Variability of oxygen affinity of blood: human subjects native to high altitude.

Whole blood O2 equilibrium curves (OEC) were measured in 46 Peruvians native to high altitude (4,540 m) and in 25 sea-level controls. A method was employed that records the entire OEC from 0 to 150 Torr with constant pH and PCO2. The data were analyzed by fitting the Adair equation describing the successive oxygenation of hemoglobin. At pH 7.4 the PO2 at which hemoglobin is half-saturated with O2 (P50) was significantly higher in the high-altitude population (31.2 +/- 1.9 Torr) than in controls (29.2 +/- 1.8 Torr, P less than 0.001). The acid-base status of the high-altitude subjects, however, was that of compensated respiratory alkalosis (plasma pH 7.439 +/- 0.065), and when the P50's were corrected to the subjects' plasma pH the values (30.1 +/- 2.2 Torr) could no longer be distinguished from the controls. We conclude that, on the average, increased P50 resulting from increased red cell 2,3-diphosphyoglycerate concentration at high altitude is offset by compensated respiratory alkalosis with the net result that the position of the OEC more closely approaches that of sea-level humans than has hitherto been thought. Considerable variation exists in P50, both at sea level and high altitude. This variation might have important consequences for acclimatization and survival under adverse environmental conditions.