Coronary Circulation, Myocardial Energetics and Pumping Efficiency under Cardiac Pacing

Summary

To evaluate the effects of electrical stimulation of the heart on myocardial energetics 15 closed-chest experiments were carried out in anesthetized dogs.

Right atrial, right ventricular and AV-sequential pacing were intraindividually compared at identical rates over a wide range (70–220 beats/min) with special reference to catecholamine-induced variations of heart rate under maintained sinus rhythm. Myocardial blood flow was measured by a pressure difference catheter in the coronary sinus. Myocardial energy demand was calculated according to Bretschneider’s equation from its hemodynamic determinants. Myocardial oxygen consumption (MVO₂) was markedly higher than the expected energy demand (E_t) under ventricular pacing. Compared to atrial pacing at identical rates MVO₂ was significantly (p < 0.001) higher with a mean oxygen wasting effect of 2.2 ml O₂/min 100 g. Under AV-sequential pacing MVO₂ was even higher as compared to ventricular pacing alone. Consequently cardiac efficiency was very low under ventricular pacing. It was markedly higher under atrial pacing with a mean increase of 63% over ventricular stimulation. Under AV-sequential pacing efficiency was only slightly increased as compared to ventricular pacing in spite of a higher cardiac output. Compared to atrial and sinus rhythm efficiency was, however, markedly decreased under sequential pacing.

The oxygen wasting effect of ventricular pacing and the deterioration of pumping efficiency under ventricular and sequential pacing probably result from the inhomogeneous contraction of the myocardium due to an abnormal spread of excitation.