Operation everest II: Gas tensions in expired air and arterial blood at extreme altitude

Measurements in alveolar gas have suggested extreme hypocapnia and alkalosis on the summit of Mt. Everest. However, tensions in both expired gas and arterial blood have not been reported for the summit of Mt. Everest (P(1O₂)=43mm Hg). To approach the problem, we examined alveolar (and end tidal) and arterial gas tensions in 6 healthy men who completed a 40-d chamber study to the simulated 'summit,' with 20 d above 6,400m and 9 d above 8,000m. In 27 simultaneous samples of alveolar air and arterial blood for inspired oxygen tensions ranging from P(1O₂) of 55mm Hg (7,380m) to 43mm Hg, the mean alveolar-arterial difference was negligible for P(O₂) (-0.8±2.4 {S.D.} mm Hg) and P(CO₂) (0.5±1.4mm Hg). For all 6 subjects at the summit, P(ACO₂) was 12.0±1.8 and P(aCO₂) was 11.4±1.6mm Hg, and for the two with the lowest values, alveolar and arterial P(CO₂), respectively, were 9.5 and 9.8mm Hg. Arterial pH averaged 7.53 units. We conclude that while acclimatization to severe hypoxia results in extreme hypocapnia, alkalosis is more moderate than previously reported. Alveolar gas tensions reflect well the values obtained in arterial blood.