Sudden cardiac death is caused when the electrical impulses in the heart become rapid, slow or chaotic, causing the heart to stop beating. Survivors can be at higher risk for another arrest. Implanted cardioverter defibrillators (ICDs) deliver an electrical shock when the heartbeat becomes erratic, causing normal beating to resume. The problem is that it's not known with certainty who actually needs a defibrillator, and the study authors estimate that 17 defibrillators need to be implanted to save one life.
Currently, physicians use ejection fraction, a measure of the heart muscle strength, to determine who might need a defibrillator.
"Unfortunately, this strategy is inadequate, as most patients expected to benefit from ICD currently aren't receiving therapy, and most patients who receive ICD never receive therapies from them," Rosenbaum said. "Using the ejection fraction alone is probably an entirely too limited and crude way of ultimately assessing risk."
Another test, the Microvolt T Wave Alternans (MTWA), measures subtle fluctuations of the T wave. The T wave is one of the three major waves of electrical signals that appear on an electrocardiogram. It records the heart's return to a resting state and is associated with the risk of sudden cardiac death.
In this new study, the MTWA was compared with the electrophysiological study (EPS), an invasive test that also carries some risk to the patient.
"This is the first trial to use MTWA to guide implantation of defibrillators," Rosenbaum said.
The study involved 43 medical centers around the world with 566 patients, none of whom had any previous arrhythmias -- abnormal heart rhythms. The patients were followed for two years.
"The predictive value was comparable. In patients who tested positive on either test, the likelihood of events at one year was double, compared to patients who tested negative," Rosenbaum said. "The MTWA is as effective as EPS but, unlike EPS, it is noninvasive, safe and relatively inexpensive, and therefore much more suitable as a screening tool to apply to large patient populations."
The two tests together were synergistic, suggesting there might be an argument for using both in some cases.
"This is another attempt to refine markers or indicators that a patient might actually need a defibrillator," said Dr. Timothy Gardner, chairman of the AHA committee on scientific sessions program and medical director of the Center for Heart and Vascular Health at Christiana Care Health Services in Wilmington, Del.
A second study also presented Wednesday at the heart association meeting compared two last-resort treatments for atrial fibrillation -- irregular heart rhythm in patients with congestive heart failure. Pulmonary Vein Antrum Isolation (PVI) proved superior on several measures and should be considered first, the study found.
"Atrial fibrillation is a very important problem and is extremely prevalent in patients with heart failure," said Dr. Andrea Natale, study senior author and head of the section of electrophysiology and pacing at the Cleveland Clinic in Ohio. "This study has shown that a cumulative strategy using catheter ablation is a better option to treat this patient population whenever atrial fibrillation becomes refractory to drugs."
More information
For more on sudden cardiac death, visit the American Heart Association.
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