Computer Modeling for Radiofrequency Bipolar Ablation Inside Ducts and Vessels: Relation Between Pullback Speed and Impedance Progress

Effect of dispersive electrode position (anterior vs. posterior) in epicardial radiofrequency ablation of ventricular wall: A computer simulation study

An epicardial approach is often used in radiofrequency (RF) catheter ablation to ablate ventricular tachycardia when an endocardial approach fails. Our objective was to analyze the effect of the position of the dispersive patch (DP) on lesion size using computer modeling during epicardial approach. We compared the posterior position (patient's back), commonly used in clinical practice, to the anterior position (patient's chest). The model considered ventricular wall thicknesses between 4 and 8 mm, and electrode insertion depths between .3 and .7 mm. RF pulses were simulated with 20 W of power for 30 s duration. Statistically significant differences (P < .001) were found between both DP positions in terms of baseline impedance, RF current (at 15 s) and thermal lesion size. The anterior position involved lower impedance (130.8 ± 4.7 vs. 146.2 ± 4.9 Ω) and a higher current (401.5 ± 5.6 vs. 377.5 ± 5.1 mA). The anterior position created lesion sizes larger than the posterior position: 8.9 ± 0.4 vs. 8.4 ± 0.4 mm in maximum width, 8.6 ± 0.4 vs. 8.1 ± 0.4 mm in surface width, and 4.5 ± 0.4 vs. 4.3 ± 0.4 mm in depth. Our results suggest that: (1) the redirection of the RF currents due to repositioning the PD has little impact on lesion size and only affects baseline impedance, and (2) the differences in lesion size are only 0.5 mm wider and 0.2 mm deeper for the anterior position, which does not seem to have a clinical impact in the context of VT ablation.

Impact of surrounding tissue-type and peri-electrode gap in stereoelectroencephalography guided (SEEG) radiofrequency thermocoagulation (RF-TC): a computational study

PURPOSE

To use computational modeling to provide a complete and logical description of the electrical and thermal behavior during stereoelectroencephalography-guided (SEEG) radiofrequency thermo-coagulation (RF-TC).

METHODS

A coupled electrical-thermal model was used to obtain the temperature distributions in the tissue during RF-TC. The computer model was first validated by an ex vivo model based on liver fragments and later used to study the impact of three different factors on the coagulation zone size: 1) the difference in the tissue surrounding the electrode (gray/white matter), 2) the presence of a peri-electrode gap occupied by cerebrospinal fluid (CSF), and 3) the energy setting used (power-duration).

RESULTS

The model built for the experimental validation was able to predict both the evolution of impedance and the short diameter of the coagulation zone (error < 0.01 mm) reasonably well but overestimated the long diameter by 2 - 3 mm. After adapting the model to clinical conditions, the simulation showed that: 1) Impedance roll-off limited the coagulation size but involved overheating (around 100 °C); 2) The type of tissue around the contacts (gray vs. white matter) had a moderate impact on the coagulation size (maximum difference 0.84 mm), and 3) the peri-electrode gap considerably altered the temperature distributions, avoided overheating, although the diameter of the coagulation zone was not very different from the no-gap case (<0.2 mm).

CONCLUSIONS

This study showed that computer modeling, especially subject- and scenario-specific modeling, can be used to estimate in advance the electrical and thermal performance of the RF-TC in brain tissue.

How far the zone of heat-induced transient block extends beyond the lesion during RF catheter cardiac ablation

PURPOSE

While radiofrequency catheter ablation (RFCA) creates a lesion consisting of the tissue points subjected to lethal heating, the sublethal heating (SH) undergone by the surrounding tissue can cause transient electrophysiological block. The size of the zone of heat-induced transient block (HiTB) has not been quantified to date. Our objective was to use computer modeling to provide an initial estimate.

METHODS AND MATERIALS

We used previous experimental data together with the Arrhenius damage index (Ω) to fix the Ω values that delineate this zone: a lower limit of 0.1-0.4 and upper limit of 1.0 (lesion boundary). An RFCA computer model was used with different power-duration settings, catheter positions and electrode insertion depths, together with dispersion of the tissue's electrical and thermal characteristics.

RESULTS

The HiTB zone extends in depth to a minimum and maximum distance of 0.5 mm and 2 mm beyond the lesion limit, respectively, while its maximum width varies with the energy delivered, extending to a minimum of 0.6 mm and a maximum of 2.5 mm beyond the lesion, reaching 3.5 mm when high energy settings are used (25 W-20s, 500 J). The dispersion of the tissue's thermal and electrical characteristics affects the size of the HiTB zone by ±0.3 mm in depth and ±0.5 mm in maximum width.

CONCLUSIONS

Our results suggest that the size of the zone of heat-induced transient block during RFCA could extend beyond the lesion limit by a maximum of 2 mm in depth and approximately 2.5 mm in width.

In Silico Modelling to Assess the Electrical and Thermal Disturbance Provoked by a Metal Intracoronary Stent during Epicardial Pulsed Electric Field Ablation

Background: Pulsed Electric Field (PEF) ablation has been recently proposed to ablate cardiac ganglionic plexi (GP) aimed to treat atrial fibrillation. The effect of metal intracoronary stents in the vicinity of the ablation electrode has not been yet assessed. Methods: A 2D numerical model was developed accounting for the different tissues involved in PEF ablation with an irrigated ablation device. A coronary artery (with and without a metal intracoronary stent) was considered near the ablation source (0.25 and 1 mm separation). The 1000 V/cm threshold was used to estimate the ‘PEF-zone’. Results: The presence of the coronary artery (with or without stent) distorts the E-field distribution, creating hot spots (higher E-field values) in the front and rear of the artery, and cold spots (lower E-field values) on the sides of the artery. The value of the E-field inside the coronary artery is very low (~200 V/cm), and almost zero with a metal stent. Despite this distortion, the PEF-zone contour is almost identical with and without artery/stent, remaining almost completely confined within the fat layer in any case. The mentioned hot spots of E-field translate into a moderate temperature increase (<48 °C) in the area between the artery and electrode. These thermal side effects are similar for pulse intervals of 10 and 100 μs. Conclusions: The presence of a metal intracoronary stent near the ablation device during PEF ablation simply ‘amplifies’ the E-field distortion already caused by the presence of the vessel. This distortion may involve moderate heating (<48 °C) in the tissue between the artery and ablation electrode without associated thermal damage.

Limitations of Baseline Impedance, Impedance Drop and Current for Radiofrequency Catheter Ablation Monitoring: Insights from In silico Modeling

Background: Baseline impedance, radiofrequency current, and impedance drop during radiofrequency catheter ablation are thought to predict effective lesion formation. However, quantifying the contributions of local versus remote impedances provides insights into the limitations of indices using those parameters. Methods: An in silico model of left atrial radiofrequency catheter ablation was used based on human thoracic measurements and solved for (1) initial impedance (Z), (2) percentage of radiofrequency power delivered to the myocardium and blood (3) total radiofrequency current, (4) impedance drop during heating, and (5) lesion size after a 25 W−30 s ablation. Remote impedance was modeled by varying the mixing ratio between skeletal muscle and fat. Local impedance was modeled by varying insertion depth of the electrode (ID). Results: Increasing the remote impedance led to increased baseline impedance, lower system current delivery, and reduced lesion size. For ID = 0.5 mm, Z ranged from 115 to 132 Ω when fat percentage varied from 20 to 80%, resulting in a decrease in the RF current from 472 to 347 mA and a slight decrease in lesion size from 5.6 to 5.1 mm in depth, and from 9.2 to 8.0 mm in maximum width. In contrast, increasing the local impedance led to lower system current but larger lesions. For a 50% fat−muscle mixture, Z ranged from 118 to 138 Ω when ID varied from 0.3 to 1.9 mm, resulting in a decrease in the RF current from 463 to 443 mA and an increase in lesion size, from 5.2 up to 7.5 mm in depth, and from 8.4 up to 11.6 mm in maximum width. In cases of nearly identical Z but different contributions of local and remote impedance, markedly different lesions sizes were observed despite only small differences in RF current. Impedance drop better predicted lesion size (R2 > 0.93) than RF current (R2 < 0.1). Conclusions: Identical baseline impedances and observed RF currents can lead to markedly different lesion sizes with different relative contributions of local and remote impedances to the electrical circuit. These results provide mechanistic insights into the advantage of measuring local impedance and identifies potential limitations of indices incorporating baseline impedance or current to predict lesion quality.

In-Silico Modeling to Compare Radiofrequency-Induced Thermal Lesions Created on Myocardium and Thigh Muscle

Beating heart (BH) and thigh muscle (TM) are two pre-clinical models aimed at studying the lesion sizes created by radiofrequency (RF) catheters in cardiac ablation. Previous experimental results have shown that thermal lesions created in the TM are slightly bigger than in the BH. Our objective was to use in-silico modeling to elucidate some of the causes of this difference. In-silico RF ablation models were created using the Arrhenius function to estimate lesion size under different energy settings (25 W/20 s, 50 W/6 s and 90 W/4 s) and parallel, 45° and perpendicular catheter positions. The models consisted of homogeneous tissue: myocardium in the BH model and striated muscle in the TM model. The computer results showed that the lesion sizes were generally bigger in the TM model and the differences depended on the energy setting, with hardly any differences at 90 W/4 s but with differences of 1 mm in depth and 1.5 m in width at 25 W/20 s. The higher electrical conductivity of striated muscle (0.446 S/m) than that of the myocardium (0.281 S/m) is possibly one of the causes of the higher percentage of RF energy delivered to the tissue in the TM model, with differences between models of 2–5% at 90 W/4 s, ~9% at 50 W/6 s and ~10% at 25 W/20 s. Proximity to the air–blood interface (just 2 cm from the tissue surface) artificially created in the TM model to emulate the cardiac cavity had little effect on lesion size. In conclusion, the TM-based experimental model creates fairly similar-sized lesions to the BH model, especially in high-power short-duration ablations (50 W/6 s and 90 W/4 s). Our computer results suggest that the higher electrical conductivity of striated muscle could be one of the causes of the slightly larger lesions in the TM model.

Low-energy (360 J) radiofrequency catheter ablation using moderate power - short duration: proof of concept based on in silico modeling

BACKGROUND

Pilot clinical studies suggest that very high power-very short duration (vHPvSD, 90 W/4 s, 360 J energy) is a feasible and safe technique for ablation of atrial fibrillation (AF), compared with standard applications using moderate power-moderate duration (30 W/30 s, 900 J energy). However, it is unclear whether alternate power and duration settings for the delivery of the same total energy would result in similar lesion formation. This study compares temperature dynamics and lesion size at different power-duration settings for the delivery of equivalent total energy (360 J).

METHODS

An in silico model of radiofrequency (RF) ablation was created using the Arrhenius function to estimate lesion size under different power-duration settings with energy balanced at 360 J: 30 W/12 s (MPSD), 50 W/7.2 s (HPSD), and 90 W/4 s (vHPvSD). Three catheter orientations were considered: parallel, 45°, and perpendicular.

RESULTS

In homogenous tissue, vHPvSD and HPSD produced similar size lesions independent of catheter orientation, both of which were slightly larger than MPSD (lesion size 0.1 mm deeper, ~ 0.7 mm wider, and ~ 25 mm³ larger volume). When considering heterogeneous tissue, these differences were smaller. Tissue reached higher absolute temperature with vHPvSD and HPSD (5-8 °C higher), which might increase risk of collateral tissue injury or steam pops.

CONCLUSION

Ablation for AF using MPSD or HPSD may be a feasible alternative to vHPvSD ablation given similar size lesions with similar total energy delivery (360 J). Lower absolute tissue temperature and slower heating may reduce risk of collateral tissue injury and steam pops associated with vHPvSD and longer applications using moderate power.

Full torso and limited-domain computer models for epicardial pulsed electric field ablation

BACKGROUND AND OBJECTIVES

Pulsed Electric Field (PEF) ablation has been proposed as a non-thermal energy to treat atrial fibrillation (AF) by ablation of ganglionated plexi using the epicardial approach. The electric field distribution at the target site (heart) and its surroundings has not yet been assessed previously, using epicardial ablation technique. Our objective was to develop computational models, incorporating the real anatomy of the heart and the patient's torso, to assess the electric field distribution when applying epicardial monopolar PEF.

METHODS

A novel 3D realistic full torso model was built with the multi-electrode ablation device placed on the epicardium and a dispersive pad on the patient's back to evaluate the electric field distribution. The 400 V/cm isoline was used to estimate the 'PEF-zone'. A 3D limited-domain model was also built including only the region of interest around the ablation device to assess its validity in comparison with the full torso model.

RESULTS

The electrical field is mainly limited to the target site (PEF-zone with lengths of 25.79 to 29.00 mm, depths of 5.98-7.02 mm and maximum widths of 8.75-10.57 mm) and is practically negligible in adjacent organs (<30 V/cm and <36 V/cm in oesophagus and lungs, respectively). The electrical currents ranged from 3.67 A to 7.44 A. The 3D limited-domain model provided a similar electric field distribution to those obtained from the 3D full torso models (differences < 0.5 mm in PEF-zone depth).

CONCLUSIONS

Computational results suggest that PEF-zone is very focused around the ablation catheter. Limited-domain models offer similar results in terms of PEF-zone size, reducing the complexity of the modelling.

Computer simulations of consecutive radiofrequency pulses applied at the same point during cardiac catheter ablation: Implications for lesion size and risk of overheating

BACKGROUND AND OBJECTIVES

To study temperature distribution and lesion size during two repeated radiofrequency (RF) pulses applied at the same point in the context of RF cardiac ablation (RFCA).

METHODS

An in-silico RFCA model accounting for reversible and irreversible changes in myocardium electrical properties due to RF-induced heating. Arrhenius damage model to estimate lesion size during the application of two 20 W pulses at intervals (INT) of from 5 to 70 s. We considered two pulse durations: 20 s and 30 s.

RESULTS

INT has a significant effect on lesion size and maximum tissue temperature (TMAX). The shorter the INT the greater the increase in lesion size after the second pulse but also the greater the TMAX. If the second pulse is applied almost immediately (INT=5 s), depth increases 1.4 mm and 1.5 mm for pulses of 20 s and 30 s, respectively. If INT is longer than 30 s it increases 1.1 mm and 1.3 mm for pulses of 20 s and 30 s, respectively. While a single 20 s pulse causes T_MAX=79 ºC, a second pulse produces values of from 92 to 96 ºC (the higher the temperature the shorter the INT). For 30 s pulses, T_MAX=93 ºC for a single pulse, and varied from 98 to 104 ºC for a second pulse.

CONCLUSIONS

Applying a second RF pulse at the same ablation site increases lesion depth by 1 - 1.5 mm more than a single pulse and could lead to higher temperatures (up to 17 ºC). Both lesion depth and maximum tissue temperature increased at shorter inter-pulse intervals, which could cause clinical complications from overheating such as steam pops.

Relationship between luminal esophageal temperature and volume of esophageal injury during RF ablation: In silico study comparing low power-moderate duration vs. high power-short duration

OBJECTIVE

To model the evolution of peak temperature and volume of damaged esophagus during and after radiofrequency (RF) ablation using low power-moderate duration (LPMD) versus high power-short duration (HPSD) or very high power-very short duration (VHPVSD) settings.

METHODS

An in silico simulation model of RF ablation accounting for left atrial wall thickness, nearby organs and tissues, as well as catheter contact force. The model used the Arrhenius equation to derive a thermal damage model and estimate the volume of esophageal damage over time during and after RF application under conditions of LPMD (30 W, 20 s), HPSD (50 W, 6 s), and VHPVSD (90 W, 4 s).

RESULTS

There was a close correlation between maximum peak temperature after RF application and volume of esophageal damage, with highest correlation (R²  = 0.97) and highest volume of esophageal injury in the LPMD group. A greater increase in peak temperature and greater relative increase in esophageal injury volume in the HPSD (240%) and VHPSD (270%) simulations occurred after RF termination. Increased endocardial to esophageal thickness was associated with a longer time to maximum peak temperature (R²  > 0.92), especially in the HPSD/VHPVSD simulations, and no esophageal injury was seen when the distances were >4.5 mm for LPMD or >3.5 mm for HPSD.

CONCLUSION

LPMD is associated with a larger total volume of esophageal damage due to the greater total RF energy delivery. HPSD and VHPVSD shows significant thermal latency (resulting from conductive tissue heating after RF termination), suggesting a requirement for fewer esophageal temperature cutoffs during ablation.

Effect of intracardiac blood flow pulsatility during radiofrequency cardiac ablation: computer modeling study

PURPOSE

To assess the effect of intracardiac blood flow pulsatility on tissue and blood distributions during radiofrequency (RF) cardiac ablation (RFCA).

METHODS

A three-dimensional computer model was used to simulate constant power ablations with an irrigated-tip electrode and three possible catheter orientations (perpendicular, parallel and 45°). Continuous flow and three different pulsatile flow profiles were considered, with four average blood velocity values: 3, 5.5, 8.5 and 24.4 cm/s. The 50 °C contour was used to assess thermal lesion size.

RESULTS

The differences in lesion size between continuous flow and the different pulsatile flow profiles were always less than 1 mm. As regards maximum tissue temperature, the differences between continuous and pulsatile flow were always less than 1 °C, with slightly higher differences in maximum blood temperature, but never over 6 °C. While the progress of maximum tissue temperature was identical for continuous and pulsatile flow, maximum blood temperature with the pulsatile profile showed small amplitude oscillations associated with blood flow pulsatility.

CONCLUSIONS

The findings show that intracardiac blood pulsatility has a negligible effect on lesion size and a very limited impact on maximum tissue and blood temperatures, which suggests that future experimental studies based on ex vivo or in silico models can ignore pulsatility in intracardiac blood flow.