Microvolt T-Wave Alternans™ (MTWA)

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 1980's 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, they developed a methodology, known as the Spectral Method, which allowed measurement of alternans at the level of one microvolt.

This method was refined to create our proprietary Analytic Spectral Method® which uses Micro-V Alternans Sensors™ that are specially designed, high-resolution, noise canceling sensors to detect minute heartbeat variations, measured at as little as one-millionth of a volt. This permits the measurement of alternans even in the presence of the noise typically encountered during exercise stress testing. These sensors allow the recording of up to four ECG signals from the same anatomical site as well as electrode-skin impedance and respiratory signal. An adaptive noise reduction algorithm combines these signals to create an enhanced ECG signal to expose previously indiscernible T-Wave alternans.

The Test

The Microvolt T-Wave Alternans test is a provocative, non-invasive, easy to perform, diagnostic test. By taking detailed ECG measurements during rest, exercise, and then rest again, the HearTwave® II System can identify the presence of Microvolt T-Wave Alternans. The appearance of Microvolt T-Wave Alternans indicates the presence of a type of cellular metabolic activity that frequently leads to ventricular tachyarrhythmias.

Following the placement of fourteen sensors - 7 Micro-V Alternans Sensors and 7 standard electrodes - in the Frank-lead configuration, the electrodes are connected to the digital ECG amplifier that leads back to the Microvolt T-Wave Alternans enabled system. At the beginning of the test, the patient is directed to begin walking on a treadmill to raise the heart rate. Patients who are unable to exercise can have their heart rates elevated pharmacologically to achieve the higher rate needed for a successful test.

The sub-maximal exercise portion of the test will be approximately 8-10 minutes. Information will be collected for the Microvolt T-Wave Alternans Test while at rest, during exercise and at rest again. Upon test completion, the system analyzes the data and provides the Alternans Report Classification, which is an automatic interpretation of the test results. The test is classified as positive, negative or indeterminate.

The Outcome

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.

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.

Patients who test MTWA positive or indeterminate for heart rate or dense ectopy (abnormal) should be referred to an electrophysiologist for further evaluation.

Patients who have an indeterminate test should be retested immediately. Studies indicate that over 50% of patients who initially test as indeterminate, become determinate if retested during the same session.