Atrial electrograms and activation sequences in the transition between atrial fibrillation and atrial flutter.

Introduction: The electrophysiologic mechanism of atrial fibrillation (AF) has a wide spectrum, and it seems that some atrial regions are essential for the occurrence of a particular type of AF. We focused on one type of AF: AF associated with typical atrial flutter (AFL), which was right atrial (RA) arrhythmia, and sought to investigate intra-atrial electrograms and activation sequences in the transition between AF and AFL. Methods and Results: Intra-atrial electrograms and activation sequences in the RA free wall and the septum were evaluated in the transition between AF and AFL in seven patients without organic heart disease (all men; mean age 57 +/- 11 years). In five episodes of the conversion of AFL into AF, the AFL cycle length was shortened (from 211 +/- 6 msec in stable AFL to 190 +/- 15 msec before the conversion, P, 0.001). Interruption of the AFL wavefront and an abrupt activation sequential change induced by a premature atrial impulse resulted in fractionation and disorganization of the septal electrograms. During sustained AF, septal electrograms were persistently fractionated with disorganized activation sequences. However, the RA free-wall electrograms were organized, and the activation sequence was predominantly craniocaudal rather than caudocranial throughout AF. In 12 episodes of the conversion of AF into AFL, the AF cycle length measured in the RA free wall increased (from 165 +/- 26 msec at the onset of AF to 180 +/- 24 msec before the conversion, P, 0.001). AFL resumed when fractionated septal electrograms were separated and organized to the caudocranial direction, despite the RA free-wall electrograms remaining discrete and sharp with an isoelectric line. Conclusion: Changes of the electrogram and activation sequence in the atrial septum played an important role in the transition between AF and AFL.