To our knowledge, this was the first study to compare the lesions generated by ablation using the new open-irrigated ablation catheter, TactiFlex, with the lesions obtained using the traditional catheter, TactiCath. This ex vivo study revealed the following points. (1) The surface area, maximum depth, maximum length, and volume increased significantly with an increasing contact force for TactiCath, and the difference due to the contact force was more pronounced than that for TactiFlex. (2) For the same ablation power and duration, the lesions tended to be significantly deeper in the TactiCath group than TactiFlex group. (3) The surface area with TactiCath was significantly smaller when used perpendicular than when used oblique/parallel, but the difference was not apparent with TactiFlex. (4) Impedance drops were larger and pops were more frequent with TactiCath than TactiFlex.

We found that the ablation lesions obtained with TactiCath were deeper than those with TactiFlex. It has been known that for the same contact force, the longer the catheter tip, the greater the area of the catheter tip exposed to saline solution and the greater the cooling effect [4]. The lesions were smaller for the TactiFlex because of its longer tip length (TactiFlex tip, 4.0 mm; TactiCath tip, 3.5 mm). TactiFlex had a lower irrigation flow rate than TactiCath. As the flow rate increased, the resistance of the cathode tip decreased, and as the resistive energy decreased, the conduction energy increased, allowing the energy to reach deeper. In this study, TactiCath had a greater impedance drop than TactiFlex. Because the variables that determine resistance heating are the applied power and impedance [5], TactiCath had greater resistive heating and deeper lesions. Additionally, the differences in the catheter tip morphology may have contributed to the differences in the ablation lesions obtained using the two catheters. The tip of the TactiCath catheter is spherical, whereas the flex tip is cylindrical, which might also have affected the size of the lesion. For the spherical tip, if the CF was small, only a small area of the tip was in contact with the tissue. Another difference in the lesion size between the TactiFlex and TactiCath could be attributed to the different cooling effects on the outside of the TactiFlex catheter due to its mesh structure. Therefore, the surface area and volume of the lesions will be small, and their size will depend on the CF. In contrast, with the cylindrical tip, the entire base is in contact with the tissue, and CF variation has only a slight effect [6, 7]. In this study, TactiFlex yielded little changes in the surface diameter or volume as the CF changed, suggesting that the tip geometry contributed to this result.

Although there were no significant differences in this study, in general, the parallel placement of the catheter is considered to increase the volume because of the larger contact surface with the tissue [8]. The small lesions obtained in the oblique and parallel applications of the catheter may have been due to the fact that the perfusion holes are closer to the tissue compared with the perpendicular application, which distributes to the force on the tissue and cools the surface.

Compared with TactiCath, the TactiFlex catheter had a significantly reduced incidence of steam pops. The TactiCath design, which has a spherical tip, may have provided a larger surface area in contact with the tissue and increased the risk of steam pops. This risk of steam pops is likely even higher with a greater applied CF due to the further increase in the surface area in contact with tissue. Other studies [6, 9] compared various catheter tip designs and found that catheters with spherical tips yielded more steam pops. TactiFlex has 56 irrigation holes and TactiCath has six irrigation holes. The increased number of irrigation holes may also directly increase the cooling effect on the electrode cooling, resulting in fewer steam pops [10]. The trend toward a lower impedance also supports the fact that the ablation lesion depth of TactiFlex is shallower than that of TactiCath and that there are fewer steam pops.

The first limitation of this study was that it was conducted in an ex vivo model; therefore, caution should be exercised when translating the results directly to clinical practice. The statistical significance of the results varied. This may be due to the limited number of ablation experiments, the fact that the same myocardium was not ablated, and the influence of the differences in the myocardial properties. Furthermore, the characteristics of the lesions in this study may also have differed from those in the clinical setting. For example, the risk of steam pops in ex vivo models did not reflect that of real lesions because of the low tissue viability and the low susceptibility to tissue boiling [11]. A clinical trial of the TactiCath catheter reported a lower steam-pop rate (0.13%) compared with this study [12]. Although we do not believe that these findings are parallel to the actual clinical practice, they may be useful in the future when further verifications are made.

Although the findings of this study may not replicate those of the beating heart in the ex vivo study, perhaps it is expected that the comparative differences between the catheters would be relatively similar. A precise understanding of the dimensions of the lesions is an essential parameter in the ablation-treatment strategy. The present study suggested the possibility of improving the success rate of ablation therapy by identifying the differences in the characteristics of the two catheters. In this study, the TactiFlex catheter provided a safer RF application with no steam pops, in addition to having a thermocouple at the distal tip end to accurately monitor the tissue temperature. High-power short-duration ablation performed with TactiFlex, which had fewer steam pops, has been reported to be more effective than conventional low-power ablation with TactiCath, with a shorter procedure time [13, 14]. In turn, TactiCath produced more steam pops but may be efficient when the lesion depth is large and requires ablation on the left ventricular wall or deeper lesions. However, the data generated in this study cannot be directly applied to the clinical setting of the beating human heart, especially with respect to the lesion size. Nevertheless, these data should assist clinicians in performing this intervention, to understand the relationships among the parameters, and to plan a strategy of experimentation accordingly.