Introduction: Anatomical studies demonstrate significant variation in cavotricuspid isthmus (CTI) architecture. We hypothesised that ablation index (AI) may further our understanding of energy delivery across the CTI.
Methods: 38 patients underwent CTI ablation at two cardiothoracic hospitals. Operators delivered 682 lesions in total with a target AI of 600 Wgs. Ablation parameters were recorded every 10-20 ms. Post hoc, VisiTags were trisected according to CTI position: inferior vena cava (IVC), middle (Mid), or ventricular (V) lesions.
Results: There were no complications. 97.4% of patients (n=37) remained in sinus rhythm at 6.6 ± 3.3 months’ follow-up. For the whole CTI, peak AI correlated with mean impedance drop (ID) (R2=0.89, p<0.0001). However, analysis by anatomical site demonstrated a non-linear relationship Mid CTI (R2=0.15, p=0.21). Accordingly, whilst mean AI was highest Mid CTI (IVC: 473.1 ± 122.1 Wgs, Mid: 539.6 ± 103.5 Wgs, V: 486.2 ± 111.8 Wgs, ANOVA p<0.0001), mean ID was lower (IVC: 10.7 ± 7.5 Ω, Mid: 9.0 ± 6.5 Ω, V: 10.9 ± 7.3 Ω, p=0.011), and rate of ID was slower (IVC: 0.37 ± 0.05 Ω/s, Mid: 0.18 ± 0.08 Ω/s, V: 0.29 ± 0.06 Ω/s, p<0.0001). Mean contact force was similar at all sites, however temporal fluctuations in contact force (IVC: 19.3 ± 12.0 mg/s, Mid: 188.8 ± 92.1 mg/s, V: 102.8 ± 32.3 mg/s, p<0.0001) and catheter angle (IVC: 0.42°/s, Mid: 3.4°/s, V: 0.28°/s, p<0.0001) were greatest Mid CTI. Use of a long sheath attenuated these fluctuations and improved ablation efficacy.
Conclusions: Ablation characteristics vary across the CTI. At the Mid CTI, operators should appreciate that higher AI values do not necessarily deliver more effective ablation; this may be explained by localised fluctuations in catheter angle and contact force.