Hemoglobin P(50) during a simulated ascent of Mt. Everest, Operation Everest II.

The amount of O(2) available to tissues is essentially the product of cardiac output, [Hb], and O(2) saturation. Saturation depends on P(O2) and the O(2)Hb dissociation curve. With altitude, increased [2,3-DPG] shifts the dissociation curve rightward, but hypocapnia and alkalosis move it leftward. We determined both standard and in vivo P(50) in 5 fit subjects decompressed over 42 days in an altitude chamber to the equivalent of the Mt. Everest summit (Operation Everest II). Arterial and venous blood was sampled at five "altitudes " (P(B) = 760, 429, 347, 282, 253 mmHg), and P(O2), P(CO2), pH, O(2) saturation, [Hb] and [2,3-DPG] were measured. As reported previously, 2,3-DPG levels increased from 1.7 (P(B) = 760) to 3.8 mmol/L (P(B) = 282). Standard P(50) also increased (from 28.2 mmHg at sea level to 33.1 on the summit, p<0.001). Alone, this would have lowered saturation by 12 percentage points at a summit arterial P(O2) of approximately 30 mmHg. However, in vivo P(50) remained between 26 and 27 mmHg throughout due to progressive hypocapnia and alkalosis. Calculations suggest that the increase in standard P(50) did not affect summit V(O2 MAX)), alveolar, arterial and venous P(O2)'s, but reduced arterial and venous O(2) saturations by 8.4 and 17.4 points, respectively, and increased O(2) extraction by 7.9 percentage points. Reduced saturation was balanced by increased extraction, resulting in no significant overall O(2) transport benefit, thus leaving unanswered the question of the purpose of increased [2,3-DPG] concentrations at altitude.