Enrique Berjano

Theoretical modeling for radiofrequency ablation: state-of-the-art and challenges for the future

Radiofrequency ablation is an interventional technique that in recent years has come to be employed in very different medical fields, such as the elimination of cardiac arrhythmias or the destruction of tumors in different locations. In order to investigate and develop new techniques, and also to improve those currently employed, theoretical models and computer simulations are a powerful tool since they provide vital information on the electrical and thermal behavior of ablation rapidly and at low cost. In the future they could even help to plan individual treatment for each patient. This review analyzes the state-of-the-art in theoretical modeling as applied to the study of radiofrequency ablation techniques. Firstly, it describes the most important issues involved in this methodology, including the experimental validation. Secondly, it points out the present limitations, especially those related to the lack of an accurate characterization of the biological tissues. After analyzing the current and future benefits of this technique it finally suggests future lines and trends in the research of this area.

Thermal-electrical modeling for epicardial atrial radiofrequency ablation

Epicardial radiofrequency ablation is increasingly being used for intraoperative treatment of atrial fibrillation. However, the effect of different parameters on the lesion characteristics has not been sufficiently characterized. We used a finite element model to calculate the temperature distribution in the atrial tissue under different conditions during a constant voltage radiofrequency ablation. Our simulation results show that although in the case of a thin atrium the lesion was less deep for a thin atrium, it was easier to achieve transmurality. While considering a thinner atrium, the location of the hottest point of the lesion shifted from the electrode tip to epicardial surface. This effect was due to the convective cooling of the circulating blood inside the atrium. This convective cooling phenomenon has almost negligible effects for atria thicker than 3 mm. The variability of the cooling values has no significant effect on the lesion, even for thin atria (1-2 mm). Increasing the electrode insertion depth (ID) in the tissue produced larger lesions. However, for thinner atria (thickness <2 mm), this increase in the ID reduced the lesion width. It was also proved that the presence of a fat layer between the electrode and the atrial tissue decreased significantly the lesion dimensions.

What affects esophageal injury during radiofrequency ablation of the left atrium? An engineering study based on finite-element analysis

Recent studies on intraoperative radiofrequency ablation of atrial fibrillation have reported some cases of injury to the esophagus. The aim of this study was to perform computer simulations using a theoretical model in order to investigate the effect of different factors on the temperature distributions in the esophagus during ablation. A three-dimensional model was built to include an active electrode, atrial tissue, epicardial fat layer and a fragment of esophagus, aorta and lung, all linked by connective tissue. The finite-element method was used to calculate the temperature distribution during a procedure of constant-temperature ablation. The lesion geometry was assessed using a 50 degrees C isotherm. Our results show that the electrical power directly applied to the esophagus is insignificant and hence the esophageal injury is exclusively due to thermal conduction from the atrium. The esophageal lesion is mainly influenced by the thickness of connective tissue. Both the programmed target temperature of the electrode and the duration of the ablation also have a significant effect on the lesion in the esophagus. In contrast, the epicardial fat layer (0.9 mm thickness) did not show a significant influence. In conclusion, this theoretical model allows us to study the effect of different factors on the thermal injury in the esophagus during intraoperative radiofrequency ablation of atrial tissue.

Modeling the heating of biological tissue based on the hyperbolic heat transfer equation

In modern surgery, a multitude of minimally intrusive operational techniques are used which are based on the point heating of target zones of human tissue via laser or radiofrequency currents. Traditionally, these processes are modeled by the bioheat equation introduced by Pennes, who considers Fouriers theory of heat conduction. We present an alternative and more realistic model established using the hyperbolic equation of heat transfer. To demonstrate some features and advantages of our proposed method, we apply the results obtained to different types of tissue heating with high energy fluxes, in particular radiofrequency heating and pulsed laser treatment of the cornea to correct refractive errors. We hope that the results from our approach will help with refining surgical interventions in this novel field of medical treatment.

A cooled intraesophageal balloon to prevent thermal injury during endocardial surgical radiofrequency ablation of the left atrium: a finite element study

Recent clinical studies on intraoperative monopolar radiofrequency ablation of atrial fibrillation have reported some cases of injury to the esophagus. The aim of this study was to perform computer simulations using three-dimensional finite element models in order to investigate the feasibility of a cooled intraesophageal balloon appropriately placed to prevent injury. The models included atrial tissue and a fragment of esophagus and lung linked by connective tissue. The lesion depth in the esophagus was assessed using a 50 degrees C isotherm and expressed as a percentage of thickness of the esophageal wall. The results are as follows: (1) chilling the esophagus by means of a cooled balloon placed in the lumen minimizes the lesion in the esophageal wall compared to the cases in which no balloon is used (a collapsed esophagus) and with a non-cooled balloon; (2) the temperature of the cooling fluid has a more significant effect on the minimization of the lesion than the rate of cooling (the thermal transfer coefficient for forced convection); and (3) pre-cooling periods previous to RF ablation do not represent a significant improvement. Finally, the results also suggest that the use of a cooled balloon could affect the transmurality of the atrial lesion, especially in the cases where the atrium is of considerable thickness.

Esophageal Temperature During Radiofrequency‐Catheter Ablation of Left Atrium: A Three‐Dimensional Computer Modeling Study

Introduction: There is current interest in finding a way to minimize thermal injury in the esophagus during radiofrequency‐catheter ablation of the left atrium. Despite the fact that the esophageal temperature is now being monitored during ablation, the influence of different anatomic and technical factors on the temperature rise remains unknown.

Research and development of a new RF-assisted device for bloodless rapid transection of the liver: Computational modeling and in vivo experiments

BackgroundEfficient and safe transection of biological tissue in liver surgery is strongly dependent on the ability to address both parenchymal division and hemostasis simultaneously. In addition to the conventional clamp crushing or finger fracture methods other techniques based on radiofrequency (RF) currents have been extensively employed to reduce intraoperative blood loss. In this paper we present our broad research plan for a new RF-assisted device for bloodless, rapid resection of the liver.MethodsOur research plan includes computer modeling and in vivo studies. Computer modeling was based on the Finite Element Method (FEM) and allowed us to estimate the distribution of electrical power deposited in the tissue, along with assessing the effect of the characteristics of the device on the temperature profiles. Studies based on in vivo pig liver models provided a comparison of the performance of the new device with other techniques (saline-linked technology) currently employed in clinical practice. Finally, the plan includes a pilot clinical trial, in which both the new device and the accessory equipment are seen to comply with all safety requirements.ResultsThe FEM results showed a high electrical gradient around the tip of the blade, responsible for the maximal increase of temperature at that point, where temperature reached 100°C in only 3.85 s. Other hot points with lower temperatures were located at the proximal edge of the device. Additional simulations with an electrically insulated blade produced more uniform and larger lesions (assessed as the 55°C isotherm) than the electrically conducting blade. The in vivo study, in turn, showed greater transection speed (3 ± 0 and 3 ± 1 cm2/min for the new device in the open and laparoscopic approaches respectively) and also lower blood loss (70 ± 74 and 26 ± 34 mL) during transection of the liver, as compared to saline-linked technology (2 ± 1 cm2/min with P = 0.002, and 527 ± 273 mL with P = 0.001).ConclusionA new RF-assisted device for bloodless, rapid liver resection was designed, built and tested. The results demonstrate the potential advantages of this device over others currently employed.

Modeling for radio-frequency conductive keratoplasty: implications for the maximum temperature reached in the cornea

Conductive keratoplasty (CK) is a new surgical technique for steepening the contours of the cornea to reduce hyperopia. It has been emphasized that during CK, tissue resistance to radio-frequency electrical current flow generates a localized heat with temperatures between 65 and 75 degrees C; however, we hypothesize that the maximum temperature reached in the cornea may be higher. For this reason, we developed a finite-element model to estimate the temperature distributions in the cornea during CK. The time evolution of the impedance obtained from computer simulations was compared to that obtained in an experimental study previously published. Our results show that during a typical CK with a 60% setting power (equivalent to 200 V peak-to-peak), the cornea may reach temperatures over 100 degrees C at the electrode tip. On the other hand, the initial impedance of the cornea has a significant influence on the temperature distribution, while the initial temperature of the cornea is not a significant parameter. The results also suggest that low power settings (30-40%) do not produce temperatures over 100 degrees C. Finally, although the actual voltage waveform during CK is exponential and pulsed, our model based on a constant voltage (with a value equal to the root mean square value) provides a better agreement between the theoretical impedance time evolution and that obtained experimentally.

Review of the mathematical functions used to model the temperature dependence of electrical and thermal conductivities of biological tissue in radiofrequency ablation

Abstract Purpose: Although theoretical modelling is widely used to study different aspects of radiofrequency ablation (RFA), its utility is directly related to its realism. An important factor in this realism is the use of mathematical functions to model the temperature dependence of thermal (k) and electrical (σ) conductivities of tissue. Our aim was to review the piecewise mathematical functions most commonly used for modelling the temperature dependence of k and σ in RFA computational modelling. Materials and methods: We built a hepatic RFA theoretical model of a cooled electrode and compared lesion dimensions and impedance evolution with combinations of mathematical functions proposed in previous studies. We employed the thermal damage contour D63 to compute the lesion dimension contour, which corresponds to Ω = 1, Ω being local thermal damage assessed by the Arrhenius damage model. Results: The results were very similar in all cases in terms of impedance evolution and lesion size after 6 min of ablation. Although the relative differences between cases in terms of time to first roll-off (abrupt increase in impedance) were as much as 12%, the maximum relative differences in terms of the short lesion (transverse) diameter were below 3.5%. Conclusions: The findings suggest that the different methods of modelling temperature dependence of k and σ reported in the literature do not significantly affect the computed lesion diameter.

Radio-frequency heating of the cornea: theoretical model and in vitro experiments

We present a theoretical model for the study of cornea heating with radio-frequency currents. This technique is used to reshape the cornea to correct refractive disorders. Our numerical model has allowed the study of the temperature distributions in the cornea and to estimate the dimensions of the lesion. The model incorporates a fragment of cornea, aqueous humor, and the active electrode placed on the cornea surface. The finite element method has been used to calculate the temperature distribution in the cornea by solving a coupled electric-thermal problem. We analyzed by means of computer simulations the effect of: a) temperature influence on the tissue electrical conductivity; b) the dispersion of the biological characteristics; c) the anisotropy of the cornea thermal conductivity; d) the presence of the tear film; and e) the insertion depth of the active electrode in the cornea, and the results suggest that these effects have a significant influence on the temperature distributions and thereby on the lesion dimensions. However, the cooling of the aqueous humor in the endothelium or the realistic value of the cornea curvature did not have a significant effect on the temperature distributions. An experimental model based on the lesions created in rabbit eyes has been used in order to compare the theoretical and experimental results. There is a tendency toward the agreement between experimental and theoretical results, although we have observed that the theoretical model overestimates the lesion dimension.