Electro-physiological parameters of hepatic radiofrequency ablation — a comparison of in vitro vs. in vivo porcine liver model

Abstract

Background: The radiofrequency ablation (RFA) uses high frequent alternating current to produce tissue necrosis. It is an inherent part of curative treatment within a multimodal therapy concept of malignant liver tumors in hepato-biliary surgery. The biggest problem is the high rate of local recurrences in tumors with a diameter of more than 3 cm. One reason of local failure is the high variability and the poor reproducibility of the necrosis (zone of ablation, ZOA). No imaging modality facilitates monitoring during intraoperativ nor percutaneous RFA. Thus the development of reliably reproducible ablation protcolls is very important. For this purpose different in vitro an in vivo models are known, which are incompletely characterized. Aim of this experimental study is to describe and compare an in vitro and in vivo porcine model by its electro-physiological parameters. Methods: The in vitro porcine model is already published [] and is used for comparison with the in vivo model, in which 22 RFA on 22 pigs (weight 81 kg) with three different RFA-systems (Rita XL 5 cm, Rita XLi7cm, LeVeen 5 cm) were performed. The pigs are larger than in previously performed studies. They underwent general anaesthesia (TIVA). Percutaneous placement of the RFA-device was guided by native CT-scan. The complete intrahepatic positioning of the applicator was afterwards document by contrast enhanced CT-scan. None of the applicator-tips were allowed to be positioned in a major vessel. The RFA was performed due to manufacturer’s algorithm. The electro-physical parameters were online (in real time) recorded by a dedicated software. Two hours after RFA a second contrast enhanced CT-scan was performed. The livers were explanted of the anaesthetised animals and sliced according to the consensus-technique []. The liver slices were digitally photographed and measured by image-J. The results were compared to the data of the already publicised in vitro experiment. Results: In vivo the delivered energy was with Rita-XL 146±52 kJoules (in vitro 135±53 kJoules); with Rita-Xli 618±261 kJoules (in vitro 159±54) and with LeVeen 218±67 kJoules. This correlates to an energy consumption per ml of necrosis with Rita XL of 5,6±3,6 kJ/ml (in vitro 6,4±3,9 kJ/ml), with Rita-Xli of 11,12±5,14 kJ/ml (in vitro 1,8±0,2 kJ/ml) and with LeVeen of 14,28±10,56 kJ/ml. The volume of ablation was in vivo 32,25±15,44 ml (Rita XL), 75,81±60,10 ml (Rita XLi) und 20,69±10,09 ml (LeVeen), in vitro 26±17 ml (Rita XL), 88±21 ml (Rita XLi) and 50±12 (LeVeen). The impedance during RFA were in vivo 39±4 Ω (Rita XL), 34±5 Ω (Rita XLi) and 32±2 Ω (LeVeen), in vitro 50±14 Ω (Rita XL) and 61±16 Ω (Rita XLi). Conclusion: The Rita XL-system shows in vivo and vitro analogue data concerning energy consumption and volume of ablation. The electro-physiological parameters are consistent with a complete ablation. The Rita-XLi-system shows volume data with wide variation, which means a little reproducibility of a single ablation. In combination with a very high rate of delivered energy (4 times higher than in vitro) and a higher energy consumption (6 times than in vitro) these are signs for heat sink effects. The loss of energy from the liver was accompanied by a rise of body temperature in these animals. The large deployment of the Rita-XLi device made the percutaneous positioning in the liver without direct contact to liver surface and major vessels impossible. The in vitro results reflect more the characteristics of the XLi-device in human tissue than the porcine in vivo model. Modern RFA-systems which generate large volume of tissue necrosis can therefore only be adequately tested in a porcine model with a liver weight of at least 2 kg. Alternatively a bovine-liver model (with a liver weight up to 10 kg) should be developed in the future.