Sudden cardiac arrest (SCA) is a major public health concern, accounting for up to 400,000 deaths each year. While SCA cases are heterogeneous in etiology, the most common underlying pathologic substrate is ischemic heart disease and the most common electrophysiologic mechanism is ventricular fibrillation (VF). Survival following VF-SCA is less than 20% in most communities with mortality due largely to fulminant injury of either the heart or brain. Accordingly, there is a compelling case to identify novel modifiable SCA risk factors, to elucidate its underlying pathophysiology, and to identify novel preventative and therapeutic targets. The mitochondrion has primary function in energetics, oxidative stress, and apoptosis, and serves as both a target and an effector of ischemia-reperfusion injury. Consequently, mitochondria have a fundamental role in arrhythmia risk and in cardiac and brain resuscitation. Indeed, we have previously demonstrated that mitochondrial DNA (mtDNA) copy number (-CN), which reflects the number of MT genomes per cell, is a novel risk factor for SCA, independent of traditional SCA risk factors. MtDNA-CN, however, captures the quantity of mitochondria. It does not capture their quality. We have recently identified a series of mtDNA haplotypes that capture functional genetic variation, and developed a computational tool to accurately assess somatic mutations in mitochondria (heteroplasmy), allowing for a comprehensive survey of mtDNA genetic variation. Thus, we propose to test the hypothesis that mtDNA variation (mtDNA-CN, inherited mtDNA haplotypes and rare mutations, mtDNA heteroplasmy [somatic variants]) will be associated with SCA risk and resuscitation outcomes. We will examine the association of SCA with mtDNA characteristics in 2,600 SCA cases from a large population- based case-control study of SCA and validate our findings in two population-based cohort studies where SCA cases have been identified prospectively and two case-control stud