At the end of the ablation procedure in both groups, AF or atrial flutter induction is attempted by isoproterenol infusion and high left atrial pacing. Induction of atrial fibrillation is registered but does not necessitate additional ablation. Induction of right, isthmus-dependent atrial flutter is dealt with cavotricuspid isthmus ablation. Left atrial flutters will be studied by means of entrainment from the mitral isthmus and the adjacent posterior wall. Since a considerable percentage of these arrhythmias disappear with time,20 they will be ablated only if sustained (> 3 minutes) and associated with 1:1 ventricular response.