Electrophysiological determinant for induction of isthmus dependent counterclockwise and clockwise atrial flutter in humans

Objective-To investigate the electrophysiological determinant underlying th e electrical induction of counterclockwise and clockwise isthmus dependent atrial butter. Patients and methods-The isthmus bordered by the inferior vena caval orific e-tricuspid annulus-coronary sinus ostium (IVCO-TA-CSO) has been assumed to be the site of both slow conduction and unidirectional block critical to t he initiation of atrial flutter. Trans-isthmus and the global atrial conduc tion were studied in 25 patients with isthmus dependent atrial flutter (gro up A) and in 21 patients without atrial flutter (group B), by pacing at the coronary sinus ostium and the low lateral right atrium (LLRA) and mapping with a 20 pole Halo catheter in the right atrium. Results-Mean (SD) fluoroscopic isthmus length between the coronary sinus os tium and LLRA sites was 28.1 (4.0) mm in. group A and 28.0 (3.09) mm in gro up B (p = 0.95), but the trans-isthmus conduction velocity of both directio ns at various pacing cycle lengths was nearly halved in group A compared wi th group B (mean 0.39-0.46 mis v 0.83-0.89 m/s, p < 0.0001). Pacing at coro nary sinus ostium directly induced counterclockwise atrial flutter in 14 pa tients and pacing at LLRA induced clockwise atrial flutter in 11 patients, following abrupt unidirectional transisthmus block. Transient atrial tachya rrhythmias preceded the onset of atrial flutter in 10 counterclockwise and six clockwise cases of atrial flutter. None of the group B patients had ind ucible atrial flutter even in the presence of transisthmus block. The intra - and interatrial conduction times, as well as the conduction velocities at the right atrial free wall and the septum, were similar and largely within the normal range in both groups. Conclusions-Critical slowing of the trans-IVCO-TA-CSO isthmus conduction, b ut not the unidirectional block or the global atrial performance, is the el ectrophysiological determinant of the induction of counterclockwise and clo ckwise isthmus dependent atrial flutter in man.