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Medical experts talk about the steps that it takes to save someone who has a cardiac arrest outside of a hospital as a "chain of survival." Witnesses who can identify the problem are the first link in that chain.

"The most important thing that a bystander can do is to recognize the issue and then act quickly by first calling 911, then doing CPR" and calling for an automated external defibrillator, or AED, says Mary Newman , president and CEO of the Sudden Cardiac Arrest Foundation, which is based in Pittsburgh, Pennsylvania.

But Newman, a researcher who developed that idea of a chain of survival, said recognition can be tricky because cardiac arrest doesn't necessarily look like a heart problem in the movies, where someone might dramatically clutch their chest and swoon.

A major source of confusion is that people often think cardiac arrest is the same as a heart attack. "They're two different things," Newman said. A heart attack is often likened to a plumbing problem, where blood flow to the heart muscle is stopped. Although a heart attack can lead to cardiac arrest, cardiac arrest is an electrical issue, where the heart stops beating properly.

"When you have a heart attack, you're awake, and your heart is beating, and you don't need CPR," Newman said. "You just need to call 911 and get help. But with sudden cardiac arrest, their heart has stopped. And if no one intervenes quickly, they will die."

She corrected herself: "When you're in cardiac arrest, you're actually dead. And the challenge is to bring the person back to life."

Even among people who know what cardiac arrest is, it can be challenging to figure out. But for anyone who does not have medical training, it comes down to two questions: Are they responsive, and are they breathing normally?

Checking responsiveness is as simple as tapping them on the shoulders. But many bystanders, also called lay responders, are confused by the abnormal breathing that accompanies cardiac arrest.

Sometimes, cardiac arrest also can cause jerking movements as if someone is having a seizure, he said, which also can slow people from responding.

If you see somebody collapse, don't waste time thinking about the possible causes. During the time it takes someone to recognize a cardiac arrest, the person's vital organs are being starved of blood. Having a bystander perform CPR can double or triple the chances of their survival.

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