Maintained stroke volume but impaired arterial oxygenation in man at high altitude with supplemental CO2.

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Abstract
Hypobaric hypoxia causes hypocapnia and alkalosis, hemoconcentration and increased hematocrit, and a decreased cardiac stroke volume. To assess the role of the hypocapnic alkalosis in causing these other changes, 5 men were exposed to hypobaric hypoxia at a barometric pressure (PB) of 440 torr with an alveolar O2 tension of 55 torr for 5 days with 3.77% CO2 added to the atmosphere to prevent alkalosis. They did not lose weight, and arterial CO2 tension, pH and cardiac stroke volume were unchanged. An unchanged hematocrit implied an unchanged plasma volume. During exercise to maximum, stroke volumes equaled sea level values but arterial hypoxemia was profound, the arterial O2 tension being 39 torr. By contrast, 3 men at high altitude without CO2 supplementation (PB = 455 torr; alveolar PO2 [O2 pressure] = 56 torr) had weight loss, hypocapnia, alkalosis and decreased stroke volume. Increased hematocrits suggested decreased plasma volumes. During exercise, arterial PO2 (48 torr) was higher than in the group receiving CO2. Maximum O2 uptakes were decreased to a similar degree in the 2 groups. Catecholamine excretion doubled in the group with CO2, but in the group without CO2, catecholamine excretion was unchanged. A normal pH at high altitude apparently maintained plasma volume, which, with the increased catecholamine excretion, may have prevented a decrease in stroke volume. The subjects with CO2 added did not have enhanced O2 transport, because their arterial oxygenation was impaired.