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.