DISCUSSION
This study assessed the relationship between ablation settings and tissue impedance drop in the context of an LSI-guided catheter ablation strategy. The main findings of the study were:
As shown in Figures 2 and 3, in our study higher powers translated into larger impedance drops regardless of ablation duration and of LSI value achieved. These data suggest that the use of LSI targets does not fully compensate for higher powers, which might be due to the plateau of lesion growth over time and to the less relevant effect of CF at lower powers. If considering impedance drop as a surrogate marker of lesion size, they are in keeping with the growing body of evidence on high RF power. Increases in RF power produce larger lesions as result of a shift to more effective resistive heating, whereas an increase in ablation duration simply gives more time for convective heating, which reaches its limit after a 30-40 second period11. High-power short-duration ablation produces broad and slightly shallower lesions which in the LA are still transmural, translating into better clinical outcomes and reduced risk of collateral damage8,12,13.Although sudden impedance rise during RF delivery was observed more frequently with use of increasing power (11% of 20W power lesions versus 24% of 40W power lesions), no steam pops were recorded in our study ablation procedures, possibly due to prompt termination of RF delivery in the case of an impedance rise.
In keeping with previous data 14-16, we generally observed a progressive increase of Max-Imp-% with use of higher mean CF in the different RF power groups. In all groups the largest increase in percentage impedance drop was observed by increasing the mean CF from less than 5 g to more than 5 g, which may suggest the importance of a minimum mean CF of at least 5 g for effective ablation. The effect of increasing CF on Max-Imp-% was progressively larger with use of increasing power, as expected as both factors play a role in maintaining a constant catheter electrode-tissue interface temperature required for RF lesion formation 17, and in keeping with previous data18. As result, higher CF values were required to achieve the same impedance drop when using lower powers. As previously demonstrated by Ullah et al 10, CFV was found to play a role on Max-Imp-%: this is not surprising considering that CFV is an indicator of catheter stability. Of note, the effect of increasing CFV on Max-Imp-% was progressively smaller with use of higher powers. Taken all together, these data might suggest that CF and CFV are more important when using lower powers. Switching to higher power could be considered in case of catheter instability or in case of difficulty to increase CF, like during sedation cases or if performing left atrial ablation without steerable sheaths.
The relation between impedance drop and LSI was found to be similar to the relation observed between impedance drop and AI by Ullah et al19, apart from the lack of an initial lag phase of impedance drop which might just not have been visible due to unavailability of LSI data in the first few seconds of RF delivery. As expected given the crucial role of RF power on Max-Imp-%, LSI showed a stronger correlation than FTI with impedance drop. However, as shown in Figure 4, each LSI value was found to correspond to different impedance drop values depending on combination of RF power, mean CF ad also CFV. Progressively higher LSI values were found to be required to achieve plateau of impedance drop with higher powers, higher mean CF and lower CFV. Target LSI values corresponding to achievement of Max-Imp-% were identified for each combination of RF power and mean CF, but they were found to correspond to different Max-Imp-% values depending on CFV. In its current formula not including CFV, LSI could represent more an indicator of lesion completeness (corresponding to plateau of impedance drop) rather than of lesion size (correlating with the value of impedance drop at plateau).