Devices to restore a normal heartbeat after cardiac arrest appear to be less useful at home than in public places, researchers said Wednesday.

The so-called automatic external defibrillators, or AEDs, are already present in airports and casinos across the U.S. But some research has hinted lately that for more and more Americans, their hearts are too far gone to be jolted back to life by an AED.

According to a new study published in the New England Journal of Medicine, however, those concerns are only partly justified.

Based on data from the world's largest registry of cardiac arrests outside of hospitals, spanning both the US and Canada, government-funded researchers found an AED could have been used far more often in public settings than in private homes.

"AEDs are of tremendous value for people who have cardiac arrest in public locations," said Dr. Myron L. Weisfeldt, of Johns Hopkins University in Baltimore, who led the new study. "It is not that they have no value at home, but they have less value there."

The researchers found that when an ambulance arrived at the scene, 60 percent of the people who collapsed in a public place while someone was watching had heart rhythms that allowed the use of an AED, so-called ventricular fibrillation and pulseless ventricular tachycardia.

By comparison, if the cardiac arrest occurred at home, only 35 percent of the people might have been helped by an AED.

"Our guess is that people who have cardiac arrest at home are people who have chronic heart disease and are on medications," said Weisfeldt.

In those cases, he added, the heart might just stop completely instead of going through the weak flutter, where AEDs are still helpful.

Overall, only 2 percent of the nearly 13,000 patients Weisfeldt and colleagues included in their study were shocked with an AED by a bystander, and a mere 7 percent left the hospital alive.

"One of the things that have been recognized for some time is that not enough members of the lay public perform CPR," said Dr. Benjamin Abella, a resuscitation expert at the University of Pennsylvania in Philadelphia who was not involved in the study.

"It certainly also is striking just how few people had AEDs applied," he added. "There needs to be much more aggressive employment and education in use of AEDs."

Abella said the new findings suggested public money would be better spent by placing more of the devices in public locations rather than private residences, although they might still be lifesavers at home in some cases.

AEDs typically cost a couple of thousand dollars apiece. A study from last year, also led by Weisfeldt, estimated they save 474 lives annually in the US and Canada. By comparison, some 300,000 Americans suffer cardiac arrest every year.

An editorial in the journal questioned the reasoning in the new study, noting that small differences in how fast bystanders call 911 could explain why people fare worse at home.

"The lone rescuer at home, who is probably less aware of the critical importance of speed, would lose the race to a public bystander," wrote Dr. Gust Bardy of the Seattle Institute for Cardiac Research.

Weisfeldt argued that time did not seem to matter much, and said more CPR education, teaching people how to do chest compressions in someone whose heart has stopped, would probably be helpful in the private setting, where most cardiac arrests still occur.