Implanted cardioverter defibrillators (ICDs) are 99 percent effective in stopping life-threatening arrhythmias and are the most successful therapy to treat ventricular fibrillation, a major cause of sudden cardiac death. An ICD is implanted under the skin near the heart and constantly monitors a patient’s heart rhythm. If it detects a very fast, abnormal heart rhythm, it delivers energy to the heart muscle causing it to beat in a normal rhythm.