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A number of different approaches have been developed for catheter ablation of atrioventricular (AV) accessory pathways.1–6 The high success rate (> 90%) and low incidence of complication (< 5%) in catheter ablation have let this modality become first-line therapy for arrhythmia associated with accessory pathways (APs). In this chapter, we describe our current approach for mapping and ablation of accessory pathways.^2,7–9

Catheter Mapping of Accessory Pathways

We perform decremental atrial pacing from 2 widely separate sites, usually at the right atrial appendage (RAA) and posterolateral coronary sinus (CS), since one atrial pacing site far from the AP may not present clear preexcitation. A change in the morphology of the preexcited QRS or the ventricular activation sequence by changing the atrial pacing site indicates the presence of two or more APs.

The presence of retrograde AP conduction is indicated by a retrograde atrial activation sequence. However, the retrograde activation sequence for anteroseptal and posteroseptal APs is similar to the activation sequence during retrograde conduction over the fast and slow AV nodal pathways, respectively. The presence of more than one V-A connection is manifested by ventricular pacing at two widely separate sites close to the base. Confirmation of retrograde AP conduction can be obtained by parahisian ventricular pacing (with intermittent His bundle [HB] capture)^10,11 and by advancing atrial activation during AVRT using a late ventricular extrastimulus delivered close to the site of earliest atrial activation.

Our approach to localize APs for ablation is based on the possibilities that: (1) an AP often has an oblique course (atrial and ventricular insertions are located at different sites along the AV groove)^8,12; and (2) multiple pathways may be present. We map the annulus during antegrade AP conduction with atrial pacing as well as during retrograde AP conduction with ventricular pacing. We perform differential atrial pacing (atrial pacing at 2 separate sites close to the annulus on opposite sides of the site of earliest antegrade ventricular activation) and differential ventricular pacing (ventricular pacing at 2 separate sites close to the annulus on opposite sides of the site of earliest retrograde atrial activation) to identify an oblique course of a single AP and presence of more than one AP connection.

For a single AP with an oblique course, differential ventricular pacing produces two different local V-A intervals (≥ 15 ms difference) measured at the site of earliest retrograde atrial activation (Figure 8.1).^7,8 The ventricular pacing site that produces a ventricular wavefront propagating from the direction of the ventricular end of the AP (concurrent direction, Figure 8.1A) results in an artificially short local V-A interval at the site of earliest atrial activation (Figure 8.1C). Because ventricular activation and AP activation are propagating simultaneously in the same direction, the ventricular potential overlaps the AP activation potential (AP potential) and may mask the atrial potential (Figure 8.1, A, C, and E). In contrast, pacing from the opposite side (reversing the direction of the ventricular wavefront: countercurrent direction) produces the ventricular wavefront propagating away from the AP shortly after activation of its ventricular end (Figure 8.1B). This pacing results in an increase of the local V-A interval along the length of the AP, exposing the AP potential and the atrial activation sequence (Figure 8.1, D and F).^7,8,13,14

For an AP with an oblique course, differential atrial pacing produces two different local A-V intervals (≥ 15 ms difference), measured at the site of earliest ventricular activation (Figure 8.2). Atrial pacing from the direction of the atrial insertion (concurrent atrial wavefront) shortens the local A-V interval at the site of earliest ventricular activation and results in overlapping atrial and AP potentials, masking the AP potential and often masking the site of earliest ventricular activation (Figure 8.2, A and C). Pacing from the opposite side reverses the direction of the atrial wavefront (countercurrent direction), resulting in the atrial wavefront propagating away from the AP shortly after activating its atrial end (Figure 8.2B). The A-V interval all along the AP is increased, unmasking the AP potential and the ventricular activation sequence (Figure 8.2D).^7,8,13,14

During AP conduction, the ability to record an isolated AP potential indicates an oblique course of AP. The absence of an oblique course should produce fusion between the atrial, AP and ventricular potentials unless the atrial or ventricular insertion of the AP is located far from the annulus, such as in Ebstein’s anomaly.

Differential atrial and ventricular pacing was performed in 114 consecutive patients with a single AP (left free-wall AP in 65 patients, right free-wall AP in 22 patients, posteroseptal AP in 21 patients, and anteroseptal AP in 6 patients).⁸ Reversing the direction of ventricular or atrial activation increased the local V-A or local A-V interval by ≥ 15 ms in 99 (87%) of the 114 patients, indicating that the majority of APs have an oblique course. Separating the atrial and ventricular potentials by differential pacing exposed an AP potential in 102 of 114 (89%) patients. Ablation was attempted in 111 of the 114 patients, and an AP potential was recorded in 99 of these 111 patients. By targeting the AP potential in these 99 patients, AP conduction was eliminated with a median of only 1 radiofrequency (RF) application (range 1–11 applications). In contrast, a median of 4.5 RF applications (range 1–18 applications) was required in the 12 patients in whom an AP potential was not recorded, despite separating the atrial and ventricular potentials using differential pacing.⁸ These data strongly support the AP potential as the optimal target for ablation.

In the same study, 60 of the 111 patients had 1 to 3 prior failed ablation procedures. The number of RF applications required to eliminate AP conduction was not significantly greater for the 60 patients who had a prior failed ablation than the 51 patients undergoing their first ablation procedure (median 1 RF application for both groups).8 AP conduction was eliminated by only 1 to 2 RF applications in 41 of the 60 patients with prior failed ablation. In these 41 patients, differential pacing demonstrated an oblique course and exposed an AP potential for the ablation target. In most cases, there were low-amplitude fractionated atrial potentials located 2 to 10 mm beyond the atrial insertion of the AP (blue hatched box in Figure 8.1, A and B), suggesting the location of previous ablation sites. These findings suggest that an oblique AP course may lead to placing unsuccessful RF applications beyond the atrial insertion of the AP when using other approaches for localizing APs (i.e., the site of earliest retrograde atrial activation and the shortest local V-A interval).^15–18 During retrograde AP conduction, atrial activation propagates rapidly in the same direction as the AP due to the parallel atrial fiber orientation (Figure 8.1F). This rapid atrial propagation may cause bipolar intracardiac electrograms (EGMs) to identify “earliest” retrograde atrial activation over a wide range, extending beyond the atrial insertion of the AP (black arrow, “Earliest” Retro Atrial Activation in Figure 8.1, A and B). RF applications in the distal half of this region (blue hatched area in Figure 8.1, A and B) are likely to fail to eliminate AP conduction.^7,8,13,14

The shortest local V-A interval for selecting an ablation site may be misleading in the presence of an oblique course. With a concurrent ventricular wavefront, ventricular activation initially precedes the atrial activation near the atrial insertion of the AP (CS₂ EGM in Figure 8.1, A and C). However, the velocity of the atrial wavefront parallel to the annulus is greater than the velocity of the ventricular wavefront. The local V-A interval shortens progressively beyond the atrial insertion of the AP as the atrial wavefront catches the ventricular wavefront (CS_d EGM in Figure 8.1, A, C, and E). The shorter local V-A interval beyond the atrial insertion of the AP may explain the location of the fractionated atrial EGMs in the majority of patients with a prior failed ablation procedure.

For most APs along the mitral annulus (posteroseptal and left free-wall pathways), the direction from ventricular insertion to atrial insertion follows a counterclockwise orientation (as viewed in the left anterior oblique projection, Figure 8.3).^7,8,12 For many of the APs along the tricuspid annulus (especially anteroseptal pathways), the direction from ventricular insertion to atrial insertion follows a clockwise orientation (as viewed in the left anterior oblique projection, Figure 8.3). Therefore, for anteroseptal and right anterior paraseptal APs, the ventricular insertion is often located toward the right free-wall, allowing ablation away from the septum to reduce the risk of AV block.

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