Timing Cycles for Biventricular Pacing

January 2002 PACE, Vol. 25 I. Goals of Biventricular Pacing Over the past several decades, pacing has become the primary treatment of patients with bradycardia. It has markedly improved the quality-of-life of individuals by preventing symptoms originating from slow heart rates. Dual chamber pacing was introduced as a method of better meeting the metabolic needs of many patients with sinus node and atrioventricular (AV) conduction abnormalities. New indications for permanent pacing have been proposed to expand the role of pacing to treat disorders like neurocardiogenic syncope and hypertrophic cardiomyopathy. Currently, pacing of the left (LV) and right ventricles (RV), biventricular pacing, is being investigated as a treatment for heart failure. Heart failure is often associated with prolonged ventricular conduction delay, like left bundle branch block that contributes to LV systolic dysfunction and poor outcome. These delays generate uncoordinated ventricular contractions that reduce pumping effectiveness. Studies of heart failure patients in normal sinus rhythm with LV conduction delay indicate that biventricular pacing can significantly improve systolic function and energetics. Biventricular pacing apparently resynchronizes RV and LV contractions and LV septal and lateral wall contractions. Another application of biventricular pacing is to correct the LV contraction delay induced by pacing only the RV, which reduces contractile function, cardiac output, and cardiac metabolic efficiency. When cardiac function is already depressed by heart disease, like dilated cardiomyopathy or atrial fibrillation, further decline from RV pacing may not be tolerated and contribute to worsening symptoms and failure progression. Initially, pacemakers intended for bradycardia therapy were adapted to provide biventricular pacing for investigational treatment of heart failure. With the successful outcome of current clinical investigations, we can expect new devices specialized to treat contractile dysfunction while concurrently treating bradycardia and tachycardia. (Fig. 1). Such devices might be called cardiac function management (CFM) devices since they will be designed to improve pumping function by altering ventricular contraction sequences while maintaining pumping rate and rhythm. A difficult challenge for CFM devices is that the goal of improving pump function by altering contraction sequences is distinct from that of bradycardia pacing. To treat bradycardia, pacing is only performed when the rate is not fast enough or the AV interval (AVI) is too long. Thus, patients with intact AV conduction and adequate ventricular rates may not be paced at all (Fig. 2A). In contrast, to improve pump function, the primary goal is to biventricular pace the ventricles continuously to correct pathological ventricular conduction delays (Fig. 2B). These distinct goals require a new set of timing cycle rules for biventricular pacing in CFM devices, which are described in this article. The timing scenarios and diagrams presented in this article represent theoretical behavior when there is total independent programmability in the two ventricular channels. The distances between the schematic complexes are not proportional to the timing in the actual systems.