Novel method for electrode-tissue contact measurement with multi-electrode catheters

Catheter-to-tissue contact angle's effect on lesion formation and characterisation using multichannel bioimpedance method

Objective.Radiofrequency (RF) catheter ablation is a standard treatment for patients with cardiac arrhythmias, providing an efficient, minimally invasive solution. However, the ablation efficiency remains suboptimal due to numerous contributed factors that are overlooked in the literature and not monitored during the procedure. This paper explores the effect of catheter-to-tissue contact angles on lesion formations and the feasibility of the multichannel bioimpedance method in characterising the angles to inform cardiologists.Approach.Two silico simulations based on a realistic human model were built to: (1) simulate lesion formations with different catheter-to-tissue angles under varying conditions of powers and convection cooling, and (2) simulate multichannel bioimpedances measured at each catheter's location and angle. 13 locations were picked in all four chambers with 3 contact conditions (catheter lies along the muscle (0° and 180°), in perpendicular to the muscle (90°) and in middle angles (45° and 135°)). 64 electrodes divided into 4 bands were placed on the thorax for multichannel bioimpedances (3-terminal) measured between the catheter's second electrode E2 (I+,V+), and each pair of adjacent surface electrodes (I-,V-). ANOVA and Tukey's Honestly Significant Difference (HSD) tests were used to evaluate the contact angle's effect on the lesion formations and the bioimpedance's capability in distinguishing between angles.Main results.The results showed that 0° and 180° configurations generated significantly different lesions from other angles. The multichannel bioimpedances could recognise 0°/180° from other angles and correlated moderately to lesion sizes at low ablation power.Significance.This paper concludes that catheter-to-tissue angles can influence the lesion outcomes significantly and the multichannel bioimpedance is able to detect the angles that matter.

Evaluation of electrode-tissue contact using multifrequency impedance analysis and Cole-Cole model fitting

Atrial fibrillation (AF) is a common cardiac arrhythmia, and ablation is the primary treatment for patients with drug intolerance. The success of AF ablation depends on the adhesion of the catheter to the tissue. Existing electrical coupling index (ECI) and electrode-interface resistance (IR) methods based on impedance measurement to evaluate the adhesion between catheters and tissues do not explore the internal changes of the tissue during the compression process. This study introduces a new method to understand these internal changes using multi-frequency impedance combined with Cole-Cole model fitting, which is critical for accurate characterization of the contact between catheter and tissue. We used four-electrodes impedance measurement, using customized circuits and compression platform, applying 5-400 g (3.6-228.2 Pa) pressure to the bullfrog thighs to collect impedance data at frequencies of 500-100 kHz. The Cole-Cole model was then used for data fitting and analysis. The customized circuit accurately detects impedance up to 2 kΩ with less than 5% amplitude error, less than 15% phase error, and less than 6% error in model component values. Correlation analysis showed a significant linear relationship between extracellular fluid resistance and applied pressure (Pearson R ≈ 0.9, p < 0.05), indicating that extracellular fluid resistance increases with compression. This suggests that there is a significant linear positive correlation between the extracellular fluid resistance and the applied pressure, meaning that as the pressure increases, the extracellular fluid resistance correspondingly rises. This may provide a new perspective for studying the degree of catheter-tissue contact during atrial fibrillation ablation procedures.

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

INTRODUCTION

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.

METHODS

PEF ablation via a 10-spline LAF catheter was used to create discrete right ventricle (RV) lesions and atrial lesion sets in 10 swine (eight acute, two chronic). Local impedance (LI) was used to assess CTC. Lesions were assigned to three cohorts using LI above baseline: no tissue contact (NTC: ≤∆10 Ω, close proximity to tissue), low tissue contact (LTC: ∆11-29 Ω), and high tissue contact (HTC: ≥∆30 Ω). Acute animals were infused with triphenyl tetrazolium chloride (TTC) and killed ≥2 h post-treatment. Chronic animals were remapped 30 days post-index procedure and stained with infused TTC.

RESULTS

Mean (± SD) RV treatment sizes between LTC (n = 14) and HTC (n = 17) lesions were not significantly different (depth: 5.65 ± 1.96 vs. 5.68 ± 2.05 mm, p = .999; width: 15.68 ± 5.22 vs. 16.98 ± 4.45 mm, p = .737), while mean treatment size for NTC lesions (n = 6) was significantly smaller (1.67 ± 1.16 mm depth, 5.97 ± 4.48 mm width, p < .05). For atrial lesion sets, acute and chronic conduction block were achieved with both LTC (N = 7) and HTC (N = 6), and NTC resulted in gaps.

CONCLUSIONS

PEF ablation with a specialized LAF catheter in a swine model is dependent on CTC. LI as an indicator of CTC may aid in the creation of consistent transmural lesions in PEF ablation.

Feasibility of Linear Irreversible Electroporation Ablation in the Coronary Sinus

INTRODUCTION

Previous studies demonstrated that the coronary sinus (CS) is an important target for ablation in persistent atrial fibrillation. However, radiofrequency ablation in the CS is associated with coronary vessel damage and tamponade. Animal data suggest irreversible electroporation (IRE) ablation can be a safe ablation modality in vicinity of coronary arteries. We investigated the feasibility of IRE in the CS in a porcine model.

METHODS

Ablation and pacing was performed in the CS in six pigs (weight 60-75 kg) using a modified 9-French steerable linear hexapolar Tip-Versatile Ablation Catheter. Pacing maneuvers were performed from distal to proximal segments of the CS to assess atrial capture thresholds before and after IRE application. IRE ablations were performed with 100 J IRE pulses. After 3-week survival animals were euthanized and histological sections from the CS were analyzed.

RESULTS

A total of 27 IRE applications in six animals were performed. Mean peak voltage was 1509 ± 36 V, with a mean peak current of 22.9 ± 1.0 A. No complications occurred during procedure and 3-week survival. At 30 min post ablation 100% isolation was achieved in all animals. At 3 weeks follow-up pacing thresholds were significant higher as compared to baseline. Histological analysis showed transmural ablation lesions in muscular sleeves surrounding the CS.

CONCLUSION

IRE ablation of the musculature along the CS using a multi-electrode catheter is feasible in a porcine model.

Multielectrode Contact Measurement Can Improve Long-Term Outcome of Pulmonary Vein Isolation Using Circular Single-Pulse Electroporation Ablation

BACKGROUND

Irreversible electroporation (IRE) ablation is generally performed with multielectrode catheters. Electrode-tissue contact is an important predictor for the success of pulmonary vein (PV) isolation; however, contact force is difficult to measure with multielectrode ablation catheters. In a preclinical study, we assessed the feasibility of a multielectrode impedance system (MEIS) as a predictor of long-term success of PV isolation. In addition, we present the first-in-human clinical experience with MEIS.

METHODS

In 10 pigs, one PV was ablated based on impedance (MEIS group), and the other PV was solely based on local electrogram information (electrophysiological group). IRE ablations were performed at 200 J. After 3 months, recurrence of conduction was assessed. Subsequently, in 30 patients undergoing PV isolation with IRE, MEIS was evaluated and MEIS contact values were compared to local electrograms.

RESULTS

In the porcine study, 43 IRE applications were delivered in 19 PVs. Acutely, no reconnections were observed in either group. After 3 months, 0 versus 3 (P=0.21) PVs showed conduction recurrence in the MEIS and electrophysiological groups, respectively. Results from the clinical study showed a significant linear relation was found between mean MEIS value and bipolar dV/dt (r²=0.49, P<0.001), with a slope of 20.6 mV/s per Ohm.

CONCLUSIONS

Data from the animal study suggest that MEIS values predict effective IRE applications. For the long-term success of electrical PV isolation with circular IRE applications, no significant difference in efficacy was found between ablation based on the measurement of electrode interface impedance and ablation using the classical electrophysiological approach for determining electrode-tissue contact. Experiences of the first clinical use of MEIS were promising and serve as an important basis for future research.

Local Electrical Impedance Mapping of the Atria: Conclusions on Substrate Properties and Confounding Factors

The treatment of atrial fibrillation and other cardiac arrhythmias as a major cause of cardiovascular hospitalization has remained a challenge predominantly for patients with severely remodeled substrate. Individualized ablation strategies are extremely important both for pulmonary vein isolation and subsequent ablations. Current approaches to identifying arrhythmogenic regions rely on electrogram-based features such as activation time and voltage. Novel technologies now enable clinical assessment of the local impedance as tissue property. Previous studies demonstrated its use for ablation monitoring and indicated its potential to differentiate healthy substrate, scar, and pathological tissue. This study investigates the potential of local electrical impedance-based substrate mapping of the atria for human in-vivo data. The presented pipeline for impedance mapping particularly contains options for dealing with undesirable effects originating from cardiac motion, catheter motion, or proximity to other intracardiac devices. Bloodpool impedance was automatically determined as a patient-specific reference. Full-chamber, left atrial impedance maps were drawn up from interpolating the measured impedances to the atrial endocardium. Finally, the origin and magnitude of oscillations of the raw impedance recording were probed into. The most dominant reason for exclusion of impedance samples was the loss of endocardial contact. With median elevations above the bloodpool impedance between 29 and 46 Ω, the impedance within the pulmonary veins significantly exceeded the remaining atrial walls presenting median elevations above the bloodpool impedance between 16 and 20 Ω. Previous ablation lesions were distinguished from their surroundings by a significant drop in local impedance while the corresponding regions did not differ for the control group. The raw impedance was found to oscillate with median amplitudes between 6 and 17 Ω depending on the patient. Oscillations were traced back to an interplay of atrial, ventricular, and respiratory motion. In summary, local impedance measurements demonstrated their capability to distinguish pathological atrial tissue from physiological substrate. Methods to limit the influence of confounding factors that still hinder impedance mapping were presented. Measurements at different frequencies or the combination of multiple electrodes could lead to further improvement. The presented examples indicate that electrogram- and impedance-based substrate mapping have the potential to complement each other toward better patient outcomes in future.

Local impedance for the optimization of radiofrequency lesion delivery: A review of bench and clinical data

INTRODUCTION

Radiofrequency catheter ablation is a cornerstone of treatment for many cardiac arrhythmias. Progression in three-dimensional mapping and contact-force sensing technologies have improved our capability to achieve success, but challenges still remain.

METHODS

In this article, we discuss the importance of overall circuit impedance in radiofrequency lesion formation. This is followed by a review of the literature regarding recently developed "local impedance" technology and its current and future potential applications and limitations, in the context of established surrogate markers currently used to infer effective ablation.

RESULTS

We discuss the role of local impedance in assessing myocardial substrate, as well as its role in clinical studies of ablation. We also discuss safety considerations, limitations and ongoing research.

CONCLUSION

Local impedance is a novel tool which has the potential to tailor ablation in a manner distinct from other established metrics.

Efficacy of multi-electrode linear irreversible electroporation

AIMS

We investigated the efficacy of linear multi-electrode irreversible electroporation (IRE) ablation in a porcine model.

METHODS AND RESULTS

The study was performed in six pigs (weight 60-75 kg). After median sternotomy and opening of the pericardium, a pericardial cradle was formed and filled with blood. A linear seven polar 7-Fr electrode catheter with 2.5 mm electrodes and 2.5 mm inter-electrode spacing was placed in good contact with epicardial tissue. A single IRE application was delivered using 50 J at one site and 100 J at two other sites, in random sequence, using a standard monophasic defibrillator connected to all seven electrodes connected in parallel. The pericardium and thorax were closed and after 3 weeks survival animals were euthanized. A total of 82 histological sections from all 18 electroporation lesions were analysed. A total of seven 50 J and fourteen 100 J epicardial IRE applications were performed. Mean peak voltages at 50 and 100 J were 1079.2 V ± 81.1 and 1609.5 V ± 56.8, with a mean peak current of 15.4 A ± 2.3 and 20.2 A ± 1.7, respectively. Median depth of the 50 and 100 J lesions were 3.2 mm [interquartile range (IQR) 3.1-3.6] and 5.5 mm (IQR 4.6-6.6) (P < 0.001), respectively. Median lesion width of the 50 and 100 J lesions was 3.9 mm (IQR 3.7-4.8) and 5.4 mm (IQR 5.0-6.3), respectively (P < 0.001). Longitudinal sections showed continuous lesions for 100 J applications.

CONCLUSION

Epicardial multi-electrode linear application of IRE pulses is effective in creating continuous deep lesions.

Local catheter impedance drop during pulmonary vein isolation predicts acute conduction block in patients with paroxysmal atrial fibrillation: initial results of the LOCALIZE clinical trial

Aims

Radiofrequency ablation creates irreversible cardiac damage through resistive heating and this temperature change results in a generator impedance drop. Evaluation of a novel local impedance (LI) technology measured exclusively at the tip of the ablation catheter found that larger LI drops were indicative of more effective lesion formation. We aimed to evaluate whether LI drop is associated with conduction block in patients with paroxysmal atrial fibrillation (AF) undergoing pulmonary vein isolation (PVI).

Methods and results

Sixty patients underwent LI-blinded de novo PVI using a point-by-point ablation workflow. Pulmonary vein rings were divided into 16 anatomical segments. After a 20-min waiting period, gaps were identified on electroanatomic maps. Median LI drop within segments with inter-lesion distance ≤6 mm was calculated offline. The diagnostic accuracy of LI drop for predicting segment block was assessed using receiver operating characteristic analysis.

For segments with inter-lesion distance ≤6 mm, acutely blocked segments had a significantly larger LI drop [19.8 (14.1–27.1) Ω] compared with segments with gaps [10.6 (7.8–14.7) Ω, P < 0.001). In view of left atrial wall thickness differences, the association between LI drop and block was further evaluated for anterior/roof and posterior/inferior segments. The optimal LI cut-off value for anterior/roof segments was 16.1 Ω (positive predictive value for block: 96.3%) and for posterior/inferior segments was 12.3 Ω (positive predictive value for block: 98.1%) where inter-lesion distances were ≤6 mm.

Conclusion

The magnitude of LI drop was predictive of acute PVI segment conduction block in patients with paroxysmal AF. The thinner posterior wall required smaller LI drops for block compared with the thicker anterior wall.

Local impedance-guided radiofrequency ablation with standard and high power: Results of a preclinical investigation

BACKGROUND

Local impedance (LI) drop measured with microfidelity electrodes embedded in the tip of an ablation catheter accurately reflects tissue heating during radiofrequency (RF) ablation. Previous studies found 15-30 Ω LI drops created successful lesions, while more than 40 Ω drops were associated with steam pops. The objective of this study was to evaluate the safety and efficacy of LI-guided ablation using standard (30 W) and high-power (50 W) in a preclinical model.

METHODS

RF lesions were created in explanted swine hearts (n = 6) to assess the feasibility of LI-guided ablation by targeting 10, 20, or 30 Ω (n = 20/group) drops. Subsequently, LI-guided ablation was evaluated in a chronic animal model (n = 8 Canines, 25-29 kg, 30/50 W). During the index procedure point-by-point intercaval line ablation and left inferior pulmonary vein (PV) isolation were performed. RF duration was at the operators' discretion but discontinued early if a 15-30 Ω drop was achieved. Operators attempted to avoid LI drops of more than 40 Ω. At 1-month, durable conduction block was evaluated with electroanatomic mapping followed by necropsy and histopathology.

RESULTS

In explanted tissue, terminating ablation at 10, 20, or 30 Ω LI drops created statistically larger lesions (p < .05; 1.8 [1.6-2.4] mm, 3.3 [3.0-3.7] mm; 4.9 [4.3-5.5] mm). LI-guided high-power ablation in vivo significantly reduced RF duration per application compared to standard-power (p < .05; intercaval: 8.9 ± 5.2 vs. 18.1 ± 11.0 s, PV: 9.6 ± 5.4 vs. 23.2 ± 10.3 s). LI drops of 15-40 Ω were more readily achievable for high-power (90.1%, 318/353) than standard-power (71.7%, 243/339). All intercaval lines and PV isolations were durable (16/16) at 1-month. Necropsy revealed no major collateral injury to the pericardium, phrenic nerve, esophagus, or lungs. There was no pericardial effusion, stroke, tamponade, or PV stenosis. Vagal nerve injury was found in two 30 W animals after using 19.7 ± 13.9 and 19.5 ± 11.8 s RF applications.

CONCLUSION

LI-guided ablation was found to be safe and efficacious in a chronic animal model. High-power ablation more readily achieved more than 15 Ω drops, reduced RF duration compared with standard-power, and had no major RF collateral injury.

A novel assessment of local impedance during catheter ablation: initial experience in humans comparing local and generator measurements

AIMS

A novel measure of local impedance (LI) has been found to predict lesion formation during radiofrequency current (RFC) catheter ablation. The aim of this study was to investigate the utility of this novel approach, while comparing LI to the well-established generator impedance (GI).

METHODS AND RESULTS

In 25 consecutive patients with a history of atrial fibrillation, catheter ablation was guided by a 3D-mapping system measuring LI in addition to GI via an ablation catheter tip with three incorporated mini-electrodes. Local impedance and GI before and during RFC applications were studied. In total, 381 RFC applications were analysed. The baseline LI was higher in high-voltage areas (>0.5 mV; LI: 110.5 ± 13.7 Ω) when compared with intermediate-voltage sites (0.1-0.5 mV; 90.9 ± 10.1 Ω, P < 0.001), low-voltage areas (<0.1 mV; 91.9 ± 16.4 Ω, P < 0.001), and blood pool LI (91.9 ± 9.9 Ω, P < 0.001). During ablation, mean LI drop (△LI; 13.1 ± 9.1 Ω) was 2.15 times higher as mean GI drop (△GI) (6.1 ± 4.2 Ω, P < 0.001). Baseline LI correlated with △LI: a mean LI of 99.9 Ω predicted a △LI of 12.9 Ω [95% confidence interval (12.1-13.6), R2 0.41; P < 0.001]. This relationship was weak for baseline GI predicting △GI (R2 0.06, P < 0.001). Catheter movements were represented by rapid LI changes. The duration of an RFC application was not predictive for catheter-tissue coupling with no further change of △LI (P = 0.247) nor △GI (P = 0.376) during prolonged ablation.

CONCLUSION

Local impedance can be monitored during ablation. Compared with the sole use of GI, baseline LI is a better predictor of impedance drops during ablation and may provide useful insights regarding lesion formation. However, further studies are needed to investigate if this novel approach is useful to guide catheter ablation.

Pulmonary Vein Isolation With Single Pulse Irreversible Electroporation

Background:

Irreversible electroporation (IRE) is a promising new nonthermal ablation technology for pulmonary vein (PV) isolation in patients with atrial fibrillation. Experimental data suggest that IRE ablation produces large enough lesions without the risk of PV stenosis, artery, nerve, or esophageal damage. This study aimed to investigate the feasibility and safety of single pulse IRE PV isolation in patients with atrial fibrillation.

Methods:

Ten patients with symptomatic paroxysmal or persistent atrial fibrillation underwent single pulse IRE PV isolation under general anesthesia. Three-dimensional reconstruction and electroanatomical voltage mapping (EnSite Precision, Abbott) of left atrium and PVs were performed using a conventional circular mapping catheter. PV isolation was performed by delivering nonarcing, nonbarotraumatic 6 ms, 200 J direct current IRE applications via a custom nondeflectable 14-polar circular IRE ablation catheter with a variable hoop diameter (16–27 mm). A deflectable sheath (Agilis, Abbott) was used to maneuver the ablation catheter. A minimum of 2 IRE applications with slightly different catheter positions were delivered per vein to achieve circular tissue contact, even if PV potentials were abolished after the first application. Bidirectional PV isolation was confirmed with the circular mapping catheter and a post ablation voltage map. After a 30-minute waiting period, adenosine testing (30 mg) was used to reveal dormant PV conduction.

Results:

All 40 PVs could be successfully isolated with a mean of 2.4±0.4 IRE applications per PV. Mean delivered peak voltage and peak current were 2154±59 V and 33.9±1.6 A, respectively. No PV reconnections occurred during the waiting period and adenosine testing. No periprocedural complications were observed.

Conclusions:

In the 10 patients of this first-in-human study, acute bidirectional electrical PV isolation could be achieved safely by single pulse IRE ablation.

Multicentre randomised trial comparing contact force with electrical coupling index in atrial flutter ablation (VERISMART trial)

Introduction

Electrical coupling index (ECI) and contact force (CF) have been developed to aid lesion formation during catheter ablation. ECI measures tissue impedance and capacitance whilst CF measures direct contact. The aim was to determine whether the presence of catheter / tissue interaction information, such as ECI and CF, reduce time to achieve bidirectional cavotricuspid isthmus block during atrial flutter (AFL) ablation.

Methods

Patients with paroxysmal or persistent AFL were randomised to CF visible (range 5-40g), CF not visible, ECI visible (change of 12%) or ECI not visible. Follow-up occurred at 3 and 6 months and included a 7 day ECG recording. The primary endpoint was time to bidirectional cavotricuspid isthmus block.

Results

114 patients were randomised, 16 were excluded. Time to bidirectional block was significantly shorter when ECI was visible (median 30.0 mins (IQR 31) to median 10.5mins (IQR 12) p 0.023) versus ECI not visible. There was a trend towards a shorter time to bidirectional block when CF was visible. Higher force was applied when CF was visible (median 9.03g (IQR 7.4) vs. 11.3g (5.5) p 0.017). There was no difference in the acute recurrence of conduction between groups. The complication rate was 2%, AFL recurrence was 1.1% and at 6 month follow-up, 12% had atrial fibrillation.

Conclusion

The use of tissue contact information during AFL ablation was associated with reduced time taken to achieve bidirectional block when ECI was visible. Contact force data improved contact when visible with a trend towards a reduction in the procedural endpoint.

ClinicalTrials.gov trial identifier: NCT02490033.

Novel Measure of Local Impedance Predicts Catheter–Tissue Contact and Lesion Formation

Background:

Coupling between the ablation catheter and myocardium is critical to resistively heat tissue with radiofrequency ablation. The objective of this study was to evaluate whether a novel local impedance (LI) measurement on an ablation catheter identifies catheter–tissue coupling and is predictive of lesion formation.

Methods and Results:

LI was studied in explanted hearts (n=10 swine) and in vivo (n=10; 50–70 kg swine) using an investigational electroanatomic mapping system that measures impedance from an ablation catheter with mini-electrodes incorporated in the distal electrode (Rhythmia and IntellaNav MiFi OI, Boston Scientific). Explanted tissue was placed in a warmed (37 °C) saline bath mounted on a scale, and LI was measured 15 mm away from tissue to 5 mm of catheter–tissue compression at multiple catheter angles. Lesions were created with 31 and 50 W for 5 to 45 seconds (n=90). During in vivo evaluation of LI, measurements of myocardium (n=90) and blood pool (n=30) were guided by intracardiac ultrasound while operators were blinded to LI data. Lesions were created with 31 and 50 W for 45 seconds in the ventricles (n=72). LI of myocardium (119.7 Ω) was significantly greater than that of blood pool (67.6 Ω; P <0.01). Models that incorporate LI drop (ΔLI) to predict lesion size had better performance than models that incorporate force-time integral ( R 2 =0.75 versus R 2 =0.54) and generator impedance drop ( R 2 =0.82 versus R 2 =0.58). Steam pops displayed a significantly higher starting LI and larger ΔLI compared with successful radiofrequency applications ( P <0.01).

Conclusions:

LI recorded from miniature electrodes provides a valuable measure of catheter–tissue coupling, and ΔLI is predictive of lesion formation during radiofrequency ablation.