Early acclimatization to high altitude: Acid-base and fluid balance dynamics during the first 2 days at 3100 m.

Immediate responses to hypoxia at high altitude are hyperventilation and successive respiratory alkalosis. Alkalosis, in turn, can affect cerebrospinal fluid pH and ventilatory control. The kidneys compensate metabolically for respiratory alkalosis. The time line and detailed sequence of these renal compensatory processes have not been explored thoroughly. We examined the initial adjustments of acid-base and fluid balances during the first 2 days at high altitude.  Twelve unacclimatized adults of either sex were transported to 3100 m a.s.l. at the Sonnblick Observatory (Austria). Measurements (fluid and acid-base balance and arterial blood gases) were performed before and 24 and 44 h after arrival. Exposure to high altitude provoked hyperventilation, resulting in hypocapnia and alkalosis. Altitude diuresis started immediately after arrival at altitude. The only metabolic response within the first 24 h was a slight reduction in plasma bicarbonate, but after 44 h distinct reductions in bicarbonate and a trend change in altitude-corrected base excess and arterial pH were observed. Hyperventilation and increased diuresis appeared immediately upon exposure to high altitude, whereas compensatory bicarbonate excretion showed the first influence on the arterial pH at the 44 h measurement. Further research is needed to explore differences in individual responses in the setting of antedated or carry-over acclimatization.