T wave alternans (TWA) is a periodic beat-to-beat variation in the amplitude or shape of the T wave in an electrocardiogram (ECG or EKG). TWA was first described in 1908. At that time, only large variations ("macroscopic" TWA) could be detected. Those large TWAs were associated with increased susceptibility to lethal ventricular tachycardias. Most modern references to TWA refer to microvolt T wave alternans (MTWA), a non-invasive heart test that can identify patients who are at increased risk of sudden cardiac death . It is most often used in patients who have had myocardial infarctions (heart attacks) or other heart damage to see if they are at high risk of developing a potentially lethal cardiac arrhythmia...