Harvard Heart Letter

A new wave in heart testing

An odd variation on the electrocardiogram could help doctors determine who would get the most benefit from an implanted defibrillator.

A test for heart signals so faint they are measured in millionths of a volt can help pick out who is and, equally important, who isn't likely to develop a deadly heart rhythm that suddenly stops the heartbeat. In March 2006, Medicare agreed to pay for an FDA-approved test for the signal pattern called microvolt T wave alternans, opening the door to wider use. One of the most important early uses of the test will be identifying who could benefit from an implantable heart defibrillator. These "shock boxes" jolt the heart with a burst of electricity to stop a potentially deadly rhythm.

At one time, defibrillators were implanted only in people who had survived a sudden cardiac arrest. But over the last several years, studies have shown that they help some heart attack survivors with damage to the left ventricle (the heart's main pumping chamber) live longer than those who don't have a defibrillator. The trouble is, doctors can't easily tell which heart attack survivors are at high risk of sudden cardiac arrest — and so would benefit from a $50,000 implanted defibrillator — and which aren't. Testing for microvolt T wave alternans may fill that need.

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