Discussion
Although IVC is known as a possible source of non-PV foci in AF patients, the prevalence is very low. Nie et al. recently reported from a single center that the incidence of AF triggered from the IVC was 0.91% (6/661).1 However, a total of only 7 isolated reports (8 cases) had been reported as of July 2022.2-8 In line with the rarity of previously published cases, there were only 2 cases (0.04%) among 4,500 AF ablation procedures in our hospital.4 Thus, the prevalence of AF triggers in the IVC is considered to be much lower in the real world than the recent report.1
Hashizume et al. described that the IVC histologically has the same arrhythmogenicity as the thoracic veins (PVs, superior vena cava [SVC], coronary sinus) with cardiac musculature extending from the RA in humans. However, the myocardial sleeve in the IVC was much shorter than that in the SVC (18 mm vs. 45 mm).9 In addition, the IVC is considered to be electrically more silent than the SVC because of the frequent absence of longitudinal myocardial fibers.10 These different electrophysiological features may relate to different developmental processes between the two venous systems; the SVC is embryologically derived from the anterior cardinal vein, whereas the terminal segment of the IVC is derived from the right vitelline vein.11
In the clinical setting, the precise localization of the APC origin in the IVC is often difficult due to the proximity of the coronary sinus ostium and lower part of the RA. In previous reports, although nine cases anatomically demonstrated the origin of the trigger from the IVC using a contact 3D-EAM system, the definition of the border between the IVC and RA was unclear.1,6-8 In the present case, the concealed IVC firings (bigeminy) observed in the IVC strongly suggested an origin inside the IVC rather than at the RA-IVC junction. This is the first case demonstrating the limited excitation area with a low voltage in the IVC by detailed 3D-EAM.