffs inflated, low-frequency gain was 9.4 (SD 4.3) ms/mmHg. There was a shift in time-domain-determined pulse interval-to-pressure lag, Tau, toward higher values (P < 0.001). None of the postflight results were mimicked during preflight venous occlusion. In conclusion, two of five cosmonauts showed abnormal orthostatic response 1 and 2 days after spaceflight. Overall, there were indications of increased sympathetic response to standing, even though we can expect (partial) restoration of plasma volume to have taken place. Preflight venous occlusion did not mimic the postflight orthostatic response.
microgravity; tilt table; baroreflex; venous pooling; hypovolemia
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