Do you know your ejection fraction? Clinical studies show that a person's ejection fraction, or EF, is the number-one predictor of risk for sudden cardiac death (SCD), a condition which claims more than 400,000 American lives each year. But a recent survey showed that most heart attack survivors don't know if they are at risk for a dangerous heart rhythm disorder that causes SCD.

Unlike a heart attack, which is caused by clogged or blocked blood vessels, SCD is caused by ventricular fibrillation, a problem with the heart's electrical system. In ventricular fibrillation, the lower chambers of the heart called ventricles suddenly begin to beat rapidly and out of rhythm. Unless immediate emergency help in the form of an automated external defibrillator (AED) is available, death follows in minutes.

"Sudden cardiac death is devastating because it strikes like a bolt out of the blue, even in apparently healthy people who have no known heart disease," says Eric Prystowsky, M.D., past president of the Heart Rhythm Society. "We now know, however, that most victims of cardiac arrest do have underlying disease or other risk factors, although they may not be aware of it. We also have the tools to identify patients at risk for SCD and can take steps to prevent it."

The tools include knowing one's ejection fraction, which can easily be measured by echocardiography, a simple, painless ultrasound exam of the heart, or by other tests.

A normal EF is 55 percent or higher, while those with an ejection fraction of 40 percent or lower should be tested further to determine if an abnormal heart rhythm puts them at high risk for sudden cardiac death.

"Most people know their blood pressure and their cholesterol level," said Dr. Prystowsky. "If heart attack survivors knew one more number, their EF, thousands of lives could be saved. If people are tested and found to be at high risk, we can treat them to prevent SCD."

Patients at high risk of SCD may be candidates for an implantable cardioverter defibrillator, which automatically monitors the heart rate to detect and correct abnormal rhythms.