[Effects of lower body negative pressure in the first and last week during 21 d head-down bed rest on orthostatic tolerance and cardiac function].

Objective: To investigate the changes of orthostatic tolerance and cardiac function during 21 d head-down tilt (HDT) bed rest and effect of lower body negative pressure (LBNP) in the first and the last weeks in humans.

Method: Twelve healthy male volunteers were exposed to -6 degrees HDT bed rest for 21 d. Six subjects received -30 mmHg LBNP sessions for 1 h/d from the 1st to the 7th day and from the 15th to the 21st day of the HDT, and the other six who did not receive LBNP served as control. Orthostatic tolerance was assessed by means of standard tilt test. The cardiac pumping function and cardiac systolic function were measured before and during HDT.

Result: Before HDT, all the subjects in the two groups completed the tilt tests. After 10 d and 21 d of HDT, all the subjects of the control group and one subject of the LBNP group could not complete the tilt test due to presyncopal or syncopal symptoms. The mean time of upright standing in the control group (15.0 +/- 3.2 min) was significantly shorter than those in the LBNP group (19.7 +/- 0.9 min). The stroke volume and cardiac output decreased significantly in the control group on days 3 and 10 of HDT, but remained unchanged throughout HDT in the LBNP group. A significant increase in preejection period (PEP)/left ventricular ejection time (LVET) was observed on days 3 and 14 of HDT in both groups. The PEP/LVET in the LBNP group was significantly lower than those in the control group on days 3 of HDT, while LVET in LBNP group was significantly higher than those in the control group on days 3, 7 and 14 of HDT.

Conclusion: It is suggested that the brief daily LBNP sessions in the first and the last weeks were effective in preventing orthostatic intolerance and the reduction of cardiac pumping function and cardiac systole function induced by 21 d HDT bed rest.