Figure 2: Relationship between peak ablation index (Wgs) and mean impedance drop (Ohms) per lesion according to CTI anatomical site (V CTI R2=0.95, p<0.0001, Mid CTI R2=0.15, p=0.21, IVC CTI R2=0.88, p<0.0001)
Whilst AI and impedance drop maintained a strong linear association at the V and IVC margins, at the Mid CTI linearity was less apparent with a qualitative plateau in impedance drop noted beyond AI of 500Wgs (Figure 2 - R2=0.15, p=0.21). Longer lesion duration was also required Mid CTI to achieve the target AI (table 2). In addition, whilst mean peak AI was highest Mid CTI, mean impedance drop was lower, and rate of impedance drop was slower Mid CTI (table 2). Mean contact force and power were similar at all sites, however mean temperature was lowest Mid CTI (table 2).
Given that these findings suggested that comparable energy applications (according to AI) produced inconsistent degrees of tissue injury (by ID) at different CTI sites despite similar mean contact force and power, further analysis was performed to examine the catheter-tissue interface.
Temporal fluctuation in contact force was determined by examining the change in recorded catheter force (g) every 50ms. A mean value for each lesion (total fluctuation divided by lesion duration) was calculated and expressed in mg/s. Variation was greatest Mid CTI (table 3) and was amplified in lesions with higher peak AI values. However, use of a long sheath significantly reduced temporal fluctuations in contact force throughout the CTI (table 3, figure 3).