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). 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. Those who are found to be at high risk would therefore benefit from the placement of a defibrillator device which can stop an arrhythmia and save the patient's life.
The TWA test uses an electrocardiogram (ECG) measurement of the heart's electrical conduction. The test looks for the presence of repolarization alternans (T-wave alternans), which is variation in the vector and amplitude of the T wave component of the EKG.