Objective: To determine the optimized protocol for wet monopolar radiofrequency ablation (RFA) using a perfused-cooled electrode to induce coagulation necrosis in the ex vivo bovine liver.
Materials and methods: Radiofrequency was applied to excised bovine livers in a monopolar mode using a 200W generator with an internally cooled electrode (groups A and B) or a perfused-cooled electrode (groups C, D, E, and F) at maximum power (150-200 W) for 10 minutes. A total of 60 ablation zones were created with six different regimens: group A-dry RFA using intra-electrode cooling; group B-dry RFA using intra-electrode cooling and a pulsing algorithm; group C-wet RFA using only interstitial hypertonic saline (HS) infusion; group D-wet RFA using interstitial HS infusion and a pulsing algorithm; group E-wet RFA using interstitial HS infusion and intra-electrode cooling; and group F-wet RFA using interstitial HS infusion, intra-electrode cooling and a pulsing algorithm. In groups C, D, E, and F, RFA was performed with the infusion of 6% HS through the perfused cooled electrode at a rate of 2 mL/minute. During RFA, we measured the tissue temperature at a distance of 15 mm from the electrode. The dimensions of the ablation zones and the changes in impedance, currents, and liver temperature during RFA were compared between these six groups.
Results: During RFA, the mean tissue impedances in groups A (243+/-88 ohm) and C (252.5+/-108 ohm) were significantly higher than those in groups B (85+/-18.7 ohm), D (108.2+/-85 ohm), E (70.0+/-16.3 ohm), and F (66.5+/-7 ohm) (p < 0.05). The mean currents in groups E and F were significantly higher than those in groups B and D, which were significantly higher than those in groups A and C (p < 0.05): 520+/-425 mA in group A, 1163+/-34 mA in group B, 652.5+/-418 mA in group C, 842.5+/-773 mA in group D, 1665+/-295 mA in group E, and 1830+/-109 mA in group F. The mean volumes of the ablation regions in groups E and F were significantly larger than those in the other groups (p < 0.05): 17.7+/-5.6 cm(3) in group A, 34.5+/-3.0 cm(3) in group B, 20.2+/-15.6 cm(3) in group C, 36.1+/-19.5 cm(3) in group D, 68.1+/-12.4 cm(3) in group E, and 79.5+/-31 cm(3) in group F. The final tissue temperatures at a distance of 15 mm from the electrode were higher in groups E and F than those in groups A, C, and D (p < 0.05): 50+/-7.5 degrees C in group A, 66+/-13.6 degrees C in group B, 60+/-13.4 degrees C in group C, 61+/-12.7 degrees C in group D, 78+/-14.2 degrees C in group E, and 79+/-12.0 degrees C in group F.
Conclusion: Wet monopolar RFA, using intra-electrode cooling and interstitial saline infusion, showed better performance in creating a large ablation zone than either dry RFA or wet RFA without intra-electrode cooling.