Diffusion at high altitude.

The problems posed by exercise at high altitude for oxygen diffusion across the blood-gas barrier have been recognized since the early part of this century. But the successful ascent of Mt. Everest (altitude 8848 m) by two climbers without supplementary oxygen in 1978 focused attention on this issue. We have therefore carried out a theoretical study of gas exchange under these conditions of extreme hypoxia. Calculations of oxygenation along the pulmonary capillaries show that, even at rest, there is an alveolar-end capillary PO2 of about 6 torr caused by diffusion limitation, and this widens rapidly on mild exercise. As the oxygen uptake is increased, the PO2 of mixed venous blood falls to very low values. If we assume a minimal value of 15 torr, a maximal oxygen uptake on the summit of less than 700 ml/min is predicted. VO2max is extremely sensitive to barometric pressure, and to a lesser extent to lung diffusing capacity.