Frequently Asked Questions

What is Sudden Cardiac Arrest?

Sudden Cardiac Arrest (SCA) is a leading cause of death in the United States, accounting for an estimated 325,000 deaths each year — more than stroke, lung cancer and breast cancer combined.¹ 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.

Out-of-hospital survival for SCA is <5%, making prediction and prevention critically important.¹ 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 US fit these clinical profiles and may be at risk.² SCA can also strike apparently healthy individuals and is the leading cause of death in young athletes.
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Can sudden cardiac arrest be prevented?

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 which 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.^3,4 MTWA testing provides an additional piece of information that may help allay such concerns by accurately stratifying a patient's risk, thereby contributing to the objective clinical decision-making process.
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What are the clinical benefits of risk stratification for SCA?

Current risk stratification paradigms for SCA are designed to identify high-risk populations based on 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.^5,6

MTWA testing is a non-invasive tool that 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.
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What is Microvolt T-Wave Alternans (MTWA)?

T-wave alternans refers to an alternating pattern in the T-wave portion of the surface electrocardiogram (ECG). Cambridge Heart's proprietary Analytic Spectral Method® is able to detect subtle, microvolt-level T-wave alternans (MTWA) not visible to the human eye. Clinical studies show that MTWA is a marker of arrhythmic vulnerability and SCA risk.

MTWA is linked to beat-to-beat alternations in the action potential duration of individual cardiac cells. Evidence suggests that intracellular calcium cycling is the underlying mechanism for cardiac alternans.⁷
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How is MTWA measured?

MTWA is a heart-rate dependent phenomenon typically measured during a non-invasive, low-level exercise stress test using multi-segment electrodes (sensors) to minimize noise and physiologic sources of artifact.

Cambridge Heart's Analytic Spectral Method? is used to detect subtle beat-to-beat variations in the T-wave of the ECG during rest, exercise and recovery.
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What clinical data supports MTWA testing?

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.⁸ 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. Most recently, a 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%).
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Are there clinical guidelines to support MTWA testing?

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).⁹
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Which patients would benefit from an MTWA test?

Patients at risk for sudden cardiac death may benefit from the MTWA test. Risk factors for SCD include heart failure, prior heart attack, coronary artery disease, left ventricular dysfunction (low EF), syncope and family history of SCA.
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Is the MTWA test reimbursed?

Yes. Cambridge Heart’s Analytic Spectral Method is the only Medicare-reimbursable technique for measuring MTWA.¹⁰ Cambridge Heart’s MTWA test is also reimbursed by a growing number of private payers including Aetna, Cigna, Humana, several BCBS plans and Harvard Pilgrim HealthCare. For specific information about reimbursement in your area, click here.
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What equipment is needed to perform a Microvolt T-Wave Alternans Test?

Cambridge Heart’s HearTwave® II and CH2000 systems are PC-based units that use the Analytic Spectral Method to measure MTWA. Both systems can be used for standard stress testing as well as MTWA assessment.

The HearTwave II and CH2000 are designed for use with Cambridge Heart’s proprietary Micro-V Alternans Sensors™, multi-segment electrodes which minimize noise and artifact to permit detection of alternans at the microvolt level.
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How is an MTWA test performed?

MTWA tests may be administered in a physician’s office, hospital or outpatient clinic setting, in much the same way as a stress test. Patients typically walk on a treadmill for 5-10 minutes with a set of 14 electrodes on the torso: seven standard 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.
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What is the significance of a negative or normal MTWA test?

Studies demonstrate that patients with a negative (or normal) MTWA test have a very low risk of experiencing SCA. The negative predictive value of MTWA testing has been shown to be 97-99% in several clinical trials. A recent meta-analysis concluded that a negative MTWA test confers an extremely low risk of experiencing SCA in the next 12-18 months (<0.3%). Since cardiac disease is progressive in nature, it may be prudent to re-test patients with a negative MTWA test on an annual or semi-annual basis, particularly if their risk profile has changed.
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What is the significance of a positive or abnormal MTWA test?

A recent 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.  The risk of SCA may be managed through treatment of underlying coronary artery disease, optimal pharmacologic therapy or lifestyle changes like smoking cessation. High-risk patients may benefit from further, invasive diagnostic testing or ICD therapy.
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References