Implantable cardioverter defibrillators (ICDs) have been developed to treat ventricular tachycardia or fibrillation (abnormal heart rhythms) by electrical shock or by pacing the heart. ICD therapy is established as highly effective for stopping life-threatening arrhythmias, but it does not preclude the use of anti-arrhythmic drugs for prevention and to decrease the frequency of ICD shocks.