Greenleaf JE,
Farrell PA, Loomis JL, Federle MJ, West J, Rössler A, Hinghofer-Szalkay H

Sodium chloride-citrate beverages affenuate hypovolemia in men resting 12 h at 2800 m altitude

Aviat Space Environ Med 1998; 69:936-43

Background: The mechanism for reduction and restoration of total body water and plasma volume (PV) during initial exposure to acute altitude (ALT) is not clear but may involve involuntary dehydration; i.e., delayed voluntary fluid intake.

Method: Ten men (24±SD 3 yr, 180.8±8.1 cm height, 78.8±12.8 kg weight, 1.99±0.19 m2 surface area, and 12.2±4.0% body fat) were in a semi-reclining position for 12 h in a chamber at 2800 m (539 mmHg) ALT or at 321 m (732 mmHg; ground). They ate a controlled breakfast (450 kcal + 3 ml/kg H2O) on the ground, and lunch and dinner at ALT (or on the ground) for a total daily intake of 2850 kcal (14% PRO, 67% CHO, 16% fat, 2.6g NaCI). At hour 10 they consumed fluid-electrolyte beverages or water (12 ml/kg, 948 ml/d ) in 4 sessions at weekly intervals. Beverage compositions were: a) 185 mEq/l Na+, 283 mOsm/kg; b) 21.6 mEq/l Na+, 365 mOsm/kg; c) water at ALT; and d) water on the ground.

Results: After 10 h at ALT plasma volume (Hb-Hct) decreased (p<0.05) by: a) 9.0±SE 1.5%; b) 6.2±1.7%; c) 7.4±2.2%; and d) by 9.0±2.4%, respectively. After drinking from 1000-1030 h, PV at l200 h changed by: a) +8.3±SE 2.0% (p<0.05); b) +2.8±2.7% (NS); c) -0.9±1.5% (NS); and d) by +0.8±3.5% (NS), respectively. The similar ground-induced hypovolemia suggests a response to confinement rather than an ALT effect and involuntary dehydration does not appear to be implicated.

Conclusion: The significant increase in PV after consuming the (a) NaCI-NaCitrate beverage indicates that drink ionic composition appears to be more important than its osmolality for restoring PV in these conditions.

Practical considerations: Because this hypovolemia was probably due to the confinement rather that reduced ambient pressure, appropriate countermeasures could be consumption of isotonic beverages, elastic stockings, leg exercise, and leg elevation.