Sudden Cardiac Arrest (SCA) is a leading cause of death in the United States, accounting for an estimated 300,000 deaths each year — more than lung cancer, breast cancer, and HIV/AIDS combined. SCA occurs abruptly and without warning; the heart’s electrical system malfunctions and blood cannot be pumped to the rest of the body. SCA is different from a heart attack, which occurs when a blockage in a blood vessel interrupts the flow of oxygen-rich blood to the heart, causing the heart muscle to be damaged.

Fig. 1 – Microvolt T-Wave Alternans (MTWA) testing: non-invasive risk assessment for Sudden Cardiac Arrest.

Out-of-hospital survival for SCA is <8%, making prediction and prevention critically important. Patients at highest risk for SCA are those with heart failure, a prior heart attack, reduced ejection fraction, prior SCA or a family history of SCA. Approximately 12 million people in the U.S. fit these clinical profiles and may be at risk. SCA can also strike apparently healthy individuals and is the leading cause of death in young athletes.

The risk of SCA can be reduced through treatment of underlying coronary artery disease, optimal pharmacologic therapy, and lifestyle changes like smoking cessation. Some patients may benefit from an implantable cardioverter defibrillator (ICD), a device that is surgically implanted to immediately administer electrical shocks to restart an arrhythmic heart.

While the ICD is an effective treatment, patients and physicians often question the need for such an intervention or have concerns about cost, surgical risks, and device reliability. As such, recent studies show that many at-risk patients, particularly women and minorities, are often not referred for evaluation or do not receive appropriate therapy.

Current risk stratification paradigms for SCA are designed to identify high-risk populations based on assessment of the heart’s pump function (ejection fraction). However, determining the risk of a particular individual remains a significant clinical challenge. In fact, most patients with implanted ICDs never require therapy from the device.

To allay such concerns, a non-invasive tool, called the Microvolt T-Wave Alternans™ (MTWA) test, has been developed by Dr. Richard J. Cohen, a professor at the Harvard- Massachusetts Institute of Technology (MIT) Division of Health Sciences and Technology, with developmental support and funding from NASA's Johnson Space Center and the National Space Biomedical Research Institute (NSBRI). In 1993, MIT licensed the technology to Cambridge Heart (Tewksbury, MA), a start-up company that Dr. Cohen helped to establish.

How it Works

Fig. 2 – American College of Cardiology (ACC), American Heart Association (AHA) and European Society of Cardiology (ESC) guidelines indicate use of MTWA testing to improve the diagnosis and risk stratification for those who are at risk of developing life-threatening ventricular arrhythmias, particularly those with heart failure, reduced ejection fraction, prior heart attack, unexplained syncope or family history.

MTWA testing provides an additional piece of information that may help accurately stratify a patient’s risk, thereby contributing to the objective clinical decision-making process. This tool may be used in conjunction with other clinical factors to help physicians more accurately assess a patient’s risk of SCA. Employing focused risk stratification protocols for SCA is cost-effective for the health care system and makes therapies like the ICD more clinically effective by targeting those at highest risk who are most likely to benefit.

The MTWA test is a non-invasive method of assessing a patient’s risk for sudden cardiac arrest (SCA). Based on technology originally developed at MIT, the MTWA test is administered in a physicians office, hospital, or out-patient clinic. Patients typically walk on a treadmill for several minutes with a set of 14 electrodes on the torso: seven standard (the same used in a conventional stress test) and seven proprietary high-resolution electrodes. Unlike stress testing, the heart rate is increased gradually and patients are not required to exercise until exhaustion. MTWA testing can also be conducted using pharmacologic agents or pacing to elevate the heart rate.

During the test, Cambridge Heart’s proprietary algorithm, the Analytic Spectral Method®, is used to measure Microvolt T Wave Alternans, a subtle beat-to-beat fluctuation in the T-wave segment of the electrocardiogram not visible to the human eye. In multiple clinical trials, MTWA has been shown to be a risk marker for SCA.

The first major study of MTWA was published in the New England Journal of Medicine in 1994 and concluded that MTWA was an independent marker of arrhythmic vulnerability equivalent to invasive electrophysiology testing. Since that time, several larger-scale studies have confirmed the predictive value of MTWA in various patient groups including those with left ventricular dysfunction, ischemic cardiomyopathy, non-ischemic cardiomyopathy and history of myocardial infarction. A 2009 meta-analysis of 13 studies (~6,000 pts) showed that patients with an abnormal MTWA result are up to 14 times more likely to experience sudden cardiac arrest than those with a normal test result. This analysis also concluded that a negative MTWA test confers an extremely low risk of experiencing SCA in the next 12–18 months (<0.3%). Results of a pooled analysis of 2,883 patients presented at the 2011 Scientific Sessions of the Heart Rhythm Society confirm the prognostic value of MTWA in patients with reduced ejection fraction (EF 35%)

According to American College of Cardiology, American Heart Association, and European Society of Cardiology guidelines issued for the Management of Patients with Ventricular Arrhythmias and Prevention of Sudden Cardiac Death, it is reasonable to use MTWA testing to improve the diagnosis and risk stratification for those who are at risk of developing life-threatening ventricular arrhythmias (Class IIa, Level of Evidence: A).

MTWA testing is indicated for any patient at risk for life-threatening ventricular tachyarrhythmias or sudden cardiac arrest, particularly those with heart failure, reduced ejection fraction (heart pumping capacity), prior heart attack, unexplained syncope (loss of consciousness or fainting), or family history of SCA.

Where it Stands

Cambridge Heart’s Analytic Spectral Method is the only Medicare-reimbursable technique for measuring MTWA. Cambridge Heart’s MTWA test is also reimbursed by a growing number of private payers including United Healthcare, Aetna, Cigna, Humana, Harvard Pilgrim Healthcare, and several BCBS plans.

In July 2010, Medicare updated its guidelines to allow MTWA to be performed during the same patient visit as a stress procedure (including stress echo and nuclear stress) with full reimbursement for each test. Lifting the previous restriction on same-day testing results in more convenience for patients and a more efficient testing schedule for physician practices.

Cambridge Heart’s HearTwave® II system is a third-generation product for measuring MTWA. The PC-based unit employs the Analytic Spectral Method to measure alternans at the level of 1 microvolt, so small that it is not detectable on a standard ECG. The HearTwave II can be used for standard stress testing as well as MTWA assessment.

Through a partnership with Cardiac Science, the market leader in stress testing equipment, MTWA is now available on the Q-Stress line of cardiac stress testing systems.

More Information

For more information about MTWA testing, visit http://info.hotims.com/40433-184.


Medical Design Briefs Magazine

This article first appeared in the May, 2012 issue of Medical Design Briefs Magazine.

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