Bipolar ablation's unique paradigm: Duration and power as respectively distinct primary determinants of transmurality and steam pop formation

Endo-epicardial Transmural Lesion Formation Utilizing Multipolar Radiofrequency Ablation

BACKGROUND

Conventional unipolar radiofrequency catheter ablation (RFCA) is limited by maximal lesion depths that fail to eliminate VTs with deep mid-myocardial critical components. The use of multipolar mapping catheters as an active part of the ablation circuit may provide a solution to these limitations.

OBJECTIVE

To evaluate a novel endo-epicardial multipolar-RFCA technique for creating transmural lesions in an in-vivo porcine model.

METHODS

Two catheter configurations were evaluated: (1) standard bipolar configuration with a 3.5-mm irrigated-tip catheter paired with an 8-mm non-irrigated-tip catheter. (2) Multipolar configuration with a 3.5-mm irrigated-tip catheter paired with a multipolar mapping catheter. In both configurations, the 3.5-mm irrigated-tip catheter was positioned on the endocardial surface of the LV with the corresponding catheter positioned perpendicularly on the epicardial surface.

RESULTS

The study included 10 subjects with a total of 30 lesions created using 6 different sets of catheter configurations and ablation parameters. Histopathological analysis revealed an average lesion depth was 10.6 ± 3.1 mm (range: 5-17 mm), corresponding to a lesion depth/tissue thickness ratio of 93.5 ± 12%. 20 out of 30 lesions (67%) achieved transmurality. No safety complications such as steam pops, fistulas, perforations, or tamponades were observed.

CONCLUSION

Endo-epicardial multipolar radiofrequency ablation can be performed effectively and safely using a multipolar mapping catheter as the return electrode in the epicardial space. Lesions created were voluminous with a high degree of transmurality, with no complications related to the application of radiofrequency recorded. This catheter configuration may present a novel solution to rapidly identify and target ventricular arrhythmias arising from the mid-myocardium.

Temperature-controlled bipolar radiofrequency ablation: An ex vivo study for optimizing efficacy and safety parameters

BACKGROUND

Clear indicators for efficacy and safety of bipolar radiofrequency catheter ablation (BRFA) remain undefined.

OBJECTIVE

This study aimed to investigate predictive indicators of transmural lesion formation and steam pop in BRFA.

METHODS

In an ex vivo model, BRFA was performed with either the QDOT Micro or ThermoCool SmartTouch SF as the active catheter and the DiamondTemp ablation (DTA) as the return catheter. Predictors of transmural lesion formation and steam pop occurrence were investigated.

RESULTS

A total of 391 BRFA applications were conducted under various catheter tip and tissue contact configurations with interelectrode distance of 6-27 mm. The ablation index (AI) adjusted for interelectrode distance was effective in predicting transmural lesion formation, with AI increasing linearly as interelectrode distance increased. Logistic regression revealed that the coefficient for AI was -0.040 (standard error, 0.0067; 95% confidence interval, -0.053 to -0.027; P < .0001); for interelectrode distance, it was 2.2 (standard error, 0.35; 95% confidence interval, 1.5-2.9; P < .0001). The AI required to achieve transmural lesion formation was calculated as AI = 54 × interelectrode distance - 260. Steam pops on the active side occurred only during power-controlled BRFA and were absent with a 45°C cutoff. On the return side, steam pops occurred above 55°C. Higher DTA temperatures resulted in deeper cracks.

CONCLUSION

An AI adjusted for interelectrode distance strongly predicted transmural lesion formation. Temperature-controlled BRFA with a 45°C cutoff for the active catheter and 55°C for the return catheter may prevent steam pops. Furthermore, steam pops induced by higher electrode temperatures may result in the formation of deeper cracks.

Lesion characteristics of long application time ablation using unipolar half-normal saline irrigation and bipolar normal saline irrigation

INTRODUCTION

Unipolar radiofrequency (RF) ablation using half-normal saline irrigation (UNIP-HNS) and bipolar RF ablation using normal saline irrigation (BIP-NS) are effective to treat arrhythmias from inside thick myocardium. However, differences between these two ablations when using a long application time had not fully been studied.

METHODS

UNIP-HNS, BIP-NS and unipolar RF ablation using normal saline irrigation (UNIP-NS) were applied for 120 s (30 W and 20-g contact) to porcine endocardial wall (≧15.0 mm thickness).

RESULTS

All ablations (30 applications each in UNIP-HNS and BIP-NS, and 20 applications in UNIP-NS) were successfully accomplished without steam-pop. Total impedance decline was largest in BIP-NS followed by UNIP-HNS and UNIP-NS. UNIP-HNS created larger surface lesions and greater maximum lesion width under the surface than those by UNIP-NS and BIP-NS. Endocardial lesion depth in UNIP-HNS and BIP-NS were deeper than that in UNIP-NS, but with no difference between UNIP-HNS and BIP-NS, when selecting non-transmural lesions. Similar results were obtained when all lesions (non-transmural and transmural) were included and endocardial lesion depth of the transmural lesions (13/30 applications of BIP-NS) was estimated as 50 % of the myocardial thickness. Lesion length in the transverse myocardial wall (endocardial plus epicardial lesions) was greatest in BIP-NS.

CONCLUSIONS

Longer application time ablation (30 W) targeting the thick myocardium was performable in UNIP-HNS and BIP-NS. Since a transmural lesion and/or a deeper lesion into the myocardial wall are created, BIP-NS is preferable if two ablation catheters can be positioned on either side of the target.

Volumetric lesion analysis and validation of various bipolar configurations in radiofrequency ablation of ventricular myocardium in a bovine model

BACKGROUND

The bipolar radiofrequency ablation(B-RFA) strategy was increasingly used to target deep intramural re-entrant foci responsible for the arrhythmia not ablated by conventional unipolar RFA / sequential unipolar RFA. Lesional characteristics of various bipolar configurations were largely unknown.

OBJECTIVE

To investigate the lesional geometry in relation to various factors to determine the most effective ablation strategy that minimises steam pops and achieves transmurality. To assess the temperatures at the return electrode.

METHODS

A custom-made validated ex-vivo bipolar ablation model was used to assess lesion formation. The myocardial sample was placed between two ablation catheters in four different orientations. Lesions were created using different power (30 W, 40 W, 50 W) and time settings(30, 40 and 50 s) with different catheter orientations. Data was analysed using binary logistic regression and multiple linear regression.

RESULTS

Among 107 lesions, The volume of the active catheter lesion (266 +/- 137 mm^3) significantly differed from their return electrode counterparts (130 +/- 91.8 mm^3) (p < 0.001), and the temperatures at the return electrode end were lower than at the active electrode (p = 0.004). Higher power and longer duration application led to more frequent steam pops (p < 0.001), while true parallel configuration resulted in fewer steam pops (p < 0.001).

CONCLUSION

A custom model without ground electrode temperature monitoring is safe and cost-effective. The safest strategy is a true parallel configuration with an inter-electrode distance of at least 15 mm and a power of 30 W to 40 W, which generates lower steam pops and better transmurality.

Bipolar endo-epicardial RF ablation: Animal feasibility study

BACKGROUND

Bipolar radiofrequency ablation (B-RFA) is a method used to treat the arrhythmia substrate resistant to unipolar ablation. Few studies have addressed endo-epicardial B-RFA.

OBJECTIVE

The aim of the study was to evaluate chronic lesions resulting from endo-epicardial B-RFA and to determine optimal settings for such procedures in an animal model.

METHODS

In 7 pigs, up to 5 radiofrequency applications per animal were performed with 2 electrodes placed on both sides of the left ventricular free wall. Current was delivered for 60 seconds by a generator dedicated for B-RFA with power settings of 25, 30, 35, 40, and 50 W.

RESULTS

At 12 weeks after ablation, 31 lesions were assessed. Their maximal cross-sectional area ranged from 7.2 to 68 mm2 and correlated with total power delivered (r = 0.53), with temperature increment at the endocardial catheter (r = 0.65), and inversely with temperature decrement at the epicardial catheter (r = 0.54). For power values between 30 and 40 W, the lesion area did not differ significantly (P = .92). Lesion depth ranged from 1.9 to 11 mm and correlated with impedance decrement (r = 0.5). Lesions were transmural in 8 cases. Lesion depth/wall thickness ratio was on average 0.6 ± 0.3, with the smallest value for 25 W (0.5 ± 0.3) and the largest for 50 W (0.8 ± 0.3). Steam pops occurred at a power range of 30-50 W, with an incidence of 1 in 5 applications, with 1 case of fatal tamponade at 40 W. Impedance decrement, endocardial catheter temperature increment, and endocardial electrogram amplitude decrement were greater during applications with steam pops.

CONCLUSION

Chronic lesions resulting from endo-epicardial B-RFA appear smaller and less often transmural compared with acute lesions described in the literature. The incidence of steam pops during endo-epicardial B-RFA is relatively high even at low powers.

Cryocure-VT: the safety and effectiveness of ultra-low-temperature cryoablation of monomorphic ventricular tachycardia in patients with ischaemic and non-ischaemic cardiomyopathies

AIMS

The ultra-low-temperature cryoablation (ULTC) ablation system using -196°C N2 cryogen has been reported to create lesions with freeze duration-dependent depth titratable to over 10 mm with minimum attenuation by scar. Cryocure-VT (NCT04893317) was a first-in-human clinical trial evaluating the safety and efficacy of a novel, purpose-built ULTC catheter in endocardial ablation of scar-dependent ventricular tachycardias (VTs).

METHODS AND RESULTS

This prospective, multi-centre study enrolled patients referred for de novo or second ablations of recurrent monomorphic VT of both ischaemic and non-ischaemic aetiologies. Primary safety and efficacy endpoints of the study were freedom from device- or procedure-related major adverse events (MAEs) up to 30 days post-ablation, acute non-inducibility of clinical VTs at the end of the procedure, and freedom from sustained VT or implantable defibrillator intervention at 6 months. Ultra-low-temperature cryoablation was performed in 64 patients (age 67 ± 11 years, 78% ischaemic, ejection fraction = 35 ± 10%) at 9 centres. The primary acute effectiveness endpoint was achieved in 94% (51/54) of patients in whom post-ablation induction was attempted. There were no protocol-defined MAEs; four procedure-related serious adverse events resolved without clinical sequelae. At 6-month follow-up, 38 patients (60.3%) remained VT-free, and freedom from defibrillator shock was 81.0%, with no significant difference between ischaemic and non-ischaemic cohorts. In 47 patients with defibrillator for at least 6 months prior to the ablation, the VT burden was reduced from median of 4, inter-quartile range (IQR, 1-9) to 0, IQR (0-2).

CONCLUSION

In this first-in-human multi-centre experience, endocardial ULTC ablation of monomorphic VT appears safe and effective in patients with both ischaemic-cardiomyopathy and non-ischaemic-cardiomyopathy.

CLINICAL TRIAL REGISTRATION

NCT04893317.

Symmetrical recovery time course between impedance and intramyocardial temperature after bipolar radiofrequency ablation; Role of impedance monitoring to estimate temperature rise

INTRODUCTION

During radiofrequency (RF) ablation, impedance monitoring has been used to avoid steam-pop caused by excessive intramyocardial temperature (IMT) rise. However, it is uncertain why the impedance decline is related to steam-pop and whether the impedance decline is correlated to IMT.

METHODS

Twenty-one bipolar ablations (40 W, 30-g contact, 120 s) were attempted for seven perfused porcine myocardium. Immediately after ablation, a temperature electrode was inserted into the mid-myocardial portion, and the recovery process of impedance and its correlation to IMT were assessed.

RESULTS

Transmural lesion was created in all 21 applications but steam-pop occurred in 5/21 applications with large impedance decline. In the 16 applications without steam-pop, impedance and IMT soon after ablation were 97.2 ± 4.0 Ω and 66.1 ± 4.8 °C, respectively. Reasonably high linear correlation was demonstrated between the maximum IMT after ablation and impedance differences before and after ablation. Recovery processes of the decreased impedance and the elevated IMT fit well to each equation of the single exponential decay function and showed symmetric shapes with no statistical difference of time constant (100.1 ± 34.5 s in impedance vs. 108.7 ± 27.3 s in IMT) and half-time of recovery (144.5 ± 49.8 s in impedance vs. 156.9 ± 39.4 s in IMT). Recovered impedance after ablation (104.8 ± 3.9 Ω) was 5.1 ± 2.0 Ω smaller than that before ablation (109.9 ± 2.7 Ω), suggesting several factors other than IMT rise participate in impedance decline in RF ablation.

CONCLUSIONS

Recovery of impedance and IMT after ablation well correlated, which supports the usefulness of impedance monitoring for safe RF ablation.

Bipolar ablation involving coronary venous system for refractory left ventricular summit arrhythmias

Background

Catheter ablation of premature ventricular complexes (PVCs) and ventricular tachycardia (VT) from the left ventricular summit (LVS) may require advanced ablation techniques. Bipolar ablation from the coronary veins and adjacent endocardial structures can be effective for refractory LVS arrhythmias.

Objective

The aim of this study was to investigate the outcomes of bipolar ablation performed between the coronary venous system and adjacent endocardial left ventricular outflow tract (LVOT) or right ventricular outflow tract (RVOT).

Methods

This multicenter study included consecutive patients with LVS PVC/VT who underwent bipolar ablation between the anterior interventricular vein (AIV) or great cardiac vein (GCV) and the endocardial LVOT/RVOT after failed unipolar ablation. Ablation was started with powers of 10-20 W and uptitrated to achieve an impedance drop of at least 10%. Angiography was performed in all cases to confirm a safe distance (>5 mm) of the catheter from the major coronary arteries.

Results

Between 2013 and 2023, bipolar radiofrequency ablation between the AIV/GCV and the adjacent LVOT/RVOT was attempted in 20 patients (4 female; age 57 ± 16 years). Unipolar ablation from sites of early activation (AIV/GCV, LVOT, aortic cusps, RVOT) failed to effectively suppress the PVC/VT in all subjects. Bipolar ablation was delivered with a maximum power of 30 ± 8 W and total duration of 238 ± 217 s and led to acute PVC/VT elimination in all patients. No procedural-related complications occurred. Over a follow-up period of 30 ± 24 months, the freedom from arrhythmia recurrence was 85% (1 recurrence in the VT group and 2 in the PVC group). PVC burden was reduced from 22% ± 10% to 4% ± 8% (P <.001).

Conclusion

In cases of LVS PVC/VT refractory to unipolar ablation, bipolar ablation between the coronary venous system and adjacent endocardial LVOT/RVOT is safe and effective if careful titration of power and intraprocedural angiography are performed to ensure a safe distance from the coronary arteries.

Bipolar ablation of ventricular arrhythmias: Step-by-step

Radiofrequency (RF) ablation of intramural ventricular arrhythmias (VAs) may require advanced ablation techniques to achieve effective energy transfer to the targeted tissue. As an alternative to standard RF ablation, catheter ablation can also be conducted in bipolar configuration when two ablation catheters participate in the RF circuit. This strategy has proved to result in deeper lesion formation and may be effective for eliminating arrhythmias that have been refractory to standard ablation. In this article, we provide a step-by-step guide on when and how to perform bipolar ablation of VAs.

Safety and efficacy of ablation index-guided high-power ablation for the treatment of atrial fibrillation

Objective

To study the safety and efficacy of high-power ablation for atrial fibrillation (AF) guided by lesion size index (LSI) and impedance cutoff.

Method

A total of 223 patients who underwent radiofrequency catheter ablation of atrial fibrillation (including paroxyparal atrial fibrillation and persistent atrial fibrillation) in the Department of Cardiology of Anhui Provincial Hospital from February 2019 to July 2020 were enrolled, and were divided into 123 patients in the high-power ablation group (HPAI) and 100 patients in the conventional power ablation group (CPAI). The HPAI group adopted high-power (40-50 W) ablation by impedance cutoff, and the CPAI group adopted conventional-power (30-35 W) ablation. Patients in both groups were ablated guided by the same LSI. For both groups, we analyzed the pulmonary vein single-circle isolation rate, ablation time, X-ray exposure, impedance drop value, incidence of complications, and recurrence rate within one year after operation.

Results

There was no significant difference in the success rate of pulmonary vein single-circle isolation, X-ray perspective time, and X-ray exposure quantity between the HPAI group and the CPAI group (88.60% vs 82.00%, P = 0.161; 8.7 ± 3.74 min vs 7.82 ± 3.86 min, P = 0.067; 54.74 ± 28 min vs 52.78 ± 39.58 min, P = 0.139); the annular pulmonary vein ablation time and total ablation time were less in the HPAI group (35.74 ± 7.25 min vs 65.49 ± 7.34 min, P < 0.01; 55.42 ± 11.61 min vs 76.9 ± 6.79 min, P < 0.01); the impedance drop values at 10-15Ω and 15-20Ω were higher in the HPAI group (25.3% vs 19.1%, P < 0.05; 24.1% vs 19.1%, P < 0.05); there was no significant difference in the recurrence rate within one year after operation between the two groups; and no serious complications occurred in the two groups.

Conclusion

High-power ablation guided by LSI and impedance cutoff could significantly shorten the AF ablation time and reduce complications.

Impedance-decline-guide power control long application time bipolar radiofrequency catheter ablation

INTRODUCTION

Bipolar (BIP) radiofrequency (RF) ablation creates deep myocardial lesions but ideal energy application modes to treat ventricular arrhythmias originating from deep inside the thick myocardium have not been well established. An experimental study was performed to clarify whether high power and long application time BIP ablation were performable by impedance-decline-guide power control (PC) and whether it could create transmural lesions in the thick ventricle with a minimum risk of steam-pop.

METHODS AND RESULTS

Perfused porcine ventricle (18.4 ± 2.3 mm) was placed in an experimental bath and BIP ablation (50 W) for 120 s was attempted with catheter contact of 30-g using two protocols; fixed power (FP) and impedance-decline-guide PC. In the latter protocol, BIP ablation was started from 50 W, while the energy was decreased to 40-20 W according to the impedance decline during RF ablation. FP ablation was attempted in 30 applications and the transmural lesion was created in all 30, although steam-pop occurred in 16/30 applications (53%). Low minimum impedance, large total impedance decline (TID), and %-TID were associated with the steam-pop occurrence. PC ablation was attempted in another 21 applications, and the transmural lesion was created in all 21 without steam-pop. PC ablation was superior to FP ablation (21/21 vs. 14/30, p < .001) in the creation of a transmural lesion without resulting in steam-pop.

CONCLUSIONS

High power and long application time BIP ablation seems to be feasible according to the impedance-decline-guide approach, which could create transmural lesions in thick porcine ventricles with minimal risk of steam-pop.

Insurance lesions: Does a second lesion make a difference?

INTRODUCTION

In radiofrequency ablation procedures for cardiac arrhythmia, the efficacy of creating repeated lesions at the same location ("insurance lesions") remains poorly studied. We assessed the effect of type of tissue, power, and time on the resulting lesion geometry during such multiple ablation procedures.

METHODS

A custom ex vivo ablation model was used to assess lesion formation. An ablation catheter was oriented perpendicular to the tissue and used to create lesions that varied by type of tissue (atrial or ventricular free wall), power (30 or 50 W), and time (30, 40, or 50 s for standard ablations and 5, 10, or 15 s for high-power, short-duration [HPSD] ablations). Lesion dimensions were recorded and then analyzed. Radiofrequency ablations were performed on 57 atrial tissue samples (28 HPSD, 29 standard) and 28 ventricular tissue samples (all standard).

RESULTS

With ablation parameters held constant, performing multiple ablations significantly increased lesion depth in ventricular tissue when ablations were performed at 30 W for 50 s. No other set of ablation parameters was shown to affect the width or depth of the resulting lesions in either tissue type.

CONCLUSION

Multiple ablations created with the same power and time, delivered within 30 s of each other at the same exact location, offer no meaningful benefit in lesion depth or width over single ablations, with the exception of ventricular ablation at 30 W for 50 s. Given the risks associated with excessive ablation, our results suggest that this practice should be re-evaluated by clinical electrophysiologists.

Perpendicular Catheter Orientation During Papillary Muscle Ablation Results in Larger, Deeper Lesions

INTRODUCTION

Ablation of papillary muscles (PM) for refractory ventricular arrhythmias can often be challenging. The catheter approach and orientation during ablation may affect optimal radiofrequency (RF) delivery. Yet, no previous study investigated the association between catheter orientation and PM lesion size. We evaluated ablation lesion characteristics with various catheter orientations relative to the PM tissue during open irrigated ablation, using a standardized, experimental setting.

METHODS

Viable bovine PM was positioned on a load cell in a circulating saline bath. RF ablation was performed over PM tissue at 50W, with the open irrigated catheter positioned either perpendicular or parallel to the PM surface. Applied force was 10 grams. Ablation lesions were sectioned and underwent quantitative morphometric analysis.

RESULTS

A catheter position oriented directly perpendicular to the PM tissue resulted in the largest ablation lesion volumes and depths compared to ablation with the catheter parallel to PM tissue (75.26±8.40 mm³ vs. 34.04±2.91 mm³ , p<0.001) and (3.33±0.18 mm vs. 2.24±0.10 mm, p<0.001), respectively. There were no significant differences in initial impedance, peak voltage, peak current, or overall decrease in impedance among groups. Parallel catheter orientation resulted in higher peak temperature (41.33±0.28°C vs. 40.28±0.24°C, p=0.003), yet, there were no steam pops in either group.

CONCLUSION

For PM ablation, catheter orientation perpendicular to the PM tissue achieves more effective and larger ablation lesions, with greater lesion depth. This may have implications for the chosen ventricular access approach, the type of catheter used, consideration for remote navigation, and steerable sheaths. This article is protected by copyright. All rights reserved.

Newer Methods for VT Ablation and When to Use Them

Radiofrequency (RF) catheter ablation has long been an important therapy for ventricular tachycardia and frequent symptomatic premature ventricular beats and nonsustained arrhythmias when antiarrhythmic drugs fail to suppress the arrhythmias. It is increasingly used in preference to antiarrhythmic drugs, sparing the patient drug adverse effects. Ablation success varies with the underlying heart disease and type of arrhythmia, being very effective for patients without structural heart disease, less in structural heart disease. Failure occurs when a target for ablation cannot be identified, or ablation lesions fail to reach and abolish the arrhythmia substrate that may be extensive, intramural or subepicardial in location. Approaches to improving ablation lesion creation are modifications to RF ablation and emerging investigational techniques. Easily implemented modifications to RF methods include manipulating the size and location of the cutaneous dispersive electrode, increasing RF delivery duration, and use of lower tonicity catheter irrigation (usually 0.45% saline). When catheters can be placed on either side of culprit substrate RF can be delivered in a bipolar or simultaneous unipolar configuration that can be successful. Catheters with extendable/retractable irrigated needles for RF delivery are under investigation in clinical trials. Cryoablation is potentially useful in specific situations when maintaining contact is difficult. Transvascular ethanol ablation and stereotactic radioablation have both shown promise for arrhythmias that fail other ablation strategies. Although substantial clinical progress has been achieved, further improvement is clearly needed. With ability to increase ablation lesion size, continued careful evaluation of safety, which has been excellent for standard RF ablation, remains important.

Safety, efficacy, and monitoring of bipolar radiofrequency ablation in beating myopathic human and healthy swine hearts

BACKGROUND

The safety and efficacy parameters for bipolar radiofrequency (RF) ablation are not well defined.

OBJECTIVE

The purpose of this study was to investigate the safe range of power, utility of transmyocardial bipolar electrogram (EGM) amplitude, and circuit impedance in ablation monitoring.

METHODS

Sixteen beating ex vivo human and swine hearts were studied in a Langendorff setup. Ninety-two bipolar ablations using two 4-mm irrigated catheters were performed at settings of 20-50 W, 60 seconds, and 30 mL/min irrigation in the left ventricle.

RESULTS

For low-power ablations (20 and 30 W), transmurality was observed in 29 of 38 (76%) and 10 of 28 (36%) ablations for tissue thickness ≤17 mm and >17 mm, respectively. For high-power ablations (40 and 50 W), transmurality was observed in 5 of 7 (71%) and 7 of 19 (37%) ablations for tissue thickness ≤17 mm and >17 mm, respectively. Steam pop occurrence for low- and high-power ablations was 11 of 66 (16%) and 16 of 26 (62%), respectively (P = .0001), respectively. Lesion depth (limited by transmurality) was 12.0 ± 5.7 mm and 12.3 ± 5.8 mm, respectively (P = 1). Transmyocardial EGM amplitude decrement >60% strongly predicted transmurality (area under the curve [AUC] 0.8), and circuit impedance decrement >26% predicted steam pops (AUC 0.75). Half-normal saline did not affect transmurality or incidence of steam pops compared to normal saline irrigation.

CONCLUSION

Bipolar RF ablation at power of 20-30 W provided an ideal balance of safety and efficacy, whereas power ≥40 W should be used with caution due to the high incidence of steam pops. Lesion transmurality monitoring and steam pop avoidance were best achieved using transmyocardial bipolar EGM voltage and circuit impedance, respectively.