Gas exchange at extreme altitude: results from the British 40th Anniversary Everest Expedition.

Since Messner and Habeler climbed to the summit of Mount Everest (8,848 m) without oxygen in 1978, there has been controversy between scientists trying to explain this feat. Field studies have suggested better respiratory performance than that found during a simulated climb in a hypobaric chamber, but the lack of data at extreme altitude has hampered the debate. We measured arterial oxygen saturation (Sa,O2) and alveolar partial pressure of oxygen (PA,O2) in nine subjects as they climbed from 3,500 to 8,000 m. Four of the climbers reached 8,000 m and one reached the summit. We also tested the effects of breathing supplementary oxygen on Sa,O2 at 6,550 and 8,000 m. At all altitudes tested, we found that both PA,O2 and Sa,O2 were higher than expected from low pressure chamber studies. We also found that standard rates of supplementary oxygen (2 L x min(-1)) were insufficient to restore Sa,O2 above 90% at 8,000 m. The respiratory performance of climbers at extreme altitude is better than expected from sea-level chamber studies, which may, in part, explain why humans can reach the summit (8,848 m) without added oxygen. The better performance is likely to be due to more appropriate acclimatization.