Comparative analysis of different methods of modeling the thermal effect of circulating blood flow during RF cardiac ablation

Our aim was to compare the different methods of modeling the effect of circulating blood flow on the thermal lesion dimensions created by radio frequency (RF) cardiac ablation and on the maximum blood temperature. Computational models were built to study the temperature distributions and lesion dimensions created by a nonirrigated electrode by two RF energy delivery protocols (constant voltage and constant temperature) under high and low blood flow conditions. Four methods of modeling the effect of circulating blood flowon lesion dimensions and temperature distribution were compared. Three of them considered convective coefficients at the electrode-blood and tissue-blood interfaces to model blood flow: 1) without including blood as a part of the domain; 2) constant electrical conductivity of blood; and 3) temperaturedependent electrical conductivity of blood (+2%/°C). Method 4) included blood motion andwas considered to be a reference method for comparison purposes. Only Method 4 provided a realistic blood temperature distribution.The other three methods predicted lesion depth values similar to those of the reference method (differences smaller than 1 mm), regardless of ablation mode and blood flow conditions. Considering the aspects of lesion size and maximum temperature reached in blood and tissue, Method 2 seems to be the most suitable alternative to Method 4 in order to reduce the computational complexity. Our findings could have an important implication in future studies of RF cardiac ablation, in particular, in choosing the most suitable method to model the thermal effect of circulating blood

A computational model of open-irrigated electrode for endocardial RF catheter ablation

Radiofrequency catheter ablation (RFCA) is an important curative treatment for cardiac arrhythmias. However, during RFCA thrombus formation can occur when the electrode-tissue interface temperature exceeds 80°C. Open-irrigated electrodes reduce the risk of such side-effect. No computational model of an open-irrigated electrode in endocardial RFCA accounting for both the saline irrigation flow and the blood motion in the cardiac chamber has been proposed yet. Our aim was to introduce the first computer model including both effects. The model has been validated against existing experimental results. Computational results showed that the surface lesion width and blood temperature are affected by the irrigation flow rate. Smaller surface lesion widths and blood temperatures are obtained with higher irrigation flow rate, while the lesion depth is not affected by changing the irrigation flow rate. Larger lesions are obtained when electrode is placed horizontally. Overall, the computational findings are in close agreement with previous experimental results

Computational Model for Prediction of the Occurrence of Steam Pops during Irrigated Radiofrequency Catheter Ablation

Radiofrequency catheter ablation (RFCA) is a curative treatment for cardiac arrhythmias. Although globally a pretty safe procedure, it may present some risk. Steam pop is a serious complication that can occur during RFCA with irrigated electrodes. Pops are caused by tissue overheating above 89oC, and may cause explosive rupture of myocardial wall. Today, it is still very complicated to predict the occurrence and location of steam pops into the tissue during RFCA. Our aim was to use a computational model to address these issues considering two irrigated catheter tip designs and different power settings. The model has been validated against existing experimental results. Computational results show no evidence of significant differences in the incidence of steam pops between the two catheter designs. Steam pops appears at powers higher than 30W at approximately 2 mm depth under the electrode tip. Overall, the computational findings were in close agreement with previous experimental results, which suggests that the proposed model could be useful to predict the occurrence of steam pops in different clinical situations.MINECO TEC2014–52383-C3-R to E.B. and Generalitat Valenciana APOSTD/2016/045 to A.G.S

Effect of the trabecular bone microstructure on measuring its thermal conductivity: A computer modeling-based study

[EN] The objective of this work is to quantify the relation between the value of the effective thermal conductivity of trabecular bone and its microstructure and marrow content. The thermal conductivity of twenty bovine trabecular bone samples was measured prior to and after defatting at 37, 47, and 57 degrees C. Computer models were built including the microstructure geometry and the gap between the tissue and measurement probe. The thermal conductivity (k) measured was 0.39 +/- 0.06 W m(-1) K-1 at 37 degrees C, with a temperature dependence of + 0.2%degrees C-1. Replacing marrow by phosphate-buffered saline (defatting) increased both the computer simulations and measurement results by 0.04 W m(-1) K-1. The computer simulations showed that k increases by 0.02-0.04 W m(-1) K-1 when the model includes a gap filled by phosphate-buffered saline between the tissue and measurement probe. In the presence of microstructure and fatty red marrow, k varies by +/- 0.01 W m(-1) K-1 compared with the case considering matrix only, which suggests that there are no significant differences between cortical and trabecular bone in terms of k. The computer results showed that the presence of a gap filled by phosphate-buffered saline around the energy applicator changes maximum temperature by < 0.7 degrees C, while including the bone microstructure involved a variation of < 0.2 mm in the isotherm location. Future experimental studies on measuring the value of k involving the insertion of a probe into the bone through a drill hole should consider the bias found in the simulations. Thermal models based on a homogeneous geometry (i.e. ignoring the microstructure) could provide sufficient accuracy.This work was supported by a grant from the "Agencia Nacional de Promocion Cientfica y Tecnologica de Argentina" (Ref. PICT-2016-2303), by the National Scientific and Technical Research Council of Argentina (Grant PIO CONICET-UNAJ 0001), and by the Spanish "Programa Estatal de Investigacion, Desarrollo e Innovacion Orientada a los Retos de la Sociedad" under Grant TEC2014-52383-C3-R (TEC2014-52383-C3-1-R).Fajardo, JE.; Carlevaro, CM.; Vericat, F.; Berjano, E.; Irastorza, RM. (2018). Effect of the trabecular bone microstructure on measuring its thermal conductivity: A computer modeling-based study. Journal of Thermal Biology. 77:131-136. https://doi.org/10.1016/j.jtherbio.2018.08.009S1311367

Numerical resolution of the hyperbolic heat equation using smoothed mathematical functions instead of Heaviside and Dirac delta distributions

The hyperbolic bioheat equation (HBE) has been used to model heating applications involving very short power pulses. This equation includes two mathematical distributions (Heaviside and Delta) which have to be necessarily substituted for smoothed mathematical functions when the HBE is solved by numerical methods. This study focuses on which type of smoothed functions would be suitable for this purpose, i.e. those which would provide solutions similar to those obtained analytically from the original Heaviside and Delta distributions. The logistic function was considered as a substitute for the Heaviside function, while its derivative and the probabilistic Gaussian function were considered as substitutes for the Delta distribution. We also considered polynomial interpolation functions, in particular, the families of smoothed functions with continuous second derivative without overshoot used by COMSOL Multiphysics. All the smoothed functions were used to solve the HBE by the Finite Element Method (COMSOL Multiphysics), and the solutions were compared to those obtained analytically from the original Heaviside and Delta distributions. The results showed that only the COMSOL smoothed functions provide a numerical solution almost identical to the analytical one. Finally, we demonstrated mathematically that in order to find a suitable smoothed function (f) that must adequately substitute any mathematical distribution (D) in the HBE, the difference D - f must have compact support. (c) 2013 Elsevier Ltd. All rights reserved.This work received financial support from the Spanish "Plan Nacional de I + D + I del Ministerio de Ciencia e Innovacion" Grant No. TEC2011-27133-C02-01 and from Universitat Politenica de Valencia (PAID-06-11 Ref. 1988). V. Romero Garcia is grateful for the support of "Programa de Contratos Post-Doctorales con Movilidad UPV del Campus de Excelencia (CEI-01-11)" and FEDER Project MAT2009-09438.Rivera Ortun, MJ.; Trujillo Guillen, M.; Romero García, V.; López Molina, JA.; Berjano Zanón, E. (2013). Numerical resolution of the hyperbolic heat equation using smoothed mathematical functions instead of Heaviside and Dirac delta distributions. International Communications in Heat and Mass Transfer. 46:7-12. https://doi.org/10.1016/j.icheatmasstransfer.2013.05.017S7124

Electrical-thermal analytical modeling of monopolar RF thermal ablation of biological tissues: determining the circumstances under which tissue temperature reaches a steady state

This is a pre-copy-editing, author-produced PDF of an article accepted for publication in MATHEMATICAL BIOSCIENCES doi:10.3934/mbe.2015003 AND ENGINEERING following peer review. The definitive publisher-authenticated version Mathematical Biosciences and Engineering (MBE) Pages: 281 - 301, Volume 13, Issue 2, April 2016 is available online at http://www.aimsciences.org/journals/displayArticlesnew.jsp?paperID=11998[EN] It has been suggested that during RF thermal ablation of biological tissue the thermal lesion could reach an equilibrium size after 1-2 minutes. Our objective was to determine under which circumstances of electrode geometry (needle-like vs. ball-tip), electrode type (dry vs. cooled) and blood perfusion the temperature will reach a steady state at any point in the tissue. We solved the bioheat equation analytically both in cylindrical and spherical coordinates and the resultant limit temperatures were compared. Our results demonstrate mathematically that tissue temperature reaches a steady value in all cases except for cylindrical coordinates without the blood perfusion term, both for dry and cooled electrodes, where temperature increases infinitely. This result is only true when the boundary condition far from the active electrode is considered to be at infinitum. In contrast, when a finite and sufficiently large domain is considered, temperature reaches always a steady state.This work received financial support from the Spanish "Plan Estatal de Investigacion, Desarrollo e Innovacion Orientada a los Retos de la Sociedad" under Grant TEC2014-52383-C3-R (TEC2014-52383-C3-1-R).López Molina, JA.; Rivera Ortun, MJ.; Berjano, E. (2016). Electrical-thermal analytical modeling of monopolar RF thermal ablation of biological tissues: determining the circumstances under which tissue temperature reaches a steady state. Mathematical Biosciences and Engineering. 13(2):281-301. https://doi.org/10.3934/mbe.2015003S28130113

Mathematical Modeling of Epicardial RF Ablation of Atrial Tissue with Overlying Epicardial Fat

The efficacy of treating atrial fibrillation by RF ablation on the epicardial surface is currently under question due to the presence of epicardial adipose tissue interposed between the ablation electrode and target site (atrial wall). The problem is probably caused by the electrical conductivity of the fat (0.02 S/m) being lower than that of the atrial tissue (0.4-0.6 S/m). Since our objective is to improve epicardial RF ablation techniques, we planned a study based on a two-dimensional mathematical model including an active electrode, a fragment of epicardial fat over a fragment of atrial tissue, and a section of atrium with circulating blood. Different procedures for applying RF power were studied, such as varying the frequency, using a cooled instead of a dry electrode, and different modes of controlling RF power (constant current, temperature and voltage) for different values of epicardial fat thickness. In general, the results showed that the epicardial fat layer seriously impedes the passage of RF current, thus reducing the effectiveness of atrial wall RF ablation

Radiofrequency-assisted transection of the pancreas vs stapler in distal pancreatectomy: a propensity score matched cohort analysis

[EN] To demonstrate the efficacy of radiofrequency for pancreatic stump closure in reducing the incidence of postoperative pancreatic fistula (POPF) in distal pancreatectomy (DP) compared with mechanical transection methods. Despite all the different techniques of pancreatic stump closure proposed for DP, best practice for avoiding POPF remains an unresolved issue, with an incidence of up to 30% regardless of center volume or surgical expertise. DP was performed in a cohort of patients by applying radiofrequency to stump closure (RF Group) and compared with mechanical closure (Control Group). A propensity score (PS) matched cohort study was carried out to minimize bias from nonrandomized treatment assignment. Cohorts were matched by PS accounting for factors significantly associated with either undergoing RF transection or mechanical closure through logistic regression analysis. The primary end-point was the incidence of clinically relevant POPF (CR-POPF). Of 89 patients included in the whole cohort, 13 case patients from the RF-Group were 1:1 matched to 13 control patients. In both the first independent analysis of unmatched data and subsequent adjustment to the overall propensity score-matched cohort, a higher rate of CR-POPF in the Control Group compared with the RF-Group was detected (25.4% vs 5.3%, p = 0.049 and 53.8% vs 0%; p = 0.016 respectively). The RF Group showed better outcomes in terms of readmission rate (46.2% vs 0%, p = 0.031). No significant differences were observed in terms of mortality, major complications (30.8% vs 0%, p = 0.063) or length of hospital stay (5.7 vs 5.2 days, p = 0.89). Findings suggest that the RF-assisted technique is more efficacious in reducing CR-POPF than mechanical pancreatic stump closure.This work was supported completely by a grant for medical research from the Catalan Surgery Society. Project PI20/00008, funded by Instituto de Salud Carlos III (ISCIII) and co-funded by the European UnionPueyo-Périz, E.; Téllez-Marquès, C.; Radosevic, A.; Morató, O.; Visa, L.; Ilzarbe, L.; Berjano, E.... (2022). Radiofrequency-assisted transection of the pancreas vs stapler in distal pancreatectomy: a propensity score matched cohort analysis. Scientific Reports. 12(1):1-8. https://doi.org/10.1038/s41598-022-11583-01812

Ring electrode for radio-frequency heating of the cornea: modelling and in vitro experiments

[EN] Radio-frequency thermokeratoplasty (RF-TKP) is a technique used to reshape the cornea curvature by means of thermal lesions using radio-frequency currents. This curvature change allows refractive disorders such as hyperopia to be corrected. A new electrode with ring geometry is proposed for RF-TKP. It was designed to create a single thermal lesion with a full-circle shape. Finite element models were developed, and the temperature distributions in the cornea were analysed for different ring electrode characteristics. The computer results indicated that the maximum temperature in the cornea was located in the vicinity of the ring electrode outer perimeter, and that the lesions had a semi-torus shape. The results also indicated that the electrode thickness, electrode radius and electrode thermal conductivity had a significant influence on the temperature distributions. In addition, in vitro experiments were performed on rabbit eyes. At 5 IN power the lesions were fully circular. Some lesions showed non-uniform characteristics along their circular path. Lesion depth depended on heating duration (60% of corneal thickness for 20s, and 30% for 10s). The results suggest that the critical shrinkage temperature (55-63degreesC) was reached at the central stroma and along the entire circular path in all the cases.Berjano, E.; Saiz Rodríguez, FJ.; Alió, J.; Ferrero, JM. (2003). Ring electrode for radio-frequency heating of the cornea: modelling and in vitro experiments. Medical & Biological Engineering & Computing. 41(6):630-639. https://doi.org/10.1007/BF02349970S630639416Alió, J. L., Ismail, M. M., Artola, A., andPérez-Santonja, J. J. (1997a): ‘Correction of hyperopia induced by photorefractive keratectomy using non-contact Ho: YAG laser thermal keratoplasty’,J. Refract. Surg.,13, pp. 13–16Alió, J. L., Ismail, M. M., andSanchez, J. L. (1997b): ‘Correction of hyperopia with non-contact Ho: YAG laser thermal keratoplasty’,J. Refract. Surg.,13, pp. 17–22Alió, J. 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