Effect of Hypoxia on Cardiac Oxygen Consumption and Coronary Flow

¹ From the Cardiovascular Department, Medical Research Institute, Michael Reese Hospital, Chicago, Illinois

The arterial O₂ content was gradually decreased in the anesthetized open-chested dog, prepared for total coronary flow measurement, while cardiac performance and myocardial O₂ consumption were explored over a wide range. The coronary venous O₂ content and the coronary arteriovenous O₂ difference decreased in proportion to the decline in arterial O₂, hence the percentage of myocardial O₂ extracted remained relatively constant. The a-v O₂ difference and the percentage of O₂ extraction remained fairly constant at each level of arterial O₂ over a wide range of cardiac performance and O₂ consumption. Coronary blood flow increased both when the a-v O₂ decreased and when the O₂ consumption increased. The coronary flow response to hypoxia was commensurate with the decreased a-v O₂—hence the O₂ consumption/O₂ availability ratio remained normal and constant throughout the period of hypoxemia. The O₂ cost of cardiac external useful work (external efficiency) was essentially unchanged, while the O₂ cost of cardiac effort, as measured by the product of blood pressure and heart rate, was only slightly altered by hypoxia.

Note:
with the technical assistance of E. Boyd

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