Increased 2,3-diphosphoglycerate during normocapnic hypobaric hypoxia.

The effect of 96 h of exposure to hypobaric hypoxia with and without 3.8% CO2 supplementation was studied in two groups of subjects. Five subjects (CO2) were exposed to 440-465 mm Hg barometric pressure (4000-4400 m), and 4 subjects (no-CO2) were exposed to 455-492 mm Hg (3500-1400 m) in order to produce similar levels of resting end-tidal PO2. After 24 h, 2,3-DPG levels of both groups significantly increased and remained elevated. The CO2 group had higher levels than the non-CO2 group after 48 and 72 h. Concurrent measurements of P50 showed similar changes over the same time course. Mean corpuscular hemoglobin concentrations remained normal for 48 h and then decreased in both groups, the CO2 group showing the larger decrease. We conclude that altitude exposure may produce an increase in 2,3-DPG without the presence of respiratory alkalosis previously thought necessary.