PURPOSE: Complete left bundle branch block is a well-established independent risk factor for mortality, but the prognostic importance of right bundle branch block is unclear. We determined whether left and right bundle branch block was associated with all-cause mortality risk after adjustment for potential confounders, including clinical, exercise, and nuclear scintigraphic variables.

SUBJECTS AND METHODS: We studied 7,073 adults who were referred for symptom-limited nuclear exercise testing. Patients with heart failure or pacemakers were excluded. The presence or absence of bundle branch block was determined from resting electrocardiograms. The main outcome measure was all-cause mortality during a mean of 6.7 years of follow-up.

RESULTS: One hundred ninety patients (3%) had complete right bundle branch block, and 150 (2%) had complete left bundle branch block. There were 825 deaths (12%). Mortality was greater in patients with complete right bundle branch block (24% [46 of 190]) or left bundle branch block (24% [36 of 150]) than in those without these findings (11% [779 of 6,883 and 789 of 6,923, respectively]; both P <0.0001). After adjustment for potential confounders, right bundle branch block was as strong an independent predictor of mortality (hazard ratio [HR] 1.5; 95% confidence interval [CI]: 1.1 to 2.1; P = 0.007) as left bundle branch block (HR 1.5; 95% CI: 1.0 to 2.0; P = 0.017). Incomplete right bundle branch block was not associated with mortality.

CONCLUSION: Complete right and left bundle branch block are independent predictors of all-cause mortality risk even after adjustment for exercise capacity, nuclear perfusion defects, and other risk factors.