Recent Advances in Human Physiology at Extreme Altitude

Abstract

There have been recent advances in the physiology of extreme altitude, especially on the barometric pressure-altitude relationship, and pulmonary gas exchange. Until recently, the only direct measurement of barometric pressure on the summit of Mt. Everest was the value of 253 Torr obtained in October 1981. During the 1997 NOVA Everest expedition, another measurement was made with a hand-held barometer and, after calibration, the value was within approximately 1 Torr of the previous measurement. In addition, weather balloons released at approximately the same time in the vicinity of Mt. Everest gave values that agreed closely. In 1998, a large series of measurements of barometric pressure were made using a weather probe placed on the South Col of Everest (altitude 7,986 m). The mean pressure in May was 283.7 Torr which agrees well with the measurements made just above the South Col in October 1981. The new data fit closely with the Model Atmosphere Equation P_B = exp (6.63268-0.1112h - 0.00149h²) where h is in km. The conclusion is that on days when the mountain is usually climbed during May and October, the summit pressure is 251–253 Torr. The inspired Po₂ is therefore 43 Torr and these data clarify expected pulmonary gas exchange on the summit. Sixty-three measurements of alveolar Po₂ for barometric pressures of 300 to 253 Torr from the Silver Hut and AMREE field expeditions gave a mean value for Po₂ of 35 Torr. Therefore for this value the alveolar Pco₂ cannot be greater than 8 Torr under steady-state, conditions when the respiratory exchange ratio (R) is 1. When R is less than 1, the alveolar Pco₂ must be even lower.