7 In the Comparison of Medical Therapy, Pacing and Defibrillation in Heart Failure (COMPANION) trial, a survival benefit of CRT over pharmacologic therapy was observed in patients with LBBB but not in those with other QRS morphologies. Improvements in other HF parameters, such as 6-minute walk, peak oxygen consumption, and ejection fraction, did not differ significantly between the groups. 6 In a retrospective, pooled analysis from MIRACLE and CONTAK-CD that compared outcomes in 34 patients with RBBB who received CRT and 27 who received medical management, New York Heart Association (NYHA) functional class was the only clinical outcome that improved significantly more after active CRT. The impact of QRS morphology on HF outcomes has been examined in four randomized trials of CRT.1, 2, 4, 5 In a substudy of CRT-D recipients from the MIRACLE (Multicenter InSync Randomized Clinical Evaluation) ICD study, RBBB was associated with less improvement in peak oxygen consumption than other QRS morphologies in univariate, but not multivariate, analysis. Cardiac resynchronization therapy (CRT) has been shown to improve outcomes in trials in which most patients with drug-refractory heart failure (HF) had intrinsic left bundle branch block (LBBB).1, 2, 3, 4 However, the benefits of CRT have not been conclusively demonstrated in patients with other native QRS morphologies or those who have undergone right ventricular (RV) pacing in the setting of a preexisting pacemaker or defibrillator due to limited representation within randomized controlled trials and variable outcomes.
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