When ablating over epicardial fat, consider the path of least resistance!

Abstract

Critical portions of ventricular tachycardia circuits sometimes include the epicardial portion of ventricular myocardium and therefore successful catheter ablation of these arrhythmias depends upon effective lesion delivery to this region. On the basis of the thickness of the left ventricular myocardium, this usually requires ablation within the epicardial space, which is often clinically challenging either due to inaccessibility or overlying epicardial fat.Successful radiofrequency ablation relies upon conversion of radiofrequency energy to heat energy via resistive heating. Resistive heating occurs from the catheter tip/tissue interface, penetrating approximately 3 to 4 mm into the tissue. Heating of tissue outside of the zone of resistive heating relies on conductive heating. Epicardial adiposity (EA) impairs radiofrequency energy delivery to the underlying myocardium for several reasons. EA physically separates the ablation catheter tip and the myocardium, potentially beyond the penumbra of resistive heating. The insulative properties of EA can also impair the conductive heating of the underlying myocardium.Finally, EA has a much greater resistance compared to the myocardium. As current (I) is related to resistance (R) by the equationis evident that the increased impedance of EA has the potential to significantly reduce current delivery to underlying myocardial tissue. In this context, Zipse et al 1 performed a series of ex vivo experiments to determine the relationship between a variety of epicardial ablation strategies on lesion formation with varying degrees of overlying EA. The authors used a freshly explanted bovine myocardium preparation similar to prior experiments performed by the authors. 2 The endocardial surface was emerged in circulating saline bath and the epicardial surface was maintained above the saline so convective cooling was only present on the endocardial surface, in an effort to mimic clinical in vivo conditions. With an openirrigated (30 cc/minute) ablation catheter oriented parallel to the epicardial surface with a target force of 10 g and 50 W of power, ablation duration was modified (1, 3, and 5 minutes) over tissue with varying degrees of EA (<1, 1-2, 2-3, and >3 mm). Although overall lesion size increased modestly with increasing duration of ablation over small amounts of EA, no lesion was observed when EA was greater than 3 mm. Subsequent experiments demonstrated that increasing forces to 20 or 30 g generally increased lesions size over small amounts of EA but even high forces were unable to create visible lesions over EA of greater than 3 mm despite 50 W of power over 1 minute. The authors further observed that neither lowtonicity irrigation (0.45% saline) nor the use of bipolar ablation could overcome the challenges of ablating over EA greater than 3 mm. No steam pops were observed with open-irrigated ablation regardless of force, duration of ablation, use of low-tonicity irrigation, or use of a bipolar configuration. The final set of experiments compared an open-irrigated ablat...