BACKGROUND AND OBJECTIVES: The mechanism by which atrial fibrillation (AF) electrically converts to sinus rhythm remains undefined. The purpose of this study was to assess in detail the electrograms recorded during cardioversion using direct current (DC) shock. SUBJECTS AND METHODS: In 23 patients with AF (chronic n=20, paroxysmal n=3, M:F=15:8, 50 - 70 years old), electrograms were recorded simultaneously from a 20-pole electrode catheters (Duo-deca, DAIG) in the right atrial free wall and the coronary sinus immediately after DC shock given transthoracically. The activation patterns following 45 trials consisting of 23 successful and 22 unsuccessful cardioversion were analyzed. RESULTS: Two distinct patterns following successful cardioversion were observed; either immediate resumption of normal sinus rhythm (n=5, 21%), or one or two activations immediately after shock preceded normal sinus rhythm (n=18, 79%). The energy levels of the two patterns were not significantly different (260 J, 250 J, respectively). Four patterns following unsuccessful cardioversion were noted; unchanged (n=10, 45%), converted to atrial flutter (n=4, 18%), production of three or four beats of more coordinated complexes and reverted to AF (n=5, 22%), and converted to sinus rhythm transiently and reinitiated AF by one or two atrial premature beats (n=3, 13%). The magnitude of the DC shock applied at these 4 different patterns was 196 J, 240 J, 264 J, and 340 J, respectively in which low energy levels made a simultaneous depolarization of the entire atria unlikely. CONCLUSION: Distinct activation patterns were identified following successful and unsuccessful cardioversion using DC shock for AF. These observations suggest that total depolarization of the entire atria is not a prerequisite for the conversion of AF into sinus rhythm.