Peak O₂ Consumption and Physical Performance at High Altitudes in Mountaineers Breathing Ambient Air

When people (and other mammals) are exposed to decreased atmospheric concentrations of O₂, they marshal vigorous physiological defenses, discussed in Boxes 23.2 and 24.5. Because of these responses, resting or moderately active people show a substantial degree of oxygen regulation as the ambient O₂ level falls with increasing altitude. Here, however, we consider the most demanding of circumstances: the capability of mountaineers for all-out physical effort at the highest altitudes on Earth.

As the figure shows, when people are asked to work hard enough that they take in O₂ at their peak rate, their maximum rate of O₂ consumption becomes a smaller and smaller fraction of their rate at sea level as altitude increases. The cost of any particular form of exercise remains the same regardless of altitude, however. Accordingly, a rate of climbing that is distinctly submaximum at low altitudes can become maximum, or even supramaximum, at high altitudes. At altitudes near the top of Mt. Everest (8848 m), the maximum rate of O₂ consumption is so low that even minimum rates of climbing require about the maximum possible rate of O₂ consumption. Work that requires 100% of maximum O₂ consumption is always extremely taxing: barely possible. When, in 1978, Reinhold Messner and Peter Habeler became the first to reach the summit of Mt. Everest without supplemental O₂, they reported climbing so slowly near the top that, even though they felt they were working at their limits, they required an hour to cover the final 100 meters!

Already in the1920s, long before the invention of modern mountaineering clothing and equipment, a few intrepid men attempted to reach the summit of Mt. Everest while breathing ambient air rather than using supplemental O₂. One of the most persistent was E. F. Norton, who left behind a series of magnificent sketches of the Himalayas at the time. Climbing without supplemental O₂ at 8500 m on Mt. Everest, he tried to keep his focus by counting steps. He reported his attempts to take 20 consecutive uphill steps. He never could! And he did not reach the top. Physiological data gathered on climbers who have reached the peak of Mt. Everest without supplemental O₂ indicate that the feat is so close to the margin of what is possible that success or failure depends on whether the barometric pressure is relatively high or low on the day of the climb (O₂ is slightly more concentrated in the air on a day of high barometric pressure).

Maximum rates of O₂ consumption of human mountaineers at increasing altitudes Values are expressed as percentages of the values at sea level. Mountaineers were breathing from the ambient air, not from O₂ tanks. (After Fulco et al. 1998.)

One of the intriguing aspects of our planet is that Mt. Everest is nearly exactly as high as a mountain could be and still be climbed by people without supplemental O₂. If a mountain were 400 m higher than Everest, maximum human O₂ consumption at its top would be 25% lower than on the top of Everest, and probably not even the most elite mountaineer could reach the top breathing ambient air, regardless of the barometric pressure on the day of the climb.

References

Fulco, C. S., P. B. Rock, and A. Cymerman. 1998. Maximal and submaximal exercise performance at altitude. Aviat. Space Environ. Med. 69: 793–801.