Hypovolemia During Early Exposure to 2,800 m Altitude: Effect of Rehydration

The mechanism for reduction of total body water and plasma volume (PV) during initial exposure to acute attitude (ALT) is not clear. Ten man (25+/-SD 3yr. 1.99+/-0.l9sq m) were confined (sitting) for 12 hr in a chamber a 2800m ALT or at 305m(ground). They ate a controlled breakfast (450 kcal + 3ml/kg H2O) on the ground, and lunch and dinner at ALT for a total daily intake of 2,850 kcal (14%PRO, 67%CHO, 16%FAT, 26gNaCl). At hr 10 they consumed fluid-electrolyte drinks or water (12ml/kg, 948ml/day) with sessions at weekly intervals: (a) 185mEq/L Na, 283mOsm/kg; (b) 21.6Na, 365mOsm: and (c) water at ALT: and (d) water on the ground. After 10 hr at ALT%(Delta)PV(Hb-Hct) decreased (P<0.05) by (a) 9.0+/-SE1.5%, (b) 6.2+/-1.7%, (c) 7.4+/-2.2%. and (d) 9.0+/-2.4%. respectively. After drinking at 1200 hr PV increased by (a) 8.3% (P<0.05), (b) 2.8% (NS), (c) -0.9% (NS). and (d) by 0.8% (NS). respectively. The ground-induced hypovolemia suggests a confinement rather than an ALT effect. The increase in PV after the NaCl-NaCitrate drink (a) indicates that drink composition is mom important than as Osm for restoring PV in these Conditions.