• cardiac electrophysiology;
  • cavotricuspid isthmus;
  • atrial flutter;
  • entrainment mapping;
  • catheter ablation

Background: We describe a new cavotricuspid isthmus (CTI) circuit.

Methods: This study includes 8 patients referred for atrial flutter (AFL) ablation whose tachycardia circuit was confined to the septal CTI and the os of the coronary sinus (CSOS) region. Entrainment mapping was performed within the CTI, CSOS, and other right atrial annular sites (tricuspid annulus (TA)). Electroanatomic mapping was available in 2 patients.

Results: Sustained AFL occurred in all patients with mean tachycardia cycle length (TCL) of 318 ± 54 (276 − 420) ms. During tachycardia, fractionated or double potentials were recorded at either the septal CTI and/or the region of CSOS in all, and concealed entrainment with post-pacing interval (PPI) − TCL ≤ 25 ms occurred in this area; but manifest entrainment with PPI > TCL was demonstrated from the anteroinferior CTI and other annular sites in 7/8 patients. In one, tachycardia continued with conduction block at the anteroinferior CTI during ablation. Up to three different right atrial activation patterns (identical TCL) were observed. The tachycardia showed a counterclockwise (CCW) pattern in 6, a clockwise pattern in 2, and simultaneous activation of both low lateral right atrium and septum in 5. Electroanatomic mapping was available in 2, showing an early area arising from the septal CTI in 1, and a CCW activation sequence along the TA in another. Radiofrequency application to the septal CTI terminated tachycardia in 4, and tachycardia no longer inducible in all.

Conclusions: We describe a tachycardia circuit confined to the septal CTI/CSOS region, and hypothesize that this circuit involves slow conduction within the CTI and around the CSOS, which acts as a central obstacle.