Unfavorable effects of ventricular pacing on myocardial energetics

The effects of ventricular pacing (90-330 beats/min) and atrial pacing (120-210 beats/min) on myocardial oxygen consumption (MVO2) and its hemodynamic determinants and on myocardial pumping efficiency were studied systematically on intact dogs. In six closed-chest experiments 158 steady states were analyzed. Myocardial blood flow was measured with a differential pressure sinus catheter, oxygen consumption (5-30 ml/min . 100g) was determined simultaneously by the Fick principle and the additive hemodynamic parameter Et. Ventricular and atrial pacing were compared with both methods at identical heart rates. Additionally, the coincidence between both methods of determining MVO2 was examined at sinus rhythm with sympathetic stimulation (norepinephrine, atropine) within each experiment. Ventricular pacing increased MVO2 overproportionally up to 50% in relation to the hemodynamic determinants. Consequently, myocardial pumping efficiency markedly decreased with increasing ventricular rate. The close relation between directly measured MVO2 and Et, found in previous studies, was maintained under sympathetic stimulation. Atrial pacing, as compared to ventricular pacing at identical rates, resulted in a decrease of MVO2 up to 25% although the expected mVO2 according to its hemodynamic determinants rather increased. The hemodynamic and metabolic mechanisms probably responsible for the energetic difference between ventricular and atrial pacing at equal heart rates are discussed.