Atrial fibrillation (AF) has long been recognized as a random phenomenon. Recent studies, however, suggest that activation sequence during atrial fibrillation is not entirely disorganized and organized activations as the transitional rhythm exists in the process of conversion from atrial fibrillation to atrial flutter. The purpose of this study is to characterize the process and significance of transitional rhythm during conversion of atrial fibrillation to atrial flutter.

In 9 patients with paroxysmal atrial fibrillation, 13 episodes that atrial fibrillation spontaneously converted to atrial flutter during electrophysiological study were analyzed. To map the right atrium (RA), 10 bipolar Halo catheter was positioned in the lateral free wall of the RA and multielectrode catheters were positioned in the coronary sinus, high RA, and His bundle area, respectively. Calculation of cycle length (CL) was performed with digital caliper during atrial fibrillation and atrial flutter. Direction of activation sequences was compared between transitional rhythm and following atrial flutter.

During conversion of atrial fibrillation to atrial flutter, characteristic findings were observed as follows. 1) There was a gradual increase in atrial fibrillation cycle length before transitional rhythm. 2) A pause appeared in atrial fibrillation immediately prior to transitional rhythm, and between transitional rhythm and following atrial flutter. 3) Direction of activation sequences of the transitional rhythm to atrial flutter was concordant in 9 episodes and discordant in 4 episodes. 4) Spontaneous termination of atrial flutter (n=3) was also preceded by organized rhythm showing different sequence of activations from those of atrial flutter.

A stereotypical pattern of activation sequence exists in the process of conversion of atrial fibrillation to atrial flutter. The occurrence of transitional rhythm influences the activation sequence of atrial flutter. Spontaneous termination of atrial flutter also preceded by transiently changing pattern of activation.