Myocardial dynamics, contractility, O2 consumption, and K+ balance during paired stimulation

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Abstract
The influence of paired stimulation (continuous postextra-systolic potentiation) on myocardial performance, O2 consumption, and net K balance was determined in the isolated supported dog heart. The results of the experiments show that at a constant heart rate, stroke volume, and aortic pressure, paired stimulation is associated with a decline in left ventricular end-diastolic pressure, i. e., the contractility of the ventricle is increased; the decline in ventricular end-diastolic pressure cannot be attributed solely to a change in myocardial extensibility, since ventricular size also decreases a second contractile event was always observed during paired stimulation if 2 waves of depolarization were produced; when a short interval exists between paired stimuli further shortening of the interval may produce little further change in ventricular performance; paired stimulation is associated with a substantial elevation of myocardial O2 concumption which cannot be attributed to the increase in the velocity of ventricular contraction per sec; paired stimulation produces a net loss of myocardial K at least as great as that seen with large subtoxic doses of acetyl strophanthidin. This finding taken in conjunction with other studies from this laboratory suggests that the improvement in ventricular performance produced by paired stimulation is importantly related to the loss of myocardial K.