KEY POINTS Idiopathic ventricular fibrillation (IVF) is defined as unexplained sudden cardiac death due to ven-tricular fibrillation (VF) without any identifiable structural or electrical cause after extensive investigations (no phenotype). Recent data show that the use of high-density electrophysiologic mapping may ultimately offer sub-clinical diagnoses of cardiac disease in about 90% of individuals with IVF. Two major conditions underlie the occurrence of VF: the presence of either depolarization abnormalities due to micro-structural myocardial alteration or Purkinje abnormalities manifesting as triggering ectopy or reentry in the peripheral network. J-wave syndromes are defined as a distinct electrocardiographic phenotype (slurring/notch) affecting the junction between the QRS complex and the ST segment in inferolateral leads. Recent data provide evidence for heterogeneous substrates, related to either delayed depolarization due to microstructural alterations or early repolarization abnormalities. IVF and J-wave syndromes are the result of a wide spectrum of pathophysiologic processes. The individual phenotypic characterization is essential given its implications in therapy, genetic testing, and risk stratification
