Goldilocks and the importance of AV intervals in cardiac resynchronization—How to best find the AV interval that is not too long, not too short, but just right for patients

Abstract

Long before cardiac resynchronization (CRT) became standard therapy for patients with QRS prolongation and drug-refractory heart failure, standard dual chamber pacing in the right atrium and right ventricle using short atrioventricular (AV) delays was undertaken. The proponents of this procedure performed in patients without traditional bradycardia indications for pacing theorized that a shortened AV delay would decrease symptoms of heart failure by decreasing mitral and tricuspid valve regurgitation. Several case reports and case studies demonstrated favorable outcomes of improvement in cardiac output and decrease in symptoms of breathlessness in patients randomized to right ventricular pacing with short AV intervals. 1,2 Right ventricular pacing solely to improve symptomatic heart failure in patients without bradycardia was no longer thought useful after the publication of a double-blind randomized crossover trial of 12 patients, which demonstrated no difference in hemodynamic parameters or symptoms of heart failure between the times spent in the backup VVI 40 beats/min mode vs the VDD mode with an AV delay of 100 ms. The importance of AV intervals was realized even in the early days of left ventricular pacing for the treatment of heart failure. In a substudy of the Pacing Therapies in Congestive Heart Failure (PATH-CHF) trial, patients with a left bundle branch block treated with an epicardial left ventricular lead had significant differences in acute hemodynamic parameters as the AV interval was varied. There were significant variations in the optimal AV interval among individual patients, but the greatest benefit was seen as improvements in aortic pulse pressure, aortic systolic pressure, and left ventricular contractility if the AV interval was not too short and not too long. 3