In this vanguard study, the investigators hypothesize that in patients with high degree AV block and normal to mild LV dysfunction (LVEF >= 50%), patients undergoing LBBP will demonstrate a significantly lower indexed LV end-systolic volume (LVESVi) as compared to those undergoing RV pacing over 24 months follow-up. The investigators will demonstrate that recruitment is feasible, LBBP can be successfully performed with reproducible and consistent results, and that LVESVi will prove to be a suitable efficacy endpoint to inform the design and proceed with the pivotal LEFT-HF study.
High burden right ventricular (RV) pacing has been shown to increase cardiovascular mortality, incidence of heart failure (HF), worsen left ventricular (LV) function and accelerate the development of atrial fibrillation (AF). High percentage ventricular pacing and wider paced QRS in the setting of normal baseline LV ejection fractions have consistently been shown to be independent risk factors for pacing-induced cardiomyopathy. Left bundle branch pacing (LBBP) has recently emerged as a potential alternative pacing mechanism that may avoid LV dyssynchrony and pacing-induced LV dysfunction by mimicking native electrical conduction.