T wave alternans

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. 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.

Microvolt T wave alternans is a variant of T wave alternans that detects T wave alternans signals as small as one-millionth of a volt. Microvolt T wave alternans is defined as an alternation in the morphology of the T wave in an every other beat or AB-AB pattern. It has long been associated with ventricular arrhythmias and sudden death. First recognized nearly a century ago, visually discernible alternans were linked to the rapid onset of ventricular tachyarrhythmias.

Research conducted in the early 1980s by Dr. Richard Cohen and his colleagues at MIT explored the idea that visually indiscernible alternans may be equally significant. These efforts established a link between visually imperceptible alternans at the microvolt level and susceptibility to arrhythmias and showed alternans to be a heart rate dependent phenomenon. In addition, Dr. Joseph Smith, working with Dr. Cohen developed a methodology known as the Spectral Method which allowed measurement of alternans at the level of one microvolt.

Microvolt T wave alternans testing acts as a risk stratifier between patients who need implantable cardiac defibrillators (ICDs) and those who do not. Patients who test negative for MTWA are less likely to require an ICD than those who test positive. In some cases, the test is equivalent to a more invasive electrophysiology (EP) study.

Multiple prospective clinical trials indicate that patients from broad groups of at risk populations who test MTWA negative will likely live ventricular event-free for 12 to 24 months after their initial MTWA test.

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