Thermomechanics of electrosurgical procedures

Abstract

Starting with Pennes’ bioheat transfer equation to model the temperature profiles observed during ex vivo experiments, we show that the apparent specific heat of the tissue increases significantly at 100°C, highlighting the important role latent heat loss plays in energy dissipation. A proposed two-scale model that couples evaporation at the microscale with energy conservation at the macroscale is able to capture evaporation losses at 100°C and prevent further temperature increase. However, to account for the coupled effects of phase change of water in the intra- and extracellular spaces to vapor, vapor transport, and mechanical deformation, a more comprehensive thermo-mechanical model based on continuum mixture theory is proposed to accurately predict temperature in in vivo and ex vivo experiments on porcine liver tissue. We highlight the effect of applied power on tissue cutting and coagulation damage by coupling this model to an Arrhenius damage model to capture dynamic tissue heating during in situ electrosurgical cutting. The mixture model is coupled to a multi-frequency model to quantify the effect of frequency-dependent electrical properties on the rate of energy generation during ex vivo and in vivo electrosurgical tissue heating with multi-frequency power modes.

The goal of this thesis is to establish the thermomechanical foundations of the response of soft hydrated tissues subjected to excitation by radiofrequency alternating current during electrosurgical procedures. Such procedures are finding increasing application in both surgery and therapeutic endoscopy to conduct various procedures including cutting, ablation, coagulation, and desiccation. However, with rapid heating that results in maximum temperatures exceeding 100°C within seconds, electrosurgery has the potential to cause thermal injuries. Existing models do not provide insight into the different modes of energy dissipation during electrosurgical procedures and often neglect important phenomena such as phase change and vapor transport. These limitations are reflected in large prediction errors when compared to experiment. They also do not offer a quantitative understanding of the effects of electrode motion and neglect the multi-frequency nature of the waveforms used in cutting and coagulation. To overcome these limitations, we have performed ex vivo and in vivo experiments on porcine liver tissue and developed a hierarchy of models of increasing sophistication to systematically capture and quantify the effects of phase change, mass transport, deformation, and damage for both stationary and moving electrodes operating with a variety of waveforms.