Background: Calcium channel-dependent tissue has been suggested to be involved in the circuit of verapamil-sensitive atrial tachycardia originating from the atrioventricular (AV) node vicinity (V-AT), but little information exists.
Methods and results: To examine the tachycardia circuit of V-AT, a single extrastimulus was delivered during tachycardia to 10 sites of the intraatrial septum: the earliest atrial activation site; His bundle (HB) site; 3 arbitrarily divided sites on the AV junction extending from the HB site to the coronary sinus ostium (CSOS) (sites S, M, and I); the internal-CSOS, inferior-CSOS, superior-CSOS, posterior-CSOS, and posteroinferior-CSOS in 10 patients with V-AT. The longest coupling interval that reset V-AT and subsequent return cycle were measured. The longest coupling interval at earliest atrial activation site was significantly longer than the longest coupling interval at the HB site, site S, M, and I, internal-CSOS, inferior-CSOS, superior-CSOS, posterior-CSOS, and posteroinferior-CSOS, respectively (P<0.001 for HB site and P<0.0001 for the remaining 8 sites). The return cycle at earliest atrial activation site did not differ from the tachycardia cycle length, whereas those at the remaining 9 sites were significantly longer than tachycardia cycle length (P<0.001). Furthermore, a single extrastimulus delivered from sites inferior to the HB site advanced His potential without resetting V-AT in 7 patients in whom AV block was not observed during tachycardia.
Conclusions: Atrial tissue within the Koch's triangle extending from the HB site to posteroinferior-CSOS is not involved in the tachycardia circuit. Verapamil-sensitive atrial tissue close to the AV node but not the AV nodal conducting system forms the tachycardia circuit of V-AT.