Same-day discharge after AF ablation procedure is becoming the preferred trend. Vascular closure devices have shortened the postprocedural bedrest, associated with increased patient satisfaction. Although this approach comes with a cost, it might benefit also the healthcare system.

The wall of unintended consequences: is the main benefit of posterior LA wall isolation simply more durable pulmonary vein isolation?Paul C. Zei, MD PhD FHRSDepartment of MedicineBrigham and Women’s HospitalHarvard Medical SchoolBoston, MAFunding: NoneConflict of Interest: None

We read, with interest, the article by Deb et al. entitled “Positive QRS complex in limb lead 2 with negative QRS in lead 3 on surface electrocardiogram: A novel predictor for anterior location of right sided accessory pathways.” 1 We would like to raise a few concerns regarding the interpretation of the electrocardiographic (ECG) sign they have highlighted.

Myocardial wall thickness is one of the crucial parameters affecting the lesion formation produced by radiofrequency current (RF) delivering. Knowing the tissue characterization is critical for improving the durability of the RF lesion. A novel dielectric based method (KODEX-EPD) has been developed for measuring the tissue thickness at the catheter-tissue interface. The authors of this study report for the first time the tissue characterization (i.e. atrial wall thickness) of the cavo-tricuspid isthmus in a series of patients undergoing common atrial flutter ablation, showing a higher thickness close to the tricuspid valve as compared to the inferior vena cava. This can affect the outcome of ablation

Lowering the Threshold for Left Bundle Branch Area PacingLukasz P. Cerbin, MD1 and Lohit Garg, MD11 Division of Cardiac Electrophysiology, University of Colorado Anschutz Medical Campus, Aurora, Colorado, USA.Relevant Disclosures: NoneFunding: NoneCorresponding Author: Lohit Garg, MDDivision of Cardiac Electrophysiology, University of Colorado Anschutz Medical Campus12401, E. 17th Ave, Aurora, Colorado-80045Email: [email protected] pacing remains the mainstay of therapy for conduction system disease and irreversible bradyarrhythmias. Due to known complications from chronic RV pacing and electromechanical dissociation1,2, there has been a growing interest in physiologic pacing. While cardiac resynchronization therapy (CRT) with LV pacing from a lead in the coronary sinus has been shown to ameliorate some of the morbidity from RV septal pacing3, activation is still via myocyte to myocyte conduction rather than engaging the specialized conduction system. Furthermore, LV lead placement can be challenging and response to CRT is heterogeneous and difficult to predict4. And finally, CRT’s role in patients with preserved LV systolic function has not been established.In this light, there has recently been a substantial growth in conduction system pacing directly engaging the native His-Purkinje conduction system. This began with a case series published in 2000 demonstrating selective his bundle capture and subsequent permanent His bundle pacing (HBP) lead placement in patients with heart failure and tachycardia induced cardiomyopathy secondary to atrial fibrillation5. Additional studies demonstrated the feasibility and clinical benefit of HBP as a strategy for CRT6,7. However, continued research revealed high capture thresholds and low R wave amplitudes, calling this technique into question8. Furthermore, HBP failed to demonstrate improved outcomes in patients with underlying LBBB, likely due to the presence of conduction disease distal to the site of capture9. Due to these concerns, the enthusiasm for HBP gradually waned, with a subsequent rise in interest in left bundle branch area pacing (LBBAP). This approach utilizes the same lead and implant technique to fix a lumenless pacing lead approximately 1.5 cm distal to the anatomical His bundle within the interventricular septum, directly engaging the left bundle.In this issue, Mehta et al. describe a single center’s experience of implantation of left bundle branch area pacing leads, complete with one year follow-up data. Their retrospective study included 65 patients over an 18 month period who received a 3830 Medtronic lead for LBBAP. Only patients with one year of follow up data were included. Importantly, they excluded 7 patients in whom LBBAP implant was unsuccessful, resulting in a reported implant success rate of 91.8%. The reported procedure time was 72.7 +/- 28.8 minutes. The mean QRS duration decreased significantly in patients with pre-existing LBBB, was unchanged in those with pre-existing RBBB, and increased significantly in patients without pre-exiting bundle branch block. At one year follow-up, they found a significantly higher capture threshold compared to threshold at implant (p<0.0001), although they noted that the absolute increase (0.2-0.3 V) was relatively modest. No short or long-term complications related to device implant were identified.This study underscores several important points regarding LBBAP. Firstly, procedural success is high, with the authors reporting an implant success rate of 91.8% in this study. Reasons for implant failure included unacceptably high pacing thresholds, inability to meet parameters used to ensure capture of the left bundle, and anatomic constraints in one patient that prohibited lead placement. This success rate is similar to that reported in other studies10. Other studies have found higher implant success rate with LBBAP as compared to HBP11. Secondly, this success rate is likely easily reproducible as the implant technique does not require additional or specialized training, only a long delivery sheath and some familiarity with the mechanism to fix the 3830 lead to the interventricular septum. With preliminary data suggesting improved outcomes compared to traditional RV septal pacing12and the relative ease of procedural proficiency, it would seem that this technique could rapidly become the gold standard for pacemaker implantation.While there is great enthusiasm for LBBAP, several key questions remain. Firstly, this study, along with a recent publication from the Geisinger-Rush Conduction System Pacing Registry12, are the first to publish one year data following LBBAP lead implant. While lead parameters were reported to be stable at one year in this study, longer term data are needed. Furthermore, in addition to data on long term lead parameters and clinical efficacy, long term safety outcomes will be important. Particularly, subclinical perforation of the helix into the LV may carry thrombotic risk and long term data on whether this increases stroke risk will be important13. One prior study found two cases of subclinical septal perforation occurring after LBBAP lead placement14. Another case report described a patient presenting with presyncope two weeks after device implant with perforation of the lead into the LV and associated lack of capture up to 3.5 V @ 0.4 ms (bipolar)15. The incidence and risk factors for these events has yet to be established and the associated morbidity, including stroke risk from thromboembolism, remains uncharacterized. Another question that needs to be addressed is that of extractability of LBBAP leads. While there have been few published reports of successful extraction of these leads16, these lumen-less leads represent unique challenges in terms of extraction tools and technique. More data is needed to characterize the approach and outcomes for extraction of LBBAP leads.Finally, the criteria for left bundle capture, as opposed to left septal pacing, has yet to be firmly established, with both being included in the catch-all term “left bundle branch area pacing.” A recent study by Wu et al. utilized temporary HBP leads and LV septal mapping with multielectrode catheters during LBBAP lead implant to identify markers of selective LBBP17. They identified several useful parameters, including paced RBBB pattern (100% sensitive), LBB potential on the lead electrogram, abrupt shortening of stim-LV activation time (LVAT) and certain stim-LVAT times (which varied based on the presence or absence of an underlying LBBB). Another study from the same group utilized a decapolar coronary sinus catheter to examine differences in LV activation time and depolarization with left bundle capture versus left septal pacing, finding that the a model combining the presence of a LBBB potential on the lead coupled with a validated stim-LVAT had excellent test characteristics (AUC = 0.985) to differentiate conduction system from septal pacing18. In the current study, the authors reported 67.7% of patients with selective left bundle capture and 32.3% with left septal pacing when applying this algorithm. A scientific consensus on how to identify left bundle capture is essential to both study the long term clinical benefits in patients undergoing LBBAP implantation as well as establishing criteria to guide implanting physicians attempting to achieve selective left bundle capture.In summary, left bundle branch area pacing remains an exciting and burgeoning area within cardiac electrophysiology. This study by Mehta et al. demonstrated stable and acceptable lead parameters one year after LBBAP implantation, adding evidence of the long term safety and efficacy of this technique. Several questions remain, including longer term efficacy, the incidence of subacute and chronic complications, the ability to extract these leads when indicated, and the effect of LBBAP clinical outcomes. Furthermore, rigorous definitions for left bundle capture and left septal pacing need to be agreed upon. However, if these data can be replicated on a larger scale and longer term data shows similar safety and efficacy, LBBAP may likely represent the gold standard for pacing for bradyarrhythmia indications in the near future.

Catheter ablation in children has evolved to become a highly effective and safe therapy. Each iterative improvement in ablation technology provides another opportunity to investigate how much incremental benefit can be made without sacrificing safety. Contact force sensing catheters represent an example of such technology that has become commonplace in adult ablation. Its capability in predicting lesion size and collateral damage to critical structures has not been meticulously explored. Backhoff and colleagues describe an animal ablation model where they quantitate lesion characteristics at the atrium, atrioventricular groove, and ventricle using low and high contact force targets, with a specific focus on assessing for coronary arterial injury. In this controlled experiment, chronic lesion characteristics were widely variable (~0-8 mm diameter) yet there was a statistically significant (albeit small) increase in lesion diameter for high (vs low) contact force lesions delivered to the atrioventricular groove. The risk of chronic sub-clinical coronary artery injury was 1-2%.

Fetal atrioventricular block (AVB) is a failure of conduction from atria to ventricles. Immune- and non-immune-mediated forms occur, especially in association with congenital heart disease. Second-degree (2°) AVB may be reversible with dexamethasone and IVIG in immune-mediated disease. However, once third-degree AVB develops, it is deemed irreversible with need for a pacemaker and risk for cardiomyopathy. Rarely, 2° AVB is a transient, benign phenomenon in the immature conduction system. Few case series of transient AVB have been reported, but a management approach has not been defined. We report four patients with self-resolving, non-immune fetal AVB and outline a management strategy.

Pulsed-Field Ablation: What Are the Unknowns and When Will They Cease to Concern Us?Nathaniel A. Steiger* and Jorge Romero, MD*

Introduction: Novel ablation catheters equipped with mini-electrodes (ME) offer high resolution mapping for target tissue. This study aimed to evaluate the mapping performance and efficacy of ME catheters in radiofrequency ablation of paroxysmal supraventricular tachycardias (PSVTs). Methods: We prospectively enrolled 136 patients who were undergoing catheter ablation of PSVT including 76 patients with atrioventricular nodal reentrant tachycardia (AVNRT) and 60 patients with atrioventricular reentrant tachycardia (AVRT) or Wolff-Parkinson-White (WPW) syndrome. Patients were randomized to the ME group (ablation using a 4.5mm tip ME catheter) or the control group (ablation using a conventional 4mm tip catheter). The number of ablation attempt and cumulative ablation time to ablation endpoints, which was defined as an emergence of junctional rhythm in AVNRT or accessory pathway (AP) block in AVRT/WPW syndrome were compared. Results: During ablation procedures, discrete SP or AP electrograms were found in 27 (39.7%) patients in the ME group and 13 (19.1%) patients in the control group. The primary study outcomes were significantly lower in the ME group (ablation attempt number: 2.0 [1–4] vs. 3.0 [2–7] in the ME and control group, p=0.032; ablation time: 23.5 [5.0–111.5] vs. 64.5 [16.0–185.0] seconds, p=0.013). According to the PSVT diagnosis, ablation time to junctional rhythm was significantly shorter in the ME group in AVNRT. In AVRT/WPW syndrome, both ablation attempt number and ablation time to AP block showed trends favoring the ME group. Conclusion: The novel ME catheter was advantageous for identifying pathway potentials and reducing initial ablation attempt number and time in PSVT ablation. (ClinicalTrials.gov number, NCT04215640)

Introduction Most currently available pacing and defibrillation leads utilize a stylet-based design that facilitates implantation. This has advantages, but also increases the lead diameter and adds the potential for metal fatigued-based conductor failure. Methods A systematic literature search was conducted, and the authors add their twenty-year experience with this lead design. Results The global experience with lumenless leads was reviewed both for “standard” positioning and with conduction system pacing. Methods for both placement and system modification are reviewed. Conclusions Lumenless leads have the potential to improve the durability of endocardial pacing and facilitate conduction system pacing.

Background: Epicardial pacemakers are known as an alternative for endocardial pacemakers in some cases such as heart block, and complex congenital heart diseases. Considering recent advances and improvement of epicardial lead subtypes, it is essential to investigate the long-term function of them. In this study, we aimed to assess the sensing and pacing characteristics, and survival of bipolar steroid-eluting and unipolar non-steroid-eluting epicardial pacemakers. Methods: We conducted an entirely concentrated search on the documents of all patients who had undergone epicardial lead implantation in the Shaheed Rajaie Cardiovascular, Medical & Research Center during 2015-2018. Implant, and follow up data were extracted. Kaplan Meier analysis and Weibull regression hazards model were applied for the survival analysis. Results: eighty-nine leads were implanted for 77 patients. Of the total leads, 52.81%, 53.93%, and 47.19% were implanted in children (under-18-year-old), females, and patients with congenital heart diseases, respectively. Bipolar steroid-eluting leads comprised 33.71% of 89 leads. Pacing threshold of unipolar non-steroid-eluting leads that were implanted on the left ventricle and right atrium increased significantly during the follow up to greater records than bipolar steroid-eluting leads. Survival analysis also revealed that bipolar steroid-eluting leads are significantly better in 48-month survival (Weibull HR: 0.13 (95%CI, 0.02-0.99), p-value, 0.049). Age, ventricular location of the lead, and acute pacing characteristics were not associated with survival. Conclusions: Bipolar steroid-eluting epicardial leads have an acceptable survival compared with unipolar non-steroid-eluting, without a significant difference regarding patients age. Therefore, they could be an excellent alternative for endocardial ones.

Title: Reply to “Pro-arrhythmia with Anti-arrhythmic Drugs in Patients with Idiopathic Ventricular Arrhythmia: A Common Problem with Vague Definitions and Complex Interactions”Jacky K. K. Tang MD1 and Marc W. Deyell MD MSC1,2Heart Rhythm Services, Division of Cardiology, University of British ColumbiaCentre for Cardiovascular Innovation, University of British ColumbiaWord Count: 538 (including references)Address for correspondence:Dr. Marc William DeyellHeart Rhythm Services, St. Paul’s Hospital200 – 1033 Davie St.Vancouver, B.C., Canada, V6E 1M7Phone: 604-806-8256; Fax: 604-806-8723Email: [email protected]@MarcDeyellCompeting Interests: Dr. Deyell reports research grants from Biosense Webster and honoraria from Biosense Webster, Medtronic and Abbott.Funding: This work was supported by the UBC Division of Cardiology Academic Practice Plan.Drs. Hasdemir and Payzin have cogently brought up one of the primary challenges in studying patients with frequent premature ventricular complexes (PVCs) and evaluating the impact of therapy. They highlight, based on their prior study,(1) that a group of patients may actually experience a significant increase in PVC burden (>50%) with medical therapy, which obviously raises concerns that this may enhance deleterious effects of PVCs, particularly in the long term.With the advent of ambulatory monitoring, it was recognized early that PVC burden could be highly variable, leading to measurement error when using a 24-hour monitor. This error is highest when performing before-and-after studies of the effect of intervention on PVC burden, using single monitoring periods of 24 hours prior to and after intervention. In particular, spontaneous reductions in PVC burden can overestimate treatment effects. This error can be minimized in two ways, through serial monitoring (repeated measures) or longer-term monitoring (>48h). Indeed, an elegant study by Dr. Mullis and colleagues, using 14 day patch monitors, showed a median absolute day-to-day fluctuation in PVC burden of almost 10% among patients with a high burden of PVC.(2) Thus, an apparent “pro-arrhythmic” effect of a medication may simply reflect inefficacy and expected variation in PVC burden.In our prior work,(3) published in this journal and referenced by Drs. Hasdemir and Payzin, we were also limited by using only 24-hour ambulatory monitors to assess PVC burden. However, we did include a control group on no medical therapy, to mitigate the effect of measurement error, by obtaining an estimate of variation in PVC burden in the absence of therapy. In our study, we observed a “pro-arrhythmic effect” (>50% increase in PVC burden), in 2.5% (1/40) of patients on no medical therapy, 7.5% (4/53) on beta blockers/calcium channel blockers and 11.1%% (3/27) on class I/III antiarrhythmic therapy. Despite the trend, these were not significantly different (p=0.28 and p=0.14 for beta blocker/calcium channel blocker and class I/III antiarrhythmics versus no therapy).This does not negate a potential pro-arrhythmic effect of medical therapy in a minority of patients. However, more definitive proof of a pro-arrhythmic effect, distinguishing this from spontaneous variation in PVC burden, would require demonstration of a decrease in PVC burden with cessation of therapy. This would best be accomplished with a blinded, cross-over trial design.Drs. Hasdemir and Payzin remind us to always critically assess, and reassess, our therapies for patients with frequent PVCs. We must always evaluate whether treatment is warranted (in the majority of cases it is not), and whether patients are at risk for adverse events, particularly from class I and III antiarrhythmics. Frequent PVCs make physicians uncomfortable but we should not rush to treatment and expose patients to unnecessary harm.References:1. Turan OE, Aydin M, Odabasi AY, Inc M, Payzin S, Hasdemir C. Therapeutic Inefficacy and Proarrhythmic Nature of Metoprolol Succinate and Carvedilol Therapy in Patients With Idiopathic, Frequent, Monomorphic Premature Ventricular Contractions. Am J Ther 2021;29:e34-e42.2. Mullis AH, Ayoub K, Shah J et al. Fluctuations in premature ventricular contraction burden can affect medical assessment and management. Heart Rhythm 2019;16:1570-1574.3. Tang JKK, Andrade JG, Hawkins NM et al. Effectiveness of medical therapy for treatment of idiopathic frequent premature ventricular complexes. Journal of Cardiovascular Electrophysiology 2021;32:2246-2253.

An 80-year-old man underwent catheter ablation for atrial tachycardia (AT), which developed after catheter ablation for atrial fibrillation. The AT was diagnosed as dual-loop tachycardia, which included peri-mitral and roof-dependent ATs. An ethanol infusion into the vein of Marshall resulted in left phrenic nerve paralysis. During the procedure, the phrenic nerve paralysis was completely relieved.

There is still uncertainity about the use of CT-scan for LAAO device sizing. The main reason for this disappointing position is likely to relate to the scarcity of robust data, since there is still difference among institutions with regards how to perform measurement of the devices. Dallan et al. (1) report their own experience on the use of a novel computed tomography angiography-based (CTA) for sizing the Watchman Flex device for left atrial appendage occlusion (LAAO) . The authors through the TruPlan software package that a pre-procedural CTA sizing protocol can be applied successfully with ICE guidance and provide excellent procedural outcomes.The applied CTA protocol is safe and can provide high success rates with the WatchmanTM FLX device reducing the number of deployment attempts and reducing the risk of complications.

Dr. Rahul George Muthalaly MBBS, MPH1,2, Dr. Roy M John MBBS, PhD, FRCP3Monash Heart, Monash Health, Melbourne VICVictorian Heart Institute, Monash University, Melbourne VICStanford University, Palo Alto, CA 94304 USACorresponding Author:Dr. Rahul G MuthalalyPh: [email protected] information for all other authors:[email protected] Count: 1238 wordsFunding: NoneConflict of Interest: NoneAblation for atrial fibrillation (AF) is an established therapy that continues to grow in scope and indication(1). The benefits of AF ablation are well recognized in heart failure and symptomatic paroxysmal AF. Additionally, the recent EAST-AFNET4 trial demonstrated benefit for an early AF rhythm control strategy even in asymptomatic patients (2). This change in paradigm from rate and rhythm control equivalence may be partially related to the increasing use of AF ablation for rhythm control. As ablation therapy continues to proliferate, questions of how to optimise procedural outcomes at a health service level arise. A key component of this optimisation is defining predictors of outcomes related to ablation procedures.One such predictor is procedural case volume. Studies outside of AF ablation have demonstrated complex relationships between institutional size, procedural volume, case difficulty and outcomes. Published data on transcatheter aortic valve replacement (TAVR) for example, suggest a durable link between hospital case volume and mortality even after adjusting for institutional learning curves and known risk factors for poor outcomes (3). These insights have relevance for AF ablation for which there is wide-ranging procedural variation across factors such as lesion sets and type of ablation energy. The relationship between AF ablation volume and outcomes have been explored in previous studies(4, 5). Tonchev and colleagues, in a meta-analysis, demonstrated a significantly lower risk of complications in centres performing >100 procedures per year (OR 0.62, 95%CI 0.53 to 0.73) (4). This carried over to mortality, which was significantly lower in high volume centres (OR 0.33, 95% CI 0.26-0.43). Ablation efficacy was also greater in higher volume centres. Importantly, the majority of procedures included in this meta-analysis of real-world data were undertaken in low volume centres (70.9% in centres with <100 procedures/year) alerting to the reality that high-volume centers are not readily available to most patients.The influence of energy types (cryoballoon ablation versus catheter based radiofrequency ablation) on the relationship between procedural volume and outcomes is less well defined. This is of relevance as cryoablation appears to have a gentler learning curve than radiofrequency ablation for centres that are newly introducing AF ablation(6). In this issue of the journal , Kanaoka and colleagues present data on the relationship between procedural volume, energy type and acute procedural outcomes based on analysis of the Japanese National Database of Insurance Claims and Specific Health Check-ups. The authors identified 270,116 patients from this database undergoing first-time AF ablation with cryoablation or radiofrequency energy. A small subset of patients who underwent hot balloon and laser ablation were excluded as the numbers were too low. Complications were identified using administrative coding for the most common diagnoses associated with ablation risks. Ablation success could only be defined using coding for repeat AF ablation or initiation of anti-arrhythmic drug within 1 year.The authors split hospitals into quartiles using case volume resulting in groups with medians of 69 (very low), 157 (low), 252 (high) and 469 (very high) procedures per year. They found that the relative risk of peri-procedural complications was approximately 10-20% lower in all other groups when compared with the very low procedural volume hospitals. Cubic spline plots demonstrated a plateau effect, with no further reduction in complications when hospitals approach 150-200 cases per year. Similarly, when considering AF recurrence assessed by repeat ablation or initiation of anti-arrhythmic drugs up to 1 year, there was approximately a 10% relative risk reduction in the low, high and very high-volume hospitals when compared to very low-volume centres. Of note, there appeared to be a similar plateau volume of approximately 150-200 cases per year above which the benefit attenuated. The relationship between procedural volume and outcomes was however, only seen with RF ablation. Among the more than 56,000 cryoablation cases, there were no significant differences in complications or recurrence in the low, high or very high procedural volume hospitals when compared with the very low volume hospitals. Of note, there was a similar burden of AF related comorbidities in the radiofrequency and cryoablation populations.These results of Kanoaka and colleagues add further evidence to the notion that, when it comes to AF ablation, there appears to be a threshold of hospital procedural volume above which the risk of complications and recurrence decreases. The finding of a threshold in the range of 150-200 procedures per year, is in keeping with previous work suggesting benefits in excess of 100 procedures per year(5). Additionally, the present study sheds new light on the effect of ablation type on this relationship. In keeping with the recognition that cryoablation appears to offer an easier learning curve, the effect of procedural volume on both safety and efficacy outcomes was eliminated when considering only cryoablation cases.Limitations of the study have to be recognized. The study is observational, retrospective and derived from a national database. These factors admit the potential for unmeasured confounders and imperfect assessment of outcomes. The definition of ablation success was based on coding for repeat ablation or use of antiarrhythmic drugs within the first year. This definition, although pragmatic, does not account for recurrence occurring outside of 1 year or recurrence that did not result in repeat ablation or anti-arrhythmic drug therapy. Furthermore, complications were defined using administrative coding. Notably, this coding was not able to identify phrenic nerve injury, which occurs in approximately 5-6% of patients undergoing cryoablation(7). In addition, significant differences existed between the RF and cryoablation groups. There was a greater proportion of paroxysmal AF in the cryoablation compared to RF group (74% versus 46%). Because RF ablation allows for more flexibility in ablation lesion set, this energy form tends to be used in those with more advanced atrial remodelling. Hence, it is very likely that the RF group comprised a more complex patient group with a greater dependence on operator experience and case volume. The observed differences in learning curve between the two energy sources may have been less striking or even absent, had the groups been matched. The study however, has the advantage of the use of a database that covers of 98% of the Japanese population and magnitude of case volume included, allow for valuable insights.Abundant work has now demonstrated that hospital procedural volume is a key component of optimising outcomes from any complex interventional procedure(3, 5). The improved outcomes observed as a result of hospital procedural volume are likely due to a wide-range of additional factors that include appropriate patient selection, an experienced electrophysiology laboratory team and standardized, guideline-directed pre- and post-procedural management(8). The Heart Rhythm Society’s 2020 AF Centres of Excellence whitepaper outlines the rationale and guidance for key components of an AF Centre of Excellence(8). However, as the increasing benefit of AF ablation creates an increasing demand for the procedure, a balance must be struck between having very few centres of excellence and abundant centres with limited experience. In this context, the results of the current study by Kanoaka and colleagues are valuable and raises the possibility of a two-tiered approach. That of lower volume centres providing less complex procedures such as cryoablation for simpler cases of AF and higher volume ‘Centres of Excellence’ providing advanced ablation procedures for more complex AF cases. Such a division of labor may strike the balance between ablation availability and optimal outcomes. This differentiation, based on the characteristics of procedure offered and type of patients treated, will likely achieve greater importance as future technologies such as pulsed field ablation emerge and promise faster, safer and easier ablation strategies for AF(9).References1. Hindricks G, Potpara T, Dagres N, Arbelo E, Bax JJ, Blomström-Lundqvist C, et al. 2020 ESC Guidelines for the diagnosis and management of atrial fibrillation developed in collaboration with the European Association for Cardio-Thoracic Surgery (EACTS): The Task Force for the diagnosis and management of atrial fibrillation of the European Society of Cardiology (ESC) Developed with the special contribution of the European Heart Rhythm Association (EHRA) of the ESC. European Heart Journal. 2020;42(5):373-498.2. Kirchhof P, Camm AJ, Goette A, Brandes A, Eckardt L, Elvan A, et al. Early Rhythm-Control Therapy in Patients with Atrial Fibrillation. New England Journal of Medicine. 2020;383(14):1305-16.3. Vemulapalli S, Carroll JD, Mack MJ, Li Z, Dai D, Kosinski AS, et al. Procedural Volume and Outcomes for Transcatheter Aortic-Valve Replacement. New England Journal of Medicine. 2019;380(26):2541-50.4. Cheung JW, Yeo I, Cheng EP, Ip JE, Thomas G, Liu CF, et al. Inpatient hospital procedural volume and outcomes following catheter ablation of atrial fibrillation. J Cardiovasc Electrophysiol. 2020;31(8):1908-19.5. Tonchev IR, Nam MCY, Gorelik A, Kumar S, Haqqani H, Sanders P, et al. Relationship between procedural volume and complication rates for catheter ablation of atrial fibrillation: a systematic review and meta-analysis. Europace. 2021;23(7):1024-32.6. Velagic V, Prepolec I, Pasara V, Puljevic M, Puljevic D, Planinc I, et al. Learning curves in atrial fibrillation ablation – A comparison between second generation cryoballoon and contact force sensing radiofrequency catheters. Indian Pacing and Electrophysiology Journal. 2020;20(6):273-80.7. Heeger C-H, Sohns C, Pott A, Metzner A, Inaba O, Straube F, et al. Phrenic Nerve Injury During Cryoballoon-Based Pulmonary Vein Isolation: Results of the Worldwide YETI Registry. Circulation: Arrhythmia and Electrophysiology. 2022;15(1):e010516.8. Piccini JP, Sr., Allred J, Bunch TJ, Deering TF, Di Biase L, Hussein AA, et al. Rationale, considerations, and goals for atrial fibrillation centers of excellence: A Heart Rhythm Society perspective. Heart Rhythm. 2020;17(10):1804-32.9. Reddy VY, Dukkipati SR, Neuzil P, Anic A, Petru J, Funasako M, et al. Pulsed Field Ablation of Paroxysmal Atrial Fibrillation: 1-Year Outcomes of IMPULSE, PEFCAT, and PEFCAT II. JACC: Clinical Electrophysiology. 2021;7(5):614-27.

We would like to thank the authors for their letter and interest in our manuscript. We appreciate their valuable comments. The authors have raised the following important points: 1. The heterogenicity of our cohort with mitral valve surgery (MVS) 2. Elucidating the characteristics and causes of ventricular arrhythmias (VA) in patients with primary versus secondary mitral disease as well as in those with ischemic versus non-ischemic heart disease 3. The paucity of cardiac magnetic resonance imaging (MRI) as a limitation of the study 4. The need for careful evaluation of patients prior to ablation since most arrhythmias did not originate from the perimitral area.

The aortic sinuses of Valsalva are an important ablation site in non-ischemic substrates and in patients with idiopathic ventricular arrhythmias. Siontis and colleagues have demonstrated that these sites should also be considered for ablation in patients with infarct-related inferior axis VT. Low voltage in the aortic sinuses of Valsalva or in the sub-aortic region should prompt further evaluation of these regions for ablation.

A 77-year-old woman with palpitations was referred for a second radiofrequency ablation for persistent atrial tachycardia (AT). She previously underwent pulmonary vein (PV) isolation for paroxysmal atrial fibrillation, linear ablation between the 3’o clock position of the mitral annulus (MA) and left PV from the endocardium, and ablation inside the coronary sinus (CS) for perimitral atrial tachycardia (PMAT) in the first procedure. A baseline 12-lead electrocardiogram in the second procedure showed stable AT with a cycle length (CL) of 250 ms. No PV reconnection was observed. The CS catheter was placed from 3:30 to 5:00 on the MA, and a proximal-to-distal pattern of CS activation during AT was observed. Activation mapping in the left atrial (LA) endocardium using a three-dimensional mapping system (CARTO3, Biosense Webster, Diamond Bar, CA, USA) revealed a sequence of counterclockwise rotations of the MA. Figures 1A and 1B show the intracardiac electrograms during high-output (20 V) and low-output (5 V) atrial entrainment pacing at a pacing CL of 240ms from CS 3,4, which corresponds to the 4’o clock position of the MA. Dose residual conduction occur across the mitral isthmus (MI) endocardium, epicardium, or both? What is the electrophysiological mechanism during high- and low-output entrainment pacing?