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:
- for any target LSI value, higher
RF powers produced progressively greater impedance drops and
prolonging ablation time when using lower powers did not compensate;
- increasing the mean CF to at least 5 g produced the largest increase
in impedance drop in all RF power groups;
- lower CF values and higher CFV translated in a lower impedance drop
especially when using lower powers;
- although LSI was found to be a better predictor of Max-Imp-% compared
to FTI, its correlation with Max-Imp-% was found to depend on
combination of RF power, mean CF and also CFV used.
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).