Adaptive Value of Respiratory Adjustments to Shunt Hypoxia and to Altitude Hypoxia1

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Abstract
Respiratory adjustments occurring in hypoxia arising from chronic exposure to a low environmental Po2 (altitude hypoxia) are compared with those occurring in hypoxia arising from a right to left shunt in the circulation (shunt hypoxia). Although an increased pulmonary ventilation is an important adaptation to altitude hypoxia, it is of negligible value in raising the Po2 of the body in shunt hypoxia. It may be of importance in the optimal regulation of acid-base balance in the latter instance however. In chronic altitude hypoxia the usual acid-base balance is one of compensated respiratory alkalosis. In shunt hypoxia there is a tendency for metabolic acidosis to be present. This may be of advantage in aiding the unloading of O2 from the blood to tissues. Polycythemia, which is usually present in both altitude and shunt hypoxia, is a more effective adaptation in the latter type because here it can raise the arterial as well as the mixed venous Po2. The basal O2 requirement of individuals with shunt hypoxia does not appear to be lower than the normal.