Patients considered at high risk of ventricular arrhythmias and thus sudden cardiac death (SCD) may be treated with drugs to suppress the emergence of arrhythmias, or may undergo implantation of an automatic implantable cardiac defibrillator (AICD), which promptly detects and terminates tachyarrhythmias when they occur. Since SCD, whether from arrhythmias or pump failure, is one of the most common causes of death after a previous MI, there is intense interest in risk stratification to target therapy.
SAECG and MTWA have been investigated as techniques of risk stratification for arrhythmic events in patients with a variety of cardiac conditions, including cardiomyopathy and prior history of MI. MTWA measures beat-to-beat variability, while SAECG measures beat-averaged conduction. Other risk factors include left ventricular ejection fraction, arrhythmias detected on Holter monitor or electrophysiologic studies, heart rate variability, and baroreceptor sensitivity.
Patient groups are divided into those who have not experienced a life-threatening arrhythmia (i.e., primary prevention) and those who have (i.e., secondary prevention). Those who have already experienced an arrhythmia are already at high risk and probably do not require testing.
SAECG is a technique involving computerized analysis of small segments of a standard EKG to detect abnormalities, termed "ventricular late potentials" (VLP), that would be otherwise obscured by “background” skeletal muscle activity. VLP reflect aberrant, asynchronous electrical impulses arising from viable isolated cardiac muscle bordering an infarcted area and are thought to be responsible for ventricular tachyarrhythmias.
MTWA refers to a beat-to-beat variability in the amplitude of the T-wave. A routine electrocardiogram (EKG) cannot detect these small fluctuations, and thus this test requires specialized sensors to detect the fluctuations and computer algorithms to evaluate the results. MTWA is a provocative test that requires gradual elevation of the heart rate to above 110 beats per minute. The test can be performed in conjunction with an exercise tolerance stress test. Test results are reported as the number of standard deviations by which the peak signal of the T-wave exceeds the background noise. This number is referred to as the "alternans ratio." An alternans ratio of three or greater is typically considered a positive result, an absent alternans ratio is considered a negative result, and anything in between is considered indeterminate. MTWA has also been investigated as a diagnostic test for patients with syncope of unknown origin and as a noninvasive test to identify candidates for further invasive electrophysiology testing of the heart.