The anatomical features of right atrial cavo-tricuspid isthmus can impact radiofrequency catheter ablation in term of success rate and complications

Background: Radiofrequency (RF) catheter ablation targeting the isthmus between the tricuspid annulus and the inferior vena cava (IVC) is the established treatment for typical atrial flutter (AFL) due to its high efficacy. Despite this high success rate, ablation of the cavo-tricuspid isthmus (CTI) can be extremely difficult, due to its highly variable anatomy. The aim of this study is to analyze how and why the great inter-individual variability can influence ablation procedure. Primary endpoint was to evaluate the impact of the underlying CTI anatomy on acute and long-term success rate. Secondary endpoint was to determine any ablation related complication and procedure time. Methods: Over a period of 54 months, 337 consecutive patients underwent CTI ablation. One 5F decapolar catheter was placed within the coronary sinus (CS) and another 7F multipolar catheter was placed anterior to the crista terminalis at the right free wall close to the tricuspid annulus. CTI anatomy was classified into: (A) simple (flat) CTI, (B) complex (pouch-like recess or concave shape) CTI. Vital parameters, such as arterial blood pressure and oxygen saturation were monitored throughout the entire procedure. All procedures were performed under conscious sedation using boluses of midazolam and fentanyl. Termination ablation was decided when bidirectional block was demonstrated. Ablation was performed using 8 mm-tip catheter with a power limit of 70 W and a target temperature of 60°C. The CTI length was measured as the shortest linear distance between the lower hinge point of the tricuspid annulus and the IVC in the frontal fluoroscopic projection. Outpatient follow-up included 12-lead ECG, and Holter ECG monitoring, scheduled at months 3, 6, and 12. We also examined 104 formalin-fixed hearts from patients who underwent routine clinical autopsy performed at the Cardiovascular Pathology Unit, University of Padua. Every heart was examined by two independent expert cardiovascular pathologists. The following features were assessed: 1. the length of the central isthmus; 2. the presence of pouch-like recesses; 3. the number of the recesses; 4. the position of the recesses. The histological analysis was performed. Results: Ablation: Mean age of all patients was 62.8 ± 10.6 years (range 28 – 84), male was 236 (70%). Deep recesses were found in 37 patients (10.9%) and were not related to aging, gender, left atrial size, and left ventricle ejection fraction (p = NS). Total mean CTI length was 23.3 ± 3.9 mm (range 10 – 35 mm). CTI length was shorter in type A anatomy as compared to type B (22.9 ± 3.2 vs 25.6 ± 6; p = 0.01). The primary endpoint of efficacy, proved by the bidirectional block, was achieved in 99.4% (335/337). Acute ablation failure (n = 2 patients) was associated with a CTI type B anatomy. Regard to the secondary endpoints, complication occurred in 1 patient (0.29%) with the CTI type B anatomy: a “pop” immediately followed by cardiac tamponade required urgent pericardiocentesis. Total mean procedure time of was 51.6 ± 14.4 minutes, no difference between two groups (p= NS), but CTI bidirectional block was obtained with a significantly longer RF application time in CTI type B anatomy as compared to the type A (8.3 min vs 10.7 min, p = 0.025); that demonstrates ablation is more difficult in case of complex anatomy. During a mean follow-up period of 36.7 ± 17.2 months, 99.7% (334/335) of the patients were free of AFL recurrences. AFL recurred in 1 symptomatic patient with CTI type B anatomy. The patient underwent second ablation procedure. Conduction across the ablation line was demonstrated. Successful ablation was performed. No new recurrences have occurred until this time. Anatomy: The mean age of the individuals was 67 ± 17 years, with a male prevalence (65%). The mean heart weight was 457 ± 102 g. The length of the central isthmus was 24 ± 4.1 mm (range 15 - 38 mm) without differences between groups (24.1 vs 23.6, p = NS), respectively. The central isthmus in the patients affected by atrial flutter was significantly longer compared to the other cardiac specimens (p < 0.01). In 10 of our cases (9.6%), a sub-Eustachian recess was present. As in the clinical series of patients undergoing ablation, the presence of recesses was not related to aging and gender. The recess was single in the majority of cases (60%) and the most frequent location was the central isthmus (60%). In the 4 cases diagnoses with atrial flutter, no recess was found in the CTI. The highest number of recesses found in one specimen was 3. The regions with the thinnest muscular wall resulted to be the anterior and middle paraseptal and middle central. Conclusions: Our investigation with both electrophysiological and anatomical study provides relevant information to clinical practice. Catheter ablation of typical AFL involving the CTI has been confirmed to be a safe, effective, and well-established ablation procedure. Nevertheless, the CTI anatomy impacts ablation parameters, such as RF energy application. Moreover, difficult procedure is also associated with longer length of isthmus. Recesses can complicate ablation, and knowing where they might be present is useful to avoid them. Therefore, success rate and ablation complications can be optimized by a deep knowledge of anatomy and centre experience.