Myocardial electrical impedance as a metric of completeness for radiofrequency ablation lesions

Radiofrequency (RF) ablation has emerged as a promising curative therapy for atrial fibrillation and other supraventricular tachyarrhythmias. Most RF ablation procedures create a pattern of linear lesions that either isolate ectopic focal triggers or divide the atria into functional regions too small to sustain macroreentry. It is thought that these linear lesions must be complete (i.e. both continuous and transmural) in order to block arrhythmic conduction and to avoid creation of a more proarrhythmic substrate. Currently, most ablation endpoints employ pacing-based metrics of lesion completeness that rely upon active tissue properties and are therefore limited by transient or delayed effects of RF on conduction. Myocardial electrical impedance is a passive tissue property that may provide an alternative intraoperative metric that overcomes certain limitations of pacing-based endpoints. Because RF ablation markedly alters the tissue, it was hypothesized that linear ablation lesions change myocardial impedance. Custom instrumentation and data acquisition software were developed to measure impedance using the four-electrode method. Computer simulations were performed to determine the effect of electrode size on four-electrode impedance measurement. Receiver operating characteristic analysis was used to assess the diagnostic accuracy of both resistivity and translesion stimulus-excitation delay to predict completeness of individual lesions. Lesions (n=50) were created in the ventricles of 18 Langendorff-perfused rabbit hearts using a linear epicardial ablation probe. Lesions were either continuous and transmural (n=25), noncontinuous (n=18), or nontransmural (n=7), which was verified histologically after staining with 2,3,5-triphenyltetrazolium chloride. Subthreshold AC current (10 μA, 1 kHz) and four electrodes in a linear array across the lesion were used to measure magnitude and phase of the tissue impedance. When a continuous and transmural lesion was produced, the resistivity increased 26-58 Ωcm (p<0.02). When a noncontinuous or nontransmural lesion was produced, changes in resistivity were markedly smaller and did not reach statistical significance. No changes in phase shift were found. The resistive component of myocardial impedance may provide a useful intraoperative metric of lesion completeness for RF ablation procedures.