The risk of stroke associated with AF is well established, and several RCTs have demonstrated the efficacy of anticoagulation for stroke prevention in patients with additional risk factors.366 Anticoagulation has also been shown to prevent stroke when administered during the weeks immediately before and after cardioversion.31,367 Whether atrial flutter carries a risk of stroke similar to that in patients with AF has been debated in the past but is now supported by limited evidence from mechanistic, observational, and prospective studies that include patients primarily with atrial flutter.365 369 Several reports have suggested that the risk of stroke in patients with atrial flutter is mitigated by anticoagulation. Meta-analysis of 13 studies of patients undergoing cardioversion of atrial flutter reported short-term stroke risks ranging from 0% to 7%, and the thromboembolism rate in patients with sustained flutter in 4 studies averaged 3% annually.365 Other studies have reported similar risk and effectiveness of anticoagulation.369 ACHD patients with nonfibrillatory atrial tachyarrhythmias appear to be at high risk, as well.367,459 Therefore, on the basis of available data, recommendations for antithrombotic therapy for ACHD patients who also have AT or atrial flutter are similar to those for patients with AF.10 I C-LD Assessment of associated hemodynamic abnormalities for potential repair of structural defects is recommended in ACHD patients as part of therapy for SVT.25,29 See Online Data Supplement 20. The development of AT, atrial flutter, or AF in ACHD patients is often associated with progressive hemodynamic deterioration of the underlying disease. Surgical treatment of the hemodynamic problems does not eliminate atrial arrhythmias, and ablation of atrial arrhythmias alone could allow significant hemodynamic issues to progress and potentially deteriorate. Successful treatment involves assessing both the arrhythmia and the contributing hemodynamic changes and addressing both when indicated and feasible. 