PHYSIOLOGICAL VARIATIONS OF THE CARDIAC OUTPUT OF MAN

Abstract

The effect of residence at high altitudes on the cardiac output and related functions was investigated by comparing the results of measurements at sea-level with those obtained on the summit of Pike''s Peak (altitude 4300 meters). The cardiac output gradually increases on the summit of Pike''s Peak, reaching a maximum of about 40% above its sea-level value on the 5th day after arrival on the Peak. It then gradually declines to its normal value. The relation of this change to the hemoglobin content of the blood was demonstrated. As the hemoglobin content of the blood increases the cardiac output decreases, the increased O capacity of the blood replacing the increased cardiac output as a means of supplying the tissues with O. When the hemoglobin content reaches its maximum, the cardiac output has returned to normal. There is thus a definite relation between the cardiac and hema-topoietic reactions to high altitudes, the former being more rapid in its development and being gradually replaced in its function by the latter. Both factors aim to furnish the tissues with their normal supply of O and overcome any deficit occasioned by the low O tension of the atmosphere. The changes of pulse rate, blood pressure, basal metabolism, alveolar gas tensions, and the effect of the ingestion of food on the cardiac output at high altitudes were also studied and the relation of these various factors to other physiological changes, discussed. After a month''s stay on Pike''s Peak, a study of the cardiovascular changes encountered on returning to a lower level (Manitou, Colorado, altitude 2000 meters) was made. The relation of the O content of the inspired air to the cardiac output was also determined by breathing gas mixtures of varying O content, for varying periods of time at sea-level. The existence of a critical concentration of O below which further reduction in the O content of the inspired air causes an immediate increase in the cardiac output was demonstrated. The relation of this immediate increase to the progressively developing increase observed at high altitudes was demonstrated. The relation of the various cardiovascular reactions to anoxemia was discussed.