Effect of 14-Day Head-Up Bedrest on Orthostatic Tolerance in Humans

The effect of 14-day head-up bedrest (HUBR) on human orthostatic tolerance was studied with participation of 23 apparently healthy male subjects aged 18 to 35 years. HUBR with the head tilted up at +9.6° was to simulate the physiological effects of lunar gravity. Orthostatic tolerance was tested on the day prior to and immediately after HUBR completion with subjects tilted up to +70° for a maximum of 20 min. The parameters recorded before, during, and after the tilt test included blood pressure, ECG, and heart rate, the health status of the subjects was also monitored. In comparison with the baseline testing, HUBR reduced the number of completed tests and increased the number (10 vs. 4) of tests terminated due to precollapse (p < 0.05). The group mean time of orthostatic test (OT) tolerance decreased by 3.8 min (p < 0.01), and the time interval between the test onset and precollapse decreased by 4.7 min (p = 0.055). In most cases, orthostatic disorders following HUBR were described as a hypotensive (vasodepressor) type of tilt-induced collapse with high activity of the chronotropic function of the heart. Several cases of vagus-mediated heart rate deceleration indicated a sharp decline of the sympathetic nervous activity, which is an important mechanism for regulating hemodynamics under altered postural conditions.