 T-wave alternans is an electrocardiographic finding that has been shown to predict the occurrence of future cardiac arrhythmias in patients who have had a heart attack. What is unknown about T-wave alternans testing is when is the best time to perform the test. In most studies, T-wave alternans testing is conducted 4 weeks or more after a heart attack. It is unknown if T-wave alternans testing performed prior to hospital discharge in heart attack patients is reliable. The objective of this project is to determine the diagnostic accuracy of T-wave alternans testing performed prior to hospital discharge and again at 30 days after hospital discharge in patients who have suffered a heart attack. 