Some defibrillators offer step-by-step voice instructions to guide bystanders offering help to cardiac arrest victims. ((CBC))

The chances of surviving when the heart stops suddenly and a bystander applies a shock are better in a large public place like a mall or hockey arena than when it happens at home, a new study suggests.

For the study in Wednesday's online issue of the New England Journal of Medicine, U.S. and Canadian researchers evaluated nearly 13,000 cardiac arrests.

Of the total arrests studied between 2005 and 2007, 34 per cent of victims survived when they received electrical shocks from a defibrillator in public places compared with 11.6 per cent of those who received the shock at home, the researchers found.

The investigators wanted to explore how the location of the arrest, type of arrhythmia and odds of survival are related.

"Our research clearly shows that the chances of surviving a shockable cardiac arrest are best when someone publicly witnesses it happening, a bystander uses CPR to keep blood flowing to the brain and other key organs, and an AED can be applied to electrically restart the heart," said cardiologist and study author Dr. Myron Weisfeldt of Johns Hopkins Hospital in Baltimore, Md.

As part of the study, researchers analyzed 4,000 cardiac arrest calls on 911 lines in 10 North American regions, including Vancouver, Ottawa and southern Ontario.

Nowadays, the chances of getting bystanders to help with CPR and using an AED in public are "pretty good," Weisfeldt said, since many people are trained in the basic life-saving techniques.

Nearly four out of five of the heart stoppages seemed to be due to a disruption in heart rhythm that responds to an automated external defibrillator, or AED, when used along with CPR, researchers said.

The victims were often people who were going about their lives in the community rather than being confined to a hospital or health care facility because of poor health. When they experienced a cardiac arrest, it was more likely to be a heart rhythm called ventricular fibrillation, the study's authors said.

That's an electrical problem in the heart that can be caused by a variety of conditions from poor circulation to drug use.

The researchers concluded that defibrillators should be made more widely available in public spaces.

But they said for those caring for cardiac arrest victims at home, the emphasis on saving lives might be better placed on calling 911 quickly to get high-quality CPR assistance rather than putting more AEDs into homes.

The research, which is set to continue through 2017, was funded by the U.S. National Heart, Lung and Blood Institute.