<qa>  What is the cause of cardiac arrest?Arrhythmias caused by congenital or acquired abnormalities of cardiac K or Na channels are increasingly recognized as a cause of syncope and sudden death. Cardiac arrest in the absence of overt structural heart disease was previously considered idiopathic ventricular fibrillation IVF. The list of causes of unexplained cardiac arrest UCA now encompasses K related abnormalities Long and Short QT, Andersens, Na related Long QT3, Brugada, Ca related Catecholaminergic Polymorphic Ventricular Tachycardia-CPVT, and latent cardiomyopathy. These underlying causes of cardiac arrest are overtly familial in 30-60 of cases. Clinical detection of the underlying phenotype is crucial to direct appropriate treatment, genetic testing and screening of family members. 