Inclusion Criteria:  Informed written consent. Recent-onset atrial fibrillation (AF) (≤48 hours of onset) and lasting ≥30min, documented by a 12-lead ECG Administration of intravenous flecainide Administration of an oral b-blocker  Exclusion Criteria:  Age <18 years Severe structural or ischemic heart disease (previous cardiac surgery, any cardiomyopathy, myocardial infarction at any time) Atrial flutter, sick sinus syndrome, high degree atrioventricular block, bifascicular block Abnormal electrolyte levels (especially hypo- or hyperkalemia) Known sensitivity to flecainide