Can noncontact mapping distinguish between endo- and epicardial foci?

Objectives  Noncontact mapping has been demonstrated to facilitate RF ablation of ventricular arrhythmias, but the reproducibility in the localization of endocardial exit sites during focal ventricular tachycardia (“VT”) originating from defined myocardial layers has not been systematically studied. Furthermore, it remains unclear whether noncontact mapping can distinguish between endo- and epicardial foci. Methods  In six dogs, constant pacing was applied through octopolar needle electrodes in the left ventricle to mimic VT of subendocardial, midmyocardial (mid1; mid2) or subepicardial origin. Using noncontact mapping, the site of origin was determined for each of 50 consecutive beats of all “VTs” and the variation between respective exit sites was measured. Exit sites were reconstructed for 50 consecutive beats of each “VT” and the time span between site of origin and exit site was measured as a parameter of intramural conduction. Results  While subendocardial and midmyocardial (mid1, mid2) foci were pinpointed with a variation of ≤2 mm, a variation of 4 mm was encountered for subepicardial foci. A gradual increase in intramural conduction was evident from endocardial towards epicardial foci, with significant differences between subendocardial (4.8 ± 0.9 ms), midmyocardial (mid1 = 11.1 ± 4.6 ms; mid2 = 11.8 ± 3.5 ms) and subepicardial (16.8 ± 3.6 ms) foci (P < 0.005). Systematic differences in the morphology of virtual waveforms depending on the site of origin could not be detected. Conclusions  Except for subepicardial foci, noncontact mapping localized focal activity in the LV with high reproducibility. In contrast to morphological parameters, the determination of intramural conduction provides a fair estimate of the depth of foci and is proposed as a novel parameter to identify a subepicardial origin. Dr. Frederik Voss and Dr. Alexander Bauer contributed equally to this work.