Conduction Velocity in the Tricuspid Valve‐Inferior Vena Cava Isthmus is Slower in Patients With Type I Atrial Flutter Compared to Those Without a History of Atrial Flutter

INTRODUCTION: In human type I atrial flutter, the electrophysiologic substrate is unclear. In order to determine if slow conduction is mechanistically important, we evaluated conduction velocity in the tricuspid valve-inferior vena cava (TV-IVC) isthmus, right atrial free wall, and interatrial septum in patients with and without a history of atrial flutter undergoing electrophysiologic study. METHODS AND RESULTS: Nine patients with (group 1) and nine without a history of type 1 atrial flutter (group 2) were studied. Conduction time (msec) in the right atrial free wall, TV-IVC isthmus (bidirectional), and interatrial septum was measured during pacing in sinus rhythm at cycle lengths of 600, 500, 400, and 300 msec from the low lateral right atrium and coronary sinus ostium. Conduction velocity (cm/sec) was calculated by dividing the distance between pacing electrodes and sensing electrodes (cm) by the conduction time (sec). Conduction velocity was slower in the TV-IVC isthmus in group 1 (range 37 +/- 8 to 42 +/- 8 cm/sec) versus group 2 (range 50 +/- 8 to 55 +/- 9 msec) at all pacing cycle lengths (P < 0.05). However, conduction velocity was not different in the right atrial free wall or interatrial septum between groups 1 and 2. Conduction velocity was also slower in the TV-IVC isthmus than in the right atrial free wall and interatrial septum in group 1 patients, at all pacing cycle lengths (P < 0.05). Atrial flutter cycle length correlated with total atrial conduction time (r > or = 0.832, P < 0.05). CONCLUSION: Slow conduction in the TV-IVC isthmus may be mechanistically important for the development of human type I atrial flutter.