People whose hearts suddenly stop beating in public places are twice as likely to survive if they are treated by a trained lay person using an automated external defibrillator and cardiopulmonary resuscitation than if they receive only CPR, a major new study has shown.

The findings of the Public Access Defibrillator Trial, performed over 21 months in Pittsburgh and 23 other cities, suggest that AEDs could play an important role in reducing cardiac arrest deaths.

Earlier studies showed that emergency responders, such as firefighters, could save lives with the AEDs. The new study confirms that ordinary people with a little training also can use them effectively, said Dr. Vincent Mosesso Jr., an emergency physician at the University of Pittsburgh Medical Center and the study's principal investigator here.

Cardiac arrest occurs when the heart stops beating effectively, as can occur following a heart attack or accident, or as the result of an underlying heart problem. A person can survive without a pulse for only a few minutes, which makes cardiac arrest particularly deadly if it occurs outside of a hospital. A quarter-million Americans die each year following "out-of-hospital" cardiac arrests.

An AED restores a normal heart rhythm by delivering an electrical shock.

More than 60 sites were involved in the study locally, including many YMCAs, Allegheny County Housing Authority complexes, the Monroeville Mall, One Oxford Centre, and Pitt's Cathedral of Learning. About 600 volunteers were trained to treat cardiac arrest.

Nationwide, almost 1,000 sites were included in the study and about 20,000 volunteers were trained. Victims were treated with CPR at half of the sites and with both CPR and AEDs at the other half.

"The study was a major frontier to cross," said Dr. Joseph P. Ornato, chairman of the study's steering committee and an emergency physician at Virginia Commonwealth University Medical Center. He presented the findings yesterday at the American Heart Association meeting in Orlando, Fla.

He reported that 29 people with cardiac arrest survived until hospital discharge after being treated with AED and CPR, compared to just 15 who survived to discharge after being treated with CPR alone.

Once attached by electrical leads to the victim's chest, an AED can determine whether a shock should be administered. None of the patients in the study received any unnecessary shocks, Mosesso said, and the AEDs never failed to shock those who needed one.

But Mosesso, co-director of the Pittsburgh-based National Center for Early Defibrillation, emphasized that the device alone cannot save lives.

"We didn't just go out and hang AEDs in these buildings," he said. Not only were volunteers trained in their use, but an emergency plan was put in place so that the volunteers would know what to do -- who should call 911, who should get the AED, etc. -- in the event of a cardiac arrest.

"This important study shows that lives can be saved by training community volunteers to use external defibrillators," Dr. Barbara Alving, acting director of the National Heart, Lung and Blood Institute, said in a statement yesterday. The institute sponsored the study.

But the study also showed that the AEDs only made a difference in the public sites, such as shopping, office and entertainment centers, and at hotels and recreational facilities. They did not improve survival rates when installed in apartment buildings and other residential centers.

About 20 percent of all out-of-hospital cardiac arrests occur in public places and the rest occur in residences.

A new trial at the University of Washington will see whether AEDs affect survival when placed in the homes of individuals who have had a heart attack and have not been implanted with automatic defibrillators.