Sudden cardiac death is the largest cause of natural death in the U.S. This cardiac event typically occurs when blockages in blood vessels — often caused by a blood clot inside one of the arteries of the heart — stop blood flow, which can lead to cardiac arrest, usually within an hour of symptoms.

An estimated 365,000 sudden cardiac deaths, or nearly 1,000 per day, occur annually outside of hospital settings, and nearly 90 percent are fatal. Of these sudden deaths, 70 percent happen in people’s homes, 18 percent in public places, and the remainder in nursing homes.

Though most people use the terms “sudden cardiac arrest” and “heart attack” interchangeably, the two events are different. A heart attack occurs when a blocked artery prevents oxygen-rich blood from reaching a section of the heart. If the blocked artery is not reopened quickly, the part of the heart normally nourished by that artery begins to die.

The longer a person goes without treatment, the greater the damage. The symptoms of a heart attack may be immediate and intense. But more often, symptoms start slowly and persist for hours, days, or even weeks before a heart attack occurs. And unlike sudden cardiac arrest, the heart usually does not stop beating during a heart attack.

On the other hand, sudden cardiac arrest often occurs without warning. It is triggered by an electrical malfunction in the heart, leading to an irregular heartbeat (arrhythmia). With its pumping action disrupted, the heart can’t get blood to the brain, lungs, and other organs. Within a matter of seconds, a person loses consciousness and has no pulse. If untreated, death occurs within minutes.