Treatment strategy and endpoint of catheter ablation for bi‐atrial tachycardia after substrate modification ablation in a low voltage zone of the left atrial anterior wall: Long‐term results

Abstract Background The termination of bi‐atrial tachycardia (BiAT) via the ablation of the Bachmann's bundle (BB) and mitral isthmus (MI) has been previously reported; however, the strategy and long‐term results of catheter ablation for BiAT remain unclear. Methods The data of nine patients with BiAT who underwent low voltage zone (LVZ) ablation of the left atrial anterior wall (LAAW) after pulmonary vein isolation were reviewed. Patients with a P wave duration <100 ms during sinus rhythm underwent MI ablation and those with a P wave duration >100 ms underwent BB ablation. Results MI ablation was performed in three patients and six patients underwent BB ablation. The difference in the P wave duration before and after ablation was significantly different between the ablation sites (MI group: 5.0 ms difference; BB group; 38.5 ms difference; P = .024). The P wave duration was prolonged by >20 ms and was 120 ms or more after ablation in 5/6 patients who underwent BB ablation. The total recurrence rate was 11.0% (mean: 26.9 months). Conclusion The recurrence of BiAT after MI or BB ablation is low. When BB ablation was performed, the P wave duration was prolonged by >20 ms and was at least 120 ms after the ablation, which may be an endpoint that can be used to measure the success of the ablation.


| INTRODUC TI ON
Catheter ablation, including pulmonary vein isolation (PVI), is a generally accepted therapy for atrial fibrillation (AF). However, recurrent AF and left atrial tachycardia (AT) are likely to occur after PVI. 1,2 Several studies have reported that additional substrate modifications after PVI, such as anatomical line and low voltage zone (LVZ) ablations of the left atrial anterior wall (LAAW), reduce AF recurrence. 3 However, a previous study revealed that LAAW substrate modification leads to bi-atrial tachycardia (BiAT), 4 a rare form of atrial macro reentrant tachycardia in which the reentry circuit is located in both the right and left atria (RA and LA, respectively). 5 Several reports have indicated that BiAT can be terminated via the ablation of the Bachmann's bundle (BB) and anatomical linear ablation, especially mitral isthmus (MI) ablation. [6][7][8] However, the treatment strategy and long-term results of catheter ablation for BiAT remain unclear. This study aimed to investigate the optimal ablation sites to treat BiAT termination and the long-term results of catheter ablation for BiAT.

| Study population
A total of 265 patients who underwent PVI from April 2017 to April 2019 at Tokyo Metropolitan Hiroo Hospital were enrolled in this retrospective, observational study. Of those patients, 34 underwent LVZ ablation of the LAAW after PVI. The LVZ was defined as that with a bipolar voltage of less than 0.5 mV. Ten (29.4%) patients who underwent LVZ ablation developed BiAT. One patient was excluded due to an inadequate evaluation of the P waves. Therefore, the final analysis included nine patients. We evaluated the precise circuits in both the LA and RA using a high-density mapping system and analyzed the optimal treatment strategy and long-term results. Informed consent was obtained from each patient. The study was approved by our hospital's institutional review board and conforms to the provisions of the Declaration of Helsinki (as revised in Fortaleza, Brazil, October 2013).

| Ablation procedure
Patients were administered an oral anticoagulant for at least 1 month prior to the ablation procedure. All antiarrhythmic drugs were discontinued for at least five half-lives before the ablation procedure.
In all patients, transesophageal echocardiography or enhanced computed tomography was performed prior to the procedure to detect an LA thrombus.

| Selection of the ablation site
Previous studies have reported that BiAT can be treated with MI or BB ablation. [6][7][8] The BB spans a wide area, and its distribution differs among patients. 9,10 Therefore, the range of the BB ablation is not clear and a method for BB ablation has not yet been reported. It can be assumed that the BB in some patients with BiAT may be damaged during previous LAAW ablation procedures. A previous study reported that a P wave duration >100 ms in the inferior leads is associated with interatrial conduction disturbances and an obstructed BB. 11 Therefore, BB ablation is more effective than MI ablation in patients with a P wave duration >100 ms, because BB is already obstructed and we speculated that BB block line could succeed easily.
We ablated any residual potentials that were obstructed during previous procedures in the area of the BB.

| Diagnosis of BiAT
Activation maps of AT in both the RA and LA were created for all patients using the CARTO3 or RHYTHMIA HDx™ systems. BiAT was diagnosed when the AT activation map indicated a reentrant circuit involving both atria and the post-pacing interval (PPI) and tachycardia cycle length (TCL) of the entrainment pacing of the circuit was <20 ms different.

| Procedure endpoint and follow-up
After the termination of the BiAT, we induced tachycardia via overdrive burst pacing. The minimum pacing cycle length was set at 200 ms, as permitted by the local effective refractory period of the atrial tissue. Successful termination of the BiAT was defined as the inability to induce tachyarrhythmia. Patients were followed up at 1, 3, 6, 12, 18, 24, 30, and 36 months after the ablation procedure. During the follow-up visits, a 12-lead electrocardiogram (ECG) and 24-hour Holter monitoring were performed. The recurrence of AF or AT was defined as an episode of either lasting more than 30 s.

| Statistical analyses
Continuous variables were described as mean values and compared using the Mann-Whitney U-test. All analyses were conducted using Statistical Package for the Social Sciences (SPSS) version 18.0J software (SPSS Inc, IBM). Statistical significance was set at P < .05.

| RE SULTS
Thirty-four patients who underwent LVZ ablation of the LAAW were enrolled in this study including 10 who developed BiAT ( Figure 1). The BiAT propagated from the MI in eight patients. MI ablation was performed in three patients with BiAT from the MI and a P wave duration <100 ms BB ablation at the LA was performed in five patients with BiAT from the MI and a P wave duration >100 ms The P wave duration was measured using the final sinus rhythm before the recurrence of BiAT in the inferior lead.
Two patients had BiAT without the involvement of the MI, including one with an implanted pacemaker due to sick sinus syndrome in whom the P waves could not be adequately evaluated before and after ablation; therefore, this patient was excluded from the final analysis.

| Patient characteristics
The baseline characteristics of patients with BiAT who underwent MI or BB ablation are shown in Table 1 (Table 1).

| MI ablation in a patient with BiAT
involving the MI and a P wave duration <100 ms The activation map created using the RHYTHMIA HDx™ system indicated a tachycardia circuit in the CS of the RA septum via the MI with a clockwise rotation that returned to the LA via the BB in patient 2 (Figure 2A, B). The PPI was equal to the TCL at the RA septum. MI ablation performed as the P wave duration during sinus rhythm was 95 ms The BiAT was successfully terminated using MI ablation.

| BB ablation in a patient with BiAT
including the MI and a P wave duration >100 ms The activation map created using the CARTO3 system indicated that the wavefront of the tachycardia circuit propagated from the LA and passed through the MI and CS into the BB on the RA side with a clockwise rotation in patient 6 ( Figure 3A

| BB ablation in a patient with BiAT not including the MI and a P wave duration >100 ms
The activation map created using the RHYTHMIA HDx™ system showed that the wavefront of the tachycardia circuit propagated from the posterior wall of the LA to the RA septum returned to the LA via the BB with a clockwise rotation, and did not involve the MI in patient 9 ( Figure 4A, B). The PPI was equal to the TCL at the RA septum and the roof of the LA. Moreover, the PPI of the posterior wall of the LA was less than the TCL by 20 ms The P wave duration was 103 ms during sinus rhythm before ablation; therefore, BB ablation, including the roof of the LA, was performed. The BB ablation successfully terminated the BiAT.

| P wave characteristics after the ablation procedure
In the MI ablation group, the P wave duration after ablation was slightly prolonged compared to that before ablation in two patients (66%). The P wave duration after ablation was prolonged compared to that before ablation in six patients (100%) in the BB ablation group. The prolongation of the P wave duration was significantly greater in the BB ablation group (38.5 ms, IQR: 16.5-62.5 ms) than in the MI ablation group (5 ms, IQR: 0-6.0 ms) (P = .024; Table 2).
The P wave duration was prolonged from 234 ms to 343 ms in patient 7 who underwent BB ablation. The P wave morphology showed a prolonged, biphasic P wave in the inferior leads on ECG ( Figure 5).
AT recurrence occurred in one patient (11%) 11 months after the ablation procedure ( Table 2). The P wave duration in this patient was not prolonged by >20 ms and was <120 ms This patient did not undergo a repeat catheter ablation and continued to receive antiarrhythmic drugs.

| D ISCUSS I ON
We compared the outcomes of ablation at the MI or BB in patients with BiAT and found that the change in the P wave duration was significantly greater in patients who underwent BB ablation than in patients who underwent MI ablation. We also found no significant difference in the ablation times between the two groups. The recurrence rate over a 26.9-month follow-up period was 11%.
Previous studies have reported that BiAT can be terminated by BB ablation. 4,[6][7][8] The BB is a muscle bundle that connects the RA and LA, spanning from the RA appendage to the LA appendage, requiring a large area of ablation. 9,10 As ablation from the endocardium at the BB is more difficult than ablation at the MI, we predicted that the ablation time in the BB group would be longer than that in the MI group. However, there was no significant difference in the ablation time between the two groups. Bayes et al reported that P wave durations over 100 ms suggest impairment of the interatrial conduction, including conduction through the BB. 9 In this study, it is considered that the BB ablation time was reduced because BB ablation was performed only for the residual potential in patients in whom the BB was already obstructed due to perform past procedures.

F I G U R E 4
Activation and voltage maps of bi-atrial tachycardia in patient 9 created using the Rhythmia mapping system. (A) Patient 9 was found to have a clockwise activation pattern of bi-atrial tachycardia (BiAT or CS) and the corresponding atrial propagation site. 13 Impaired interatrial conduction may be seen as the prolongation of the P wave on a 12-lead ECG due to the conduction disturbance along the BB.
In addition, advanced interatrial block caused by the obstruction of the BB may be seen as a prolonged, biphasic P wave in the inferior leads on ECG. Bayes et al also reported that the prolonged (≧120 ms) monophasic and biphasic P waves on ECG indicate interatrial blocks such as the obstructed BB. 11 The P wave duration was prolonged by >20 ms and was 120 ms or more after the ablation in five patients in the BB ablation group.
In patient 7, the P wave duration was >120 ms prior to the ablation due to the conduction disturbance caused by past procedures. The P wave morphology of patient 7 changed to a biphasic wave that remained prolonged, as indicated by an advanced interatrial block ( Figure 5). Patient 5 had a prolonged P wave duration of 106 ms after ablation, which was insufficiently prolonged. We believe that BB ablation might have been insufficient in this patient, leading to a recurrence, as the P wave duration was not prolonged by <20 ms and was <120 ms Determining the endpoint of the BB ablation was challenging as the BB covers a wide area leading from the RA to the LA and the ablation points were unclear. Therefore, the prolongation of the P wave duration by >20 ms and a duration ≥120 ms after ablation was used to determine the success of the BB ablation procedure. One possible complication of the BB ablation procedure is a worsened interatrial conduction disturbance, causing heart failure. In this study, no patients developed heart failure after BB ablation.
The mean follow-up period in this study was 26.9 months. To our knowledge, this is the longest follow-up period reported for patients with BiAT. One patient (11%) experienced recurrent BiAT after the ablation procedure, which is consistent with a previous study that reported a recurrence rate of BiAT after ablation of 12.5%. 5  14,15 However, another previous study reported that LA ejection fraction (LAEF) is a more sensitive marker of early LA remodeling than LA enlargement. 16 Impairment of LAEF precedes LA dilatation in patients with paroxysmal AF, and early LA remodeling can be detected by a decrease in LAEF without LA enlargement. 16 Moreover, LAEF dysfunction after ablation in patients with AF has been reported as a risk factor for the recurrence of AF. 17 In this study, the LA diameter before and after ablation was evaluated. All F I G U R E 5 Changes in P wave duration and morphology before and after ablation. In patient 7, the P wave morphology changed to an obvious biphasic wave in the inferior leads of a 12lead electrocardiogram and its duration is prolonged from to 343 ms after ablation from 234 ms before ablation patients in this study had a history of ablation procedures. The LA diameter increased in all patients, except for patient 8. However, in this study, we did not measure LA volume and cannot evaluate LAEF or LA function.

| CON CLUS IONS
Ablation of either the MI or BB would be an effective strategy to treat patients with BiAT in which baseline duration of the P wave may suggest it as a proper choice. Both P wave prolongation by >20 ms and a P wave duration of ≥120 ms after BB ablation may an indicative of a successful BB ablation for BiAT.

ACK N OWLED G M ENTS
None.

CO N FLI C T O F I NTE R E S T
Authors declare no conflict of interests for this article.