Unusual Atrioventricular Reentry Tachycardia in Congenitally Corrected Transposition of Great Arteries: A Novel Site for Catheter Ablation.

Presentation A 19-year-old male with congenitally corrected transposition of the great arteries (ccTGA; S, L, L–situs solitus, l-loop, l-transposition) presented with recurrent frequent palpitations, fatigue, and effort intolerance. Previous history was significant for initial presentation with supraventricular tachycardia (SVT) at the age of 14 years. Transthoracic echocardiography at that time led to diagnosis of ccTGA. He reportedly underwent a posteroseptal accessory pathway ablation, and subsequently a second ablation the same year for recurrent SVT, again targeting a posteroseptal pathway. At the age of 17 years, he had a third procedure and reportedly linear ablation between the right atrioventricular valve (mitral), and inferior vena cava was performed for inducible right atrial flutter. Physical examination was unremarkable except for loud aortic closure on cardiac auscultation. The baseline ECG showed the presence of septal Q waves and the absence of lateral Q waves, findings characteristic of ccTGA (Figure 1A). The chest radiograph showed mesocardia without cardiomegaly or pulmonary congestion. Transthoracic echocardiography showed ccTGA, no atrial or ventricular septal defect, no pulmonary stenosis, normally functioning competent atrioventricular valves, and preserved function of both ventricles. Ambulatory monitoring revealed tachycardia, sometimes regular but at other times with an alternating variation in QRS axis and RR interval (Figure 1B). He was brought to the electrophysiology laboratory, and catheters were positioned in standard positions, including high right atrium, His-bundle, subpulmonic ventricle, and coronary sinus (CS). Dissociated signals were noted in the posteroseptal region/CS ostium, presumably related to previous ablations. With premature atrial beats, a distinctly different QRS complex with loss of notching in the inferior leads and a slightly more superior axis was noted. This second more superiorly directed QRS morphology was associated with a shortening in the recorded HV interval. There was no VA conduction. SVT was easily inducible with atrial extrastimuli and terminable with atrial burst pacing. During SVT, there were more atrial than ventricular complexes (2:1 or 3:2 with alternating QRS axis) (Figure 2), and 1:1 atrioventricular conduction during SVT was achieved with isoproterenol intravenous infusion 1 to 2 μg/min. What is the differential diagnosis and how will you approach mapping and ablation in this case? Initial Considerations There is a possibility for the presence of 2 atrioventricular conduction axes with 2 distinct atrioventricular nodes and His bundles in this case with ccTGA. It is important to recognize that any maneuvers that distinguish between different mechanisms of SVT like atrioventricular nodal reentry tachycardia (AVNRT), atrial tachycardia, or junctional tachycardia based on relationship between His bundle/ventricular activation with atrial activation will be unreliable because of the presence of 2 separate His bundles. As an example, AVNRT or junctional tachycardia originating from 1 of the 2 atrioventricular nodes and conducting to the ventricle using the other atrioventricular node–His bundle, would seem like an atrial tachycardia if the His bundle that is passively conducting to the ventricle is being mapped. Abrupt onset and offset of SVT with pacing maneuvers in this case was suggestive of a reentrant mechanism. During tachycardia, fewer ventricular than atrial complexes effectively excluded atrioventricular reentry, including reentry using an atriofascicular (Mahaim) connection. The differential thus included AVNRT, atrial tachycardia, or junctional tachycardia. An automatic junctional tachycardia was felt to be unlikely because of the abrupt onset and offset of tachycardia with atrial pacing maneuvers, more A than V during tachycardia with underlying VA conduction block, and an AHA response with the last atrial overdrive pacing impulse. Entrainment of the tachycardia showed postpacing intervals much longer than the tachycardia cycle length from the right and left atrial–free walls. The shortest postpacing intervals were obtained when entrainment was performed from the interatrial septum (postpacing interval−tachycardia cycle length ≈20 ms when atrial overdrive pacing 10 ms shorter than tachycardia cycle length). Furthermore, an activation map during tachycardia with point-to-point mapping of both atria (Carto 3 system, Biosense Webster) showed the earliest site of activation at the midseptal region of the putative fast pathway exit to the atrium. Furthermore, only 100 ms of the 340 ms tachycardia cycle length was mapped. In summary, this is a reentrant SVT close to the septum, with a large part of the cycle length not registered with complete mapping; and the earliest site at the fast-pathway exit of the atrioventricular node. Although a septal atrial tachycardia cannot be conclusively excluded, these findings are highly suggestive of AVNRT.