The purpose of the study is to determine the usefulness of Microvolt T Wave Alternans (MTWA) testing of patients that have had a heart attack and have decreased pumping ability of the lower chamber of their heart. MTWA testing is a non-invasive test used to detect the likelihood of developing abnormally fast rhythms in the lower chambers of the heart. The objective of the study is to prove that if a patient has a positive MTWA test, they will be more likely to develop abnormally fast and dangerous rhythms in the lower chambers of the heart. The results of the study will help doctors to determine which patients would benefit the most from having an Implantable Cardioverter Defibrillator (ICD) implanted.

Inclusion Criteria:

• Patients with a history of heart attacks
• LVEF >0.30 and <= 0.40 as measured by a radionuclide study, angiography or quantitative echocardiography within 3 months prior to enrollment.

Exclusion Criteria:

• Patients with electrocardiographically documented sustained ventricular tachycardia, ventricular fibrillation or prior cardiac arrest (Class I indications)
• Patients who have undergone an electrophysiologic study with ventricular stimulation or a MTWA test within 1 year prior to enrollment
• Patients with medical conditions that preclude the testing required by the Investigational Plan or limit study participation
• Patients unable to complete MTWA testing requirements due to current atrial fibrillation or flutter
• Patients unable to exercise and for which alternative modalities (pacing or pharmacological stress) used for MTWA testing are not available
• Patients who have experienced an MI within 1 month prior to enrollment
• Patients with advanced cerebrovascular disease per physician's assessment
• Patients classified as NYHA Class IV at time of enrollment
• Patients who have undergone cardiac revascularization within 3 months prior to enrollment or are scheduled for cardiac revascularization at the time of enrollment
• Patients with a life expectancy of less than 1 year