Disclosed herein is the use of a potassium channel blocker selected from the group consisting of bretylium, sotalol, ibutilide, dofetilide, azimilide, bretylium clofilium, E-4031, nifekalant, tedisamil, sematilide, fampridine and tertiapin, and one or more late sodium channel blockers of Formula I in the manufacture of a medicament for treating or preventing atrial fibrillation and/or atrial flutter or for prolonging atrial effective refractory period (ERP). Also disclosed are pharmaceutical compositions comprising the combinations.