Conceptual overview of electrical activation of the cardiac His-Purkinje fibers

Conceptual overview of electrical activation of the cardiac His-Purkinje fibers for (A) normal conduction, (B) with left bundle branch block, (C) with biventricular pacing, and (D) with fusion pacing. In all 4 panels, the length of the x-axis is the biventricular activation time reflected by the width of the QRS complex recorded on a surface electrocardiogram.

(A) With normal conduction, biventricular activation is rapid (ie, 90 ms) when impulses propagate antegrade over intact right and left bundles.

(B) In a patient with complete left bundle branch block, electrical activation is blocked antegrade due to the underlying disease state and proceeds more slowly following transseptal activation via the right bundle occurring via cell-to-cell connections. The total biventricular activation time of 170 ms is a typical intrinsic QRS duration for a patient with complete left bundle branch block.

(C) The grey thunderbolts depict left and right ventricular pacing stimulation resulting in electrical activation largely along myocardial cell-to-cell connections that are typically slower than intact His-Purkinje fibers. The total activation time of 140 ms is a typical paced QRS duration for a patient with biventricular pacing.

(D) The concept of fusion pacing is that intrinsic electrical activation proceeds antegrade over an intact right bundle and collides with electrical activation resultant from the left ventricular pacing stimulus depicted by the grey thunderbolt. The total activation time of 110 ms is afforded by rapid physiologic conduction over the native right bundle. The white thunderbolt depicts the optional delivery of right ventricular pacing stimulation that is delayed and offset from the left ventricular stimulus according to vendor-specific algorithms. As an example, such pacing stimulation could be applied in scenarios when the native His-Purkinje disease is more complicated than an isolated left bundle branch block.