Unusual variants of pre‐excitation: From anatomy to ablation: Part I—Understanding the anatomy of the variants of ventricular pre‐excitation

Abstract

The famous quotation of Winston Churchill, made in his radio broadcast of 1939 regarding Russia's next move, specifically “A riddle wrapped up in a mystery, inside an enigma,” perfectly fits the current understanding of unusual accessory atrioventricular pathways, including the variants producing ventricular pre‐excitation. It was many decades after their original descriptions that we came better to begin to understand most of their structure‐function relationships. Their mysterious pathophysiology was sometimes unveiled after invasive treatments, such as surgical ablation of the atrioventricular conduction axis instead of the accessory pathway itself. Speculations made on this basis have largely been validated by subsequent clinical experience. Most of the names suggested for description of the pathways have stood well the test of time. For some of them, however, this is not the case, with the initial names becoming confusing. In a series of reviews, we re‐visit those accessory pathways producing ventricular pre‐excitation other than classical Wolff‐Parkinson‐White syndrome. To set the scene, in this initial review, we describe the development and anatomy of the normal atrioventricular conduction axis, along with the insulating tissues of the atrioventricular junctions. We have sought to illustrate our explanations by using virtual dissection of computerized tomographic datasets, since they retain the intact heart within the setting of the body. These images illustrate well the value of attitudinally appropriate terminology. Thereafter, we discuss the electrophysiological manifestations of the abnormal anatomical pathways which provide the potential for both accessory atrioventricular and intraventricular conduction.

Incessant narrow-QRS tachycardia mediated by ventriculo-fascicular accessory pathway

“…Fibers connecting the AV bundle with the ventricular septal crest (namely fasciculo-ventricular AP) have been described in anatomical studies, and they usually account for a distinctive form of ventricular pre-excitation. 1 , 8 Conversely, the active involvement of a VF AP in reentrant tachycardias, as demonstrated in our case, has been rarely reported so far. 7 Moreover, the electrophysiological properties of these APs are not well known 1 , 8 ; slow conduction seems relatively intuitive (prolonged VH interval during reentrant tachycardia), but the coexistence of decremental conduction is less clear.…”

“… 1 , 8 Conversely, the active involvement of a VF AP in reentrant tachycardias, as demonstrated in our case, has been rarely reported so far. 7 Moreover, the electrophysiological properties of these APs are not well known 1 , 8 ; slow conduction seems relatively intuitive (prolonged VH interval during reentrant tachycardia), but the coexistence of decremental conduction is less clear. The increasing pattern of tachycardia reset curve obtained in our case strongly suggests decremental conduction along the bypass tract ( Figure 2D ).…”

“…Nonetheless, whether this behavior is related to unique structural features of the AP (ie, nodal-like cell types, asymmetrical architecture with nonuniform anisotropic conduction properties) or to functional changes in refractoriness owing to rate is unknown. 1 , 8 Finally, catheter ablation of VF AP is challenging, since the exact localization of the proximal insertion of the bypass tract cannot be established according to reliable electrophysiological markers, especially when the retrograde atrial activation is missing. Hence, the correct diagnosis of the AP type through careful HB activation mapping and pacing maneuvers is key to establishing the proper target for catheter ablation.…”

Variants of accessory pathways

“…These tracts are thought to be extremely rare and often not discovered until aborted sudden cardiac death. A recent publication questioned the existence of these pathways …”

Comparison of electrophysiological characteristics of right‐ and left‐sided Mahaim‐type accessory pathways

“…The atria are electrically isolated from the ventricles and the conduction system develops in the early stages of embryological development. The myocardium of the atrioventricular (AV) canal surrounds both the right and left atrioventricular junctions, but the so‐called “primary ring” that forms the AV node and the atrioventricular conduction axis contributes only to the tricuspid annulus 1–3 . This embryological development process explains why atypical accessory pathways (APs), which act as accessory AV nodes, are almost always located around the tricuspid annulus.…”