Class III drugs have a risk of ventricular proarrhythmia, particularly torsades de pointes VT, and are not used in patients with torsades de pointes VT Class IV Antiarrhythmic Drugs Class IV drugs are Nondihydropyridine calcium channel blockers These drugs depress calcium-dependent action potentials in slow-channel tissues and thus decrease the rate of automaticity, slow conduction velocity, and prolong refractoriness.