Discussion
Our study demonstrated the efficacy of new ablation catheters equipped with MEs compared to conventional ablation catheters in RFA of PSVTs. The ME catheter was advantageous to the conventional ablation catheter in terms of the number of RF attempts and RFA time to achieve effective ablation endpoints. The advantage of the ME catheter was shown for both SP and AP ablation, although the difference of the primary study outcomes did not reach statistical significance when analyzed separately. Notably, the ME catheter was more than twice as effective in visualization of the pathway electrograms as the conventional ablation catheter (27/68 vs. 13/68). However, the overall procedure results were successful in all patients without a significant difference in total procedure time or acute SVT reinduction rates.
High-density mapping catheters enable the precise identification of local electrical signals with minimization of far-field signals and background noise that confers higher resolution mapping in low-voltage zones and scar areas.6, 7 Currently, high-resolution electroanatomical mapping is widely used in catheter ablation of ventricular tachycardias and atrial arrhythmias.8-11The IntellaTip MIFI OI catheter is an ablation catheter that shows only three additional bipolar ME signals. This catheter may not be suitable to map the entire arrhythmia circuit in detail, but it can help to accurately localize catheter tip and to assess ablated and viable tissue. Previous clinical studies showed that the ablation catheter equipped with ME was effective in the visualization of local gaps and avoiding unnecessary ablation during RFA of CTI and ATs.4, 5, 12 However, Iwasawa et al. reported that the ME catheter showed worse efficacy compared to the conventional ablation catheters for CTI ablation in a prospective clinical study. The ME catheter group had higher RF application numbers and longer ablation time compared to the 8 mm dumbbell-shaped irrigated-tip catheter or cryothermal catheter group.13 In that study, there was a significant difference in the average RF power between the ME group and conventional ablation catheter group (31.3 ± 9.1W vs. 38.6 ± 7.6W in the ME and conventional ablation catheter group, respectively). For CTI ablation, effective transmural lesion formation would be more important than precise electroanatomical mapping. Although the ME signal showed viable ablation targets and signal attenuation after ablation, the temperature-controlled ablation via the small electrode may have resulted in less RF power and shallow ablated lesions compared to the irrigated tip ablation catheter. In contrast, RFA of PSVT requires relatively less power delivery, and detailed mapping with accurate localization of the ablation target is a more important factor for a successful procedure.1, 2 Our study first showed that use of a high-resolution ablation catheter can be helpful in RFA for PSVTs. The ME catheter showed improved ability to identify pathway potentials, as well as to localize actual ablation site. SP or AP potentials are often small and unclear in large electrodes, because it can be overlapped with far-field atrial and ventricular electrograms. In our study, ME catheters were highly effective in the recognition of pathway electrograms (overall 39.7%), compared to the conventional bipolar ablation catheter (overall 19.1%). The higher effectiveness of MEs in differentiating pathway potentials is most likely to have driven the difference in the primary study outcomes.
It is important to point out that the ME groups showed higher average RF power and lower average temperature in total ablation parameters, despite identical RF generator setting in the two groups. Because the IntellaTip MiFi OI catheter was an irrigated-tip catheter, continuous saline infusion was required to maintain patency. Although the flow rate was minimalized (2 mL/min) to balance with the Blazer II HTD catheter which has no irrigation function, it may have affected the temperature sensed in the ablation tip that would result in altered temperature-controlled RF power delivery. Irrigated-tip catheter ablation has been shown to be more effective in AP ablation via increased power delivery, but previous studies mostly reported that increased efficacy was observed when more than 17 mL/min of saline irrigation was applied.14, 15 With the minimal irrigation in our study, initial RF efficacy does not seem to have been largely influenced by this difference. Even if the difference in RF power contributed to the initial ablation time, it would have little effect on ablation attempt number to achieve ablation endpoint, which mostly depends on the accuracy of ablation location rather than the speed of RF energy rising. Traditionally, irrigation mode is not preferred in AVNRT ablation due to the risk of atrioventricular nodal injury but irrigated-tip ablation is occasionally used in complex AP ablation. The use of ME catheters with moderate saline irrigation could more improve RF efficacy in AVRT/WPW syndrome.
Limitation
The operators could not be blinded to the ablation catheters used, and awareness of the study group may have biased the study results. Although the primary outcomes were significantly different between the two groups in the entire population, the study did not have enough power to validate statistical significance in the AVNRT population or the AVRT/WPW syndrome population alone. However, there were similar trends favoring the ME catheters in both the AVNRT and AVRT/WPW syndrome groups. As discussed above, although we minimalized the irrigation flow rate in the ME group, it could have affected the ablation efficacy, especially for the initial ablation time to achieve the ablation endpoints.