INTRODUCTION

Radiofrequency (RF) catheter ablation is the treatment of choice in patients with accessory pathway and Wolff-Parkinson-White (WPW) syndrome. Endocardial catheter ablation has limitations, including the inability to map and to ablate intramural or subepicardial accessory pathways. Some of these difficulties can be overcome using an epicardial approach, performed through the epicardial venous system or by percutaneous catheterization of the pericardial space.

When a suspected left inferior or inferoseptal accessory pathway is refractory to ablation or no early activation is found at the endocardium, a transvenous approach via the coronary sinus is warranted, because such epicardial pathways can be in close proximity with the coronary venous system. Associated congenital abnormalities like right atrial appendage–right ventricle diverticulum, coronary sinus diverticulum, and absence of coronary sinus ostium may also hamper a successful outcome.

Percutaneous epicardial subxiphoid approach should be considered when endocardial or transvenous mapping and ablation fails. Epicardial mapping may be successful; epicardial mapping can guide and enhance the effectiveness of endocardial ablation; and the finding of no epicardial early activation lead to a more persistent new endocardial attempt. When both endocardial and epicardial ablation are unsuccessful, open-chest surgery becomes the only option to eliminate the accessory pathways.

ACCESSORY PATHWAY ABLATION

RF catheter ablation is currently the treatment of choice in patients with accessory pathways and WPW syndrome and has been shown to have a success rate higher than 95%.¹

Accessory pathways are epicardial structures but usually with endocardial ventricular and atrial insertions, located close to the atrioventricular valve rings, making most endocardial catheter ablation procedures relatively straightforward and yielding a high success rate. However, this is not the case when the accessory pathway’s distal end is closer to the epicardial surface, or its atrial or ventricular insertion is located far from the atrioventricular groove; and a small subset of patients, however, will fail ablation procedures using a conventional endocardial approach.^2,3

ENDOCARDIAL ABLATION

Endocardial catheter ablation has limitations, including the inability to access intramural or epicardial portions of arrhythmia circuits. Epicardial accessory pathway location was pointed as the cause of 8% of prolonged and failed accessory pathway ablation attempts.⁴ Technological improvements, such as cooled-tip, larger-tip ablation catheters, steerable sheaths, contact-force technology, and different energy sources for tissue ablation, have not completely solved the problem and some arrhythmia substrates might not be accessible from the endocardium.⁵

Several other factors may contribute to RF ablation failure: difficulties with catheter manipulation, including inability to reach the appropriate accessory pathway site; catheter instability (particularly in right-sided accessory pathways) or inadequate tissue contact; inaccurate mapping, related to accessory pathway slanting and localization away from an endocardial positioned catheter, or in the setting of Ebstein’s anomaly; proximity of the accessory pathways to vital structures, such as a coronary artery or the atrioventricular node; or associated structural abnormalities, as congenital venous system anomalies, or acquired coronary system stenosis developed in consequence of previous unsuccessful ablation attempts.4

Some of these difficulties can be overcome during cardiac surgery (open-chest surgery or thoracoscopy), an epicardial approach performed through epicardial vessels of the coronary sinus system, or through percutaneous catheterization of the pericardial space, as described by Sosa et al.^5,6

INTRAVENOUS MAPPING AND ABLATION

Accessory pathways located in the posteroseptal and left posterior areas, in the so-called pyramidal space, may be difficult to ablate, because of the relative epicardial localization, the thickness of the myocardium, the anatomic complexity of this area, and also coexistence of coronary sinus (CS) diverticulum that contains a pouch and a neck.^7,8 The coronary sinus anatomy should be carefully assessed, either by venography or computer tomography, to rule out diverticulum, which may be present in 15–20% of refractory posteroseptal accessory pathways. Cooled-tip catheter ablation inside the coronary sinus venous system and middle cardiac vein is effective in most epicardial posteroseptal accessory pathways. However, one has to be aware that a fast conducting accessory pathway may become a decremental accessory pathway after an ablation attempt. In this instance, the ECG may change, lacking overt preexcitation during sinus rhythm. Its correct identification would be possible if a thorough programmed electrical stimulation would be carried out after the ablation attempt.^3,4,8,9

In 1992, Haïssaguerre et al. reported the effectiveness and safety of radiofrequency catheter ablation of left lateral accessory pathways via the mid or distal coronary sinus when endocardial approaches are unsuccessful.¹⁰ They had no significant complications, except a marked nonspecific pain during radiofrequency energy application.¹⁰ Langberg et al., in 1993, evaluated a group of patients with left-sided accessory pathways difficult to ablate from the endocardial surface. The absence of an accessory pathway potential during endocardial mapping in combination with a relatively large accessory pathway potential within the coronary sinus may be a useful marker of a subepicardial pathway, localized in the atrio-ventricular groove. In this select group of patients, radiofrequency application from within the coronary sinus appears to enhance ablation efficacy.¹¹

Morady et al. reported a series of difficult catheter ablation cases. In 3 patients who were initially thought to have right or left posteroseptal accessory pathways, it was found that the effective ablation site was up to 3 cm within the coronary sinus or within a posterior interventricular branch of the coronary sinus. In 2 patients thought to have a left free-wall accessory pathways, the accessory pathways site was mapped within the coronary sinus, in the region of the lateral mitral annulus. In each of these patients, accessory pathway potentials were absent or small in amplitude from the endocardium, but a relatively large potential was recorded within the coronary sinus.⁴

The coronary sinus has a myocardial coat with extensive connections to the left and right atria. An extension of this coat through the posterior coronary vein, the middle cardiac vein, or a diverticulum neck, can connect to the left ventricular epicardium and form epicardial posteroseptal and left posterior accessory pathways.^1,12 Coronary sinus accessory pathways (defined by earliest activation within the venous system) were identified in 36% of the patients with posteroseptal or left posterior accessory pathways in a study with a selected group of patients where most had failed previous attempts at ablation, and actual incidence of such pathways should be much smaller.¹² Usually the coronary sinus accessory pathways have an oblique course because of the oblique orientation of the fibers connecting the CS myocardial coat with the left atrium.¹³ Between these patients, coronary sinus angiography revealed a CS diverticulum in 21% and fusiform or bulbous enlargement of the small cardiac vein, middle cardiac vein, or coronary sinus in 9% of patients.¹² These venous anomalies mostly arise 1.5 cm away from the CS and before the middle cardiac vein, but they can originate from the middle or posterior cardiac veins as well.¹ Successful ablation of these pathways may be achieved while ablating in the diverticulum neck.^1,14

A precise knowledge of the coronary sinus anatomy and its potential abnormalities, like the presence of diverticulum or persistent left superior vena cava, as well as coronary sinus electrograms recordings, are essential for a successful RF catheter ablation in patients with a prior history of multiple ablation failures or in whom successful ablation cannot be achieved.¹ The presence of a negative delta wave in lead II is suggestive of an epicardial localization of the accessory pathways (identifying a coronary sinus accessory pathway), with a sensitivity of 70%.¹ Takahashi et al.¹⁵ reported that the combination of a steep positive delta wave in lead aVR and a deep S wave in lead V6 (R wave ≤ S wave) during maximal preexcitation had the highest specificity for identifying epicardial coronary vein posteroseptal accessory pathways while the highest sensitivity is provided by a negative delta wave in lead II.

Ablation in posteroseptal diverticula has lower success rates and is correlated with more procedural complications, due to the close proximity of the epicardial coronary arteries, risk of venous perforation, tamponade, venous occlusion, or heart block.1 Results can be improved by targeting the neck of the diverticulum, applying irrigated-tip catheters, using cryoablation, or performing the subxiphoid epicardial approach.¹⁶ Although RF ablation can be done safely inside the CS, cryoablation could be a safer alternative, especially if the best ablation location is in close proximity to a coronary artery, although a higher rate of recurrences have been reported.^9,17

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