Defibrillators help some heart attack survivors who have damage to the left ventricle live longer. The problem is that doctors can't easily tell which survivors are most likely to suddenly develop lethal heart rhythms -- and so would benefit from a defibrillator -- and which aren't. The new test, called the T wave alternans test, may fill that need. Originally designed for the space program, the test detects beat-to-beat changes in the heartbeat that are too small to be seen on a standard electrocardiogram (EKG).

Medicare recently announced it would pay for the T wave tests for people "at risk of sudden cardiac death." Right now that mostly means heart attack survivors with damage to the left ventricle. Researchers estimate that use of the test to choose the best candidates for implanted defibrillators could save Medicare more than $600 million a year. But it may also be used for other people at high risk of a sudden cardiac arrest.