Remote magnetic‐guided ablation for three origins of idiopathic ventricular arrhythmias with right bundle branch block and superior axis

Abstract Background Idiopathic ventricular arrhythmias (IVAs) with right bundle branch block (RBBB) and superior axis commonly originate from posterior mitral annulus (PMA), the left ventricular (LV) posterior fascicle (LPF), and the LV posterior papillary muscles (PPM). Hypothesis Remote magnetic navigation (RMN)‐guided ablation might be safe and effective for these three origins of IVAs. Methods Thirty consecutive IVA patients with RBBB and superior axis (11 MPA‐IVAs, 11 LPF‐IVAs, and 8 PPM‐IVAs) were included in this study. Electrical mapping and ablation with RMN were performed in the LV through a trans‐septal approach. Navigation index, defined as the ratio of total radiofrequency (RF) time and the time from first burn to last burn, was used to determine the efficiency of RMN‐guided ablation. Results The overall acute success rate was achieved in 93% (PMA, 100%; LPF, 91%; PPM, 88%; p > 0.05). No complication occurred in this study. The procedure time of PPM‐IVAs group was 34 and 14 min longer when compared with MPA‐IVAs and LPF‐IVAs group, respectively, without an increase of X‐ray time. The mean navigation index was 0.45 ± 0.20. The PPM‐IVAs group had an underperforming navigation index value (0.29 ± 0.11) (p < 0.01), as longer RF time was required in the PPM‐IVAs group. Conclusions RMN‐guided ablation can achieve a high acute success rate for IVAs with RBBB and superior axis. The lower navigation index for PPM‐IVAs indicated that increasing the RF time and improving the catheter contact should be considered when using RMN.


| INTRODUCTION
Idiopathic ventricular arrhythmias (IVAs), including frequently premature ventricular contractions (PVCs) and ventricular tachycardias (VT), originate predominately from the right and left ventricular outflow tracts. 1 Generally, IVAs have been regarded as non-lethal arrhythmia, 2 however, recent studies 3-5 report a subgroup of IVAs with significant mortality, and are a challenge to be successfully eliminated by traditional catheter ablation. 6 One of the most significant indicators of such arrhythmias is the electrocardiographic (ECG) morphology of right bundle branch block (RBBB) and superior axis. This type of IVA predominately originates from the posterior mitral annulus (PMA), the left ventricular (LV) posterior fascicle (LPF) or the LV posterior papillary muscles (PPM), and rarely from the cardiac apical crux. 7 Despite some similarities, it is required to distinguish these IVAs from each other, for the differences in prognosis and clinical presentation. 8 Moreover, in an effort to obtain better ablation outcomes and a shortened procedure time, it is recommended to fully appreciate the precise origin of the tachycardia utilizing algorithms in advance. The remote magnetic navigation (RMN) system has emerged as an alternative to traditional catheter ablation for difficult to treat arrhythmias. 9,10 Briefly, this robotic system is designed to remotely control the magnetic field generated by the external magnetic poles to guide catheter ablation. Compared with manual operation, it offers precise and flexible catheter maneuver making it easier to reach traditionally challenging target sites. Moreover, reduced radiation exposure and peri-procedure complications have been observed proving its general safety and broadening its scope of application. Our previous work 11 has demonstrated the advantages of RMN system in ablation for certain complex arrhythmias. We hypothesize these advantages can be retained on this subgroup. The aim of this study was to investigate the ECG and electrophysiological characteristics and the procedure outcomes of RMN-guided ablation for IVAs with RBBB and superior axis.

| Studied patients
Patients who underwent catheter ablation of symptomatic IVAs with RBBB and super axis in our center, were consecutively included from January 2017 to January 2020. Patients with structural heart diseases including ischemic, valvular, or congenital heart disease were excluded. All patients signed an informed consent before the procedure.

| Electrocardiographic analysis
The 12-lead ECG of VAs was recorded at a paper speed of 25 mm/s in all patients. Analysis of QRS morphology focused on the following characteristics: QS pattern or magnitude of R wave in lead V6; QS pattern or rS pattern in the inferior leads; and monophasic R wave or qR pattern in lead aVR. Waves with a relatively high amplitude (>0.5 mV) were marked with capital letters (Q, R, or S) and vice versa.
QRS duration was measured as the interval between the earliest rapid deflections of the ventricular complex in any lead to the latest offset in any lead. The QRS transition zone was determined by the R-wave amplitude in the precordial leads.

| Electrophysiological procedure
After withdrawal of antiarrhythmic drugs for more than five half-lives, the patients underwent electrophysiological study in fasting and conscious state. A deca-polar catheter and a bi-polar catheter (St Jude Medical, Inc, St. Paul, MN) were placed within the coronary sinus and at the right ventricle (RV) apex, respectively. A programmed stimulation protocol from multiple RV/LV sites at the 500, and 400 ms drive cycle with up to 3 extra stimuli in decrements down to 200 ms or ventricular refractoriness, was applied to induce VT. Intravenous isoproterenol infusion (1-10 μg/min) was administered to induce PVCs, if they failed to occur spontaneously. Activation mapping was always performed to identify the earliest activation site of the VAs. We selected potential ablation sites of PVCs where local activation was at least 20 ms pre-QRS with a QS wave in the unipolar electrogram. Points with QRS morphology during pace-mapping identical to VAs were also annotated. Entrainmentmapping techniques were applied trying to characterize the arrhythmic circuit in patients with VTs.

| Mapping and ablation strategy
Radiofrequency energy was delivered in the temperature control mode with target catheter temperature of less than 43 C. Power was set at 30-40 W with a flush rate of 17 mL/min. After catheter ablation, the same stimulation protocol used previously was applied to induce tachycardia. Any induced sustained monomorphic VT was targeted with further mapping and ablation, and the inducible protocol of VT was repeated subsequently until no further VT was inducible.
Acute ablation success was defined as the elimination and noninducibility of clinical VAs with isoproterenol infusion after at least a 30-minute waiting period.

| Definition of procedural parameters of RMNguided ablation
Procedure time was defined as the total time from the Navigant™ respectively. RF applications and RF time reflected the total sum number and minutes of ablation burns during the procedure, respectively. 12 Navigation index, defined as the ratio of total radiofrequency time to the time from first burn to last burn, was utilized to indicate the efficiency of RMN-guided ablation in this study. The higher the navigation index, the greater percentage of time was spent delivering RF treatment versus locating desired RF treatment locations.

| Complications
Complications were divided into two categories: major and minor.
Major complications included cardiac tamponade, acute myocardial infarction, stroke, major bleeding. Minor complications were defined as pericarditis and inguinal hematoma.

| Follow-up
Continuous telemetry monitoring was performed for all the patients after the procedure for 24 hours. Patients were then scheduled for outpatient clinic the first 3 months and every 6 months thereafter.
The follow-up 24-hour Holter recording was performed within 6 months. Recurrence of arrhythmia was defined as either symptomatic recurrence with documented PVCs or asymptomatic frequent PVCs of 5000 per day. 11

| Statistical analysis
The data are expressed as means ± SD for the continuous variables and as numbers and percentages for the categorical variables. Continuous variables were compared using the Mann-Whitney U test, a p value <0.05 was considered as statistically significant. All statistical analyses were performed using the SPSS 19.0 (IBM Corp, Armonk, NY). Their key characteristics were listed in Table 1. The mean patient age was 51.5 years and 83% were male. Patients with LPF-IVAs were younger (45.1 ± 13.8, p < 0.05) and presented more frequently with sustained VT (91% p < 0.01) as compared to those with PMA-IVAs and PPM-IVAs. No significant difference was detected in gender, hypertension, LVEDd, LVEF, anti-arrhythmic drugs administration, and VAs burden among the three groups.

| ECG and electrophysiological findings
For each group, ECG manifestation demonstrated a morphology of RBBB and superior axis during onset of IVAs (shown in Figure 2).
There were considerable differences of R/RS pattern presence in lead  Table 1.

| Acute success and procedural outcomes
Overall, the mean acute success rate was achieved in 93%. Examples of successful ablation of IVAs with CRBBB and superior axis is shown in Figure 3. Although, no significant difference was observed in abla- LPF-IVAs (Table 2). While no significant difference was observed in total X-ray time and control room's X-ray time between groups.

| Complications and follow-up
The average follow-up duration was 22.1 ± 5.9 months after the pro-

| ECG and electrophysiological characteristics of VAs arising from PMA, LPF, and PPM
In order to obtain optimal procedure efficacy and shortened procedure and X-ray time, it is crucial to precisely understand the origin site to specify treatment strategy prior to the procedure. 13

| Implications of procedural outcomes of RMN-guided ablation
Overall procedure time (89.2 ± 38.9) and total X-ray time (4.2 ± 2.4) were remarkably low in this study. PPM-IVAs procedure parameters that included procedure time, clinical time, mapping time and RF time were all longer when compared with LPF-IVAs and PMA-IVAs procedure times. This might be related to the anatomy heterogeneity and complexity of papillary muscles. 10 Interestingly, the longer procedure time did not extend X-ray time. In fact, no significant difference was observed in total X-ray time and control room's X-ray time in this study.
In this study, a novel parameter (navigation index) was utilized to assess the accuracy and efficiency of the ablation protocol. The navigation index of PPM-IVAs ablation was much lower than either LPF-IVAs or PMA-IVAs in this study, indicating that increasing the RF time and area should be considered when using RMN for PPM-IVAs. For all that, however, we have to acknowledge the inherent advantages of RMN as verified by reduced X-ray time and low complication and recurrence rate.
Cryoablation has been proposed to meet the stability of catheter in ablation for PPM-IVAs, while this comes with its own limitation in flexibility and ablation depth. 22 Circumferential PM ablation is another direction 23 and this high demand for catheter control is also a chance for RMN-guided ablation to shows its fascinating advantages.

| LIMITATIONS
Our impressive initial report on RMN guided-ablation for IVAs with RBBB and superior axis needs to be further confirmed by larger randomized controlled trials. A more accurate mapping and ablation procedure could be facilitated by combing intracardiac ultrasound with RMN. IVAs arising from apical crux, as rare cases, were not enrolled in this study. Further study may demonstrate if ablation strategies employed here can be successfully applied to this type of IVA as well.
T A B L E 2 Procedural outcomes and acute success

| CONCLUSIONS
RMN-guided ablation for IVAs with RBBB and superior axis achieved a high acute success rate in most selected patients, proving its efficacy and safety in these cases. The lower navigation index for PPM-IVAs indicated that increasing the RF time and improving the catheter contact should be considered when using RMN.