Effectiveness of Cardiac Resynchronization Therapy by QRS Morphology in the Multicenter Automatic Defibrillator Implantation Trial–Cardiac Resynchronization Therapy (MADIT-CRT)

Study Questions:

Is the response to cardiac resynchronization therapy (CRT) affected by QRS morphology in patients enrolled in the MADIT-CRT trial?


The baseline electrocardiogram was analyzed in 1,817 patients (mean age 65 years) with cardiomyopathy, an ejection fraction (EF) ≤30%, New York Heart Association class I-II symptoms, and QRS duration ≥130 ms. Sixty percent were randomly assigned to receive an implantable cardioverter-defibrillator (ICD) with CRT (CRT-D), and the remainder received an ICD. The mean duration of follow-up was 29 months. The primary endpoint was a heart failure event or death.


Seventy percent of patients had a left bundle branch block (LBBB) and the remainder had a non-LBBB (right bundle branch block [RBBB] in 13% and nonspecific interventricular conduction delay [IVCD] in 17%). In the LBBB group, CRT-D therapy was associated with a 53% reduction in the primary endpoint compared to the ICD therapy. There was not a significant reduction in the primary endpoint with CRT-D in the non-LBBB group. CRT was associated with a greater reduction in left ventricular end-diastolic volume in patients with LBBB (23%) than with non-LBBB (16%), and a greater absolute improvement in EF (12% vs. 9%, respectively).


CRT-D therapy is associated with a significant reduction in the risk of a heart failure event or death in patients with an EF ≤30% and class I-II heart failure who have an LBBB, but not in those who have a non-LBBB QRS morphology.


The greater benefit of CRT in patients with LBBB most likely reflects the greater degree of left ventricular dyssynchrony associated with LBBB than with RBBB or nonspecific IVCD.

Clinical Topics: Arrhythmias and Clinical EP, Heart Failure and Cardiomyopathies, Implantable Devices, EP Basic Science, SCD/Ventricular Arrhythmias, Acute Heart Failure

Keywords: Follow-Up Studies, Heart Failure, Bundle-Branch Block, Electrocardiography, New York, Heart Ventricles, Defibrillators, Implantable, Cardiac Resynchronization Therapy

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