Evidence for central venous pressure resetting during initial exposure to microgravity.

We measured central venous pressure (CVP); plasma volume (PV); urine volume rate (UVR); renal excretion of sodium (UNa); and renal clearances of creatinine, sodium, and osmolality before and after acute volume infusion to test the hypothesis that exposure to microgravity causes resetting of the CVP operating point. Six rhesus monkeys underwent two experimental conditions in a crossover counterbalance design: 1) continuous exposure to 10 degrees head-down tilt (HDT) and 2) a control, defined as 16 h/day of 80 degrees head-up tilt and 8 h prone. After 48 h of exposure to either test condition, a 120-min course of continuous infusion of isotonic saline (0.4 ml. kg(-1). min(-1) iv) was administered. Baseline CVP was lower (P = 0.011) in HDT (2.3 +/- 0.3 mmHg) compared with the control (4.5 +/- 1.4 mmHg) condition. After 2 h of saline infusion, CVP was elevated (P = 0.002) to a similar magnitude (P = 0.485) in HDT (DeltaCVP = 2.7 +/- 0.8 mmHg) and control (DeltaCVP = 2.3 +/- 0.8 mmHg) conditions and returned to preinfusion levels 18 h postinfusion in both treatments. PV followed the same pattern as CVP. The response relationships between CVP and UVR and between CVP and UNa shifted to the left with HDT. The restoration of CVP and PV to lower preinfusion levels after volume loading in HDT compared with control supports the notion that lower CVP during HDT may reflect a new operating point about which vascular volume is regulated. These results may explain the ineffective fluid intake procedures currently employed to treat patients and astronauts.