Catheter–tissue contact optimizes pulsed electric field ablation with a large area focal catheter

Pulsed electric field (PEF) ablation relies on the intersection of a critical voltage gradient with tissue to cause cell death. Field‐based lesion formation with PEF technologies may still depend on catheter–tissue contact (CTC). The purpose of this study was to assess the impact of CTC on PEF lesion formation with an investigational large area focal (LAF) catheter in a preclinical model.

Pulmonary vein isolation (PVI) remains one of the most effective treatment strategies for atrial fibrillation. 13][4] After being introduced in oncology over 30 years ago, 5,6 pulsed electric field (PEF) ablation has been developed as a primarily nonthermal modality for cardiac tissue ablation, achieved by applying short-duration bursts of high-voltage electric fields.Cardiac myocytes within a critical voltage gradient area undergo cell death due to destabilization of the cell equilibrium. 7F ablation treatment size is dependent on several factors, including waveform characteristics, electrode configuration, and field interaction with target tissues. 7Although it has been proposed that, unlike thermal modalities, PEF ablation may not be dependent on catheter-tissue contact (CTC) because of its field-based nature, 8 computational modeling data and ex vivo bench studies using a bipolar PEF system have demonstrated a profound treatment size dependence on catheter-tissue proximity. 9Proximity and contact dependence for PEF ablation, however, has not been thoroughly examined in vivo.
We investigated the relationship between CTC and monopolar PEF ablation in a preclinical porcine model.This study sought to determine, (1) if CTC is necessary for effective PEF treatment and (2) how CTC can be used to optimize PEF ablation workflows and efficacy.

| METHODS
In 10 animals, preablation CTC between a unique investigational large area focal (LAF) ablation catheter and cardiac targets was evaluated using a custom local impedance (LI) system, with guidelines established in previous pilot studies using intracardiac echocardiography (ICE), electrogram amplitude, and electroanatomic mapping (EAM).Isolated ventricular lesions and atrial lesion sets were created with varying amounts of CTC.Treatment size and isolation of targeted structures were examined at acute and chronic timepoints using EAM and histology.

| ABLATION SYSTEM
An 8.5 Fr, bidirectional, irrigated catheter (6 mL/min) with a 10 mm diameter spheroid tip comprised of 10 nitinol composite splines (Figure 1A) was connected to an investigational PEF system consisting of a generator and connection box (Figure 1B) (Galvanize Therapeutics).The system is designed to create large focal lesions (Figure 1C) that reduce the number of applications required to create a typical lesion set.Monopolar PEF energy is delivered asynchronously using a proprietary biphasic waveform (25 A, 1.6 ms) that minimizes microbubble formation and muscle contraction. 10The system measures LI between each spline and a central ring electrode.LI information is displayed using a graphical representation of the catheter tip where color and bar length changes represent increasing impedance on each spline (Figure 1D).The system is compatible with multiple EAM systems and is capable of creating electroanatomic maps of the heart and monitoring local electrograms (Figure 1E).

| PRECLINICAL PROCEDURE
The study protocol was approved by the Institutional Animal Care

| STATISTICAL ANALYSIS
Eight samples (2 s) of LI data from the spline with the highest delta from baseline were averaged to determine CTC before and after each ablation.Continuous variables are expressed as mean ± standard deviation or median with interquartile range (IQR).
Categorical variables are presented as counts or percentages.
Analysis of variance was performed to determine significance between variable groups where appropriate, with p < .05considered significant.Statistical analyses were performed with Minitab (v21.1.1).

| Ventricular lesion analysis
In eight acute animals, 41 lesions were attempted using LI and ICE to In two chronic animals, N = 8 lesions were attempted (NTC: n = 2, LTC: n = 3, HTC: n = 3).All LTC and HTC lesions were identified and cross-sectioned and no NTC lesions were identifiable after 30 days.
Of the six identified lesions, four were transmural and, therefore, not measurable (Figure S1).No adverse events were observed during the 30-day waiting period.
Representative acute lesions following TTC staining are provided for each cohort (Figure 3B).LTC and HTC lesions were similar in appearance and size (consistently >3 mm depth), while NTC yielded superficial lesions, moderate endocardial blanching, or no visible effect.H&E and Masson's trichrome (Figure 1C), revealed necrotic myocardium, mild edema, and myocytolysis on ventricular lesions with no visible impact to surrounding vessels.4).

| DISCUSSION
As new ablation modalities are introduced, it is important to develop tools that enable safe and effective treatment.For RF and Cryo, there This study sought to evaluate the utility of CTC for PEF ablation using a unique investigational LAF catheter, LI system, and PEF system.Ventricular lesions and atrial lesion sets were successfully created in 10 swine with CTC assessed using LI.
Treatment size, conduction block, and electrical isolation of target structures were examined acutely (N = 8) and chronically (N = 2) using EAM, pacing maneuvers, and tissue histology.No adverse events, collateral damage, microbubbles, or treatment-related movement were observed.Analysis of LI, tissue characteristics, and EAM demonstrated that with a unique LAF catheter (1) CTC is critical for consistent and effective PEF treatment, (2) above a minimum contact threshold, increasing CTC does not increase treatment size, and (3) LI can be an effective tool for assessing CTC during PEF ablation, its application will differ from that of focal RF.

| IS CONTACT NEEDED FOR PEF ABLATION?
Proximity of the field origin to targets and tissue homogeneity both directly influence the impact of a critical PEF voltage gradient. 7ward et al 9 demonstrated ex vivo that maximum treatment size is achieved when a catheter is in apposition to target tissue while the displacement of the electric field source from the tissue surface reduced lesion depth.Contact force and LI are both well-established indicators of CTC. 13,14In this study, with ICE guidance, LI consistently >Δ10 Ω from baseline was found to be correlated to stable CTC.PEF treatments delivered with >Δ10 Ω LI resulted in maximum treatment size and transmurality while treatments delivered with ≤Δ10 Ω LI (even with close proximity to tissue) resulted in reduced lesion size or no discernable lesion acutely and chronically.Although LI is a measurement of electrode surface area in contact with resistive myocardium and not a direct measurement of contact force, LI has been shown to correlate with contact force over relevant operational ranges for focal catheters. 19Similarly in this study, increased LI correlated with increased CTC; however, in contrast to a traditional solid-tip catheter, the LAF spheroid tip compresses in response to increased contact rather than indenting into the myocardium (Figure S2).Additionally, solid-tip catheters produce small electric fields that are more susceptible to differences in contact than bigger LAF catheter electric fields. 7These mechanical behaviors and field-size discrepancies may explain the differing impacts of increased contact force on PEF treatment sizes for compressible LAF catheters versus solid-tip catheters.This study also indicates that the number of splines in contact and the amount of each spline in contact with myocardium do not influence lesion size if the spline(s) have stable CTC (Figure 4).With RF ablation, increased contact force leads to larger treatments, but also increased safety risks. 20Increased CTC with the LAF catheter did not yield any additional safety risk.No damage to collateral structures, steam pops, perforations, or incidences of char were noted acutely or chronically for any CTC cohort.The spheroid catheter tip inherently lends itself to reduced risk of perforation and the flexibility of the nitinol splines allows force to be absorbed by the catheter rather than tissue, reducing tissue trauma.However, because the structure of the LAF catheter creates sizeable lesions even at LTC, splines that are in proximity to critical structures (e.g., AV node, HIS) should be closely monitored to prevent inadvertent damage.Tissue thinning and remodeling were noted after 30 days at ventricular lesion sites; these observations are similar to previous work with a different LAF catheter 21 and require further investigation (Figure S1).
This study determines that, in relation to PEF treatment size and safety, the amount of CTC does not matter once CTC is established with an LAF catheter.These results may extend to "single-shot" catheters where apposition to myocardium, rather than embedding a catheter tip into tissue, is the primary mode of operation. 22It is critical to note that both catheter electrode configuration (e.g., electrode spacing, size) and system configuration (e.g., monopolar, bipolar) play important roles in contact dependence.Monopolar modalities have the ability to create deeper lesions for comparable energy outputs relative to bipolar configurations, making them more robust to intermittent contact. 7Contact detection systems will need to be harmonized with PEF platforms to ensure appropriate feedback for the catheter design and delivery mechanism.

| HOW SHOULD LI BE USED WITH PEF ABLATION?
4][25] When used in a clinical RF workflow, targeting a >20 Ω LI drop effectively predicted lesion efficacy and durability. 26,27For PEF ablation, LI is a useful way to measure CTC and confirm optimal catheter placement for lesion formation.However, LI may be limited in informing real-time PEF lesion formation.PEF ablation does not rely on electrode-tissue coupling to conduct current to tissue and cause resistive heating for cell death. 7erefore, starting impedance as an indicator of resistive potential or electrode coverage is not critical in PEF ablation.Once contact was established, no correlation was observed between starting impedance and treatment size in this study.Additionally, inconsequential impedance drops immediately post-PEF delivery (1-3 Ω) indicate negligible resistive heating is occurring from energy delivery (Figure 7).These PEF-specific differences limit the utility of impedance drop as an acute metric of therapy success.The effects of the induced electric field cause a cascade of cellular activity which impacts cell homeostasis and viability over the course of hours to days.LI, a wholistic measurement of tissue properties (e.g., temperature and relative health), changes with the evolution of myocardial damage and fibrosis. 28In previous RF studies, tissue impedance was reduced in more fibrotic tissue compared to healthy myocardium. 29Similarly, in this study, there was a substantial decrease in LI response on treated myocardium that presented evidence of transmural fibrosis at 30 days.When considering LI as an indicator of CTC, the decrease in feedback on scar tissue poses a potential shortcoming.Determining contact with previously treated myocardium or in fibrotic tissue will be challenging with LI and warrants further investigation.

| LIMITATIONS
This study was conducted in healthy swine and results may vary in humans or in scarred or diseased tissue.Lesion assessment was done on fresh TTC-stained tissue and all images were retrospectively analyzed for lesion dimensions using methods that are well-established preclinically but impossible to validate clinically.Steps were taken to ensure measurement systems were calibrated correctly and TTC procedures were identical in all animals.All findings in this study use a proprietary biphasic waveform and investigational catheter design that may not be generally applicable for predicting treatment results of different PEF waveforms, systems, and catheters.

F
I G U R E 1 Large area focal (LAF) catheter with an investigational pulsed electric field (PEF) ablation and local impedance (LI) measurement systems.(A) LAF catheter featuring 10 splines for energy delivery and four additional electrodes for sensing and tracking.(B) Investigational PEF system: generator and connection box.(C) Representative lesion with a single application of 25 A PEF energy.C left:Triphenyl tetrazolium chloride (TTC) stained lesion, C right: Fixed lesion with Masson's trichrome staining).(D) LI graphical user interface featuring an en face representation of the LAF catheter tip.(E) Post ablation electroanatomic map created using the LAF catheter.

and 11 5. 2 |
Use Committee of GMD Laboratories.Discrete ventricular lesions and atrial lesion sets were studied using a Yorkshire hybrid swine model (N = 10, 50-70 kg, male and female) in acute (N = 8) and chronic procedures (N = 2).Chronic procedures included recovery post-index procedure and invasive remapping at 30 days to analyze lesion durability and pathology.All catheters were introduced via femoral access under inhaled anesthesia (isoflurane, 1%-5%) and without paralytics.Intravenous heparin was administered to maintain an activated clotting time ≥350 s before catheter introduction.Operators guided the LAF catheter into bloodpool to determine a no-contact location and establish a LI baseline.The catheter was then navigated to target locations by the physician and LI was used to determine CTC at each location.Data from prior studies was used to define three distinct CTC cohorts using LI data.No catheter-tissue contact (NTC) with close proximity to tissue (approximately <2 mm) induced ≤Δ10 Ω from baseline, low catheter-tissue contact (LTC) induced Δ11-29 Ω, and high catheter-tissue contact (HTC) induced ≥Δ30 Ω (Figure 2).Each lesion was delivered at one of the three predefined contact states.To ensure LI measurements were consistent throughout a case, ICE, electrograms, and tactile feedback were used as confirmation of LI CTC feedback.5 | EXPERIMENTAL WORKFLOW 5.1 | Ventricular lesions Ventricular lesions were created in the right ventricle (RV) (four to six applications/chamber) with lesions assigned to one of the three CTC cohorts before ablation.ICE and EAM were used to position lesions ≥20 mm apart to prevent lesion overlap.Acute lesions were allowed to dwell for ≥2 h before euthanasia to enable sufficient maturation.Atrial lesion sets Lesion sets were created in the right atrium (RA) and left atrium (LA) in all animals.A posterior line was created from the superior vena cava to the inferior vena cava using 10 mm center-to-center interlesion spacing.Similarly, PVI was performed around the inferior common pulmonary vein.Lesion sets were assigned to one of the three CTC cohorts before treatment.For LTC and HTC cohorts, all lesions were assigned LTC or HTC, respectively.For the NTC cohort (acute intercaval only), the beginning of an intercaval line was started with HTC, followed by one to two applications with NTC (using ICE to confirm close proximity to tissue and targeting LI response <10 Ω), and the remainder completed with HTC.EAM, pacing from the coronary sinus, and entrance and exit block pacing were used to assess acute block across intercaval lines and isolation of pulmonary veins.Pacing was completed using the investigational LAF catheter or the Boston Scientific Orion Mapping catheter.In chronic animals, remapping was performed at 30 days to confirm durability of all lesion sets.

F I G U R E 2 5 . 3 |
Predefined catheter-tissue contact cohorts: (A) No tissue contact (NTC) example: intracardiac echocardiography (ICE) image of the large area focal (LAF) catheter (outlined in white) in close proximity to the tissue surface (dotted yellow).No color change in local impedance graphical user interface (GUI) as impedance change is ≤Δ10 Ω threshold.(B) Low tissue contact (LTC) Example: ICE image of catheter touching the tissue surface and light green GUI tip graphic corresponding to light contact.(C) High tissue contact example: ICE shows some deformation of the spheroid tip while pushing into tissue and dark green GUI tip graphic corresponding to heavy contact.All lesions were assigned to one of the three catheter-tissue contact cohorts before energy delivery.DOSHI ET AL. | 767 Gross necropsy and histology Deeply sedated swine were perfused intravenously with 1% TTC, then euthanized with an intravenous bolus of 130-150 mg/mL saturated potassium chloride.Hearts were explanted, examined, and photographed.Ventricular lesions were cross-sectioned and individually assessed and measured by a blinded operator using a digital microscope (Dino-Lite Edge, AnMo Electronics).Ventricular lesions and atrial lesion sets were formalin-fixed and processed for histological evaluation with hematoxylin and eosin (H&E) and Masson's Trichrome stains.

Figure 5
Figure 5 displays representative acute and chronic maps and chronic histology for LTC and HTC intercaval lines.In both cohorts, the low-voltage area (<0.1 mV) produced acutely corresponded to persistent low voltage and expansion of the lowvoltage area after 30 days, reflected in gross pathological width measurements (LTC acute: 21.8 ± 4.5 vs. chronic: 37.8 mm; HTC acute: 23.8 ± 3.4 vs. chronic: 43.2 mm).No statistical difference was identified between acute LTC and HTC intercaval widths (p = .62).Histologically, sections showed similar morphological changes and tissue composition for LTC and HTC chronic atrial lesion sets.Acute atrial tissue showed necrotic cardiomyocytes and sparing of blood vessels (Figure 5, 1B,2B).Chronic histological sections revealed complete transmural lesions and clear distinction between healthy and ablated myocardium at the border zone (Figures 5, 1D,2D) with complete fibrotic and fatty tissue replacement of the cardiomyocytes within the ablated region (Figures 5, 1E,2E).

Figure 6
Figure 6 shows an example of the NTC intercaval line.Preablation LI was significantly different between NTC and HTC regions (Δ10 ± 4 vs.Δ35 ± 8 Ω, p = .03).The site of the NTC lesion shows a distinct unaffected high-voltage area (≥1 mV) in comparison to the surrounding low-voltage ablated regions (<0.1 mV) (Figure6A).Gross pathology of all NTC intercaval lines revealed unaffected myocardium in the areas of NTC lesions (Figure6C).Histological analysis revealed necrotic myocardium noted by contraction band necrosis, cell shrinkage, and irregular morphology on the transmural ablated HTC region (Figure6B) versus healthy myocardium on the NTC region (Figure6D).
Abbreviations: HTC, high tissue contact; NTC, no tissue contact; PVI, pulmonary vein isolation.a NTC Intercaval line (HTC lesions interposed with one to two NTC lesions) workflow was only completed in the RA in acute subjects.b NTC workflow only completed in the RA.

F I G U R E 5
Representative electroanatomic maps and histology.(1A and 2A) Pre-ablation, acute, and chronic voltage maps (0.1-1 mV) of RA intercaval lines treated with low tissue contact (LTC) and high tissue contact (HTC).(1B and 2B) Gross pathology of representative intercaval lines (outlined in dotted yellow) show areas of damage and samples of this tissue (white boxes) with Masson's tichrome stain demonstrate pink, healthy myocardium on unablated border regions (1C,D and 2C,D), and transmural blue fibrosis for both LTC and HTC lesions (1E and 2E).RA, right atrium.DOSHI ET AL. | 771 Intercaval lines containing one to two lesions with ≤Δ10 Ω LI resulted in gaps.Therefore, the data indicate CTC can optimize PEF treatment similar to how thermal ablation feedback tools have improved workflows and patient outcomes. 15-1710 | DOES AMOUNT OF CONTACT MATTER FOR PEF ABLATION?While research indicates a relationship between contact force and PEF treatment depth with focal catheters, 18 the LAF catheter investigated in this study did not demonstrate increased treatment depth with increased contact.LTC and HTC yielded similar lesion dimensions (approximately 6mm depth, 16mm width across all CTC ranges >Δ10 Ω).Unique catheter and electric field geometry contribute to this difference.

F I G U R E 6
No tissue contact (NTC) intercaval line (A).Electroanatomic mapping of an NTC RA intercaval line shows low voltage where high tissue contact (HTC) lesions were placed and high voltage where the NTC lesion was placed (A).Tissue treated with HTC showed signs of edema and myoctyolysis in histology with Masson's Trichrome staining (yellow arrows) (B) and visible damage in gross pathology (C).Tissue treated with NTC showed no signs of significant damage in gross pathology (C), or in histology with Masson's trichrome staining (D).ICE, intracardiac echocardiography; RA, right atrium.(A) (B) F I G U R E 7 Local impedance (LI) drop and LI on treated tissue.(A) Comparison of pre-ablation and post-ablation local impedance by cathetertissue contact (CTC) cohort.(B) LI response at 30 days by bipolar electrogram voltage.HTC, high tissue contact; LTC, low tissue contact; NTC, no tissue contact.

13 |
CONCLUSION This study confirmed the necessity and utility of CTC during monopolar PEF ablation of cardiac tissues via an investigational LAF catheter in a swine model.Using LI as an indicator, consistent CTC resulted in effective PEF ablation across a range of cardiac targets suggesting that contact, per se, is more important than the degree of contact.These results diverge from previous research using focal catheters, where increased contact resulted in increased lesion size and indicate LAF catheters require unique research and workflows.Lesion characteristics were optimized and reproducible across a wide range of LI changes and, notably, acute lesion characteristics were preserved chronically.These results suggest an important role for contact detection in the guided delivery of PEF energy.
3C further represents the relationship between lesion size and CTC.Average RV lesion size was not significantly different between LTC and HTC cohorts (depth: 5.7 ± 2.0 vs. 5.7 ± 2.1 mm,

Table 1
summarizes all atrial lesion sets by CTC cohort.LTC and HTC pre-ablation LIs were within the intended ranges for the attempted lesion sets.All LTC and HTC intercaval lines