Missing pouches in high‐density mapping of atrial tachyarrhythmia in congenital heart diseases

Abstract Background The use of high‐density electroanatomical mapping in the Chinese population for congenital heart disease (CHD) is not well reported. Methods Retrospective review of consecutive transcatheter ablation of atrial tachyarrhythmia using high‐density mapping for CHD patients (at least moderate complexity) in the only tertiary congenital heart center in the territory from January 2017 to January 2019 was conducted. Orion mapping catheter in Rhythmia system (Boston Scientific) was used to create activation and voltage maps. Parameters including mechanism of arrhythmia, acute success, and follow‐up data were recorded. Results Eight patients were identified (median age 35.5 years) who underwent transcatheter ablation of atrial arrhythmia. More than one reentry circuits of IART were identified in five patients. It took a median of 32.4 minutes with 15,952 (IQR 13,395‐18,530) mapping points per map. Cavo‐annulus isthmus‐dependent mechanism was the predominant reentry mechanism. Acute success with the elimination of all inducible tachycardia was achieved in six patients (75%), and partial success in two patients. There was recurrence of atrial arrhythmia in four patients (50%), in which three patients could be maintained in sinus rhythm with low‐dose antiarrhythmic medication. Targeted substrate ablation was performed in six patients with multiple IART circuits. Critical anatomical pouches were identified in three patients, which were missed in the initial mapping using Orion basket mapping catheter. Conclusions High acute success rate of atrial arrhythmia ablation can be achieved using high‐density anatomical mapping in CHD. Substrate ablation was required with multiple IART circuits identified. Vigilance should be sought to identify anatomical pouches.


| INTRODUC TI ON
Transcatheter ablation for atrial tachyarrhythmia in congenital heart disease (CHD) has now been recognized as one of the major treatment options but remains challenging. The challenges include the presence of significantly dilated and/or hypertrophied atrium due to pressure-and volume loading, the potential abnormally located or otherwise impaired conduction system, and the arrhythmia substrate altered by fibrosis and/or surgically acquired scars. 1 Several technical advances including three-dimensional (3D) image integration, 3D mapping, and remote magnetic navigation, have been employed to facilitate the accurate localization of arrhythmia substrate and improve ablation success. [2][3][4] A new mapping system (Rhythmia, Boston Scientific) that uses a 64-mini-electrode small basket array (IntellaMap Orion, Boston Scientific) enables rapid high-density mapping in a short time. We aim to report the use of this advanced technology in our Chinese CHD patients.

| ME THODS
This is a retrospective review of consecutive transcatheter ablation using high-density electroanatomical mapping, for atrial tachyarrhythmia of CHD (at least moderate degree of complexity) patients, 5 from January 2017 to January 2019, in the only tertiary CHD center in the territory. Computed tomography of the heart was performed before the procedure and the scans were reconstructed on the Rhythmia mapping software and merged with the 3D chamber of interest. All antiarrhythmic medication was discontinued for at least five times the elimination half-life, except one patient with active intraatrial reentrant tachycardia (IART) forgot to stop amiodarone until the day before ablation. The procedures were performed under local anesthetic, sedation, or general anesthesia where appropriate. A 5Fr 4-pole electrode catheter was placed into the ventricle (retrograde approach in univentricular patients). A 7Fr duodecapolar electrode catheter was placed into the coronary sinus, or in the free wall of the right atrium and it was used for pacing maneuvers and as a timing reference for the high-density mapping. High-density electroanatomical mapping was created using Orion mapping catheter in Rhythmia system (Boston Scientific). Before mapping using the Orion catheter, intravenous heparin was used to maintain the activated clotting time between 250 and 350 seconds. Maps and intracardiac signals were acquired predominantly through the Orion, but additional mapping was added if necessary using the ablation catheter. IART was diagnosed as an intraatrial circus movement tachycardia with a constant cycle length inducible by programmed atrial stimulation or burst atrial stimulation. Focal atrial tachycardia (FAT) was defined as tachycardia with centrifugal spread of atrial activation from a focus other than the sinoatrial node. Voltage mapping was performed simultaneously to map scar tissue. During sinus and during tachycardia bipolar signals with filters were used. The activation time at each site was displayed in color relative to the timing reference. Voltage value was recorded at each point and represented in color scale (gray <0.1 mV, purple >0.5 mV). Scar area was defined as tissue with voltage <0.1 mV.
A bidirectional mapping and ablation catheter (IntellaNav, Boston Scientific) was used for ablation after identification of the arrhythmia substrate. For FAT, RF was applied to the earliest atrial activation.
For IART, after verification of the critical parts of the reentry circuit, the ablation lesions were created point-by-point. For substrate mapguided ablation, lesions were placed along the entire path between two nonconducting barriers in order to anatomically transect the critical isthmus. Testing for isthmus block was left to the discretion of the treating electrophysiologist and was not required for the definition of success. RF generator power setting was 25-50 W in the atrium.
Successful catheter ablation was defined as the termination of IART/ FAT during ablation and failure to reinduce tachycardia at the end of catheter ablation. Partial success was defined as elimination of ≥1 but not all inducible tachycardias. The proof of conduction block along the induced RF line was required as an additional endpoint.
After conclusion of the procedure, patients were monitored in the hospital for at least 24 hours. Any recurring symptoms prompted immediate review and documentation of further arrhythmia was carried out. Major complications were defined as situations requiring additional diagnostic procedures or any specific therapy beyond standard procedural care. Follow-up data include need for repeat catheter ablation procedures and time to recurrence after the final catheter ablation procedure.
All statistical analyses were performed using SPSS version 24.0 (IBM Somers). Data are presented as mean ± standard deviation (SD) or as median with the respective interquartile range (IQR). Kaplan-Meier survival analysis was conducted to study freedom from atrial arrhythmia recurrence. P-values <.05 were considered to be significant.

| RE SULTS
A total of eight patients with CHD (median age at ablation 35. 5  with IART, all except one patient had cavo-annulus isthmus-dependent mechanism as one of the reentry mechanism. For noncavo-annulus isthmus-dependent IART, lateral or posterolateral right atrium was the most common site of critical isthmus location. This area corresponds to surgical atriotomy incision in most of the cases. Acute success with the elimination of all inducible tachycardia was achieved in six patients (75%), and partial success in three patients (25%).
In three patients, anatomical pouches were nearly missed in the initial mapping, which were part of the arrhythmia substrate: The first patient (Case 1) was a 36-year-old male with atriopulmonary Fontan. Initial high-density mapping suggested the origin of the focal atrial tachycardia was located over anterior right atrium ( Figure 1A). However, no good unipolar signals could be identified. With careful comparison with the preprocedural CT, a small pouch was found medial to the earliest activation area ( Figure 1B).
Deflation of the Orion basket catheter was required to enter the pouch and 4609 electrograms were recorded inside the right atrial pouch ( Figure 1C). Radiofrequency ablation was performed at the site with earliest activation, with the termination of atrial tachycardia.
Both case 3 and 6 demonstrated similar pouches not identified during the cavo-annulus isthmus ablation. Case 6 was a 31-year-old male with pulmonary atresia with intact ventricular septum s/p surgical total repair, tricuspid ring annuloplasty and pulmonary valve replacement, and previous surgical right-sided Maze procedure.
High-density mapping suggested anticlockwise cavo-tricuspid isthmus (CTI)-dependent atrial flutter (Figure 2A). CTI ablation line was created and atrial flutter terminated upon completion. However, atrial flutter could still be induced. High-density propagation mapping suggested there was conduction delay over the created CTI ablation line, but there was a site a few millimeters septal to the line from which the atrial activation propagated ( Figure 2B and video S1).
Angiogram identified an anatomical pouch over the septal inferior aspect ( Figure 2D). Further mapping required dedicated deflation of the Orion mapping catheter to enter the pouch, septal to the CTI line ( Figure 2E). Further ablation was performed inside the pouch area ( Figure 2C), and validation map postablation confirmed complete blockage.
Voltage mapping was performed in six of eight patients, in which more than one atrial tachyarrhythmia mechanisms were identified.  of non-CTI-related IART, the most frequent location of IART isthmus was the lateral or posterolateral wall of the venous atria. [10][11][12] Recurrent circuits around caval orifices, crista terminalis, or the atrial appendage are also observed, but less frequently. 13 We also demon- site. 13 These pouches should be carefully looked for as they may form part of the critical isthmus, or hinder the contact of the ablation catheter in creating effective lesions. The initial experience by

| D ISCUSS I ON
Ernst et al suggested the acquired 3D map was smaller than the 3D reconstructed chamber anatomy by preprocedural imaging, in half of their procedures. 9 The bulky basket mapping catheter may hinder entry into these small pouches of atrial tissues, which can be the critical area of anatomical isthmus. Intraprocedural angiograms and adequate preprocedural imaging help the operators to ensure the entire chamber has been mapped, which is also our current practice after gaining the above-mentioned experience. Sometimes, conventional ablation catheter may also be utilized to negotiate through the neck of the pouch, to complete the pouch mapping.
In the study of Roca-Luque et al, a cut-off voltage of 0.5 mV could identify 95.4% of the substrates in non-CTI-related IART. 12 Unexcitable sites can be displayed together with the caval veins, valve annuli, and lines of double potentials. Double potentials may be located between the caval veins and the posterolateral systemic venous atrium consistent with the crista terminalis but may also indicate atriotomy and suture lines. After delineation of all boundaries that create intervening isthmus potentially related to atrial tachyarrhythmia, these isthmuses can be targeted by a liner RF ablation lesion connecting the unexcitable boundaries during sinus rhythm, like in our cases. 14 Empiric cavoannulus and intercaval ablation may be particularly useful in patients with multiple tachycardias, irregular tachycardias or where the tachycardia is noninducible or not hemodynamically tolerated. 15 This could have contributed to the high acute success rate in our cohort, with the identification of these isthmuses using substrate mapping.
The advantage of using the Rhythmia system is the speedy acquisition of multiple mapping points with the mini basket and its small electrodes. In the study by Rottner et al, Rhythmia-guided ablation approach, when compared to Carto-guided ablation, was observed F I G U R E 3 A 41-year-old male with congenital corrected transposition of great arteries. (case 2) A and B, Electrophysiological study identified multiple atrial tachycardia with different cycle lengths. C, No single mechanism of atrial tachycardia could be identified. Voltage mapping identified low-voltage area (red: <0.125 mV; purple >0.5 mV). RF ablation line (black dots) was created to connect scar and low-voltage region, after which the AT changed. D and E, Focal atrial tachycardia. Earliest atrial activation was identified over the crista terminalis, with local atrial activation 43 ms ahead of the onset of P wave. RF ablation at this region successfully terminated the tachycardia. F, High-density mapping of another AT suggested clockwise cavo-annular-dependent intraatrial reentry tachycardia. G, RF ablation line was created (pink dots) and IART was terminated to enable multiple times of mapping points to be collected. 16  The follow-up period was relatively short for our cohort. However, as 75% of all recurrence occurred within the first year for previously published cohort, 18 we believe presenting early recurrence could adequately reflect the effectiveness of our atrial tachyarrhythmia ablation in CHD. We encountered a higher recurrence of atrial tachyarrhythmia for our first few cases, and this is expected as CHD ablation and the use of the system required accumulation of experience.
In fact, the AT recurrence rate (50%) is comparable to other similar CHD cohort, 14,19 in which usually more than one ablation procedures are required. Recently, clinical arrhythmia severity score was used to assess the efficacy of transcatheter ablation in CHD cohort, instead of recurrence rate. 20 All our patients with recurrence had less severe Therefore, extra fluoroscopic guidance was used to ensure complete mapping of the large atrium, to ensure the Orion basket catheter could reach the border. Difficulty to locate hidden anatomical pouches in our cohort also contributed to the extra fluoroscopic use.

| S TUDY LIMITATI ON S
This was a retrospective study with heterogeneity and is subject to the consequent limitations. The relatively short follow-up duration also hindered long-term outcome to be analyzed with our study.

| CON CLUS ION
The initial experience of using high-density mapping in transcatheter ablation of atrial tachyarrhythmia is presented, in a cohort of Chinese CHD patients with at least moderate in complexity. High acute success rate was achieved. Vigilance should be sought to identify anatomical pouches using high-density mapping in this special cohort of patients.

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

AUTH O R CO NTR I B UTI O N
SY Kwok was involved in concept/design, data collection, data analysis/interpretation, and drafting article. TC Yung and HF Tse were involved in critical revision of article, approval of article. NL Ho was F I G U R E 5 A 31-year-old male with double outlet right ventricle, subaortic ventricular septal defect, and pulmonary stenosis posttotal surgical repair (case 8). High-density activation mapping (left) suggested clockwise IART around a line of block over the right lateral atrium (black dots). This represented previous atriotomy scar. With the aid of the voltage mapping (right), RF substrate ablation was performed to create lines of block and connected the low-voltage areas: from the superior vena cava to the right atriotomy scar; from the atriotomy scar to the inferior vena cava; and from the tricuspid annulus to the lateral line of block involved in data analysis/interpretation, statistics. JJ Hai and S Tsao were involved in critical revision of article.

E TH I C A L A PPROVA L
This study received ethics approval from the Institutional Review Board of the University of Hong Kong/Hospital Authority Hong Kong Western Cluster.