Indication for lesion creation in HP strategy
Several attempts have been conducted to increase ATA-free survival rate, including various guidance during PVI (CF, FTI, AI, or impedance drop), or confirmation of acute PVR using isoproterenol infusion or ATP after PVI, which was considered responsible for durable lesion creation. Most importantly, however, all of acute PVRs were eliminated by touch-up procedure in the same session, and there had been enough waiting time (20 min) after the final ipsilateral PVI, which rendered the lesion creation robust. In these respects, we could not entirely expect additional effect of HPSD-PVI on the late outcome as previously described;[1, 2, 16] however, further examination of acute PVR is required due to its substantial association with longer RF time and larger RF energy delivery, which in turn leads to overheating of tissue. Thus, optimal measurements for the durable lesion creation is needed for HPSD-PVI.
Although AI was a reliable indicator for lesion creation in PVI at normal power and duration, whether it was also suitable in HPSD-PVI remains uncertain. Some researchers reported the efficacy of HPSD-PVI guided by AI value without any in-procedure major complications.[1, 11] In recent literature, the same finding was noted in lesion size index (LSI)-guided HPSD-PVI [17]. However, neither AI nor LSI could directly exacerbate tissue damage; suggesting these were device-related parameters. Furthermore, since CF depends on the contact sensor at the distal tip of the ablation catheter, sensor error occurrence is relatively common. Indeed, while with specificity of 90% AI of 405 and 334 au for the segments other than SAE and SAE, respectively, were associated with durable lesion creation in the present study, such values were remarkably low compared with that applied in previous studies. [1, 11]Similarly, discrepancy between AI and impedance drop in the gap site were also observed, which could be explained by lower reliability of AI value. Figure 3 showed the comparison of VisTags of AI value and impedance drop, the latter of which clearly demonstrated gap site whereas the former could not. Interestingly, cases of low AI value but sufficient impedance drop after the RF application had been observed and resulted in absence of gaps. Figure 4 showed the relevant parameters in a durable site where sufficient impedance drop and moderate AI value was found. These examples suggest AI value to be a less reliable measurement than impedance drop. Since existing AI or LSI-guided HP strategy could deliver excessive RF energy to tissues, impedance drop might be a more sophisticated indicator of lesion creation, especially in the HP strategy.
In the present study of HPSD-PVI guided by USM, impedance drop solely and significantly predicted acute PVR after adjusting for AI and ILD. USM could directly enhance cellular necrosis without any collateral damages and impedance drop appeared to behave similarly both experimentally and clinically.[7], [18, 19]Figure 1 showed an example of a durable site in the procedure, demonstrating that R-wave pattern in unipolar signal was immediately (5 s) achieved after the RFA and reached a plateau; however, generator impedance gradually decreased, finally resulting in an absolute change of 13 ohm 9 s after the RFA. This example could demonstrate the importance of continuous application after the achievement of USM to create durable lesion.