Ablation of Accessory Pathways

CLASSIC ATRIO-VENTRICULAR ACCESSORY PATHWAYS (BUNDLE OF KENT)

The classic atrio-ventricular (AV) AP (bundle of Kent) is a muscle fiber bridging the AV groove and inserting into the base of the ventricle near the tricuspid or mitral annulus. Access to the mitral annulus and a left-sided AP is achieved either by a transeptal (patent foramen ovale, transeptal puncture) or transaortic approach. With the transaortic approach, the tip of the ablation catheter is curved into a “pigtail” to avoid damaging the coronary arteries, prolapsed retrogradely across the aortic valve into the left ventricle, and positioned along the mitral annulus using posterior and counterclockwise torque. Mapping of a left-sided AP is facilitated by a recording catheter in the coronary sinus (CS) that provides a useful reference landmark and whose electrodes can bracket the AP in the left AV groove. While an equivalent venous structure is absent for the tricuspid annulus, a multipolar “Halo” catheter positioned along the endocardial aspect of the tricuspid annulus can guide ablation of a right-sided AP. The tricuspid annulus and a right-sided AP can be accessed either from the inferior vena cava (IVC) (inferior approach) or from the superior vena cava (SVC) (superior approach).

MAPPING

Antegrade

The earliest site of ventricular activation during manifest preexcitation (preexcited sinus rhythm, antidromic reciprocating tachycardia [ART]) identifies the ventricular insertion site of the AP. Target site criteria for AP ablation during antegrade mapping are 1) AP potentials (Kent potentials) and 2) earliest site of ventricular activation relative to delta wave onset (pre-delta) (Figs. 12-1, 12-2, 12-3, 12-4, 12-5, 12-6, 12-7, 12-8 and 12-9).¹,² AP potentials reflect rapid local AP activation and are sharp, high-frequency deflections sandwiched between atrial and ventricular electrograms preceding delta wave onset during preexcited sinus rhythm and atrial activation during orthodromic reciprocating tachycardia (ORT) (Fig. 12-10).¹,²,³,⁴ They can be recorded along the length of the AP and are differentiated from high-frequency components of the atrial or ventricular electrogram by demonstrating its dissociation from the atrium and ventricle, respectively (Kent validation) (see Fig. 10-20). The earlier the local ventricular electrogram precedes delta wave onset (pre-delta), the higher the probability of success, and a pre-delta value >10 ms has a 50% probability of successful ablation.² Annular (atrial and ventricular) electrograms are often fused and sometimes difficult to separate individually. Atrial or ventricular pacing maneuvers (rapid pacing/extrastimulation) to cause AV or VA block, respectively (“pacing to block”), or pacing from different sites to reverse the wavefront of activation, help to identify the atrial and ventricular components of the map electrogram.³,⁵ AV fusion as a target site criteria can be misleading because of a slanting (oblique) AP. AV fusion occurs at sites downstream to the AP when the activation wavefront is in the direction of the slant (concurrent).³ Conversely, fusion is absent at the AP (site recording an AP potential) when the activation wavefront is opposite the direction of the slant (countercurrent).

Retrograde

The earliest site of atrial activation during retrograde conduction over the AP (ventricular pacing, ORT) identifies the atrial insertion site of the AP. One limitation of mapping during ventricular pacing is rapid fast pathway (FP) conduction preempting AP conduction. This is particularly difficult for anteroseptal AP located near the His bundle. Potential solutions include 1) pacing at a fast rate (to cause delay/block in the FP), 2) pacing from a parahisian location with right ventricle (RV)-only capture (to delay conduction over the FP), 3) administration of negatively dromotropic

medications (to slow FP conduction), and 4) mapping during ORT (where retrograde conduction occurs exclusively over the AP). Target site criteria for AP ablation during retrograde mapping are 1) AP potentials, 2) earliest site of atrial activation during AP conduction, and 3) site terminating ORT by an extrastimulus with nonglobal capture (Figs. 12-6 and 12-10 to 12-20).¹,²,³,⁴,⁵,⁶,⁷ When mapping retrogradely, analyzing electrograms during sinus rhythm or “pacing to block” helps define their atrial and ventricular components. VA fusion can be misleading for slanting APs and be absent for slow-conducting APs.

FIGURE 12-1 Ablation of a left free wall AP. The earliest site of atrial activation during retrograde AP conduction is at the lateral mitral annulus (white), which also records the earliest site of ventricular activation during preexcited sinus rhythm and CS pacing (pre-delta × 30 ms). Application of RF energy causes loss of preexcitation in 3.8 sec (black arrow). White arrowheads outline the CS during contrast angiography.

FIGURE 12-2 Ablation of a left posterolateral AP. The earliest site of atrial activation during retrograde AP conduction is at the posterolateral mitral annulus (red), which records the earliest site of ventricular activation during manifest preexcitation (pre-delta × 28 ms). Application of RF energy causes loss of preexcitation in 3.0 sec (arrow).

FIGURE 12-3 Ablation of a left posterolateral AP. The earliest site of atrial activation during retrograde AP conduction is at the posterolateral mitral annulus (white), which also records the earliest site of ventricular activation during manifest preexcitation (pre-delta × 21 ms). Application of RF energy causes loss of preexcitation in 7.7 sec (arrow).

FIGURE 12-4 Ablation of a left posteroseptal AP. The earliest site of atrial activation during retrograde AP conduction is at the posteroseptal mitral annulus (red), which records a tiny AP potential (vertical down arrowheads) between atrial and ventricular electrograms during manifest preexcitation. Application of RF energy causes loss of preexcitation after 1 beat (arrow).

FIGURE 12-5 Ablation of a left posterior AP. The earliest site of atrial activation during retrograde AP conduction is at the posterior mitral annulus (white), which also records the earliest site of ventricular activation (arrowheads) during manifest preexcitation (pre-delta × 32 ms). Application of RF energy causes loss of preexcitation in 5.6 sec (arrow).

FIGURE 12-6 Ablation of a right posterior AP. The earliest site of atrial activation during retrograde AP conduction is at the inferior tricuspid annulus (red), which also records the earliest site of ventricular activation (arrowhead) during preexcited sinus rhythm (pre-delta × 20 ms). Application of RF energy causes immediate loss of preexcitation (arrow). Note that the ablation catheter is on the eustachian ridge.

FIGURE 12-7 Ablation of a right anterior AP. The earliest site of atrial activation during retrograde AP conduction is at the anterior tricuspid annulus (white), which records continuous electric activity (arrows) between atrial and ventricular electrograms during manifest preexcitation. Application of RF energy causes loss of preexcitation in 5.3 sec (arrowhead).

FIGURE 12-8 Ablation of a right anterolateral AP. The earliest site of atrial activation during retrograde AP conduction is at the anterolateral tricuspid annulus (white), which records continuous electric activity (arrows) between atrial and ventricular electrograms during manifest preexcitation. Application of RF energy causes loss of preexcitation after 1 beat.

FIGURE 12-9 Ablation of a right posterolateral AP. The earliest site of ventricular activation during manifest preexcitation is at the posterolateral tricuspid annulus (white), where the local ventricular electrogram precedes delta wave onset by 26 ms. Application of RF energy causes loss of preexcitation in 6.9 sec.

FIGURE 12-10 AP potentials during manifest preexcitation (top) and ORT (bottom). Top: The ablation catheter is positioned at the posterior mitral annulus where it records a sharp AP potential between atrial and ventricular electrograms during CS pacing. Application of RF causes loss of preexcitation (arrow). Bottom: The ablation catheter is positioned at the posterior mitral annulus where a sharp AP potential is recorded between ventricular and atrial electrograms during ORT. Application of RF energy causes loss of preexcitation in 5.9 sec (arrow).

FIGURE 12-11 Ablation of a concealed left posteroseptal AP. The earliest site of atrial activation during ORT is at the posteroseptal mitral annulus (red). Application of RF energy causes termination of tachycardia with block in the AP (arrow). Note the proximity between the ablation catheter along the endocardial mitral annulus to the epicardial CS on intracardiac echocardiography (ICE).

FIGURE 12-12 Ablation of a concealed left free wall AP. The earliest site of atrial activation during ORT is at the lateral mitral annulus (white). Application of RF energy terminates ORT with block in the AP (arrow). Note the increase in tissue echogenicity seen on intracardiac echocardiography (ICE) during RF delivery.

FIGURE 12-13 Ablation of a concealed left free wall AP. The earliest site of atrial activation during ORT is at the lateral mitral annulus (white). Application of RF energy during RV pacing causes retrograde block in the AP (asterisk) in 4.2 sec. Note the increase in tissue echogenicity seen on intracardiac echocardiography (ICE) during RF delivery.

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