Metabolic Adjustments to Alterations of Cardiac Work in Hypoxemia

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Abstract
Results are given from a series of experiments on a dog heart preparation with a relatively intact circulation in which the operation of nervous and humoral mechanisms was maintained. The preparation permitted the accurate measurement of cardiac output, blood pressure, total coronary flow and cardiac O2 consumption. When the data from all experiments were pooled, good correlations could be seen in the relationship of cardiac O2 consumption to cardiac work, left cardiac output and mean systemic arterial blood pressure. The increase of cardiac O2 consumption for each unit rise of cardiac output was greater when this rise was accomplished at higher blood pressures; similarly, the increase of O2 consumption was greater for a given rise of blood pressure at higher levels of cardiac output. Cardiac efficiency was found to be directly correlated with cardiac work, left cardiac output and mean systemic arterial blood pressure; but independent of cardiac O2 consumption. The correlation between efficiency and blood pressure was better at lower cardiac output; similarly, that between efficiency and cardiac output was somewhat better at lower blood pressure levels. Cardiac efficiency was higher when arterial O2 content was low. Cardiac O2 consumption decreased, for the same range of cardiac work, when arterial O2 content was low. This mechanism has obvious survival value. In individual experiments of this series exceptions to the classical laws of the heart were seen. For example, in several cases the work of the heart increased with a fall in O2 consumption. In this preparation, left atrial pressure was more closely related to left cardiac output than to either systemic arterial blood pressure or total cardiac work. Further, changes of left atrial pressure, both mean and diastolic levels, failed to correlate with changes of cardiac O2 consumption. In several cases of "spontaneous" changes in coronary flow it was possible to show that cardiac efficiency increased. This would indicate the operation of a mechanism which permits an increase in the useful work obtained from a given amount of O2. Importance of such an adjustment is apparent. The regulation of cardiac dynamics by end-diastolic volume is but one of many mechanisms acting in the intact animal to control the work performance of the heart and the manner by which the energy requirements for this work are met.

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