Identifying specific intermediate phenotypes ("endophenotypes") associated with defined clinical courses in AF represents a potential method to systematically subtype patients by underlying mechanism and represents a much-needed clinical advance. Clinical endophenotypes that have been studied include atrial fibrillatory rate, prolonged signal-averaged P-wave duration, and biomarker profiles. The endophenotype we will study here is right precordial ST segment elevation, seen not only in Brugada syndrome (BrS) (where it is unmasked by sodium channel blocking drugs) but also commonly in lone AF and in patients with AF-associated rare variants in genes encoding the cardiac sodium channel α- or β-subunits. Taken together these data suggest the hypothesis to be tested in this study, that variants in multiple genes can culminate in a similar AF-prone substrate by reducing sodium current that can be identified by screening for baseline or manifest right precordial ST segment elevation endophenotype after sodium channel block with intravenous procainamide.