Bipolar ablation with an additional dispersive electrode – differences in lesion geometrics and lesion formation in an ex vivo model

Background:
Bipolar ablation is considered as bail-out strategy for treating refractory ventricular arrhythmias. In bipolar ablation, current flows between an active and a passive catheter, overcoming the need for a dispersive electrode as in unipolar ablation. However, high values of impedance between the active and passive catheters can lower rates of sufficient lesion creation, e.g. in the LV summit. An option for overcoming this issue is adding an dispersive electrode to the bipolar setting, resulting in an tripolar ablation approach.

Purpose:
This study aimed to investigate feasibility, lesion geometry and lesion formation characteristics of bipolar and tripolar ablation in an ex vivo model.

Methods:
An ex vivo model was created using container filled with saline. Porcine cross sections were placed in the water bath and the clinical ablation setup was imitated by adjusting the impedance to values of approximately 120 Ω, the use of a water pump and a thermostat. 40 lesions were created in several power settings (5 lesions in each setting with 20, 30, 40, 50W) either in a bipolar setting (20 lesions) or a tripolar setting (20 lesions). All parameters (lesion depth, diameter, lesion volume, impedance, temperature, contact force) were collected once per second during ablation.

Results:
In total, 40 lesions were created, mean thickness of the porcine preparations was 20.9 ± 3.9 mm. Baseline impedance was significantly lower in TA (213.1 ± 20.6 Ω vs. 242.7 ± 23.6 Ω, Figure 1) and time to reaching transmurality was numerically higher in TA (29.3 ± 16.4 s vs. 22.0 ± 10.7 s, p = 0.1). Lesion geometry is different in BA and TA: When reaching transmurality, lesions at the active catheter are deeper (higher ratio of lesion depth at the active and passive catheter 1.25 ± 0.30 vs. 1.03 ± 0.12; p = 0.005) and wider (1.22 ± 0.20 vs. 1.06 ± 0.12; p = 0.007). Higher lesion volumes were also observed during the complete application (Figure 2). In both ablation settings, lesion diameters were still increasing after reaching transmurality (Figure 3). In 45% of lesions (n=9 in BA, n = 9 in TA), a steam pop occurred after reaching transmurality, average time to steam pop was depending on the power level (45.5 ± 27.6 s in 30W, 32.2 ± 9.6 s in 40W, 20.7 ± 10.9 s in 50W).

Conclusion:

Lesion geometrics and ablation parameters differ in bipolar and tripolar ablation. Time to transmurality was numerically shorter in bipolar ablation. In tripolar ablation, lesions at the active catheter are relatively wider and deeper and the proportion of lesion volume at the active catheter is higher. The findings indicate an alternated current flow in tripolar ablation compared to bipolar ablation and the observed differences in lesion geometrics can be used for an optimized ablation approach in complex ventricular arrhythmia.