Ablation Methods and Catheter Settings for Ventricular Tachycardia Ablation: A Bench to Bedside Review: Part 1

Ventricular tachycardia (VT) carries high morbidity and mortality in patients with structural heart disease.¹ Despite advances, ablation outcomes remain modest, with recurrence rates of 30–70%.^2,3 The main reasons for recurrence are incomplete substrate identification, progression of disease substrate and, importantly, ineffective ablation lesions. An armamentarium of ablation technologies (Figure 1 ) has emerged at the disposal of the VT operator, including radiofrequency (RF)-based approaches, cryoablation, ethanol and pulsed field ablation (PFA). Given the breadth of this subject, we decided to divide our findings into two parts. Part 1 describes the principles and technical considerations of RF ablation, including catheter designs, energy delivery parameters and ablation indices. Although this part draws on an abundance of bench and animal data, differences between experimental models and human physiology, such as tissue composition, perfusion and wall thickness variability, may limit the direct translatability of these findings into clinical practice. In the accompanying part 2, we discuss alternative ablation technologies aimed at overcoming the limitations of RF ablation, namely bipolar RF ablation, PFA, ultra-low-temperature cryoablation, venous ethanol ablation and needle-based intramural ablation.⁴ A deep understanding of various techniques is essential because successful VT ablation may require more than one source of ablation energy. Both parts include tables summarising levels of evidence and discuss potential applications. The goal of the overall review is to provide a synthesis of the current ablation technologies.

Here, in part 1, we describe principles of RF ablation and ablation metrics. In the accompanying part 2, we focus on alternative ablation energies.⁴

Methods

A narrative search was conducted across the PubMed and Embase databases, covering the period from inception until November 2025. The search used the following terms: (ventricular tachycardia or VT) and (ablation). Key studies are summarised in Tables 1 and 2. Ex vivo and in silico studies were categorised as bench studies, whereas large animal studies included in vivo experiments in dogs, sheep, pigs and cattle. Additionally, Table 1 provides columns for feasibility studies, non-randomised studies, case series, case reports and randomised controlled trials.

Results and Discussion

Principles of Radiofrequency Catheter Ablation

RF catheter ablation (RFCA) delivers current that generates heat to create lesions, leading to tissue necrosis. In the standard unipolar set-up, current passes from the generator through the tissue to a ground patch. Lesions form first by resistive heating, followed by conductive (diffusive) heat spread.⁵ Lesions are created when irreversible tissue damage occurs at temperatures exceeding 52–55°C.⁶ Effective ablation lesions can be confirmed by the absence of local capture during high-output pacing.⁷ Several factors influence lesion formation, including catheter tip size and orientation in relation to the tissue, power output, irrigation settings and tissue characteristics. Tip size significantly affects electrogram (EGM) resolution, impedance, power delivery and tissue temperature. Contemporary ablation catheters typically use 3.5–4 mm distal tip sizes to balance power delivery and impedance stability. Unfortunately, this concentrates high current over a small area. Applying Ohm’s simplified power equation, P=I²R, where P is power (in watts), I is current (in amperes), and R is resistance (in Ω).

With a standard irrigated linear catheter surface area of 27–31 mm², the average current density is approximately 20 mA/mm².⁸ To increase current output without raising current density (and thus reducing the risk of overheating), the current can be distributed over a larger electrode surface area. The lattice catheter (Sphere-9, Affera; Medtronic) features a large expandable tip that alters the contact mechanics and irrigation dynamics, allowing for a roughly 10-fold increase in current output while maintaining a relatively low current density.^8,9 According to preclinical studies, the surface area of the Sphere-9 increases from 28 to 275 mm², enabling a maximum current of up to 3,700 mA with a lower maximum current density of 13 mA/mm².⁸

Irrigated catheters produce larger lesions than non-irrigated catheters. However, high irrigation rates can cause the temperature at the catheter tip to inaccurately reflect the tissue temperature, reducing the usefulness of temperature-based power control.¹⁰ Conversely, low irrigation rates or low-flow environments reduce cooling, causing the tip to reach the temperature limit sooner and deliver less RF energy.¹¹ Catheter orientation also affects tip cooling. A perpendicular orientation to the tissue provides better cooling and results in larger lesions compared with a parallel orientation.^12,13 Lesion characteristics are also influenced by the choice of irrigant. The most common irrigant, 0.9% sodium chloride (normal saline [NS]), disperses RF energy due to its high ionic concentration. In contrast, 0.45% (half-normal) saline (HNS) and 5% dextrose produce larger and deeper lesions because these solutions disperse RF energy less effectively. However, this evidence is primarily based on bench and animal studies, which are limited by laboratory conditions, such as perfusion, catheter contact, angulation or tissue composition.^14,15 Nguyen et al. demonstrated the efficacy of HNS irrigation in treating 94 patients refractory to standard RF ablation.¹⁰ During the procedure, ablation was delivered at 30 W, increasing to 50 W, or until a 10–15 Ω impedance drop was observed. At 50 W, the benefits of HNS over NS were less pronounced because the higher power settings seemed to negate the ionic properties of NS.¹⁰ In terms of safety considerations, ablation with HNS has been linked to a higher rate of steam pops, making it advisable to perform this procedure under intracardiac echocardiography (ICE) guidance and careful impedance monitoring.^10,14,15 A major challenge in ventricular ablation is treating regions of increased wall thickness, where higher resistance reduces RF penetration. As a result, longer applications, typically from 60 seconds to 5 minutes, are often required.^16,17

Power- Versus Temperature-controlled Radiofrequency Catheter Ablation

Traditional irrigated catheters operate in a power control mode. This reduces the precision of thermal feedback at the catheter tip, making energy adjustment during ablation more difficult.¹⁸ Lesion size depends on the current delivered, which is driven by power output and impedance. Conventional settings use 25–50 W. With stable contact force (CF) and catheter position, ablation duration becomes the main factor influencing lesion depth.¹⁹ High-power short-duration (HPSD) ablation produces a larger zone of direct heating than conventional settings, and lesions continue to expand during cooling due to thermal latency. An in silico experimental study showed that HPSD creates lesions with greater diameters but shallower depths compared with standard ablation.²⁰

QMODE+ (Biosense Webster) delivers 90 W over 4 seconds (a very HSPD). An in vivo sheep study (n=6) demonstrated more transmural and contiguous linear lesions with this method compared with standard power control ablation.²¹ However, that finding contrasts with a bench study by Nakagawa et al., who found that lesions from the 90-W/4-second setting were actually smaller than 50-W/10-second or 30-W/30-second settings.²² A clinical study by Heeger et al. (n=24) showed that, compared with standard power control ablation, QMODE+ significantly reduced RF time (52 versus 350 seconds; p<0.0001) while achieving similar efficacy in terms of 6-month recurrence rates of premature ventricular contractions (PVCs) originating from the right ventricular outflow tract (RVOT) and left ventricular (LV) outflow tract (21% versus 17%, respectively; p=0.712). The number of applications was similar in both settings, supporting the non-inferiority of HPSD.²³

Recent technological advances have enabled the delivery of irrigated RF in a temperature control mode using externalised thermocouples that provide real-time temperature feedback and power modulation. Catheters such as the DiamondTemp (Medtronic), Sphere-9 (Medtronic), INTELLANAV MIFI OI (Boston Scientific) and QDOT (Biosense Webster) now offer temperature-controlled irrigated RFCA. These systems can deliver high-power RF energy up to 50 W and allow for a 200- to 400-fold increase in thermal energy transfer compared with platinum–iridium tip catheters.^24,25 During RF delivery, the power is modulated to maintain a preset tip tissue temperature of 50–60°C with an irrigation rate of 8 ml/min.²⁴

The QDOT ablation catheter is built on the Thermocool SmartTouch Surround Flow (STSF) platform. QDOT operates in two settings: QMODE, a temperature control mode and QMODE+ HPSD mode. QMODE delivers lesions with a temperature up to 55°C by titrating the power up to 50 W.²³ Compagnucci et al. conducted a prospective observational study comparing QMODE and Thermocool catheters in patients with ischaemic cardiomyopathy (ICM), finding no significant differences in outcomes between the two catheters both in the short term and at a 7-month follow-up.²⁶ Kobza et al. demonstrated that QMODE resulted in shorter RF application times and a trend towards reduced procedure times compared with other studies using STSF, specifically for VT and PVC ablation.²⁷ Another temperature-controlled catheter, INTELLANAV MIFI OI, is discussed in detail later in this review.²⁸ The Sphere-9 catheter delivers both RF and PFA, and continuously measures impedance between each mini electrode and the central electrode to assess tissue contact.²⁹ Ablation starts at the maximum preset current and automatically adjusts to maintain the surface temperature limit, with an additional current cap to restrict maximum energy delivery.²⁹

In an in vivo swine study, the Sphere-9 (maximum temperature 60°C, 60 seconds) produced significantly larger RF lesions than a standard CF catheter (40 W, 60 seconds), with CF of 10–29 g. The lesions created by the Sphere-9 and standard catheter showed a mean depth of 10.5 ± 1.4 versus 6.5±0.8 mm, respectively (p<0.001); a width of 27.3 ± 5.2 versus 9.2 ± 1.3 mm, respectively (p<0.001); and a volume of 4.3 ± 1.8 versus 0.3 ± 0.1 mm³, respectively (p<0.0001).⁸ However, variations in tissue composition and flow mean these findings may not fully translate to clinical practice. Ventricular ablation typically uses 30-second RF applications with a 30 ml/min irrigation rate.³⁰ Although temperature-controlled irrigated RF ablation is a major advance, sensor-detected temperatures remain imprecise. In vivo porcine data show that surface readings can substantially underestimate deeper tissue temperatures, and this discrepancy increases with longer RF applications.³¹

Radiofrequency Ablation Lesion Metrics

Ablation lesion metrics are essential to ensure that ablation is delivered safely and efficiently. They provide real-time feedback on lesion formation, helping clinicians achieve optimal outcomes by balancing the extent of tissue necrosis while minimising complications. Many studies have shown that these metrics improve procedural efficiency and patient outcomes. This section outlines the main types of ablation metrics (Table 2).

Electrogram Attenuation

Successful RF ablation is indicated by EGM attenuation. In animal models of atrial ablation, a 50–80% decrease in bipolar EGM amplitude corresponds to the creation of a transmural lesion. However, this observation does not reliably apply to ventricular tissue.^32,33 The degree of EGM attenuation during VT ablation can vary due to the biological characteristics and depth of the substrate. In a human study, Bates et al. observed median percentage attenuations of 29.3% (interquartile range [IQR] 4.4−53.3%) for bipolar and 9.48% (IQR 3.15–23.14%) for unipolar EGM during endocardial VT ablation.³⁴ Notably, bipolar attenuation showed only a weak correlation with the percentage impedance drop (R=0.151; p<0.002) and no correlation with other ablation delivery indices, such as mean CF, ablation index (AI), or the force–time integral (FTI).³⁴ Histological tissue assessment was not performed in that study. Recently introduced ablation catheters that feature microelectrodes, such as QDOT and DiamondTemp, are capable of recording high-resolution EGMs, but data on the degree of EGM attenuation correlating to clinical outcomes are lacking.^24,35

General Impedance

General impedance (GI) reflects the resistance to the alternating electrical current flow at specified sampling frequencies. The electrical circuit comprises the myocardial tissue, the tip of the ablation catheter, cabling and a return electrode (a skin patch). In humans, baseline RF circuit impedance typically ranges from 100–120 Ω.³⁶ Animal studies indicate that tissue type and thoracic structures (muscle, lung, bone) strongly affect GI, but these findings may not fully translate to human physiology.³⁶ A study using an ex vivo swine heart model demonstrated that transmural impedance values were lower in scarred myocardium (ranging from 107 ± 34 to 92 ± 27 Ω) compared with healthy tissue (ranging from 278 ± 81 to 146 ± 41 Ω), showing a mean value difference of 52% (p<0.0001).³⁷ Baseline impedance affects RF lesion size, because GI has a strong negative correlation with the square of the current (I²).

In another ex vivo study, Bourier et al. showed that RF applications performed at low impedance (80 Ω) resulted in significantly larger lesions than those performed at sites with a higher impedance (120 Ω).³⁸ Furthermore, in a study involving infarcted swine, baseline impedance in scar tissue was found to be lower than that in healthy myocardium.³⁹ The electrical resistance of scar tissue is approximately 20–40% lower than that of healthy myocardium, which theoretically should result in a larger thermal effect compared with normal myocardium.^40,41 However, in animal studies, RF lesions are consistently shallower in infarcted tissue than in healthy myocardium.³⁹ Consequently, the effect of RFCA on cardiomyocyte islands within scar tissue is diminished compared with healthy myocardium, with several mechanisms postulated to account for this finding: current applied to collagen results in reduced heating due to the lower resistivity of collagen compared with cardiomyocytes; cardiomyocytes are protected by layers of connective tissue, which have higher denaturation thresholds (the collagen denaturing threshold is 70°C); adipose tissue, often present in remodelled scar, serves as a heat capacitor, diverting current from cardiomyocytes to fat; and increased shunting of current to blood caused by the reduced resistivity of scar tissue.³⁹

In a human study, Shapira-Daniels et al. demonstrated that lowering baseline impedance can enhance RFCA.⁴² This was achieved by placing an additional return electrode on the left flank, scapula or sternum (wherever impedance values were lowest). By reducing the circuit size, the baseline impedance dropped from ≥120 Ω to between 100 and 120 Ω. As a result, current output increased compared with the baseline impedance setting (0.6 ± 0.02 versus 0.56 ± 0.02 A; p<0.00001).⁴² Where this approach did not effectively create lesions, HNS irrigation was used. However, in the modified impedance group, steam pops occurred more frequently (OR 1.28; 95% CI [1.18–2.21]; p=0.03), particularly with rapid impedance drops (≥14 Ω in 10 seconds).⁴² To address high baseline impedance within the coronary sinus, high-rate irrigation has been used. Chen et al. reported successful ablation in the great cardiac vein with the following settings: impedance limit 250–300 Ω, temperature 43°C, power 25–35 W and saline flow rate 30–120 ml/min.⁴³ The acute success rate was 85.9%, with a further 2.25% of patients experiencing arrhythmia recurrence within the first 3 months.⁴³

The rate of impedance drop is critical. A gradual drop reflects normal tissue heating and lesion formation, whereas an abrupt fall may indicate tip erosion or tissue penetration.⁴⁴ Conversely, a rapid impedance rise (1 Ω/s) signals overheating and may precede a steam pop, as demonstrated in ex vivo animal studies.⁴⁵ A drop of 5–10 Ω is ideal, whereas an impedance drop approaching 20 Ω is associated with overheating and risk of steam pop in humans (Table 3).^46–48 A relatively recent approach focuses on the percentage impedance drop (%Δimp), which is calculated as the impedance change divided by the maximum impedance. Because scar responds differently to RF, this approach allows for adjusted impedance targets.³⁴ Although there is good correlation between %Δimp and lesion size, validated cut-off values for VT ablation are not yet available, so values must be interpreted in context.^49,50 RFCA follows a monoexponential curve that levels off, indicating a state of thermodynamic equilibrium. By determining the plateau phase, Bates et al. attempted to define the optimal ablation endpoints for %Δimp across different scar regions.³⁴ The endpoints differed in healthy and scarred tissue, with a 10% drop (12.1 Ω) in high-voltage myocardium (>1.5 mV), an 8% (8.9 Ω) drop in border zones (0.51–1.5 mV) and a 5.5% (5.9 Ω) drop in dense scar zones (<0.5 mV).³⁴ Researchers have recently explored combining impedance with power and duration to create new ablation metrics. In an ex vivo porcine experiment, Iwakawa et al. examined the relative impedance drop (*%Imp-drop), impedance drop (Δimp), and ablation energy (AE; calculated as RF power multiplied by duration and measured in joules).⁵¹ The combined metric AE*%Imp-drop showed the strongest correlation with lesion depth, surface area and volume for both NS and HNS irrigation.⁵¹ Clinical data are pending.

Local Impedance

GI involves a large circuit and is therefore influenced by extracardiac tissues. In contrast, local impedance (LI) is measured only from the cardiac tissue. The first LI-enabled catheter, INTELLANAV MIFI OI, uses three mini electrodes and a non-stimulatory current (5.0 µA at 14.5 kHz) to generate a local field at a sampling frequency of 20 Hz. In ex vivo porcine LV studies, LI values (132–199 Ω) were comparable to GI (88–115 Ω) and were strongly correlated with lesion size.^52,53 In a swine model, LI also tracked scar density and transmurality: low LI identified dense, transmural scar with wall thinning, whereas intermediate LI corresponded to border zone corridors.⁵⁴ Clinical studies show that LI can distinguish healthy tissue from scar where GI cannot. Within the LV (n=31), Martin et al. reported mean LI values of 93±13 Ω in the blood pool, 141±13 Ω in healthy myocardium and 74±5 Ω in dense scar.⁵⁵ LI was also affected by the direction of the activation wave front, whereas GI mainly reflected global direction effects.

The maximum LI drop during ablation was linearly correlated with baseline LI. For successful LV lesions resulting in the absence of local tissue capture, the median LI drop was 16.0 Ω (range 12.1–19.8 Ω), which was greater than the median drop for unsuccessful lesions, which measured 9.4 Ω (range 5.4–15.6 Ω; p=0.001). Furthermore, the LI percentage drop was significantly larger for successful than unsuccessful lesions (17.1 [range 14.0–19.6] versus 10.6 [range 7.1–16.5] Ω; p=0.002). In contrast, no relationship was observed between the initial GI and subsequent drop in GI during ablation.⁵⁵ However, that study had limitations, including a small sample size and the absence of ICE, which made it impossible to determine whether low LI values indicated poor tissue contact or an area of fibrosis.

Münkler et al. (n=28) reported that a ΔLI ≥16 Ω was necessary to achieve ventricular loss of capture, whereas terminating lesions showed a median ΔLI of 18.5 Ω (IQR 10.20–31.25).²⁸ The GI drop did not distinguish between terminating and non-terminating lesions. ΔLI and ΔGI were higher in patients with non-ICM than ICM: ΔLI 16.0 (IQR 11.0–20.0) versus 11.0 (IQR 7.9–17.0) Ω, respectively; ΔGI 9.0 (IQR 7.0–12.0) versus 8.0 (IQR 5.0–10.0) Ω, respectively.²⁸ However, that study was limited by its small, single-centre design and reliance on fluoroscopy and tactile feedback for catheter contact assessment. Preclinical data further suggest that a ΔLI of ≥40 Ω is associated with an increased risk of steam pops.^53,56

LI has also been combined with other indices to guide ablation. Takigawa et al. introduced the concept of %LI drop, defined as ΔLI divided by the initial LI.⁵⁷ In their ex vivo porcine LV study, Takigawa et al. demonstrated that %LI drop, when paired with AE or FTI, was strongly correlated with lesion depth, surface and volume, regardless of the catheter orientation (parallel/perpendicular).⁵⁷ However, the experiment was an ex vivo study and used non-infarcted tissue. In humans, Schillaci et al. reported that combining CF and LI yielded better outcomes for treating PVCs.⁵⁸ In that study, the CF+LI group had significantly lower risk of PVC recurrence than the standard ablation group (HR 0.22; 95% CI [0.07–0.71]; p=0.01). In addition, Schillaci et al. found that LI drop and RF time were the only predictors of successful lesions.⁵⁸

Contact Force

CF provides real-time feedback of catheter–tissue contact. In a study by Jesel et al., a general linear mixed model identified optimal CF thresholds for achieving electrogram amplitudes >1.5 mV: 7 g in the LV endocardium (80% sensitivity, 75% specificity), 9 g in the right ventricular endocardium (65% sensitivity, 83% specificity) and 4 g in the epicardium (83% sensitivity, 64% specificity).⁵⁹

Calzolari et al. found no correlation between CF and lesion depth.⁵ Experimental studies suggest that a mean CF ≥10 g (range 5–40 g) produces electrical inexcitability.^33,60 Low CF may be offset by higher power, whereas excessive CF increases the risk of perforation, steam pops and thrombus.⁶¹ CF is influenced by sensor technology, catheter orientation, anatomical variability, cardiac motion and respiration. In clinical practice, achieving good CF is particularly challenging in basal LV septum and anterobasal regions.⁵⁹ The access route also affects CF: retrograde aortic access improves CF on the anterior wall, whereas transseptal access is superior for the lateral wall and apical septal/inferior regions.^59,62 CF is less reliable on papillary muscles, where ICE can help, and in the epicardium due to pericardial constraints, which tend to orient the catheter tip parallel to the surface.^1,33,63,64 Jesel et al. also noted that a CF <10 g was still adequate for creating epicardial lesions.⁵⁹ The prognostic value of CF has not been strongly proven.^63,64

Force–Time Integral

CF is a reliable marker of catheter–tissue interaction ex vivo, but in vivo variability from cardiac and respiratory motion reduces its consistency. To account for beat-to-beat variation, the FTI was developed. The FTI formula calculates the time-varying amplitude of CF by quantifying the area under the curve over a 60-second period. The Force–Power–Time Index (FPTI) builds on the FTI by including power. In an ex vivo swine skeletal model, Huang et al. demonstrated that both FTI and FPTI have a high predictive value for steam pops.⁶⁵ Specifically, FTI ≥700 gs and FPTI ≥31,000 gWs were found to predict steam pops, with sensitivity 83.3%, specificity 74.2%, positive predictive value 46.9% and negative predictive value 94.2% for FTI, compared with sensitivity 80.6%, specificity 97.7%, positive predictive value 80.6% and negative predictive value 94.7% for FTPI. Time contributed most to lesion formation, whereas excessive force was more closely associated with steam pops. Overall, FPTI outperformed FTI for predicting lesion formation and complications.⁶⁵

In vivo, Sacher et al. found that FTI >500 gs was required to create durable endocardial lesions in sheep.³³ In humans, Bates et al. reported a logarithmic relationship between FTI and Δ%Imp, with plateau points depending on myocardial voltage (1,007 gs for voltage >1.5 mV; 761 gs for 0.5–1.5 mV; 540 gs for dense scar <0.5 mV).³⁴ This indicates diminishing predictive value of FTI in more fibrotic tissue. FTI was not correlated with bipolar or EGM attenuation.³⁴ A limitation of both FTI and FPTI is their dependency on catheter stability, tip orientation and the lack of consideration of the underlying tissue scar content.

Ablation Index

AI is a lesion quality index that combines power, duration and CF with weighted contributions, available on the CARTO VISITAG platform (Biosense Webster).⁶⁶ In a swine study, Younsis et al. reported maximal lesion depth at AI values of 700–800 in healthy ventricular tissue, whereas infarct border zones showed wide variability (AI 500–900).⁶⁷ Preclinical findings on AI and steam pops are inconsistent: some report no association, whereas others note increased risk when AI is >550 and RF exceeds 30 seconds.^67,68

Animal studies have demonstrated that AI may be unreliable due to factors such as small contact angle, unstable catheter position and catheter drift, as well as the use of HNS.^67,69 In human studies, AI and FTI correlate with %Δimp, although this relationship weakens in fibrotic myocardium.³⁴ Bates et al. found no added lesion benefit beyond AI 763, and EGM attenuation did not correlate with ablation metrics. This AI threshold was derived from 402 lesions in 15 patients using an STSF catheter and Carto3 system.³⁴ AI-guided ablation has shown procedural advantages. In RVOT PVC ablation, Gasperetti et al. reported fewer lesions, reduced fluoroscopy and lower 6-month recurrence using target AI values of 590 (free wall) and 610 (septum), with reported ORs of 6.61 (95% CI [1.95–22.35]; p=0.001) for the earliest activation point, 5.99 (95% CI [1.21–29.65]; p=0.028) for the RVOT septum and 11.86 (95% CI [1.12–124.78]; p=0.039) for the RVOT free wall.⁷⁰ Casella et al. identified mean AI values of 489–630 as predictive of ablation success.⁷¹ The maximum and mean AI values were significantly higher in the success group than in the acute failure/6-month failure group (p=0.001 and p=0.04, respectively) and right ventricular free wall (p=0.007 and p=0.01, respectively) PVC subgroups.⁷¹ The SURFIRE-VT study found no further impedance drop above AI 550–600, but showed that targeting AI 550 shortened procedures and RF time without compromising outcomes.⁷² However, long-term clinical outcomes (>6 months) showed no prognostic benefit of AI-guided procedure.

Lesion Size Index

Similar to AI, the Lesion Size Index (LSI) is a multiparametric index that incorporates CF, impedance, power and duration of RF delivery. However, the LSI formula differs from AI by accounting for the non-linear nature of lesion formation, the transition from resistive to diffusive heating and thermal latency.⁵ LSI is integrated into the EnSite Precision system (Abbott) and TactiCath ablation catheter (Abbott). In a study using an ex vivo porcine heart model, Huo et al. demonstrated that LSI could effectively predict the risk of steam pops, with values exceeding 5.65 yielding a sensitivity of 94.1% and a specificity of 46.1%.⁷³ Additional in vivo swine data suggest that LSI outperforms other metrics, such as FTI, impedance drop, average temperature/power, CF and energy, in predicting lesion size.⁶⁹ However, clinical studies on the use of LSI in VT are still pending.

Time after Temperature 60°C

Irreversible myocardial injury begins to occur within a temperature range of 52–55°C.⁷⁴ The concept of ‘time after temperature 60°C’ (TAT₆₀) was introduced by Dhanjal et al. using the temperature-controlled irrigated RF DiamondTemp in an ex vivo study on healthy porcine myocardium (280 applications).⁷⁵ TAT₆₀ was developed to guide DiamondTemp ablation, accounting for lesion maturation principles: thermal latency and evolution from resistive to conductive heating, reaching a plateau stage where the risk of steam pop increases. Analysis of TAT₆₀ revealed that lesion depths plateaued at 4.6±0.8 mm between 20 and 30 seconds after reaching 60°C. Thus, ablation beyond 20 seconds of reaching 60°C does not yield additional lesion depth.⁷⁵ In a clinical feasibility study, ablation was delivered at a 60°C set-point for up to 60 seconds, stopping earlier if bipolar EGM amplitude fell by 75–80% or impedance dropped by more than 20 Ω.⁷⁶ One steam pop occurred when the temperature reached 60°C within 10 seconds and ablation continued for 34 seconds beyond the limit. The authors noted this would likely have been avoided with the TAT60 protocol. Patients were followed for over 6 months and showed an 88% reduction in VT, but the absence of a comparator group prevents conclusions about the prognostic value of TAT₆₀.⁷⁶ Based on the aforementioned ex vivo study, TAT₆₀ of 20 seconds was recommended as a safe and effective endpoint for VT ablation cases.⁷⁵ This ablation metric has been tested prospectively within the DTinVT randomised controlled trial (NCT06028919), which has yet to be reported.

Conclusion

RFCA remains the cornerstone for treating VT in patients with ICM. Despite the significant evolution of that technology, limitations such as lesion penetration and durability persist. Conventional unipolar RF relies on resistive and conductive heating of tissue, with lesion size influenced by tip size and orientation, irrigation type and tissue composition. Newer solutions like lattice-tip catheters allow for high current delivery with reduced current density, whereas temperature-controlled systems (QDOT and DiamondTemp) provide real-time modulation based on the temperature sensor feedback. HPSD allows faster lesion creation but produces shallower lesions.

Ablation metrics have been developed to guide the ablation process. Among these, AI, CF, LI and impedance drop metrics have shown a correlation with human clinical outcomes in VT cohorts.^{42,43,58,70,71} However, except for AI, the prognostic value is limited.^70–72 In contrast, metrics such as LSI, FTI and FPTI are primarily supported by surrogate correlations with lesion size rather than prospective data on patient outcomes.^{5,33,34,65,67,73,77,78} Additional indices, like %Δimp, AE*%Imp-drop and %LI drop, along with CF+LI, remain limited to bench or animal studies and have not been validated in patients with structural VT.^{34,49–51,57,58} TAT₆₀ is being evaluated in a clinical trial (NCT06028919), with results pending. The pursuit of larger lesions may enhance substrate modification. Larger lesions can be achieved by increasing power, extending ablation duration, modifying GI, or using low-osmotic irrigation.^{10,14–17,21,42} However, these strategies carry the risk of serious complications, such as steam pops and perforations. Table 3 summarises the values of ablation indices associated with the risk of steam pops. The strategies to mitigate the risk of steam pops include: close monitoring of GI and favouring a slow, progressive impedance drop; maintenance of moderate but not excessive CF; incremental power titration, especially in thick tissue; limiting RF duration; particular vigilance if using HNS or 5% dextrose as irrigant; the use of ICE in particular when ablating papillary muscles, in apical or basal septal regions; and the use of a perpendicular catheter orientation to improve cooling.^{12–15,19,33,44–46,60,61} Table 4 outlines the risks associated with various ablation techniques and provides practical guidelines regarding metrics.

This review highlights the need for personalised ablation strategies that account for myocardial substrates, anatomy and other procedure variables. The current evidence is drawn from bench studies, animal models and emerging clinical trial data. The future research priorities should include: validation of ablation metrics (such as LI, AI, LSI, TAT₆₀ and combined indices) in large, multicentre human VT cohorts, including structural VT, non-ICM and PVC populations; randomised controlled trials comparing temperature-controlled versus power-controlled RF ablation to determine the optimal lesion formation and safety; prospective safety studies comparing HNS/5% dextrose with NS irrigation; comparison of HPSD and standard ablation to assess efficacy, complication rates; as well as long-term outcome studies linking ablation metrics and technologies to recurrence rates.

In conclusion, part 1 of our review highlights the complexity of RFCA, underscores the biophysical properties of the technique and provides an evidence-based overview of RF ablation metrics.