Class 1 antiarrhythmic drugs--characteristic electrocardiographic differences when assessed by atrial and ventricular pacing.

Class 1 antiarrhythmic drugs have been subdivided into 1a, 1b and 1c according to their effect on the action potential duration. The effects on the surface electrocardiogram of one drug from each subgroup were investigated in nine patients. Electrocardiographic recordings were taken during sinus rhythm and at identical atrial and ventricular paced rates. Disopyramide (1a) significantly prolonged the QT interval during sinus rhythm and at the identical paced rates, by increasing both the QRS duration and JT interval. Lignocaine (1b) significantly reduced the QT interval during sinus rhythm and at the identical paced rates, by reducing the JT interval. Lignocaine had no effect on the QRS duration. Flecainide (1c) significantly prolonged the QRS duration during sinus rhythm, but not the QTc. However the QT interval at the paced rates prolonged significantly, due entirely to an increase of the QRS duration. Flecainide had no effect on the JT interval. These characteristic electrocardiographic differences support the differentiation of class 1 drugs into three separate subgroups.