Effect of Hypoxia on Cardiac Oxygen Consumption and Coronary Flow

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Abstract
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.