Additional Study Information: The purpose of this study is to test whether a device called an implantable cardiac defibrillator (ICD)can increase the likelihood of survival in patients at risk of heart rhythm irregularities as determinedby the analysis of a 24 hour monitor that is performed within 2 to 15 months after a heart attack. AnICD is a device that is implanted under the skin near the collarbone, which contains a battery,computer, and one or two leads (wires) that are placed in the heart. The ICD can detect a rapiddangerous beat and terminate it by either rapid pacing or administering an electric shock to the heart.If the heart beat is too slow, the ICD system can also pace the heart. The Medtronic ICD and leads used in this study are FDA approved and routinely used in patients who have had a large heart attack and have very weak hearts, but are considered investigational for this study, in which patients don't have a very weak heart but have an abnormal heart monitor result.