The overall hypothesis of this study is that subtle interactions between structural (substrate) and functional (trigger) abnormalities of the heart, some of which are genetically-determined, can be used to identify patients at high risk of sudden cardiac death (SCD). Such information may be used to better define patients most likely to benefit from replacement of an internal defibrillator (ICD). The prospective, observational study to enroll, categorize and follow patients who receive an ICD pulse generator replacement for primary prevention of SCD (PROSe-ICD) was established to :