QRS Duration or QRS Morphology: What Really Matters in Cardiac Resynchronization Therapy?

The beneficial effects of cardiac resynchronization therapy (CRT) have been well established in large, randomized trials. Despite the documented success of this treatment strategy, a significant proportion of patients with heart failure do not achieve the desired response. The aim of this review was to delineate factors contributing to a successful CRT response, emphasizing the interrelated roles of QRS morphology and QRS interval duration. More data are available on QRS duration, as this factor has been used as an enrollment criterion in clinical trials. Response to CRT seems to increase as the QRS duration becomes longer, with greatest benefit in QRS duration ≥150 ms. Recent data have placed more emphasis on QRS morphology, demonstrating variability in clinical response between patients with left bundle branch block, non-left bundle branch block, and right bundle branch block morphology. Notably, myocardial scarring and cardiac dimensions, among other variables, may alter heterogeneity in ventricular activation. Understanding the electrophysiological underpinnings of the QRS complex has become important not only to predict response but also to facilitate the patient-specific delivery of resynchronization therapy.