THE EFFECTS OF 7-DAY BED REST ON CARDIOVASCULAR AUTONOMIC REGULATION

To examine the alterations in cardiovascular autonomic function and orthostatic tolerance induced by Simulated Weightlessness (SWL), the changes in Heart Rate Variability (HRV) spectra are studied in the way that 20min, + 75℃ Head UP Tilt (HUT) tests were conducted before and after a 7-day, -6° head-down bed teat for six healthy young male subjects. After bed rest, two of the six subjects could not complete the 20min HUT test that all have completed before. During bed test, the Low Frequency (LF) and High Frequency (HF) powers of HRV spectra were all significantly decreased, the ratio of low-to high-frequency power of HRV spectrum [(LF:HF)HRV] appeared to be increased at the end of bed rest. During the first 6min period of HUT after bed rest the heart rate was faster, but the (LF:HF) HRV, were similar to the data during HUT before bed rest in all subjects. The results suggest that the peripheral vascular sympathetic activity and the cardiac parasympathetic activity are decreased with a gradual increase in the cardiac sympathetic activity during bed rest but the cardiovascular autonomic responsiveness during HUT after bed rest seems to be unimpaired.