Mechanisms of atypical flutter wave morphology in patients with isthmus-dependent atrial flutter.

Sixty-three episodes of isthmus-dependent atrial flutter (AFL) in 55 patients were studied to characterize variations in flutter wave morphology and to investigate the mechanisms of the atypical flutter waves on surface ECG. The activation patterns of coronary sinus (CS) and their relationship with flutter wave morphology on the ECG were analyzed. In 46 episodes of counterclockwise AFL (CCW-AFL), there were four types of flutter waves on ECG. Typical and atypical flutter waves were found in 47.8% and 13.0% of the episodes, respectively. Atypical flutter waves had broad positive terminal portion or entirely positive wave in the inferior leads and in V(1), with a distal-to-proximal or fused activation pattern in the CS, and an average activation time of 21.3 +/- 11.4 ms. In 17 episodes of clockwise AFL (CW-AFL), typical and atypical flutter waves were identified in 41.2% and 41.2%, respectively. Atypical flutter waves had negative waves in the inferior ECG leads and in V1, a proximal-to-distal activation pattern in the CS, and an average activation time of 42.4 +/- 14.4 ms. We conclude that atypical flutter waves are common in the isthmus-dependent AFL. The clockwise or counterclockwise conduction in the right atrium, and the activation patterns or conduction sequences between the right and the left atrium, are associated with the variations in the flutter wave morphology on body surface ECG.