How does radiofrequency ablation efficacy depend on the stiffness of the cardiac tissue? Insights from a computational model

Objective. Radiofrequency catheter ablation (RFCA) is an effective treatment for the elimination of cardiac arrhythmias, however it is not exempt from complications that can risk the patients’ life. The efficacy of the RFCA depends on several factors and uncertainties during the treatment process. In this paper, we explore the effect of the cardiac tissue stiffness in RFCA. Methods. We use our previously developed RFCA computational model that accounts for the tissue elasticity. The tissue stiffness is described by the Young’s modulus of elasticity. Results. Our numerical simulations provide insights on the efficacy of the RFCA, by measuring the lesion dimensions over a wide range of values of the modulus of elasticity that appear during the cardiac cycle and for different cardiac conditions, using a fixed ablation protocol, commonly used in clinical practice. Conclusion. The stiffness of the cardiac wall affects the power dissipated in the tissue and, as a consequence, has a marked effect on the dimensions of the generated lesion. The heart wall elasticity changes due the cardiac cycle can affect the resulting lesion and can lead to potentially dangerous complications. Pathological conditions can stiffen the cardiac wall, thus reducing the size of the resulting lesion and potentially leading to insufficient treatment. Significance. A relation of the lesion size dimensions for different tissue stiffness and contact force is presented and correlated to different pathological conditions of the heart, showing the direct relation of the tissue stiffness with the efficacy of the RFCA treatment

A computational model of open-irrigated radiofrequency catheter ablation accounting for mechanical properties of the cardiac tissue

Radiofrequency catheter ablation (RFCA) is an effective treatment for cardiac arrhythmias. Although generally safe, it is not completely exempt from the risk of complications. The great flexibility of computational models can be a major asset in optimizing interventional strategies, if they can produce sufficiently precise estimations of the generated lesion for a given ablation protocol. This requires an accurate description of the catheter tip and the cardiac tissue. In particular, the deformation of the tissue under the catheter pressure during the ablation is an important aspect that is overlooked in the existing literature, that resorts to a sharp insertion of the catheter into an undeformed geometry. As the lesion size depends on the power dissipated in the tissue, and the latter depends on the percentage of the electrode surface in contact with the tissue itself, the sharp insertion geometry has the tendency to overestimate the lesion obtained, especially when a larger force is applied to the catheter. In this paper we introduce a full 3D computational model that takes into account the tissue elasticity, and is able to capture the tissue deformation and realistic power dissipation in the tissue. Numerical results in FEniCS-HPC are provided to validate the model against experimental data, and to compare the lesions obtained with the new model and with the classical ones featuring a sharp electrode insertion in the tissue.La Caixa 2016 PhD grant to M. Leoni, and Abbott non-conditional grant to J.M. Guerra Ramo

Effect of Tissue Elasticity in Cardiac Radiofrequency Catheter Ablation Models

Radiofrequency catheter ablation (RFCA) is an effective treatment for different types of cardiac arrhythmias. However, major complications can occur, including thrombus formation and steam pops. We present a full 3D mathematical model for the radiofrequency ablation process that uses an open-irrigated catheter and accounts for the tissue deformation, an aspect overlooked by the existing literature. An axisymmetric Boussinesq solution for spherical punch is used to model the deformation of the tissue due to the pressure of the catheter tip at the tissue-catheter contact point. We compare the effect of the tissue deformation in the RFCA model against the use of a standard sharp insertion of the catheter in the tissue that other state-of- the-art RFCA computational models use.La Caixa 2016 PhD grant to M.L

Modelling studies on biological tissue properties and mechanical responses under external stimuli

PhDBiological tissues maintain their homeostasis by remodelling under external mechanical stimuli. In order to understand the tissue remodelling process, it is important to characterize tissue properties before detailed mechanical responses can be investigated. This project aims to develop a computational modelling framework to characterise mechanical properties of biological tissues, and to quantify tissue responses under mechanical loading. The thesis presents, first, mechanical responses of articular cartilages under different loadings using a poroelastic model. Unique in this study, collagen fibrils are treated separately from the rest of ECM, as they only resists tension. This leads to a fibril-reinforced poroelastic model. Effects of the distribution of the collagen fibrils and their orientation on tissue mechanical responses are investigated. Most of the effort has been on the mechanical stress distribution of the human left atrium and its correlation to electrophysiology patterns in atrial fibrillation. Detailed mechanical responses of the atrial wall to a step pressure increase in the left atrium are calculated. The geometry of the left atrium is based on patient specific images using cardio CT and incorporates variations of the atrial wall thickness as well as unique fibre orientation patterns. We hypothesize that areas of high von Mises stress are correlated to foci of abnormal electrophysiology sites which sustain cardiac arrhythmia. Results from this study show a positive correlation between them. To our knowledge, this is the first study that establishes the relationship between the atrial wall stress distribution and the atrial abnormal electrophysiology sites. The project also investigates hyperelastic properties of endothelial cells and the overlying endothelial glycocalyx, based on data from AFM micro-indentation. Both endothelial cells with & without the glycocalyx layer (i.e. following enzymatic digestion) are used. This is the first time that the mechanical property of the glycocalyx is estimated using an inverse biomechanical model

Finite element simulations: computations and applications to aerodynamics and biomedicine.

171 p.Las ecuaciones en derivadas parciales describen muchos fenómenos de interés práctico y sus solucionessuelen necesitar correr simulaciones muy costosas en clústers de cálculo.En el ámbito de los flujos turbulentos, en particular, el coste de las simulaciones es demasiado grande sise utilizan métodos básicos, por eso es necesario modelizar el sistema.Esta tesis doctoral trata principalmente de dos temas en Cálculo Científico.Por un lado, estudiamos nuevos desarrollos en la modelización y simulación de flujos turbulentos;utilizamos un Método de Elementos Finitos adaptativo y un modelo de ¿número de Reynolds infinito¿para reducir el coste computacional de simulaciones que, sin estas modificaciones, serían demasiadocostosas.De esta manera conseguimos lograr simulaciones evolutivas de flujos turbulentos con número deReynolds muy grande, lo cual se considera uno de los mayores retos en aerodinámica.El otro pilar de esta tesis es una aplicación biomédica.Desarrollamos un modelo computacional de Ablación (Cardiaca) por Radiofrecuencia, una terapiacomún para tratar varias enfermedades, por ejemplo algunas arritmias.Nuestro modelo mejora los modelos existentes en varias maneras, y en particular en tratar de obteneruna aproximación fiel de la geometría del sistema, lo cual se descubre ser crítico para simularcorrectamente la física del fenómeno

Experimental and numerical investigation of wire waveguides for therapeutic ultrasound angioplasty

Therapeutic ultrasound angioplasty is an emerging minimally invasive cardiovascular procedure for disrupting atherosclerotic lesions using small diameter wire waveguides. The lesions are damaged through a combination of direct ablation, pressure waves, cavitation and acoustic streaming caused by distal-tip displacements at ultrasonic frequencies. Numerical and experimental methods are used to investigate the outputs of the wire waveguides during ultrasonic activation. A commercially available generator and acoustic horn are used in combination with Nickel-Titanium (NiTi) wire waveguides in this study. A laser sensor is used to measure the frequency and amplitude output of the distal tip of the wire waveguide, and this is compared to amplitude estimations obtained using an optical microscope. Power is observed to affect both amplitude and frequency. Clinical devices will require long, flexible waveguides with diameters small enough to access the coronary arteries. A finite element model is used to design tapered sections in long wire waveguides in order to achieve low profile distal geometry, and improve ultrasonic wave transmission. These tapered sections reduce the wire waveguide diameter in two stages, firstly from 1 to 0.35mm and then from 0.35 to 0.2, while increasing the amplitude of the ultrasonic wave by factors of 2.85 and 1.75, respectively. The numerical model also showed damping could potentially be a significant problem in long untapered wire waveguides (>l.5m). Experimental ablation trials were conducted using the tapered long wire waveguides, including assessment of the effect of various combinations of bend radii and bend angles. The waveguide was found to perform well, but increased power levels were required to transmit ultrasound through tortuous waveguide configurations