Assessment of radiofrequency ablation effect from unipolar pacing threshold

Methods for determining if an ablation lesion has been created by RF current application are limited, but needed. This study sought to determine if a change in pacing threshold at the ablation site might be used to assess creation of an ablation lesion. Peak-to-peak amplitude of the bipolar electrogram (EGM) and the unipolar pacing threshold were determined before and after creation of RF lesions using irrigated tip (63 lesions in 11 patients) or conventional ablation catheters (33 lesions in 9 patients) in infarct scars for ablation of ventricular tachycardia. The threshold was measured during continuous pacing at a cycle length of 600 ms by a decrementing output current at a pulse width of 2 ms. The unipolar pacing threshold increased by 254 +/- 248% (from 5.7 +/- 3.5 to 15.1 +/- 6.7 mA, P<0.001) after irrigated tip ablation and by 155 +/- 144% (from 5.9 +/- 3.4 to 12.3 +/- 5.7 mA, P<0.001) after conventional ablation (P<0.05 for irrigated tip vs conventional). EGM amplitude decreased by 17 +/- 27% (from 0.39 +/- 0.32 to 0.30 +/- 0.21 mV) after irrigated tip ablation and by 16 +/- 24%(from 0.48 +/- 0.27 to 0.41 +/- 0.20 mV) after conventional ablation (irrigated tip vs conventional, P=NS). There was no correlation between the change in bipolar EGM amplitude and the pacing threshold. An increase in unipolar pacing threshold is a marker of lesion creation. In regions of infarction, the relative change in threshold produced by ablation is substantially larger than the change in bipolar electrogram amplitude. The greater increase in pacing threshold after irrigated tip ablation compared to conventional ablation suggests that the magnitude of change reflects lesion size.