Tom Petty's Death: What Is Cardiac Arrest?
Tom Petty, shown here performing with Mudcrutch in LA on June 25, 2016.
Credit: Harmony Gerber/WireImage

Update Tuesday, Oct. 3, at 6:54 a.m. ET:

Musician Tom Petty died last night (Oct. 2) at UCLA Santa Monica Hospital, where he had been put on life support following a cardiac arrest, according to news reports.

He was found Sunday night (Oct. 1) in full cardiac arrest, TMZ reported. Petty, who was 66, was found unconscious and not breathing, TMZ reported. When his family learned Petty had no brain activity, they decided to take him off life support, according to TMZ. He reportedly died while surrounded by family and friends, including his bandmates.

But what is cardiac arrest?

Cardiac arrest is different from a heart attack, according to the American Heart Association (AHA). It describes a condition in which the heart abruptly stops functioning. Cardiac arrest is unexpected, and it occurs instantly or shortly after symptoms appear, the AHA says.

The condition is caused by a malfunction of the heart's electrical system, according to the AHA. These malfunctions can cause the heart to beat irregularly, the AHA says. One type of irregular heartbeat that's common in cardiac arrest is called ventricular fibrillation and occurs when the two lower chambers in the heart suddenly start beating chaotically or don't pump blood. [Heart of the Matter: 7 Things to Know About Your Ticker

Death occurs within minutes of the heart stopping, according to the AHA. Without the heart beating, blood isn't pumped to the brain, lungs and other organs in the body, the AHA says. "Seconds later, a person loses consciousness and has no pulse," according to the organization.

"What happens immediately is that as soon as the heart stops, because there's no blood getting into the brain, the brain stops functioning within moments, almost instantaneously," said Dr. Sam Parnia, the director of critical care and resuscitation research at NYU Langone School of Medicine in New York City.

One reason that doctors know that the brain is no longer functioning is because a person loses all brain-stem reflexes, Parnia told Live Science. These reflexes include the gag reflex and pupil reflex, he said. When "all that is gone ... that tells you the brain is no longer working."

Sudden cardiac arrest is a leading cause of death in the United States, the AHA says.

The condition can be reversed if it's treated within a few minutes of it taking place, according to the AHA. It's treated with cardiopulmonary resuscitation (CPR) and by shocking the heart back to a normal rhythm using a defibrillator, the AHA says.

A heart attack, on the other hand, takes place when a blockage in a person's blood vessels prevents blood from flowing to the heart, leading to the death of the heart muscle. Heart attacks can lead to cardiac arrest, depending on which part of the heart muscle is affected.

Originally published on Live Science.

Editor's Note: This article was first published on Oct. 2 when Tom Petty was hospitalized.