The odds of surviving cardiac arrest are higher for people given an electric shock to their hearts in a public place than for those shocked at home, a new study suggests.

The study, which appears in this week's New England Journal of Medicine, found the chances of surviving the cardiac arrest are high if the event is witnessed in a large public venue, such as an airport, sports arena or shopping mall. Those going into cardiac arrest at home and have access to an automated external defibrillator (AED) have lower survival odds.

The reason, say researchers, is that almost four out of five cardiac arrests that happen in public are due a survivable type of heart rhythm disruption. Those whose hearts stop at home, on the other hand, are more likely to be suffering from an ongoing illness that leads to heart failure.

What's more, large venues with lots of people are more likely to have an AED device handy as well as someone who knows how to use one, explains sutdy researcher Dr. Laurie Morrison, an emergency physician at St. Michael's Hospital in Toronto.

"You're more commonly seen if you have a cardiac arrest in a public place," Morrison told CTV Canada AM Thursday. "There's lots of people around and at least one person will do the right thing, which is call 911, start CPR and send someone to run and get an AED."

The study found that only a third of arrests that occurred in homes were witnessed, while more than half of those in public locations had witnesses who could immediately call 911 and provide assistance.

The study focused on more than 14,000 cardiac arrests in men and women and found that 79 per cent of those who had their cardiac arrest in the community had the kind of disrupted heart rhythm that could be corrected by an electrical shock from an AED device.

These heart rhythm disruptions are known as VT and VF. The first is ventricular tachycardia, in which the electrical impulses in a heart become rapid, while "VF" refers to ventricular fibrillation, when the impulses become chaotic.

Among those who were actually shocked in a public place, 34 per cent survived and recovered well enough to be discharged from hospital.

In comparison, only 36 per cent of those who went into cardiac arrest at home had a VT/VF disturbance. And even if a relative or bystander at home used an AED found in the home, only 11.6 per cent survived to leave the hospital.

The study authors note that demographics play a role in the findings. People who have an AED at home tend to be older and more likely to have underlying chronic disease than the type of patient who is active enough to be out attending a major sports event or going shopping.

The study's lead author, Dr. Myron "Mike" Weisfeldt, a cardiologist at Johns Hopkins, emphasizes that people at risk of sudden cardiac death and who have an AED at home may still benefit.

"But the survival rate is really not that much different than with a bystander practicing CPR without an AED and quickly calling 911," he told CTV.

The authors conclude that defibrillators should be made more widely available in public spaces. Morrison notes that it takes no expertise to use a defibrillator and is simply a matter of following of turning the machine on and following the voice commands.