Circus movement atrial flutter in the canine sterile pericarditis model. Activation patterns during initiation, termination, and sustained reentry in vivo.

The mechanisms of single-loop reentry in a syncytium without anatomically predetermined pathways have not been shown. Using a "jacket electrode" with 111 bipolar electrodes in a nylon matrix, we mapped in situ the atrial epicardial surface during atrial flutter in dogs with sterile pericarditis. Of 21 episodes of reentrant atrial flutter, only four showed double-loop ("figure-eight") reentry, whereas in 17 episodes a single loop was present. During initiation of single-loop reentry, an arc of functional block extended to the atrioventricular (AV) ring. This forced activation to proceed as a single wave around the free end of the arc, before breaking through the arc close to the AV ring. Activation continued as one loop around an arc close to the AV ring (in eight episodes) or around a combined functional and anatomic obstacle (in nine episodes) when the arc joined an atrial vessel. A zone of slow conduction was consistently bordered by the arc of block and the AV ring or by the anatomic obstacle and the AV ring. Spontaneous termination occurred when conduction failed in this area and the arc rejoined the AV ring. High-density recordings (2 mm) along the arc of block showed double potentials separated by an isoelectric interval, interpreted as local activation and electrotonus due to activation on the opposite side of the arc. Histologically, a diffuse inflammatory reaction involved 50-80% of the atrial wall. A transitional layer of myocardial bundles with preserved cross striation, but separated by edema and inflammatory cells, was enclosed between an epicardial layer of fragmented myocytes and an endocardial layer of grossly intact myocardium. There were no distinctive features at sites of functional conduction block or slowed conduction. In conclusion, single-loop reentry is the common pattern during atrial flutter in this model. Its induction depends on an interaction of the AV ring, a functional arc of block, and a zone of slow conduction. The location of the inferior vena cava predisposes the lower right atrium to this type of reentry.