Background: The inherent risk of thromboembolism after cardioversion of atrial fibrillation with a duration of more than 48 hours is well established. However, the potential increased risk of these complications after cardioversion of recent-onset episodes of atrial fibrillation has been more controversial. Thus, the aim of this dissertation was to evaluate the safety and efficacy of cardioversion of acute (< 48 hours) atrial fibrillation. 

Methods: The FinCV study is a multicenter (n=3) retrospective study of 3143 patients who underwent 7660 cardioversions for acute atrial fibrillation. Of those procedures, 5362 were performed without, and 2298 with, anticoagulation protection. 

Results: The success rate of electrical cardioversions was 94.2%. After successful procedures, atrial fibrillation recurred in 17.3% of cases within 30 days. The rate of thromboembolic events (mainly ischemic strokes) was 0.7% in non-anticoagulated patients after successful cardioversion of acute atrial fibrillation. Significant independent predictors of these complications were old age, female sex, heart failure and diabetes, along with a cardioversion delay of 12 hours or longer. The risk of thromboembolism was as high as 9.8% in patients with both heart failure and diabetes. The incidence of thromboembolic complications also increased significantly from 0.4% in nonanticoagulated patients with CHA2DS2VASc score of ≤ 1 to 2.3% in those with a score of ≥ 5. Overall, the incidence of thromboembolism was significantly lower after cardioversions performed during anticoagulation (0.1% vs. 0.7%). Altogether, 0.9% of electrical cardioversions resulted in bradyarrhythmia, and 44.4% of those patients underwent pacemaker implantation later. 

Conclusions: The cardioversion of acute atrial fibrillation does not increase the risk of thromboembolism in anticoagulated patients. However, this risk is unacceptably high in non-anticoagulated patients with conventional risk factors for stroke. High CHA2DS2VASc score and a delay to cardioversion of 12 hours or longer are significant predictors of thromboembolism. Overall, electrical cardioversion is an effective procedure and immediate arrhythmic complications are rare after these procedures.