The risk of SCD is increased among patients with adult congenital heart disease compared with the general population, with the median age at death ranging from 30 to 49 years of The risk of SCD is highest among patients with moderate or severe complexity congenital heart disease, and accounts for approximately 25% of cardiac causes Patients with septal defects and a positive fam- ily history of septal defects, cardiomyopathy, or bundle-branch block/conduction defects may have the gene mutation NKX2.5, which portends an increased risk of early SCD; genetic testing and early consideration of ICD implantation if positive Patients with repaired complex forms of congenital heart disease have undergone multiple intracardiac surgeries in the first few decades of life with resultant hypertro- phy and risk for subendocardial ischemia as well as scar formation contributing to VT/VF. Risk fac- tors for SCD include increasing complexity of heart disease, VA, SVT, progressive increase in QRS duration, systemic ventricular dysfunction, and subpulmonary ventricular dysfunction.S10.8-1,S10.8-5, Extrapolation of data regarding specific measures of ventricular function warranting implantation of primary prevention ICDs from adult patients with NICM is unrealistic. 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. 