Optimizing CTI ablation: assessing time and lesion size index at unipolar electrogram transition

Background and Purpose Radiofrequency (RF) ablation of the cavotricuspid isthmus (CTI) is a standardized and effective treatment for typical atrial flutter (AFL). Achieving durable bidirectional block may be challenged by variable atrial wall thickness and heterogeneous lesion formation. The transition of the unipolar electrogram (UE) from biphasic to a purely positive morphology has been proposed as a marker of transmural lesion creation. This study aimed to characterize the timing and lesion size index (LSI)—a composite metric integrating contact force, power, and catheter stability—at the point of UE transition and compare these parameters with conventional ablation endpoints. Methods A retrospective, point-by-point analysis of 220 RF applications was performed in consecutive CTI ablation procedures for typical AFL. Ablations were guided by 3D electroanatomical mapping using an irrigated 4 mm contact force–sensing catheter delivering 35 W of power. For each application, RF time and LSI at UE transition were recorded and compared with total RF duration and final LSI. Paired comparisons were assessed using the Wilcoxon signed-rank test. Results UE transition occurred before ablation completion in 195/220 (88.6%) applications. Median LSI and RF time at transition were 4.4 (IQR 3.9–4.9) and 10 s (IQR 8–15), respectively—significantly lower than final LSI [5.6 (IQR 5.1–6.0)] and total RF time [26 s (IQR 19–33); both p < 0.001]. A purely positive UE was observed in 208/220 (94.6%) points at ablation completion. At six-month follow-up, no AFL recurrence was observed, while atrial fibrillation occurred in 15.8% of cases. Conclusion UE transition occurs significantly earlier than conventional ablation endpoints, suggesting that fixed LSI targets may lead to excessive RF delivery. Integrating real-time UE transition monitoring could enhance procedural efficiency and lesion durability. Prospective validation is warranted.