The development of unex- plained syncope in patients with moderate or severe complexity adult congenital heart disease may be a harbinger of risk for SCD; electrophysi- ological study with consideration for an ICD as primary prevention can be beneficial. 10. ICDs implanted in patients with adult con- genital heart disease, who are in their 40s and 50s, for primary prevention indications now account for >40% to 67% of implanted devices in patients with adult congenital heart In these patients, appropriate shocks are delivered in 14% to 22% of patients in the first 3 to 5 years of fol- In patients with con- genital heart disease and severely depressed ventricular function, or single ventricle anat- omy, defibrillation threshold testing may pose excessively high risk. In patients without vas- cular access or prior Fontan repairs, the risk of reoperation with sternotomy for epicardial ICD implantation may outweigh the potential ben- efits, and consideration for transplant evaluation may be preferable. Subcutaneous implantable CLINICAL STATEMENTS AND GUIDELINES cardioverter-defibrillator implantation may be an appropriate option for some patients.S10.8-42,S10.8-53 11. Adult patients with complex adult congenital heart disease typically have hypertrophy and ventricular dysfunction of varying degrees, increasing their risk for worsening ventricular function with antiarrhythmic medications. In the only large study of antiarrhythmic medi- cations for congenital heart disease, the use of flecainide was associated with proarrhyth- mia in 5.8% of patients and SCA in 3.9% of The use of amiodarone is gener- ally reserved for refractory symptomatic VA or asymptomatic VA that can aggravate ventricu- lar dysfunction, due to the high risk of adverse effects including thyroid dysfunction, particularly among females and patients with univentricular 11. DEFIBRILLATORS OTHER THAN TRANSVENOUS ICDS 11.1. Subcutaneous Implantable Cardioverter-Defibrillator Recommendations for Subcutaneous Implantable Cardioverter- Defibrillator References that support the recommendations are summarized in Online Data Supplement 55. 