The concept that acquired comorbidities may permit the occurrence of fatal cardiac arrhythmia and SCD upon use of Nav1.5 current blocking drugs was discovered in the Cardiac Arrhythmia Suppression Trial, in which patients randomized to the Nav1.5 current blockers flecainide or encainide suffered excess SCD rates compared to placebo-treated patients [10]. This evidence concerns the gene SCN5A and cardiac rhythm disease.