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Automated external defibrillators (AEDs) are used to restart hearts after cardiac arrest and restore normal heartbeats, but a new study found only about one-third of U.S. states require schools to have the life-saving devices.

Those requirements vary by state, said the study's lead author. For example, one state may require AEDs to be installed in all schools while another only requires the devices to be in public schools.

"There are very few states that require both public and private schools to have them," said Dr. Mark Sherrid, a cardiologist and professor of medicine at NYU Langone Medical Center in New York City.

Out of every 100,000 children and adolescents, two to six will have a sudden cardiac arrest each year, Sherrid and colleagues write in the Journal of the American College of Cardiology.

During a sudden cardiac arrest, a person's heart suddenly stops beating. Death occurs within minutes if the person is not properly treated with cardiopulmonary resuscitation (CPR) and an AED.

"Without an AED, the chances of surviving and leaving the hospital as a child are only 7 to 10 percent," Sherrid told Reuters Health. "So, these kids tend not to make it."

But if the heart has the kind of abnormal rhythm that can sometimes be corrected with a defibrillator - known as a shockable rhythm - then "your chances go up to 60 to 70 percent," Sherrid said.

His team found that as of February 2016, 17 of the 50 states required at least some schools to have an AED installed. In all but four of the 17, the cost of the devices is covered in some way by the state or local boards of education.

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"These laws tend to get started because of private lobbying by families who have had deaths," said Sherrid. "The families get the ear of the legislator and the legislator brings it to a floor."

The specifics of those laws varied. Only one state required AEDs in public and private schools along with colleges. Three states required AEDs in schools that participate in interscholastic sports.

Other states require AEDs only in public grade schools, only in colleges, or a mix.

The researchers estimate 25 percent of schools are in states where AEDs are required.

Sherrid said the next step is to determine how many schools across the U.S. actually have AEDs.

In Japan, he said, where most schools installed AEDs, sudies show children and adolescents benefited from the devices.

"A study we cite in the paper showed two in 50 high schools in the U.S. can expect a sudden cardiac arrest each year," said Sherrid. "That's high enough to warrant doing something about it."

AEDs should be part of the school's routine and be available in the building and on the athletic field, he said. Many states already require AEDs to be installed in buildings other than schools.

"If they've made the point of legislating to have AEDs in gambling facilities and race tracks, it would seem timely to move public access of defibrillation into schools as well," Sherrid said.