Rick Schreurs

23 Exploring CRT INTRODUCTION Patients with heart failure combined with left bundle branch block (LBBB) have broad QRS complexes and an impaired cardiac function. In these patients it is unclear whether heart failure leads to LBBB or vice versa. Animal models however, have shown that induction of LBBB reduces cardiac contractility [1]. Similarly, inadvertent induction of LBBB during Transcatheter Aortic Valve Replacement worsens outcome [2]. In LBBB cardiac function is impaired because the left (LV) and right ventricle (RV) and various regions within the LV are not activated simultaneously, leading to dyssynchronous contraction and reduced cardiac pump function [3]. Animal studies have shown that isolated LBBB causes reduction in ejection time, slower rates of rise and fall of the LV pressure and prolonged duration of isovolumic contraction and relaxation [4]. Cardiac resynchronization therapy (CRT) aims to restore synchronous contraction of the heart by either LV free wall or biventricular (BiV) pacing. The first studies started during the late 90s to evaluate the effect of CRT [5-7]. According to the current guidelines a CRT device is indicated for patients with heart failure and prolonged QRS duration, preferably with LBBB morphology on the electrocardiogram (ECG) [8]. CRT reduces mortality and improves quality of life [9], but additional research is needed to optimize this therapy. To this purpose investigations have also been performed in animal models of dyssynchrony. Animals have long been used to study hemodynamics and electrophysiology in heart failure. As reviewed by Strik et al [10]. the canine appears to be the best species to study ventricular conduction abnormalities that occur during pacing and LBBB, because the anatomical structures of the bundle branches are comparable to those in human. Percutaneous radiofrequency ablation of the LBBB results in a model perfectly suited to study the effects of CRT [1, 4, 11, 12]. This model enables the investigation of the role of LBBB and CRT in the absence of heart failure, but if needed, also in combination with heart failure (created by chronic tachypacing) or myocardial infarction [10]. The aim of this article is to review the electrophysiological and hemodynamic effects of CRT in animal models and patients with LBBB. 2