During a sudden cardiac arrest (SCA), heart function ceases – abruptly and without warning – most often due to the onset of a ventricular tachyarrhythmia . Because out-of-hospital survival is less than 8 percent, prediction and prevention are critically important . Microvolt T-wave alternans (MTWA), a subtle alternating pattern in T-wave morphology on the surface electrocardiogram is a marker of arrhythmic vulnerability and SCA risk [3, 4], demonstrating prognostic value in various patient populations including those with ischemic and non-ischemic cardiomyopathy [5-7]. We report a case of a patient who experienced SCA one year after a positive MTWA test.
An asymptomatic 64-year-old male had a history of coronary artery disease and coronary bypass surgery in 1992. A computed tomography (CT) angiogram performed in September 2006 revealed subtotal occlusion of the right coronary artery; however, angioplasty could not be completed due to anatomical complexity. An echocardiogram showed a left ventricular ejection fraction (LVEF) of 45-50 percent and significant left ventricular hypertrophy secondary to moderate aortic stenosis. Given his clinical profile, an MTWA test was ordered to further clarify his risk of ventricular arrhythmias and SCA.
MTWA Study and Follow-up
In October 2009, MTWA testing was performed with treadmill exercise using the Analytic Spectral Method to measure alternans (HearTwave II, Cambridge Heart, Tewksbury, Mass.). At the time of the test, the patient was in normal sinus rhythm with intermittent ventricular ectopy. The MTWA result was positive with sustained alternans present in several leads with an onset heart rate of 106 bpm (Fig. 1). The positive MTWA study along with the patient’s other risk factors prompted an evaluation by an electrophysiologist.
An electrophysiology study (EPS) was performed resulting in inducible ventricular tachycardia with two extrastimuli, and a dual-chamber implantable cardioverter defibrillator (ICD) (Secura DR, Medtronic, Minneapolis, Minn.) was implanted in November 2009. One year later while resting at home, the patient reported feeling a “flutter” in his heart followed by a shock from the device. Review of the ICD electrogram (Fig. 2) confirmed that the patient experienced an episode of ventricular fibrillation which was successfully terminated by a 23.7 J defibrillation shock on Dec. 2, 2010.
Reduced left ventricular ejection fraction is an established risk factor for cardiac arrest and is currently the primary determinant of ICD eligibility; however, EF lacks sensitivity  due to the fact that the majority of SCA deaths occur in patients with an EF more than 30-35 percent [9, 10]. This patient did not have significant LV dysfunction, but had several other clinical factors associated with increased risk of SCA including ischemic cardiomyopathy , left ventricular hypertrophy  and aortic stenosis .
MTWA has been cited as a useful adjunct tool in such patients with moderate to preserved LVEF for whom SCA is a “real but difficult-to-ascertain risk” . MTWA can help guide SCA risk management strategies which may include treatment of underlying ischemia, medical optimization, tighter glucose control, lifestyle changes – or, when appropriate, EPS referral and possible EPS or ICD implantation.
“Per current primary prevention guidelines, this patient would not have received life-saving ICD therapy if he had not undergone an MTWA study,” noted Keith Boman, M.D., the patient’s cardiologist. “MTWA is a tool to help clarify the overall arrhythmic risk, particularly for patients who have known risk factors but who fall into this ‘gray area’ in terms of ICD eligibility.”
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This case study was supplied by Cambridge Heart
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