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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.
What is microvolt T wave alternans?
An electrocardiogram (ECG) gives a picture of the electrical activity that drives the heart's cycle of contraction and relaxation. It has three main blips - the P wave, the QRS complex, and the T wave (see "Catching the wave").
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Catching the wave
In a normal electrocardiogram, each T wave is the same as the one before it and the one after. With T wave alternans, every other T wave is larger or has a different shape. |
The P wave registers the spread of the "contract now" signal as it travels from the heart's natural pacemaker across the left and right atria. The sharp QRS complex records the burst of electrical activity responsible for making the lower ventricles contract, sending blood to the body and lungs. The gentle rise and fall of the T wave reflects the relaxation of the heart as it prepares to repeat the cycle.
In most people, every T wave has the same size and shape. In some people, though, alternate T waves are a bit larger or have a slightly different shape. This every-other-beat variation is microvolt T wave alternans. The changes are too small to be seen on a standard ECG, but they can be detected with sophisticated equipment that filters and processes electrical signals from the heart.
The test was developed by Dr. Richard J. Cohen, the Whitaker Professor in Biomedical Engineering in the Harvard-MIT Division of Health Sciences and Technology, and his colleagues. Its initial use was to check astronauts' hearts for early warning signs of the cardiac irregularities that tend to happen in zero gravity. Its ultimate use will be more down to earth.
How is the test done?
Having a test for microvolt T wave alternans is a lot like taking an exercise stress test. A series of wires are attached to 14 stickers or small suction cups on your chest. The wires connect to a device that records your heart's electrical activity for a few minutes while you are seated, for a few minutes as you walk on a treadmill, and finally for a few minutes as you recover and relax. The test takes about 30 minutes and doesn't require any special preparation.
What does it measure?
Once written off as an electrical oddity, microvolt T wave alternans actually mirrors subtle changes in how heart cells handle calcium. Tiny alterations in calcium uptake can make cardiac cells lose their wonderful coordination and instead beat out of sync.
People with T wave alternans are at increased risk of developing ventricular tachycardia or ventricular fibrillation - potentially lethal changes in the heart's rhythm. In studies that included more than 2,000 people, most of them with some form of heart disease, 19% of those with microvolt T wave alternans had an episode of ventricular tachycardia or ventricular fibrillation within 21 months. Just as important, a whopping 97% of those without signs of microvolt T wave alternans were free of these potentially deadly rhythms.
University of Michigan researchers estimate that use of the test to choose the best candidates for implanted defibrillators could save Medicare more than $600 million a year.
Who needs the test?
Medicare has agreed to pay for microvolt T wave alternans tests, which cost $300-$400, for people "at risk of sudden cardiac death." In general, that now covers heart attack survivors with damage to the left ventricle (technically, an ejection fraction under 30%). It may also be used for other people at high risk of a sudden cardiac arrest.
A positive test could very well put you on the road to getting an implanted defibrillator, while a negative test indicates you probably don't need one, at least for now.
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| Last updated: | August 21, 2006 |
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Medical content reviewed by the Faculty of the Harvard Medical School. Harvard Health Publications, Copyright © 2007 by President and Fellows of Harvard College. All rights reserved. Used with permission of StayWell.
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