Data analysis
The tissue impedance data recorded during RF delivery were processed
offline using the R statistical software (version 4.0.3) to remove noise
(see supplementary materials section).
Ablation lesions were excluded if any of the following occurred: short
duration of less than 5 sec (as no LSI value is generated before 5 sec
of RF delivery); sudden and progressive impedance rise, suggestive of
catheter tip or tissue overheating with charring
formation9.
After processing of the raw data, the maximum absolute impedance drop
(Max-Imp-∆), the maximum percentage impedance drop (Max-Imp-%), the
mean CF, and the contact force variability (CFV) were calculated for
each study ablation lesion. The maximum absolute impedance drop
(Max-Imp-∆) was calculated as the difference between maximum impedance
value per lesion, which corresponded to the impedance at the beginning
of the ablation, and minimum impedance reached during ablation. The
maximum percentage impedance drop (Max-Imp-%) was calculated by
converting the Max-Imp-∆ into a percentage of the starting impedance to
minimise any influence from the initial impedance value. The contact
force variability (CFV) was calculated as difference between mean of the
peaks and mean of the troughs of CF during ablation, as previously
described 10 (Figure 1).
The ablation lesions were grouped by RF power used (20, 30 and 40W), by
mean CF used (< 5, 5-10, 10-15 or >15 g) and by
CFV (≤ 5g or > 5 g) and the Max-Imp-% values in each group
were compared.
The variation of impedance drop over ablation time was investigated by
dividing each ablation delivery into consecutive, cumulative 0.5 sec
time intervals and calculating the mean and the standard deviation (SD)
of all lesions Max-Imp-% at each interval compared to the initial
impedance at the start of ablation. The relative plateaus of Max-Imp-%
and the corresponding times at plateau for each group of study ablations
were identified by finding the point after which the SD of the
Max-Imp-% at each time interval became negligible (less than 0.1) and
the difference between Max-Imp-% at that point and Max-Imp-% at the
end of ablation was very low (less than 0.25).
The relation between impedance drop and FTI was assessed as described in
the supplementary section.
The relationship between impedance drop and LSI was assessed by dividing
the ablation lesions into consecutive, cumulative 0.1 LSI intervals and
calculating the Max-Imp-% of each interval compared to the initial
impedance at the start of ablation. The mean and SD of Max-Imp-% were
calculated for each interval. The relative plateaus of impedance drop
and the corresponding LSI values at plateau were identified by finding
the point after which the SD of the Max-Imp-% at each LSI interval
became negligible (less than 0.1) and the difference between Max-Imp-%
at that point and Max-Imp-% at the end of ablation was very low (less
than 0.25). To confirm the accuracy of the technique used to identify
the plateau values of the FTI/LSI over time curves, both per-patient
analyses and combined analyses of all-patients lesions were performed
and their results were compared.