Ibutilide Fumarate is the fumarate salt form of ibutilide, a class III antiarrhythmic agent. Ibutilide exerts its effect by activating a slow, inward, predominately sodium current rather than by blocking outward potassium currents. This results in prolongation of atrial and ventricular action potential duration and refractory periods. Ibutilide slightly decreases the sinus rate and atrioventricular (AV) conduction and produces a dose-related prolongation of the QT interval.