Disopyramide (DP) [4-diisopropylamino-2-phenyl-2-(2-pyridyl)butyramide] is a widely used antiarrhythmic drug with class Ia activity, according to the Vaughan and Williams classification.1,2 It has pharmacological effects on the heart that are qualitatively similar to those of quinidine and procainamide.3 Because of a relative narrow therapeutic range (2 -5 µg ml -1 ) in serum or plasma, the monitoring of DP in blood is essential.