Cardiac resynchronization therapy (CRT) is effective for the treatment of patients with heart failure and a wide QRS duration, particularly left bundle branch block. However, subjects with right bundle branch block (RBBB) do not appear to benefit from CRT. The aim of this study was to determine whether patients with specific RBBB conduction patterns responded to CRT in the Multicenter Automatic Defibrillator Implantation Trial with Cardiac Resynchronization Therapy (MADIT-CRT) trial. In the present post hoc analysis, patients with RBBB who received CRT with an implantable cardioverter defibrillator (n = 132) were divided into 2 groups according to the baseline QRS morphology in the inferior and high lateral leads: group 1: left anterior fascicular block (LAFB) pattern (n = 80; 60.6%); and group 2: non-LAFB pattern (n = 52; 39.4%). Subjects with RBBB who received an implantable cardioverter defibrillator served as the control group (n = 87). The primary end point was echocardiographic response to CRT, defined as percent change in left ventricular (LV) and left atrial volumes from baseline to 1 year. The secondary end point was heart failure or death. The non-LAFB group demonstrated a significantly larger percent reduction in LV end-diastolic volume, LV end-systolic volume, and left atrial volume compared to controls (-11%, p <0.0001; -17%, p <0.0001; -15%, p <0.0001, respectively) and LAFB (-5%, p = 0.028; -7%, p = 0.019; -6%; p = 0.022; respectively) by multivariate analysis. No difference was found in the 3-year crude event rates for death or heart failure among the LAFB (22%), non-LAFB (21%), or ICD-only (20%) groups (p = NS). In conclusion, conduction patterns influence echocardiographic response to CRT in patients with RBBB, with favorable reductions in the LV and left atrial volumes in those without an LAFB conduction pattern. This echocardiographic benefit did not translate into a reduction in heart failure or death during a 3-year follow-up period.
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