Pharmacological conversion in patients with complex anatomy, ventricular dysfunction, or prolonged QTc intervals may result in significant proarrhythmia or acute hemodynamic deterioration. Early assessment for cardiac thrombus with transesophageal echocardiogram followed by synchronized cardioversion may be preferable to prolonged or multiple attempts to achieve pharmacological cardioversion. Because of frequent coexistent sinus node dysfunction and ventricular dysfunction, the need for chronotropic or inotropic intervention should be anticipated, with appropriate personnel and support present. Modification of electrode pad placement may be necessary, by using an anterior-posterior configuration in patients with marked atrial enlargement or positioning strategically according to the individual arrhythmia and anatomic substrate, including consideration of the possibility of dextrocardia. IIb B-NR Oral dofetilide or sotalol may be reasonable for acute treatment in ACHD patients and AT and/or atrial flutter who are See Online Data Supplement 20. Small observational studies support the use of oral dofetilide474 to acutely convert AT to sinus rhythm in 70% to 85% of episodes of acute AT or atrial flutter. Proarrhythmic events are more commonly reported with dofetilide. One small study of 20 patients reported that 10% experienced torsades de pointes after dofetilide474 ; both proarrhythmic events occurred in patients with single-ventricle physiology. The risk of proarrhythmia or worsening ventricular function requires arrhythmia monitoring with use. Recommendations for Acute Treatment of SVT in ACHD Patients (Continued) COR LOE Recommendations IIa B-NR Preoperative catheter ablation or intraoperative surgical ablation of accessory pathways or AT is reasonable in patients with SVT who are undergoing surgical repair of Ebstein anomaly.479 485 See Online Data Supplement 20. The prevalence of SVT among patients with Ebstein anomaly was 33% in 1 large series, the highest noted among ACHD and increased with age. AT, atrial flutter, or AF develops in 50% of patients with Ebstein anomaly and significant tricuspid regurgitation. 