Bundle-Branch Block

Importance: Heart failure (HF) affects more than 64 million individuals worldwide, and acute HF is associated with 1-year mortality rates of 23.6% in North America and Europe. Cardiac dyssynchrony from conduction system disease may cause HF progression, particularly in patients with left ventricular (LV) systolic dysfunction.

Observations: Electrical dyssynchrony in HF most commonly presents as left bundle-branch block and affects 20% to 30% of patients with reduced LV ejection fraction (LVEF). Cardiac resynchronization therapy, which includes biventricular pacing and conduction system pacing, restores synchronous ventricular contraction and is recommended for patients with symptomatic HF despite optimal medical therapy, LVEF of 35% or less, and left bundle-branch block. Delayed referral for device therapy in this population is associated with worse clinical outcomes. Resynchronization should also be considered for patients requiring chronic ventricular pacing, typically due to symptomatic bradycardia because right ventricular pacing for atrioventricular block may result in ventricular dyssynchrony and increased risk of LV systolic dysfunction. Biventricular pacing uses 2 leads to stimulate the right ventricle and LV simultaneously; conduction system pacing uses a single lead to stimulate the His bundle or left bundle branch. An individual patient-level meta-analysis of 5 randomized clinical trials (N = 3872) reported biventricular pacing was associated with lower all-cause mortality compared with medical therapy or implantable cardioverter-defibrillator over a median follow-up of 23.7 months (13.7% vs 20.8%; hazard ratio, 0.66 [95% CI, 0.57-0.77]). A meta-analysis of 7 small randomized clinical trials, including 408 patients with HF and LVEF of 40% or less, reported that compared with biventricular pacing, conduction system pacing was associated with improvement in LVEF (mean difference, 2.06%; P = .03). An observational study of 1778 patients with LVEF of 35% or less undergoing cardiac resynchronization therapy reported conduction system pacing was associated with lower rates of HF hospitalization (12% vs 19%; hazard ratio, 0.67 [95% CI, 0.52-0.86]). A trial of 249 patients without HF undergoing permanent pacing for atrioventricular block-related bradycardia reported lower rates of pacing-induced cardiomyopathy, defined as a decrease in LVEF of at least 10% to less than 50% after conduction system pacing vs right ventricular pacing (6% vs 15%; P = .01).

Conclusions and relevance: Cardiac dyssynchrony due to conduction system disease occurs in 20% to 30% of patients with HF and systolic dysfunction. Cardiac resynchronization therapy restores synchronous ventricular activation in patients with HF, reduced LVEF, and left bundle-branch block, or in those requiring chronic ventricular pacing, and may improve LV function, decrease HF hospitalizations, and reduce mortality.