Oxygen Cost of Electrical Activation of the Heart

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Abstract
The present study was undertaken to define the O2 requirements of electrical activation of the heart. Thirteen isolated canine hearts were perfused with whole blood from which calcium had been removed with an exchange resin and to which the disodium salt of ethylenediaminetetraacetic acid had been added. Spontaneous depolarizations were suppressed by raising the plasma potassium to an average concentration of 7.5 mEq/liter, and the right ventricle was stimulated electrically at controlled frequencies. Although the stimuli produced propagated depolarizations, neither high-sensitivity strain gauge arches sutured to both ventricles, nor careful visual observation, showed any evidence of associated contractile activity. Ten of the hearts were studied with repetitive single stimuli applied in the conventional fashion, while the remaining three hearts were subjected to paired electrical stimulation. Changes of myocardial O2 consumption (MV·O2) were measured at a constant coronary blood flow and arterial O2 content by determining changes of venous O2 content from a continuous recording of venous PO2. Increases of the frequency of depolarization were uniformly accompanied by small increases of MV·O2, averaging 0.40 ± 0.04 (SEM) µliter/activation/100 g. The increases were of the same order of magnitude in the hearts subjected to paired electrical stimulation as in the hearts studied with single stimulation, suggesting that the altered frequency and rhythm of depolarization in paired electrical stimulation cannot account for the marked increase of MV·O2 which this intervention produces in the intact heart. It is concluded that the amount of O2 required for electrical activation of the heart is less than 1% of the total O2 consumption of the normally working heart.