The effects of verapamil and diltiazem on the electrocardiogram and on cardiac rhythms are similar to those of the beta-adrenergic blocking agents. These include: Slowing of sinus rate, particularly with exercise. Slowing of AV nodal conduction causing prolongation of the PR interval or slowing of the ventricular response when then rhythm is atrial fibrillation or flutter. Slowing, interruption and/or prevention of AV nodal reentrant tachycardias. Treatment and prevention of exercise induced tachycardias