Limiting factors to oxygen transport on Mount Everest.

The effect of a sudden increase of the inspired oxygen tension on the maximal aerobic performance was studied on 23 subjects acclimated to high altitude (5,350-8,848 m above sea level) in the course of a 4-mo expedition to Mt. Everest. Inhalation of 100% O2 at 390 mmHg or a rapid descent (20 min) by helicopter to 2,850 m (Pio2 = 117 mmHg) raised maximal oxygen consumption (Vo2 max) from an average 0.7 of the control sea-level value, respectively, to 0.92 and 0.97. The failure of acclimated subjects to increase markedly or even to resume the preexisting sea-level Vo2mxa while breathing O2 in the presence of a 40% increase of blood Hb concentration and of a limited reduction of maximal cardiac output (Qmax), is attributed to changes in the peripheral circulation. These may consist of a) a hindrance of O2 diffusion due to erythrocytes packing secondary to increased hematocrit (Hct up to 70%); b) a bypass of arterial blood from the high-resistance working areas of the body aimed at reducing the load on the heart caused by increased blood viscosity. A 11.6% increase above normal controls of Hb concentration still found in 13 subjects 25 days after leaving altitude does not increase significantly maximum O2 consumption.