Ablation of an Epicardial Accessory Pathway: Review of Posteroseptal Accessory Pathways

Introduction

A 24-year-old-male presented to the ER with palpitations and lightheadedness while playing soccer. A 12-lead electrocardiogram (Figure 1) revealed a short RP supraventricular tachycardia (SVT) that terminated with 12 mg of IV adenosine. 

His baseline ECG showed cardiac pre-excitation, Wolff-Parkinson-White (WPW) syndrome, with a negative delta wave in inferior leads and positive in V1 (Figure 2). He had undergone a failed catheter ablation at the age of 12 at another hospital after which he was started on a beta-blocker. Since then, he experienced few arrhythmic episodes that terminated with Valsalva maneuvers until just recently. After the risks and benefits of electrophysiology study and catheter ablation were discussed, he opted for having another ablation procedure.

ELECTROPHYSIOLOGY STUDY AND ABLATION

The following catheters were placed in the heart under mild sedation: three quadripolar catheters were placed in the atrium, AV junction and right ventricle. A decapolar catheter (Polaris XT, Boston Scientific) was placed in the coronary sinus.

Baseline intracardiac intervals were as follows: AH = 64 ms; HV = 16 (Figure 3A). Parahisian pacing demonstrated there was retrograde conduction through a septal accessory pathway (AP). SVT was induced with atrial pacing (S1 = 600 ms; S2 250), coinciding with the anterograde effective refractory period (ERP) of the AP (Figure 3B). The differential diagnosis of short RP tachycardia with VA interval >70 ms includes AVNRT, AVRT and atrial tachycardia.

VAHV response during entrainment from the right ventricular apex (RVA) excluded a septal atrial tachycardia. A post-pacing interval following entrainment of 62 (<115 ms) and the stimuli to A minus the VA interval = 50 ms (<85 ms) favored a pathway-mediated tachycardia (Figure 4). A premature ventricular contraction at the time of His being refractory advanced the tachycardia with the same atrial activation as during the tachycardia (Figure 5) (i.e. an accessory pathway was present and likely participating in this tachycardia). Prolongation of the VA interval during functional right bundle branch block is proof that a pathway is present and participating in this tachycardia.

After confirmation of an orthodromic SVT using a posteroseptal AP, mapping of the atrial insertion site was performed with a 4 mm tip ablation catheter (St. Jude Medical). The right and left posterior septum were mapped carefully, but a good site could not be found. By placing the ablation catheter within the CS, an earlier site was found where the atrial electrogram preceded the earliest atrial activation recorded by the proximal CS electrodes by 35 ms, and a possible pathway potential was found (Figure 6).

Radiofrequency energy at this site terminated the tachycardia within five seconds by eliminating retrograde conduction through the AP. Also note there was absence of cardiac pre-excitation on the first beat after termination of tachycardia (i.e. anterograde conduction was also abolished). His prior ablation likely failed because of failure of mapping within the cardiac venous system.

ANATOMY OF THE SEPTAL REGION: THE PYRAMIDAL SPACE 

In order to successfully ablate septal APs, it is imperative to understand the anatomy of this complicated region. There are three areas within the heart considered to be part of the septum (i.e. separates the left from the right heart chambers): the interatrial and interventricular septum that separates the respective paired heart chambers, and the atrioventricular (AV) septum interposed between the right atrium and the left ventricle.

Because APs connect the atrium with the ventricle at the atrioventricular level, the AV septum is of great interest for the electrophysiologist. Based on surgical experience, this region has been divided into anterior, mid and posterior septum, although now it is clear that only the mid septum is in fact a truly septal structure.¹ Absence of the AV septum is the key feature of “AV canal malformations.”

A catheter recording of His bundle potential and a catheter placed in the coronary sinus routinely used in EP procedures provide the anatomical landmarks for the AV septum (Figure 7).

The antero-septum, above the His catheter on fluoroscopy, corresponds anatomically to the area between the supraventricular crest and the membranous septum, and thus is truly off the septum. A large His potential is usually recorded at this site. APs located in this region should be referred to as superoparaseptal.² The mid septum is a true septal space between the His bundle and CS ostium, and corresponds anatomically to the triangle of Koch. Ablation at this site is associated with the highest risk of AV block. Below or at the CS ostium is the postero-septum area. It is part of the parietal structure that lies between the cavities of the heart and the pericardium. This is why Cosio et al renamed this area “inferoparaseptal.”² It has a conical shape, known as the “pyramidal space” whose vertex is formed by the central fibrous body, posteriorly is defined by the crux of the heart where the atrial and ventricular chambers diverge and superiorly is bound by the coronary sinus (Figure 8). Posteroseptal APs can be ablated from the right and left atrium — right and left posteroseptal — but also through the CS and middle cardiac vein, i.e. epicardially as it was described in this case report. A diverticulum of the CS has been described in 30% of patients with PS pathways, and its neck is usually the location of the successful ablation site when present.³ There was no CS diverticulum in this patient.

Valderrábano et al⁴ reported two patients with posterior APs that required ablation via percutaneous approach of the pericardium.

ELECTROCARDIOGRAPHY AND EP FEATURES OF POSTEROSEPTAL ACCESSORY PATHWAYS 

Up to 25% of APs are located in the pyramidal space. A negative delta wave in leads III and V1 with R/S ratio in lead V2 is characteristic of a PS location.⁵ Arruda showed that 50% of PS pathways with a negative delta wave in lead II were successfully ablated within the cardiac venous system.⁶ An ECG clue to a left PS origin is a positive delta wave in V1 and a leftward axis of greater than -60. PS pathways had shorter anterograde ERP and higher incidence of induced atrial fibrillation.⁷

COMPLICATION FROM ABLATION OF THE POSTEROSEPTAL PATHWAYS 

Ablation of septal pathways has been associated with longer procedures, an increase in radiation exposure, and more complications than pathways located in other areas.⁸ Acute injury of the coronary arteries⁹ is a potential and dreadful complication when ablating within the coronary venous system. A right coronary angiography should be done in these cases. Chronic occlusion has also been described in the literature.¹⁰ Ablation can be safely delivered when there is a distance of at least 5 mm from the coronary artery.¹¹ When it is extremely close, cryoenergy can be used instead of RF. Cryoablation, although safer and with good acute success rates, can also be associated with higher recurrences.¹²

WHEN TO THINK ABOUT EPICARDIAL ACCESSORY PATHWAYS

Epicardial posteroseptal APs should be considered when an early site is not found during endocardial mapping of an AP, a negative delta wave is present in lead II, and there is a history of prior failed ablation.

Disclosures: Dr. Navarrete reports consultancy with St. Jude Medical. Dr. Iqtidar has no disclosures to report.  

References

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