A heart attack can damage the heart, creating an area of scar tissue. If large enough, this scar can cause the heart to pump ineffectively and cause heart failure or can unexpectedly create a potentially lethal heart rhythm that may cause the heart to quiver and stop pumping blood altogether. This condition is called sudden cardiac death and can strike without warning.

Doctors can help determine a patient's risk for sudden cardiac death with a simple test that measures the fraction of blood pumped or ejected from the heart with each beat. Doctors call this measurement the ejection fraction, which can be measured with a simple and painless heart ultrasound called an echocardiogram or "echo."

A healthy heart pumps about one-half of the blood it holds with each beat. A normal ejection fraction is 50-55%. After a heart attack, the damaged heart has a reduced ejection fraction. An ejection fraction of less than 35% dramatically raises the risk for sudden cardiac death. A reduced ejection fraction indicates that a cardiomyopathy is present.

People with a low ejection fraction may be eligible to receive a pacemaker-like device that can detect and treat fatal heart rhythms. This device, known as an implantable cardioverter defibrillator (ICD), is similar to the type of device implanted in U.S. Vice President Dick Cheney in 2001. The defibrillator is placed beneath the skin near the collarbone and monitors every heartbeat for potentially fatal heart rhythms. If such a rhythm is detected, the defibrillator can deliver a lifesaving shock, restoring normal heart rhythm and preventing sudden cardiac death. In many clinical trials ICDs have been shown to prolong life in patients with a reduced ejection fraction after a heart attack.