In contrast, the population of children, adolescents, and young adults has an overall annual risk of 1 per 100000, and there is somewhat a higher risk of SCD at the younger end of that age range.S2.2.2-41 An age- associated transition range, from the mid-20s to 35 to 40 years of age, is characterized by a steep increase in risk from that of the adolescent group to the middle- aged group, corresponding to the emergence of isch- emic heart disease. Although ischemic heart disease remains the most common underlying substrate associated with SCD, the incidence of ischemic heart disease-related SCD appears to be decreasing,S2.2.2-52 with various forms of cardiomyopathy associated with myocardial fibro- sis and LV hypertrophy increasing.S2.2.2-53 In addition, a trend over time has suggested that out-of-hospital cardiac arrest patients who are admitted alive to a hospital are becoming more likely to have high-risk clinical profiles, as opposed to manifest disease.S2.2.2-54 The younger population children, adolescents, and young adults is affected by a series of disorders that manifest earlier in life, including the genetic structural disorders and cardiac channelopathies, myocarditis, congenital heart disease, and other rare During the transition range, from the mid-20s to the mid-30s, causes of SCA and SCD in- clude a lower proportion of inherited diseases and in- creasing proportion of ischemic heart disease (>40% Despite the small progress that has been made in risk prediction of SCA and SCD, the greatest challenge is to identify the relatively small, high-risk subgroups con- cealed within the large general population who have no identified disease but are at risk of SCA as their first cardiac event (Figure 1).S2.2.2-50 3. MECHANISMS OF VA 3.1. 