Valvular leaflets and epicardial fat have a significant effect on lesion size in radiofrequency ablation

Introduction
Radiofrequency (RF) current is the classic ablation technology in the EP lab. By now it is mainly the combination of RF power, duration and contact force that is known to determine RF lesion size. The aim of this ex vivo study was to analyze a by now underestimated factors - valvular tissue and epicardial fat - on RF lesion size.

Methods
N=180 RF ablations were performed in an ex vivo model of porcine cardiac preparations. The model was adjusted to provide standardized and comparable conditions (temperature, impedance, convective cooling) during all RF ablations using a thermostat, a heating element and a circulation pump. The CARTO3 mapping system and the ST SF force sensing ablation catheter were used to create RF lesions at systematically varied levels of RF power (30W, 50W), duration (30sec, 60sec) and contact force (5g, 20g). All settings were applied in a first scenario in which RF ablation was performed on valvular tissue (mitral valve leaflet, Figure A) to underlying left ventricular myocardium and in a second scenario in which RF ablation was performed directly on left ventricular myocardium (Figure B) next to mitral valve leaflets. Identical experimental settings were conducted on and next to epicardial fat. Created lesions were then dissected to measure their depth and diameter using a digital precision caliper for statistical analysis.

Results
The presence of valvular tissue (mitral valve leaflets and aortic cusps) between the RF ablation catheter and targeted left ventricular myocardial tissue resulted in significantly lower lesion sizes when compared to RF lesions ablated directly on left ventricular myocardium (p<0.01, Figure). Similar findings were observed on and next to epicardial fat in epicardial RF ablation. Lesion volume was up to 10-fold smaller when ablations were performed on valved when compared to ablations directly below valves (p<0.01). In 30W, 30sec, 5g ablations, only small and shallow traces of RF applications were identifiable in left ventricular myocardium when applied on valves (depth range 0.5-1mm). Lesion depth was significantly smaller when ablated on valvular tissue (4.4±1.2mm) when compared to below valvular tissue ablation (7.7±1.6) at 30W, 30sec, 5g, p<0.01. Lesion depth was significantly smaller when ablated on epicardial fat (6.5±1.8mm), p<0.01.

Conclusions
Valvular tissue has a significant, by now underestimated effect on RF lesion size. RF ablations performed on valvular leaflets become significantly smaller than lesion ablated on left ventricular myocardium directly. This effect may be the reason for arrythmia recurrence originating from the valvular annuli such as accessory pathways of tricuspid valve PVCs. Similar but less distinct effects are found in epicardial fat tissue.