[Effects of 24 h -6 degrees head-down tilt bed-rest on cardiovascular function and response to orthostatic stress]

Objective. To investigate the effects of 24 h -6 degrees head-down tilt bed-rest (HDT) on cardiovascular function and response to orthostatic stress. Method. -6 degrees head-down tilt bed rest for 24 h in 6 healthy males, aged 22-23, were observed. The changes of cardiac function were observed by electrical impedance instrument before and at 0th, 6th, 12th, 18th and 24th hour of HDT, and blood pressure were recorded and urine were collected. The changes of cardiovascular response to head up tilt (HUT 90 degrees, 10 min) before and post HDT were recorded. Result. During HDT, HR reduced significantly than that of pre-HDT (standing). HR at 6th, 12th, and 18th hour of HDT reduced significantly than that at 0th hour of HDT. Stroke output (SO) and stroke index (SI) [correction of CI] at 0th, 6th, 12th, and 24th hour of HDT were higher than pre-HDT value. Cardiac output (CO) and cardiac index (CI) at 18th hour of HDT were significantly lower than those at 0 h of HDT. Total peripheral resistance (TPR) at 18th hour of HDT were higher than that at 0th of HDT. During HUT before and post HDT, HR, MBP, TPR were significantly increased, SO, [correction of SV] SI, CO and CI were significantly reduced. DBP were significantly increased and PP were reduced significantly during HUT post-HDT. Hourly average value of urine during 0-4 h were higher than those during 4-12 h and 12-24 h. Conclusion. 24 h -6 degrees head-down tilt bed rest has significant effects on cardiovascular function and response to HUT in humans.