When sequential unipolar ablation fails, as is frequently the case with deep intramural substrates or dense scars, which act as impedance barriers, BRFA has emerged as a well-established bailout strategy and, increasingly, as a planned first-line approach at experienced centers. Bipolar ablation is a second-line advanced RFA technique for refractory ventricular arrhythmias that aims to achieve deep, transmural lesion formation by passing a current between two catheters positioned on opposing myocardial surfaces, rather than between a single catheter tip and a skin patch [4]. The circuit is assembled by replacing the standard dispersive ground patch with a second ablation catheter serving as the return electrode (Fig. 1). Even in the era of contact force and irrigated catheters, unipolar RFA results in limited lesion depths, whereas bipolar delivery concentrates energy across the full thickness of the targeted wall. Catheter orientation matters: a larger-tip return electrode (e.g., 8 mm) positioned parallel to the myocardial wall provides superior electrical grounding. Power delivery is typically initiated at a low setting (approximately 15 W) to test the circuit, then titrated to achieve an impedance drop of 20–30 Ω, which serves as a reliable surrogate for effective deep tissue heating (Fig. 2, Supplemental Video) [5].

Fig. 1

Schematic representation of intramyocardial scar and positioning of ablation catheter during unipolar and bipolar radiofrequency ablation (RFA); (A) intra-myocardial scar; (B) unipolar RFA with ablation catheter on one side of the scar and return electrode is a cutaneous patch (not demonstrated); (C) bipolar RFA with ablation catheter positioned at LV side of the septal scar and return electrode / catheter on RV side of the scar

Fig. 2

72-year-old male patient with ischemic cardiomyopathy with inferior-lateral scar, mapped VT at the basal inferior-septal location targeted using bipolar radiofrequency ablation (RFA) after failed unipolar RFA. The RV catheter depicted as return electrode and LV ablation catheter located at inferior-septal scar border

The safety and feasibility of BRFA were demonstrated in vivo as early as 1989, when Ring et al. performed unipolar and bipolar ablation in a closed-chest canine model [6]. With widespread adoption of unipolar RFA, the electrophysiological foundations and practical implications of BRFA were studied further throughout the past two decades. Early ex vivo studies comparing unipolar to bipolar ablation showed BRFA to achieve lesions more than twice as deep as unipolar ablation [7]. This allowed for transmural lesions and adequate reach of deep, intramural ablation targets. Early clinical adoption was reported in a small number of patients with VT refractory to unipolar ablation, in whom 83% of patients with interventricular septal re-entry circuits and 50% of patients with ventricular re-entry circuits achieved successful termination via BRFA. These studies suggested that successful termination via BRFA is related to simultaneous heating, leading to thermal synergy between catheter tips and increased current density resulting in greater thermal injury and thereby improving ablation efficacy [8]. Comparing lesion characteristics between sequential unipolar, simultaneous unipolar, and bipolar RFA on ex vivo myocardial tissue, the lesion volume and depth produced by both simultaneous unipolar and bipolar RFA across thick myocardial tissue were comparable and significantly larger than sequential unipolar RFA [9]. However, lesion geometry differed, with bipolar RFA yielding a denser necrotic core whilst the geometry of sequential and simultaneous unipolar RFA was alike. Furthermore, the necrotic cores of bipolar RFA were transmural, extending the full thickness of the tissue and involving mid-myocardial layers as compared to the hourglass-shaped lesion narrowing in the mid-myocardium seen with simultaneous RFA [9]. Another study compared unipolar and bipolar lesion characteristics in a post-infarct sheep model and found BRFA to require significantly fewer applications, less total ablation time and less energy to achieve transmural lesions [10]. For two catheter approaches, including BRFA, lesion size and geometry are determined by a multitude of factors, including catheter tip size, orientation and choice of irrigation. A comprehensive comparison of these parameters showed catheter orientation and tip size to be highly interrelated in determining lesion volume [11]. Furthermore, modulation of impedance, such as through catheter tip irrigation, can be used to achieve deeper and larger lesions [4].

A 2019 expert consensus statement described BRFA as a non-standard alternative bailout strategy only after failure of conventional unipolar RFA and did not provide a formal recommendation classification [12]. The guidelines discussed difficulties of setup and instrumentation, which were improved upon in following years, with novel techniques such as a radio frequency generator allowing simultaneous monitoring of power delivery, temperature at catheter tips and impedance at both catheters. Furthermore, this dedicated system allowed for alternating between unipolar and bipolar without the hassle of changing cable connections [13]. Other studies aimed to expand the use of BRFA beyond bailout procedures by evaluating bipolar ablation as the first ablation procedure in a defined group of patients with the decision guidance sought not through previous procedural failure but instead through electroanatomic mapping for intramural substrate [14]. The first prospective pilot cohort study evaluating the safety and efficacy of BRFA in patients with nonischemic dilated cardiomyopathy with drug-refractory VT of confirmed septal origin concluded that extra-interventricular septum substrate and inflammatory etiology are independent predictors of adverse outcome and BRFA is feasible and safe for septal VT in nonischemic cardiomyopathy [2]. The largest registry study for BRFA to date consisted of 94 procedures in 91 patients across 16 European centers. All included patients had at least one previously failed unipolar ablation for frequent premature ventricular contractions (PVCs), monomorphic VT, electrical storms or PVC-triggered ventricular fibrillation. The authors concluded that BRFA can be feasible, safe and effective in most patients with PVC or VT [15].

These studies provide a comprehensive understanding of the efficacy of BRFA and affirm its feasibility in the majority of patients with refractory intramural ventricular arrhythmias by demonstrating consistently high acute success rates and promisingly low recurrence rates with an acceptable complication profile, especially in the hands of experienced operators prioritizing safety precautions and standardized delivery protocols. The long-standing challenge of the inability to simultaneously monitor both catheters has been resolved with technological advancements in dedicated RFA generators and custom-made cable setups. These developments have allowed for improvements in safety and reproducibility across centres, both of which are essential for widespread acceptance and application. Nevertheless, the clinical impact of BRFA remains constrained by several limitations. Freedom from long-term arrhythmia is modest, lesion formation is severely hindered by epicardial adipose tissue, and large-scale randomized controlled trials comparing BRFA with other advanced ablation strategies remain absent.

Taken together, BRFA has an established role in the management of refractory intramural VT at high-volume centers. Its optimal integration as a planned primary strategy versus a bailout technique and the standardization of procedural parameters remain active areas of investigation.

Pulsed Field Ablation

Pulsed field ablation (PFA) for VT is an emerging technology with promising preclinical data and early clinical results, but current evidence is limited to small observational studies or case series using PFA catheters with significant heterogeneity in catheter footprint as well pulse parameters, such as bipolar versus unipolar PFA. Additionally, no randomized controlled trials have been completed.

Multiple swine infarct models have demonstrated key advantages of PFA over RFA in ventricular scar tissue: PFA produces deeper, more uniform, and more transmural lesions than RFA in scarred myocardium (Fig. 3). In one study, PFA lesions were significantly deeper (6.4 vs. 5.4 mm) and more frequently transmural (72% vs. 30%) compared with RFA. This is because RFA generates heat that is attenuated by intervening collagen and fat within scar, whereas PFA’s nonthermal electroporation mechanism penetrates through these layers to ablate surviving myocyte islands [16, 17]. PFA preserves the endothelial lining, coronary arteries, nerve fascicles, and connective tissue due to its tissue selectivity, which preferentially destroys cardiomyocytes while the extracellular matrix is spared [18, 19]. Furthermore, PFA lesions are less dependent on blood flow velocity and catheter contact compared with RFA, which may be advantageous in trabeculated regions and papillary muscles [18, 20]. Bipolar PFA across the interventricular septum or LV free wall has the potential to create transmural lesions up to 14.3 mm deep, significantly deeper than monopolar epicardial lesions [21]. However, lesion transmurality across the interventricular septum in unipolar PFA may be limited by both depth and complexity of substrate. Specifically, no evidence of PFA lesion transmurality was observed in patients with septal substrate and NICM undergoing ablation using a lattice-tip catheter (Sphere-9, Medtronic) in the CLEAR-VT registry [22]. These patients required biventricular septal ablation to achieve full electrogram elimination on both sides of the septum. QRS-gated delivery appears to eliminate the risk of PFA-induced ventricular arrhythmias observed with ungated applications [19].

Fig. 3

78-year-old man with ischemic cardiomyopathy with recurrent VT after radiofrequency ablation inferior-basal scar area. Activation mapping performed shows the location of mid-diastolic substrate location (blue dot) where delivering pulsed field ablation terminated VT

The VCAS trial is the most important clinical PFA study to date, investigating a purpose-built high-voltage focal PFA catheter (> 10 kV, monophasic, QRS-synchronized) for scar-related VT in 26 patients (42% VT storm, 42% prior VT ablation) [3]. It reported an acute procedural success rate of 92%, with a median of only 21 lesions and 31 min of ablation time. Clinical VT was inducible in 88% pre-ablation, but only 6% post-ablation, and VT/VF burden decreased by 98% post-ablation. Freedom from recurrent VT/VF or ICD shock at six months was 81.8% and primary safety events were recorded in 3/26 patients (11.5%), including cardiogenic shock, HF hospitalization, and retroperitoneal bleeding. A recent meta-analysis comprising six studies and 149 patients, 65 of whom had VT, reported an acute success rate of 90% for VT, but mid-term freedom from recurrence of only 63% [23].

Despite clinical success, PFA has been associated with several key limitations and safety concerns. Lesion durability remains the primary concern, with the gap between high acute success (~ 90%) and modest mid-term VT freedom (~ 45–63%) suggesting incomplete or non-durable lesion formation in some patients [23,24,25]. Conduction system block has been observed in multiple studies, sometimes caused by current leakage from proximal shaft electrodes contacting the basal septum. Other complications, including stroke/thromboembolism, have been reported [24, 25]. Catheter maneuverability in the LV is challenging, particularly with pentaspline catheters designed for atrial use and early loss of contact can lead to insufficient energy penetration [26]. Additionally, mitral or tricuspid valve apparatus entrapment of a lattice-tip catheter has been reported in the CLEAR-VT registry [22]. Proarrhythmic effects including irritative ventricular firing during PFA delivery have been observed in up to 55% of cases [27].

No randomized comparisons with RFA exist for VT, and all current evidence is from single-arm studies with considerable heterogeneity (Table 1) [23]. Hence, PFA for VT is not yet included in formal guideline recommendations and remains largely investigational. Current clinical use is primarily as a bailout after RFA failure or for large-footprint substrate modification in patients with extensive scar [28]. The VCAS trial’s purpose-built high-voltage catheter represents the most promising dedicated VT-PFA platform, but larger trials with longer follow-up are needed before PFA can be considered a standard alternative to RFA for VT.

Table 1 List of small studies with acute outcomes using pulsed field ablation as energy source for VT managementHigh-Volume Center Experience

Intramural VT and PVCs arising from deep myocardial substrates represent one of the most challenging scenarios encountered in complex electrophysiology programs. While this substrate is classically associated with non-ischemic cardiomyopathy, it poses an equally significant challenge in ischemic disease. High-volume centers with dedicated VT ablation programs routinely confront cases where conventional endocardial mapping and ablation prove insufficient, largely because standard unipolar radiofrequency energy achieves lesion depths of only 5–6 mm, often inadequate to reach intramural or mid-septal circuits.

Pre-procedural planning is central to the approach at experienced centers. Cardiac MRI with T1/T2 weighting and late gadolinium enhancement remains the preferred imaging modality for characterizing substrate location and depth, particularly in nonischemic cardiomyopathies. CT-based delayed enhancement imaging serves as a useful alternative, particularly when MRI is contraindicated. Electrophysiological assessment may incorporate intracoronary wire mapping via septal perforators or coronary sinus tributary catheters, with mapping strategies tailored to the arrhythmia mechanism, including diastolic corridor and entrainment mapping for re-entrant VTs and activation mapping for focal arrhythmias. Unipolar substrate mapping from opposing endocardial surfaces (e.g., right and left ventricular or endocardial and epicardial) with identification of 5–6 mV far-field signal zones can help localize and verify intramural targets.

Substrate and VT Characterization for Intramural Targets

Pre-procedural cardiac MRI (T1/T2-weighted and LGE) remains the preferred modality for defining substrate location and depth, particularly in nonischemic cardiomyopathy; CT delayed enhancement is a practical alternative when MRI is contraindicated. Where available, pixel signal intensity maps and heterogeneous tissue channel identification from MRI can be integrated into the mapping system to improve targeting precision.

Intra-procedurally, high-density electroanatomic mapping with paired unipolar and bipolar voltage maps from opposing endocardial surfaces is essential. A unipolar voltage of 5–6 mV corresponds to the border zone of intramural substrate and identifies potential ablation targets. Complementary strategies include intracoronary arterial or venous wire mapping, entrainment mapping when VT is hemodynamically stable, pace-mapping, and dominant frequency analysis each contributing incremental substrate localization.

Modality Selection: BRFA, PFA, and ULTC

When sequential unipolar RF fails to reach deep intramural substrate, BRFA is the recommended next step per current evidence and consensus guidance. Irrigated-tip catheters are preferred, with the active catheter positioned on the LV aspect of the septum and the return electrode oriented parallel to the tissue on the opposing RV surface. Energy delivery is initiated at 15 W to verify circuit integrity, then titrated to 30–50 W for 60–120 s per application. In our experience, 35 W is typically sufficient to achieve the target impedance drop of 20–30 Ω, at which point transmural lesion formation can be confirmed with intracardiac echocardiography. Temperature exceeding 50 °C, steam pops, or an abrupt impedance rise, or fall should prompt immediate energy termination. Multiple contiguous applications are delivered to ensure adequate substrate coverage. Acute endpoints include unipolar voltage remapping, loss of capture on pacing, and non-inducibility of VT.

PFA is preferred when substrate is densely fibrotic or dense fat on MRI, when prior RF ablation has failed, or when intracavitary anatomy limits sustained catheter contact and its short energy delivery time is an advantage in these settings. We have employed a hybrid approach combining right-sided PFA with ethanol infusion through a coronary venous perforator when BRFA is limited by conduction system proximity. For dense scar unlikely to be penetrated by RF energy, PFA or ULTC are the preferred alternatives.

Collateral Damage Prevention

Pre-procedural imaging should assess proximity of major coronary arteries to the ablation target high septal substrates may lie adjacent to the LAD-circumflex bifurcation, particularly from the RVOT approach. Prophylactic intracoronary nitroglycerin should be considered before PFA delivery per published literature. Septal perforator injury remains a risk with BRFA, and the risk-benefit balance should be explicitly reviewed.

Conduction system injury is the most common serious complication and requires a frank pre-procedural discussion: if complete heart block occurs, the decision to accept pacing dependency potentially with cardiac resynchronization therapy given the underlying cardiomyopathy must be weighed against halting the procedure. Right-sided PFA or ULTC with the intention of deeper penetration, while avoiding left-sided delivery near the His-Purkinje axis, is an alternative strategy. HV interval assessment after ablation is mandatory.

Steam pops carry a risk of ventricular septal defect (LV-RV) or acquired Gerbode defect ( LV–RA shunt) when ablation is performed near the membranous septum. Continuous intracardiac echocardiographic monitoring of lesion formation and impedance behavior is essential, and follow-up echocardiography should be performed to exclude late structural complications.

Finally, PFA catheter selection influences lesion footprint and dimensions; operators must be familiar with the specific delivery characteristics of the catheter platform in use.

Bipolar Versus Pulse Field Ablation for the Treatment of VT

Deep intramural VT remains one of the most refractory substrates in electrophysiology, and two advanced energy modalities - BRFA and PFA - have emerged as the primary strategies beyond conventional unipolar RF. A direct head-to-head randomized trial does not yet exist, and current evidence rests on preclinical models, computational studies, and early clinical series.

The central advantage of both modalities over unipolar RF is greater transmural penetration. BRFA achieves transmural lesions in the interventricular septum at wall thicknesses up to 15 mm, where sequential or simultaneous unipolar modes fail. PFA appears to provide even greater lesion penetration with bipolar PFA delivered across the septum and LV free wall generating lesions with a mean depth of 14.3 ± 4.7 mm, significantly deeper than monopolar epicardial lesions, with uniform chronic fibrosis and no tissue disruption. Computational modeling similarly predicts that for equivalent lesion depth, PFA lesions are wider and occupy a greater volume than RFA lesions in both monopolar and bipolar configurations [29].

BRFA has the more established clinical record, with acute success rates of 75–89% in patients with intramural VT refractory to unipolar ablation, though VT recurrence approaches 44% at 12 months. PFA for ventricular arrhythmias is at an earlier stage, but a recent meta-analysis of six studies reported an acute success rate of 90% (95% CI 80–97%) for VT specifically. Early case reports confirm the feasibility of bipolar PFA for refractory septal VT after RF failure, and PFA has shown improved penetration through heterogeneous and superficial fibrosis compared to secondary RF applications[23, 30]. A summary comparing indications, lesion characteristics, acute success, outcomes, complications, limitations and regulator status of BRFA vs. PFA are shown in Table 2.

Table 2 Bipolar Radiofrequency Ablation (BFRA) vs. Pulsed Field Ablation (PFA) for Ventricular Tachycardia: A structured comparison of two advanced energy modalities for intramural and refractory VT substrate

Bipolar RF ablation carries established risks of complete heart block and coronary artery injury from resistive heating adjacent to septal structures. PFA’s non-thermal, electroporation-based mechanism offers theoretical cardioselectivity, though right bundle branch block has been reported with septal PFA delivery, and long-term ventricular safety data remain limited.

BRFA remains the current standard for intramural VT refractory to unipolar approaches, supported by a larger body of clinical evidence. Bipolar PFA shows compelling preclinical depth superiority and early clinical promise, particularly in previously ablated fibrotic substrates. Prospective trials are needed to determine whether this translates into improved long-term outcomes.

Future Directions

The management of intramural VT continues to evolve along two parallel axes: refining substrate identification and expanding the armamentarium of energy delivery strategies capable of reaching deep myocardial targets.

Pre-procedural and intraprocedural imaging will play an increasingly central role. Cardiac MRI-guided VT substrate ablation, incorporating pixel signal intensity maps and heterogeneous tissue channel identification, has already demonstrated improved procedural efficiency and long-term outcomes over conventional electroanatomic mapping alone [31]. Complementing this, intraprocedural mapping strategies - including high-density isochronal late activation mapping, dominant frequency analysis, unipolar voltage mapping, timing maps, and precision pace mapping - are being refined to better localize and confirm critical intramural circuit sites that lie beyond the reach of surface electrograms.

Once the target substrate is precisely defined, the optimal energy modality remains an active area of investigation. Current options under evaluation include BRFA as the established platform, dual-energy hybrid approaches combining sequential RFA and PFA to leverage the complementary tissue penetration profiles of each modality, sequential biventricular bipolar PFA, and monopolar PFA using large-footprint catheter designs. Each strategy aims not only to reach deep substrates but to achieve durable lesion formation through dense scar - a fundamental requirement for long-term VT suppression that remains incompletely addressed by any single existing approach.

Ultra-low temperature cryoablation (ULTC), via the vCLAS system using near-critical nitrogen at − 196 °C, has demonstrated the capacity to create transmural, contiguous lesions through ventricular scar in early clinical experience and the pivotal FULCRUM-VT trial, positioning it as a promising emerging platform for this substrate [32]. For the most refractory cases where catheter-based approaches have failed, stereotactic arrhythmia radioablation (STAR) has emerged as a non-invasive complementary modality, with growing evidence supporting its use as a targeted radiation strategy for intramural and other anatomically inaccessible VT circuits [33]. Finally, cardiac sympathetic denervation - either surgical or percutaneous - represents an important adjunctive approach for refractory ventricular arrhythmias driven by sympathetic activation and may have a particularly meaningful role in patients with recurrent intramural VT storms not adequately controlled by catheter-based interventions alone.

Taken together, the future of intramural VT management lies in the integration of multimodal imaging-guided substrate definition with individualized energy delivery strategies, moving the field toward a precision-ablation paradigm tailored to the depth, composition, and location of each patient’s arrhythmogenic substrate.

Key Unknowns are anchored around the absence of RCTs, the PFA durability gap, the untested clinical translation of PFA’s preclinical depth advantage, undefined patient phenotypes, and absent cost/HF outcome data.

Optimal Patient Selection BRFA favored for IVS ≤ 15 mm, NICM with confirmed septal origin, and failed unipolar; PFA favored for fibrotic/previously ablated substrate, large-footprint scar, trabecular targets, and NICM septal cases requiring biventricular delivery per CLEAR-VT.

Standardized BRFA Dosing from experienced centers are current empirical parameters which include ~ 15 W start, 20–30 Ω impedance drop target positioning parallel 8 mm return electrode, may enhance tissue penetration without significant steam pops.

PFA Lesion Non-durability: Five proposed mechanisms (reversible electroporation at scar borders, contact failure, pulse-parameter heterogeneity, sub-optimal catheter platform, ungated delivery) and five strategies to improve permanence (high-voltage QRS-gated catheter, bipolar configuration, biventricular sequential delivery, hybrid RF + PFA, intraprocedural re-mapping validation).

Cost-effectiveness: Procedural cost is primarily driven by dual-catheter setup for BRFA and, at centers with dedicated BRFA generators, the incremental capital cost of the generator no additional per-case disposable cost beyond standard ablation catheters. VT recurrence ~ 44% at 12 months drives downstream costs with repeat ablation, ICD therapy, HF hospitalization and true cost-effectiveness depends critically on long-term arrhythmia suppression, which is only modestly established but largely unknown with PFA, except procedure time efficiency (VCAS: 31 min median) vs. unknown downstream costs from recurrence.

LV Function/HF Outcomes flags the VCAS 7.7% cardiogenic shock/HF events, BRFA’s untracked LVEF trajectory, PFA’s theoretically favorable extracellular matrix preservation, and the need for serial echo and clinical outcomes in future registries.

Emerging Modalities (ULTC, STAR, SERF needle, Coil/Ethanol) may be helpful in several ways to manage these complex intramural substrates. ULTC by FULCRUM-VT trial data suggest efficacy by creating contiguous lesions. Coil embolization although is limited by case reports by data, may have an advantage of limiting the lesion target if precisely identified in already compromised LV function. Alcohol ablation as primary modality recently is promising especially by limiting lesion by double balloon technique using coronary venous system coronary venous tributaries are available. In addition to these techniques, SERF needle which could be potential bail-out strategy in very deep substrate when access is difficult may be helpful. STAR strategy with radiation is particularly helpful when ablation techniques have failed and patient has high risk for procedures or comorbidities in a center with adequate experience these deep substrates may be targeted non-invasively.