Cardiac performance in relation to altitude ∗

Abstract

Decrease of atmospheric air density causes reductions of alveolar and arterial oxygen pressure and of arterial oxygen saturation. Acute exposure to lowered barometric pressure necessitates an increased blood flow, with or without a rise in pulmonary ventilation. When compensating for decreased arterial oxygen content by increased flow, the heart reaches performance limits at lower total energy expenditures. In chronic hypoxia the stimulation of erythropoiesis and the resulting greater arterial oxygen content tend to “normalize” cardiac work, at least as judged by the return of heart rates to more normal values. General capillary dilatation and decrease in peripheral resistance also may help to reduce the heart's work load. During acclimatization to altitude, an augmentation of total blood volume is usually observed. A substantial part of the additional blood volume is apparently absorbed by the lungs. Due to a resistance developing somewhere peripheral to the pulmonary alveolar capillaries, an arteriolar dilatation occurs, the quantity of blood in the lungs increases, pulmonary artery pressure goes up and hypertrophy of the right ventricle becomes evident. The greater demands on the right heart, in hypoxia, must be reflected in an increase of total cardiac work required for identical conditions of rest and physical work. An attempt was made to estimate the work of the heart, its oxygen requirements and the demands on coronary blood flow from measured and/or estimated parameters such as blood pressure, stroke volume, heart rate, hemoglobin and arterial and myocardial oxygen tensions. Resting values of coronary flow thus estimated (at sea level) compared favorably with “text book figures.” Similar evaluations of work situations under normal and reduced atmospheric air pressures lead to the conclusion that limitations in maximal coronary blood supply determine the range of cardiac performance capacity and, therefore, the individual range of physical working capacity, under normal atmospheric conditions as well as at altitude.