Effects of rapid saline infusion on orthostatic intolerance and autonomic tone after 20 days bed rest.

To test whether acute volume expansion can normalize orthostatic intolerance and autonomic tone after prolonged bed rest (BR), 23 men were subjected to 20 days BR. Left ventricular (LV) echocardiography was performed during the lower body negative pressure (LBNP) test before and after BR with and without preceding rapid infusion of saline (1,500 ml/30 min). Saline infusion restored heart rate, LV dimension, and stroke volume during LBNP, increased cardiac output (from 4.1 +/- 1 to 5.3 +/- 1 L/min), and normalized LBNP tolerance time (from 11 +/- 4 to 23 +/- 6 minutes). In 9 men, a Holter electrocardiogram was recorded on the day before BR, the fourth and twentieth days of BR, and the day after BR. The high-frequency component of heart rate variability during sleep gradually decreased and reached the lowest level on the day after BR (100%, 66 +/- 16%, 39 +/- 18%, 10 +/- 8%). Thus, restoring decreased blood volume is an effective countermeasure for orthostatic intolerance after BR. However, decreased vagal tone persisted, suggesting reset autonomic tone.