New Insights Regarding the Atrial Flutter Reentrant Circuit

Background-We studied atrial activation during induced atrial flutter in the canine sterile pericarditis model to test the hypothesis that the atrial flutter reentrant circuit includes a septal component. Methods and Results-We studied 10 episodes of induced, sustained (>5 minutes) atrial flutter in 9 dogs. In all episodes, the reentrant circuit included a septal component. In 6 episodes, there were 2 reentrant circuits, one in the right atrial free wall and the second involving the atrial septum, Bachmann's bundle, and the right atrial free wall; both circuits shared a pathway in the right atrial free wall (figure-of-eight). The direction (superior or inferior) of the septal wave front of the second circuit correlated with the direction (clockwise or counterclockwise, respectively) of the right atrial free-wall circuit. A line of functional block in the right atrial free wall was part of both reentrant circuits. In the other 4 atrial flutter episodes, only 1 reentrant circuit was present, with activation in an inferior-to-superior direction in the septum and a superior-to-inferior direction in the right atrial free wall in 2 episodes and in the opposite direction in the other 2 episodes. In all atrial flutter episodes, the flutter wave polarity in ECG lead II was determined by the direction of activation in the left atrium; polarity was positive when the direction was superior to inferior and negative when the direction was inferior to superior. Conclusions-In this model of atrial flutter, the reentrant circuit (1) always included a septal component, (2) did not always require a right atrial free-wall reentrant circuit, (3) demonstrated figure-of-eight reentry when a reentrant circuit was present in the right atrial free wall, and (4) was associated with a line of functional block in the right atrial free wall.