Альтернация зубца T: диагностика и оценка риска внезапной сердечной смерти.

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. The TWA test uses an electrocardiogram (ECG) measurement of the heart's electrical conduction using electrodes attached to one's torso. It takes approximately a half hour to perform on an outpatient basis. 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. The amount of variation is small, on the order of microvolts, so sensitive digital signal processing techniques are required to detect TWA. See also wikidoc article on TWA.

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. MTWA results are given as positive, negative, or indeterminate. Those with indeterminate results can be tested again. Otherwise, positive and indeterminate results are often lumped together when making clinical judgments about the likelihood of sudden cardiac death. In patients who have a negative (normal) MTWA test the risk of sudden cardiac death is very low. The Negative Predictive Value of MTWA testing has been shown to be 98% accurate for follow up periods of 12–24 months in various clinical studies. Negative patients should be retested every 12 months as cardiac function can change over time. and the ABCD study. An additional 3145 (28% of total) subjects were enrolled in Spectral Method studies that did not predict outcomes, including the SCD HeFT TWA substudy, the MASTER study, and the CARISMA study. Over 5000 patients have been enrolled in MMA studies, with >3500 in the FINCAVAS exercise based series. The remaining 1500 patients were studied during ambulatory ECG monitoring. All MMA based TWA studies have predicted outcomes. A head to head comparison of the Spectral and MMA methods revealed similar hazard ratios, kappa statistics, and areas under the receiver operator characteristic curve. The MTWA consensus guideline, authored by 11 experts in both methods, described both methods, their history, and their utility. A trial of MTWA guided ICD implantation, REFINE ICD (NCT00673842), is underway. MTWA testing has been recommended for ventricular arrhythmia risk assessment by the American College of Cardiology, American Heart Association and European Society of Cardiology and by CMS in National Coverage Analysis for Implantable Cardioverter Defibrillators (CAG 00157N).

This test is significant for insurance companies because it can potentially save thousands of dollars per patient by reducing the implantation of unnecessary implantable cardiac defibrillators (ICD's). Use of the spectral method for measuring Microvolt T wave Alternans has been approved for reimbursement by Medicare, as well as major insurers such as Aetna, Cigna, and Humana. The Spectral Method was FDA approved in 2001 and approved in 2006 for federal reimbursement by the Center for Medicare and Medicaid Services (CMS), (CAG 00293N). The MMA method received federal approval for local CMS contractor reimbursement in 2015 (CAG 00293R2). Both methods use CPT code 93025.

In 2004 & 2005, NASA's Glenn Research Center and Cleveland's MetroHealth Medical Center, Case Western Reserve University, teamed to investigate the value of MTWA testing for astronauts both pre and during space flight. Experiments were performed both on the ground and on NASA's KC 135 aircraft.