New SVT After Previous Ablation of a Left Lateral Accessory Pathway: What Is the Mechanism?

A 49-year-old male patient underwent a redo procedure for supraventricular tachycardia. He had a history of left accessory pathway (AP) ablation in an outside hospital 2 years ago. Dual AV nodal physiology was absent during atrial pacing, while programmed RV extrastimuli showed fixed VA interval with the earliest A on His catheter. Narrow QRS tachycardia could be induced with either RV or atrial pacing, showing the identical concentric atrial sequence to RV pacing. RV overdrive pacing was performed during tachycardia (Figure 1). Is this a “new” posteroseptal AP or a posteroseptal AP missed during the previous ablation? The differential diagnosis of narrow QRS tachycardias with long VA intervals included orthodromic reciprocating tachycardia (ORT), atypical atrioventricular nodal reentrant tachycardia (AVNRT), atrial tachycardia (AT), and rare types such as nodoventricular/nodofascicular-related tachycardias. The response to RV overdrive pacing was VAV, and the PPI-TCL was basically “in” the circuit with a corrected PPI-TCL of 95 ms, which essentially excluded AVNRT and AT, leading to the diagnosis of ORT. Mapping along the tricuspid annulus during RV pacing showed earliest A at anteroseptal region, while lateral tricuspid annulus was even later (Figure 2A). It is noteworthy that there was a small far-field deflection present on the distal CS (arrow in Figure 2A, also seen in Figure 1). This mid-diastolic potential was earlier than the atrial signal on His. This is most compatible with a far-field left atrial potential. Based on the history of previous ablation, split atrial potentials, including near and far-field electrograms at previous ablation site should be considered. The mitral annulus, together with the aortic sinuses, was thereafter mapped during RV pacing. The timing was late at lateral mitral annulus while remarkably earlier at aortomitral continuity, which coincided with the far-field potential, suggesting double atrial potentials on distal CS. The activation sequence in the right atrium (RA) was presumably resulted from the preferential conduction from the left atrium via Bachmann bundle (Figure 2B). Due to the very frequent induction of the tachycardia, activation mapping was then continued during ORT. Widely-split double atrial potentials were observed at the lateral mitral annulus, indicating a conduction block. A line of mitral isthmus block was present with the earliest atrial activation site slightly more distal (Figure 2C). Late and low-voltage fractionated atrial signal was present at the proximal side of the line, consistent with the ablation site in the index procedure (Figure 2D). By coherent mapping, we were able to demonstrate the activation in the atrium, which propagated from the AP insertion site preferentially to the anterior wall, septum, anterior roof, and left superior pulmonary vein, and to the right anterior septum of the RA, leading to the concentric activation pattern (Figure 2E,F and Video 1). The AP was ablated at the earliest atrial activation site. Mitral isthmus block can be observed during left atrial ablation which changes the activation sequence [1, 2]. This phenomenon should be suspected when apparently “new” SVT is present after ablation of a left AP. The iatrogenic atrial-detour may occur following a previous procedure or in the index procedure after multiple radiofrequency applications. The cycle length of the “new” tachycardia is usually the same as the original SVT before any ablation or a little longer due to increased transit time in the atrial part of the circuit. In this setting, a widely split atrial electrogram as observed on the distal CS in this case, is frequently observed. Further atrial mapping distal to the previous atrial site is diagnostic. The atrial activation sequence in the RA depended on the earliest activation site and conduction pattern in the left atrium. The preferential conduction between the two atria includes Bachmann bundle, fossa ovalis, and coronary sinus musculature. As the detailed atrial activation was demonstrated in this case, the preferential conduction over the Bachmann bundle was considered as the cause of the right atrial breakthrough at the anterior septum. We appreciate Mr. Xiangliu Tong from Biosense Webster Inc. (China) for his work in data acquisition and processing. This work is supported by the Shanghai Municipal Health Commission (Grant No. 202340005). The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.