Figure 3.

Schematic Demonstrating the Concept of Longitudinal Fascicular Dissociation. Histological studies have demonstrated that the His-Purkinje (HP) system is composed of multiple fibers with few transverse connections between fibers due to insulating collagen that runs along the longitudinal axis. A significant proportion of “infra-Hisian” block is in fact “intra-Hisian”. This explains how patients with conduction system disease (bundle branch block) can experience normalization of conduction when pacing at a more distal location within the HP system. HB = His bundle; RB = right bundle; LAF = left anterior fascicle; LPF = left posterior fascicle.