His–Purkinje system longitudinal dissociation: From bench to bedside. A case of output-dependent fascicular capture

Introduction: Histological studies reported that the His bundle (HB) is partitioned into narrow cords by collagen running in its long axis, providing the anatomical setting necessary for its longitudinal dissociation. Further confirmations came from the demonstration that direct HB pacing normalizes the QRS axis and duration in subjects with proximal HB lesions causing bundle branch block. However, there is no evidence of the possibility of selective HB partitions pacing destined to the composition of branches and fascicles. Methods and Results: We describe a case of intra-Hisian left bundle branch block in which permanent distal HB pacing corrects left ventricular delay and produces different QRS morphology at different voltage outputs, as an expression of different selective HB compartments recruitment. Conclusion: This case would strengthen the limited data in the literature about HB longitudinal dissociation.