Multiple observational and multicenter studies have demon strated acute success rates between 65% and 100% for treatment of SVT associated with ACHD.452,454,455,457,489,493,494 Acute success rates vary by tachycardia mechanism and type of congenital heart disease and repair. Success rates are highest for SVT associated with AVNRT (>80%),490,494 accessory pathways (75% to 89% among patients with Ebstein or focal AT (80% to 100%).492,494,495 Success rates for catheter ablation of AT or atrial flutter are significantly lower than that reported in patients without ACHD, with overall 65% to 82% acute success in mixed anatomic substrates,452,454,455,457,493,494 but success rates have improved with advanced mapping and ablation techniques.494 Acute success rates in ablation of AT or atrial flutter varies significantly by type of congenital heart disease, ranging from 93% to 100% in patients with repaired ASD,330,449,458 85% to 100% in atrial baffle repairs of transposition of the great arteries,490,492 and 54% to 75% of univentricular heart or Fontan repairs.457,490,495 Older age and presence of univentricular heart physiology was associated with decreased acute success rates in a series of 193 ablations of AT.457 Recurrent AT has been reported in 20% to 85% of patients during short-term follow-up, with development of AF reported in 7% to 30% of patients.330,449,454,493 Recurrent SVT may represent the same or a new reentrant circuit, and arrhythmia burden may be decreased by ablation procedures. Ablation procedures in patients with complex congenital heart disease are best performed in centers with advanced mapping techniques and expertise in congenital heart disease.25,29,496 IIa B-NR 4. Surgical ablation of AT or atrial flutter can be effective in ACHD patients undergoing planned surgical repair.497 508 See Online Data Supplement 20. 