Physiological mechanisms of QRS narrowing in bundle branch block patients undergoing permanent His bundle pacing

• In this review article, we discuss known and putative mechanisms of conduction block within the His bundle, resulting in bundle branch block;
• Discuss physiological mechanisms by which His bundle pacing narrows or normalizes the QRS interval;
• Discuss the differences and physiological bases for direct vs. indirect His bundle capture;
• Discuss the physiological mechanisms for QRS narrowing vs. normalization following His bundle pacing.

His bundle pacing is increasingly used to avoid chronic right ventricular pacing, and electrically resynchronize ventricular activation by narrowing or normalizing the QRS interval in left and right bundle branch block. The mechanisms by which this occurs remain poorly understood. In this review, the proposed mechanisms and evidence supporting them are discussed. Also discussed are aspects of mechanisms that are not completely supported by the evidence. We also review the differences and physiological bases for direct vs. indirect His bundle capture, and the physiological mechanisms for QRS narrowing vs. normalization following His bundle pacing.