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Right-Sided Accessory Pathways

Anne M. Dubin, MD

Introduction

Right-sided accessory pathways (APs) account for a minority of accessory connections but can be some of the most difficult pathways to eliminate, with ablation success rates usually reported in the 90% range.¹ Catheter stability placement is often cited as a major issue in right free-wall pathways and can be quite challenging. Posteroseptal pathways can be difficult to eliminate, as the pathway can straddle the septum and/or involve the CS. Anteroseptal and midseptal pathways have a higher incidence of complete heart block associated with ablation.² Decremental pathways are more commonly found on the right side of the heart and can add an additional layer of complexity. This chapter will address each of these issues and potential techniques used when encountering the right-sided pathway.

Classifications

Right-sided pathways can be classified according to position as well as by pathway characteristics (decremental vs. nondecremental conduction). Anatomically, APs are divided into free-wall, anteroseptal, midseptal, and posteroseptal locations (Figure 9.1). The distribution of these pathways is not equal, with free-wall and posteroseptal being the most common and midseptal the least.¹

Most right-sided pathways exhibit nondecremental conduction and have a discrete conduction velocity and refractory period. The majority of right-sided APs can conduct in both directions, while right free-wall pathways are more likely than septal pathways to conduct only in the antegrade direction.³

Some right-sided pathways can exhibit decremental conduction (conduction that slows as the frequency of excitation increases). These pathways often have unidirectional conduction. As both limbs of tachycardia (the atrioventricular node [AVN] and AP) can have varying conduction velocities, tachycardia can often be frequent and incessant in these situations. These pathways tend to be sensitive to adenosine, which can complicate diagnosis. Many of these connections may not be atrioventricular but can also be atriofaciscular, nodofascicular, or even fascicularventricular (Figure 9.2).

General Considerations

Right-sided APs can be obliquely oriented, branched, or have multiple insertion points.⁴ Thus, it is important to use both antegrade and retrograde mapping when faced with a right-sided pathway. Oblique pathways may require mapping from a retrograde approach when attempted from the atrial side and an antegrade approach when attempted from the ventricular side. Branching may require multiple ablation lesions at different sites. It is important to perform a full EP study before and after ablation to ensure that no additional pathways remain.

Right Free-Wall Pathways

Catheter ablation of right free-wall pathways is the least successful, with a success rate averaging only about 90%.1 The difficulty with these may be attributed to the tricuspid valve anatomy as opposed to the mitral valve (MV) anatomy. The MV tends to be smaller, and the mitral–aortic fibrous continuity is an area that is devoid of AP connections. Right-sided pathways may be several millimeters from the fibrous annulus and have been associated with a higher prevalence of branching pathways. Finally, the right AV groove is not as easily seen on fluoroscopy, as the coronary sinus (CS) typically outlines the left AV groove. Several possible approaches and techniques have been advocated to try and improve success rates with these pathways.

Multiple approaches have been advocated when faced with a right free-wall pathway. In general, a typical inferior vena cava (IVC) approach to the atrial side of the tricuspid valve is useful when faced with a right posterior or posterolateral pathway. However, if the pathway is anterior or anteroseptal, many authors have found a superior vena cava (SVC) approach more useful, as the catheter can be more easily stabilized (Figure 9.3).⁵ Right lateral and anterolateral pathways are often the most difficult pathways to ablate and tend to have the highest recurrence rates.⁶ There are several possible reasons for this, but it is often difficult to achieve catheter stability and adequate temperatures in this location.

One important way to stabilize and improve success in right-sided pathways is to use a long sheath (Figure 9.4). These sheaths (Daig, Minnetonka, MN; Agilis, St. Jude Medical, St. Paul, MN) come in a variety of preformed or steerable curves, which relate to areas on the tricuspid valve annulus. The reduced-radius series allows these sheaths to be used in smaller children as well. The sheath can provide catheter stability and improve torque transmission to the tip. These sheaths were judged to contribute directly to success in 43% of pathways in one pediatric study.⁷

One of the major issues with catheter ablation along the right free wall is difficulties with the tricuspid valve anatomy. The right coronary artery (RCA) runs along the ventricular aspect of the right epicardial AV groove, and RCA mapping has been reported as an aid to precise mapping and ablation of right free-wall pathways. This technique has been shown to be extremely helpful in pediatric ablation, resulting in 100% success in the two studies in which it has been used.^8,9 The RCA was visualized using angiography. A 6-F guiding catheter was advanced to the RCA where the os was engaged. A 2.3- to 2.5-Fr quadripolar or octapolar microcatheter (Pathfinder, Cardima, Fresno, CA) was then advanced into the RCA to follow the tricuspid valve annulus (Figure 9.5). This allowed for earliest activation either retrograde or antegrade and easy recognition of the AV annulus. No complications were reported in either study. Follow-up coronary artery angiography demonstrated no abnormalities in any of these patients. Unfortunately, at the time of writing this chapter, these wires are no longer available.

Another method for improved mapping of the tricuspid valve uses a 20-pole HALO catheter (Biosense Webster, Diamond Bar, CA). This catheter is commonly used to map the tricuspid valve annulus to guide ablation of the cavotricuspid isthmus (CTI) for AFL; however, it has also been useful in right-sided APs. Use of this catheter has allowed for successful ablation in 100% of patients following a prior failed ablation in one study.10

Anteroseptal Pathways

Pathways are classified as anteroseptal if an AP potential as well as a His bundle (HB) potential is simultaneously recorded from a catheter placed at the HB region. These pathways tend to run anteriorly along the central fibrous body, along the right anterior free wall. Ablation of these pathways can be difficult, as there is a risk of AV block. Optimally, the ablation catheter should be placed where only a tiny HB potential is seen. This can be accomplished with an approach from the right internal jugular (the superior approach as mentioned in Figure 9.3). The catheter is usually advanced into the right ventricle and then curved back to ablate the pathways from the ventricular aspect of the tricuspid annulus. Some authors feel that this superior approach allows for a more stable position and suggest that higher success rates can be found using this method.

Anteroseptal pathways may require an approach from the noncoronary cusp of the aortic valve.¹¹ The aortic valve is quite central and is anatomically related to both the atria and the ventricles. The noncoronary cusp lies directly adjacent to the atrial septum. Electrically active myocardial sleeves, which can constitute an AP, have been described that cross the aortic valve plane and extend into the cusp.¹² Thus, APs can extend into the cusp itself. Several subtle differences in the surface ECG preexcitation pattern have been suggested to help differentiate AP found in the noncoronary cusp from a conventional right anteroseptal pathway.¹¹ Typically, a right anteroseptal pathway can be characterized by an isoelectric or negative delta wave in lead V₁ with transition to a positive delta wave in lead V₂ and a strongly positive delta wave in the inferior surface leads (II, II, aVF). In patients in whom a noncoronary cusp location was found, a small positive delta wave already was seen in lead V₁ and an isoelectric delta wave in lead III (Figure 9.6).¹¹

Midseptal Pathways

An important technique, which can be crucial when trying to differentiate between retrograde conduction across a septal AP and the AV node, is parahisian pacing.13 This technique consists of pacing the RV close to the HB. When pacing output is increased, it is often possible to capture the HB and directly stimulate the HPS; this should result in a relatively narrow QRS complex. As the output is decreased, the insulated His is no longer excited, and the local ventricle is stimulated instead. This results in a wider QRS pattern. The presence or absence of a change in the atrial activation sequence and VA timing identifies whether retrograde conduction uses an AP or the AV node (or both) (Figure 9.7).

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