Biatrial activation in isthmus-dependent atrial flutter

Background-The aim of this study was to determine the biatrial activation p attern in isthmus-dependent atrial flutter (AFL) to understand the function al interatrial connections and the activation pattern of the left atrium (L A). Methods and Results-Biatrial activation was performed, using an electroanat omic mapping system, in 10 patients undergoing right atrial isthmus ablatio n for counterclockwise (n=7) or clockwise (n=3) AFL. The AFL circuit was pe ritricuspid and propagated slowly (0.5 +/-0.2 m/s) through the isthmus. LA was activated by two wave fronts, with discrete breakthroughs in the superi or, mid, or inferior atrial septum. The activation of LA overlapped 50 +/- 16% of the AFL cycle length. In counterclockwise AFL, at least one breakthr ough was located in the inferior atrial septum. LA activation began immedia tely after the exit of the flutter wave from the isthmus and was directed i nferosuperiorly in all patients, being synchronous with the atrial septal a ctivation. The septal breakthroughs in patients with clockwise AFL were var iably located. The direction of LA activation was superoinferior in 2 and i nferosuperior in 1 patient. Conclusions-The circuit of isthmus-dependent AFL was entirely in the light atrium. LA activation was a bystander and followed trans-septal conduction across the inferior coronary sinus-LA connection, Bachmann's bundle, and/or fossa ovalis.