Ventricular septal defect characteristics influence presence of septal anatomical isthmuses in patients with repaired tetralogy of Fallot

Aims: In repaired tetralogy of Fallot (rTOF), the septal anatomical isthmuses (AI), AI 3, between the ventricular septal defect (VSD) and pulmonary annulus, and AI 4, between the VSD and tricuspid annulus, are important ventricular tachycardia (VT) substrates when slow conducting. Our aim was to assess the influence of VSD characteristics, specifically the presence of muscular or fibrous tissue at its border, on the presence or absence of septal AIs, potentially related to VT. Methods and results: All consecutive rTOF patients who underwent electroanatomical mapping between January 2005 and March 2023 with an available surgical report providing VSD details (n = 91) were included. The majority of patients had an outlet perimembranous VSD (n = 76, 84%), 6 (7%) an outlet muscular VSD, and 7 (8%) a doubly committed juxta-arterial VSD. In patients with an outlet perimembranous VSD, AI 3 was present in almost all (97%), whereas no AI 4 was observed. In patients with an outlet muscular VSD, AI 3 and AI 4 were present in 83% and 33%, respectively. In all patients with a doubly committed VSD, where the outlet septum is hypoplastic/fibrous, AI 3 was absent. Among patients with a doubly committed VSD with a muscular postero-inferior rim, 50% had AI 4, whereas none of those with a fibrous postero-inferior rim had AI 4. Conclusion: Ventricular septal defect characteristics aid in determining the presence of septal AIs potentially related to VT. In patients with doubly committed VSDs, septal VT substrates are unlikely. Detailed knowledge of anatomical VSD characteristics is desirable for understanding VT in rTOF.