INTRODUCTION
Reliable creation of durable
transmural lesions is the key for successful catheter ablation
procedures. To this aim, acute
markers of lesion formation are needed for real-time estimation of
lesion width and depth.
When using radiofrequency energy (RF) for catheter ablation,
tissue impedance is traditionally
assessed as a real-time surrogate marker of lesion formation. Local
tissue heating during ablation produces increased ion mobility and
reduced myocardial resistivity that translate into a drop in tissue
impedance 1,2. The impedance reduction is
typically rapid in the first seconds of RF delivery, as result of faster
resistive heating of the superficial tissue layers from the catheter
tip; it then progressively slows down thereafter, due to slower
conductive heating of deeper tissue layers, until a plateau is reached.
The magnitude of impedance drop at the plateau point (maximum impedance
drop) strongly correlates with lesion size in in-vitro studies3. Whether maximum impedance drop also correlates with
lesion transmurality is largely unknown to date, but it certainly seems
a reliable indicator of RF lesion “completeness”, as no further lesion
enlargement is expected with more prolonged RF delivery once plateau has
been reached 4.
Ablation indexes such as Force Time Integral (FTI), Ablation Index (AI)
and Lesion Size Index (LSI) are now available in clinical settings as
markers of lesion formation 5,6 . When using Abbott
Medical contact force (CF)-sensing catheters, both FTI and LSI are
continuously displayed and can be monitored during ablation. Compared to
FTI, which includes CF and time, LSI combines in a non-linear formula
all three main determinants of RF lesion formation: power, CF and
ablation duration. In in-vitro studies, LSI is highly predictive of
lesion width and depth, exhibiting a stronger correlation with lesion
dimensions compared to FTI. Moreover, the correlation between LSI and
lesion size is linear and independent from the power used, meaning that
it is theoretically possible to achieve the same LSI ―and therefore
possibly the same lesion size― with lower powers by using a higher
contact force or by ablating for longer 6. However,
in-vitro data also suggest that ablation lesions reach maturity after
20-30 sec of energy delivery without any further increase thereafter
despite more prolonged ablation 7.
This study was conducted to
investigate the relationship between ablation parameters and impedance
drop in the context of an LSI-guided left atrial (LA) catheter ablation
strategy. We hypothesised that impedance drop, as a surrogate marker of
lesion size, could help to establish ideal ablation settings and optimal
target LSI indices for ablation.