Physiologic Pacing: More Answers, More Questions

Right ventricular apical (RVA) pacing is deleterious and may result in left ventricular dysfunction, heart failure, and increased mortality.1-5 Pacing from the RVA results in a left bundle branch block morphology, due to depolarization of the right ventricle prior to the left ventricle and from the apex to the base.6,7 It has been suggested that pacing the heart in a manner closer to the normal physiologic pattern of activation may reduce the incidence of these negative clinical outcomes.8,9 The determination of the best site(s) to pace has been a question that remains unanswered. Before we abandon right ventricular pacing altogether, it seems worthwhile to consider if there are other RV sites that should be evaluated. One of the potential sites that can be paced and results in near-normal depolarization of the heart is the septal aspect of the right ventricular outflow tract (RVOT).10-12 Septal RVOT pacing results in a reduced QRS duration when compared with RVA pacing.13-17 This translates into a shorter ventricular activation time and possibly less ventricular dyssynchrony. Furthermore, whereas RVA pacing has been shown to result in myofibrillar disarray and detrimental ultrastructural changes, septal pacing in canine hearts does not.18 All of these findings suggest a potentially less harmful role of RVOT pacing than RVA pacing. However, in order to draw any clinical conclusions, a comparison between different pacing sites is needed. In this issue of the Journal, Muto et al. report the results of the Effect of Pacing the Right Ventricular Mid-Septum tract in Patients with Permanent Atrial Fibrillation and Low Ejection Fraction study.19 This retrospective analysis of singlechamber right ventricular mid-septal (RVMS) pacing compared with RVA pacing in patients with an ejection fraction (EF) of less than or equal to 30% and permanent atrial fibrillation (AF) is the tipping point and aids in our understanding of “physiologic pacing.” In this study, patients with pacing of the RVMS experienced a significant improvement in NYHA class, EF, and quality of life measured at 18 months follow-up when compared with patients with RVA pacing. This study’s patient population, with over 100 subjects in each arm followed for 1.5 years, is an improvement from prior studies that have attempted to address this question