Letter by Jackson et al Regarding, "PR Interval Identifies Clinical Response in Patients With Non-Left Bundle Branch Block: A Multicenter Automatic Defibrillator Implantation Trial-Cardiac Resynchronization Therapy Substudy" by Kutyifa et al.

Letter by Jackson et al Regarding, “PR Interval Identifies Clinical Response in Patients With Non-Left Bundle Branch Block: A Multicenter Automatic Defibrillator Implantation Trial-Cardiac Resynchronization Therapy Substudy” by Kutyifa et al Kutyifa et al present an interesting post hoc analysis of the Multicenter Automatic Defibrillator Implantation Trial-cardiac resynchronization therapy database where they determined that patients who had non-left bundle branch block (non-LBBB) pattern only benefitted from cardiac resynchronization therapy (CRT) if they had a prolonged PR interval at baseline. The authors postulate that this benefit may relate to improved atrio-ventricular timing and cited a small case series where patients with dilated cardiomyopathy may have benefitted from a shortened atrio-ventricular delay. Given that improved atrio-ventricular timing has not been shown to be critical for CRT response in patients with an LBBB pattern, it is not clear why improved atrio-ventricular synchrony alone would have such a profound effect on CRT response for non-LBBB patients. The majority of evidence for CRT response comes from patients with left ventricular dyssynchrony because of LBBB. Therefore, it seems equally possible that the benefit derived from CRT by non-LBBB patients with a prolonged PR interval in this study was because of the presence of masked LBBB. A mean PR interval of 254 ms (range, 230–360 ms) in these patients, would allow room for significant conduction delay in the left bundle branches to still be masked by more significant conduction delay in the right bundle. The significantly increased left ventricular end diastolic volume indices in this subgroup of patients (mean, 115 mL/m) would also be consistent with delayed left ventricular basal and lateral activation. Confirming the presence of a masked LBBB in patients before CRT implantation would require a more invasive approach to map left ventricular activation with an electrophysiology study. This approach may, however, increase our understanding of CRT response in patients with non-LBBB QRS patterns.