Devices known as automated external defibrillators, or AEDs, are increasingly common in public places. It's likely you've seen one at an airport, gym, government building or large office building.

An AED can check a person's heart rhythm, recognize when it's out of whack, then advise the rescuer how to shock the heart back to normal. It can be a lifesaver. AEDs boost survival of a condition called sudden cardiac arrest (SCA), when the heart stops beating. SCA usually causes death within minutes if not treated. That's why AEDs have become so common.

Each year in the U.S., about 420,000 such cardiac arrests are assessed by emergency medical personnel, according to the American Heart Association. An AED isn't going to save everybody, of course, but, says Michael Sayre, MD, professor of emergency medicine at the University of Washington in Seattle and a spokesperson for the AHA. "Using an AED roughly doubles the chances of survival."

If you found yourself needing to help someone with an AED, would you be able to use it? The short answer is yes. If you can read and stay relatively calm, you should be able to operate an AED. But it never hurts to be prepared, so in advance of that possibility, here's some background about AEDs.

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AED: How simple is it?

Although AEDS are made by several manufacturers, the devices have certain key features in common, according to the American Heart Association (AHA). They use voice prompts, lights and text messages to guide the rescuer about what to do.

"The devices are smart and will talk you through," says Sayre, who advises following these steps.

  • Make sure the person in distress is in sudden cardiac arrest. One sign of SCA is a lack of responsiveness. For example, if you lightly tap him on the shoulder and he doesn't react. "If someone collapses in front of you suddenly, it's almost always sudden cardiac arrest, eighty percent of the time," Sayre adds.
  • Call 9-1-1.
  • Follow the directions on the AED. The device will walk you through with spoken commands. "Ideally, there should be two people [involved in the rescue]," Sayre says, with one doing CPR and chest compressions and the other following the directions on the [AED] box. The device instructs users in how to place the electrode pads on the chest, then analyzes heart rhythm and with a voice prompt says whether a shock is needed, then delivers it as the rescuer stands back. It also tells when to do CPR and how to do so. The device also tells the rescuer when to stop.

The NIH offers additional step-by-step directions.

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Advanced AED

Sayre encourages business owners and those who buy the devices for public buildings to train some of their staff how to use an AED. Good candidates for training, he says, would be the security staff or the receptionist at the front desk, as well as emplyees who come into contact with large numbers of people and so would be more likely to witness an emergency.

The training also would help remind the staff the device is available, Sayre says. Many errors he's seen don't involve mistakes using an AED, he says, but rather not remembering it was available.

It's also important to store an AED in a place that's accessible. Canadian researchers looked at the medical cases of 451 residents of Toronto who went into cardiac arrest within 100 meters (the length of a football field) of an AED. They found that in one quarter of cases, the devices were there, but behind locked doors. The events often occurred when the businesses weren't open, the researchers concluded. Experts say these findings point to the need to be sure AEDs are accessible at all times. For that reason, companies are trying to develop weather-proof and vandal-proof containers for the devices.

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Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.