A Comparative Experimental Study of the In-vitro Efficiency of Hypertonic Saline-Enhanced Hepatic Bipolar and Monopolar Radiofrequency Ablation

RFA using monopolar techniques has been used for the treatment of focal liver malignancies, renal tumors, and breast tumors (1, 2). RFA has great potential, but the use of monopolar RF techniques and currently available RF devices fails to create a thermally-mediated zone of ablation which is 3-5 cm in diameter and thus large enough to cover the target tumor. This limitation is related to tissue desiccation and charring at the electrode tip, phenomena attributed primarily to the rapid increase in electrical impedance and marked alterations in of electrical current that typically occur when tissues are heated to greater than 100℃ (19). To overcome this problem, one of the most effective approaches, since it increases electrical conductance and thermal conductivity, is infusion of a saline solution (11, 12, 19). In addition, several studies have demonstrated that bipolar RFA can create larger lesions than monopolar RFA: in the bipolar mode, the current is more concentrated between the probes than in monopolar mode, in which current travels outward toward a dispersive pad (13, 14). In this study, therefore, we hypothesized that a bipolar technique combined with HS infusion could more efficiently create thermal lesions than could monopolar modes, and compared the in-vitro efficiency of the former and the latter.

Our results showed that the bipolar saline-enhanced method created larger lesions than the monopolar method. Interestingly, however, the energy accumulated during RFA was less in the bipolar mode than in monopolar modes. The larger lesions created using less accumulated RF energy output in the bipolar mode than in monopolar modes could, therefore, be attributed to the greater energy efficiency of the former, which might be explained by the high concentration of current between the two electrodes and the thermodynamic effects observed in the bipolar mode (20, 21). In monopolar modes, heat is diverted from the ablation site in all directions. In the bipolar mode, in contrast, one electrode is thermally shielded by the opposing second electrode, which also actively heats the tissue in its proximity (21); there is, furthermore, since heat is trapped between the two electrodes and higher temperatures are thus achieved, less cooling in the direction of the collateral electrode than is the case with monopolar ablation. With the probes in the same positions, this results in a lesion larger than the two lesions produced sequentially by monopolar ablation.

In our study, impedance suddenly rose rapidly during bipolar RFA to more than 700Ω, resulting in reduced power output (26675±3047 Watt·s) compared to the monopolar mode (28778±1300 Watt·s), a fact which can be explained by the balance between the current in the proximity of the electrode, and tissue temperature. While current does contribute to the increased size of a thermal ablation zone up to 100℃ (without causing tissue boiling), a current which is too high might induce such boiling due to rapidly increased tissue impedance. This phenomenon could be supported by the rapid rise in tissue temperature occurring earlier (approximately four minutes after the instillation of RF energy), compared with the monopolar mode (Fig. 3). In order to minimize impedance rises during bipolar RFA, current density between the electrodes should therefore be decreased to within the range within which current causes increases in tissue temperatures, without boiling. We speculate that changing the electrical conductance of the tissue by infusing NaCl solution could be a good solution, and there is a much room for further optimization of the concentration of hypertonic saline and the infusion rate.

During monopolar RFA using an open-perfused electrode, however, the impedance did not change rapidly, suggesting that HS-enhanced monopolar RFA using a perfusion electrode could effectively prevent desiccation or charring around the electrodes. The increased electrical conductance induced by the infusion of hypertonic saline through an open perfusion electrode could prevent desiccation, but to enlarge the zone of ablation using a monopolar Berchtold® system, it appears necessary to increase total current deposit around the electrode by increasing the duration of RF application or maximum power output, as with other commercially available RF systems. In this present study, the greater efficiency of the bipolar RF mode using the Berchtold® system was demonstrated, but to determine whether this is so where other monopolar RF systems with larger maximum power output (including Radionics™, Radiotherapeutics™ and RITA® systems) are used at their maximum outputs, further comparative studies are needed.

Since McGahan et al. (13) reported that bipolar RF ablation appeared to offer increased tissue coagulation in fresh bovine liver, compared with the monopolar technique, reports have described the greater efficiency of bipolar RF ablation in this aspect (14, 20). In our study, bipolar RF application for 10 minutes created ablation zones with a mean width of 49.2±6.0 mm and a minimum diameter of 31.2±7.6 mm, dimensions which are comparable with those of a previous in-vitro experimental study involving bovine liver, in which lesion width was approximately 1-1.5 cm greater than needle length, and width was approximately 1.5-2 cm greater than inter-needle distance (13). In an in-vivo study by Curley et al. (14), however, the diameter of bipolar RF-induced ablation zones ranged from 2.0 to 2.5 cm, which was much less than in our study. We speculate that this difference could be attributed to perfusion-mediated cooling in in-vivo liver tissue.

Jang et al. (22) recently reported that simultaneous RFA using two-needle electrodes could produce greater coagulation necrosis than two successive applications of monopolar ablation using single-needle electrodes. However, the theoretical disadvantage of the simultaneous monopolar mode is that there is a large area of relatively constant electric potential between the two electrodes, resulting in reduced power deposition in this area due to the small field gradient between the electrodes, and less desirable deep waist formation midway between the electrodes (20). Bipolar RFA, on the other hand, has the potential to create more concentrated current flow between the two electrodes, and more extensive well-defined RF-induced coagulation necrosis is therefore a possibility. Since focal liver lesions are usually round or oval shaped, this ability of bipolar RFA to produce a well-defined oval-shaped lesion could be valuable for clinical RF application in cases involving liver tumors, and could also be beneficial when the target tumor lies close to structures which are vulnerable to heat, such as the bile duct, or bowel.

Although hypertonic saline-enhanced bipolar RFA more efficiently created coagulation necrosis than did monopolar RFA, it has certain drawbacks. One of these is that only two electrodes can be used at the same time, and another is that all the current originating from one electrode must also enter the second electrode (20). When using the bipolar mode there is no way to independently control the amount of heat generated in the vicinity of each electrode. When the degree of cooling at the site of each electrode is different due to differences in perfusion, one electrode thus reaches a higher temperature than the other and this can lead to boiling and a rapid rise in impedance.

Our experimental study suffers certain limitations. First, it was performed in-vitro, and the extent to which the results can be transposed to human liver is limited. In living tissue, there is a cooling "sink" containing, for example, blood vessels, and within the treated area, rapid heat exchange can thus arise (13). Furthermore, all ablations involved normal liver parenchyma, not tumor tissue. Despite the foregoing, however, our model provides a reliable basis for a comparative study of the efficiency of different RF modes. Second, following the procedure employed in an earlier study, we tested only 5% NaCl solution infused at a rate of 1 mL/min. Given that sudden rapid impedance rises occur during bipolar RFA, we believe that further experimental studies to determine the optimun concentration and amount of hypertonic saline solution are warranted. Third, and last, tissue temperature was measured at only one site.

In conclusion, using an open-perfused. electrode to create thermal coagulation, bipolar RFA performed better than monopolar ablation and during a single application may kill more tumor cells more quickly.