Sustained VT and VF Not Associated With ACS Patients with structural heart disease are at an in- creased risk for sustained VT and VF. Sustained VT that is not associated with an ACS is often monomorphic as it is usually due to scar-related reentry, but it may degenerate to VF.S2.2.2-34 The risk and predictors of VT in patients with structural heart disease depend on the type, severity, and duration of structural heart disease, increasing with the severity of ventricular dysfunc- tion and the presence of symptomatic HF. Monomor- phic VT occurring in the absence of structural heart disease is commonly referred to as idiopathic VT and is often due to an automatic focus in a characteris- tic location, giving rise to typical electrocardiographic appearances. Polymorphic VT and VF occurring in the absence of structural heart disease are rare and may be due to a cardiac channelopathy,S2.2.2-35,S2.2.2-36 med- ication-induced long QT syndrome,S2.2.2-36 or they may 2.2. Sudden Cardiac Death 2.2.1. Incidence of SCD SCA and its most common consequence, SCD, constitute major public health problems, account- ing for approximately 50% of all cardiovascular with at least 25% being first symp- tomatic cardiac events.S2.2.2-1,S2.2.2-40,S2.2.2-41 In addition, analyses of the magnitude of SCD are limited, in part because of the broad range of estimates of the risk based on different epidemiological methods.S2.2.2-42 During the past 20 to 30 years, SCD accounted for in the United States, with a range of <170 000 to >450 000, depending on epidemiological methods, data sources, and inclusion criteria.S2.2.2-41,S2.2.2-43 The lowest of these extremes came from national extrapo- lation of data from specific local programs, while the highest rates included noncardiac causes of sudden death such as pulmonary embolism or intracranial bleeding. 