When that drama takes place in an airport or shopping mall, bystanders using battery-powered defibrillators may be saving more than 500 lives every year in the United States and Canada alone, researchers reported on Monday.

'Good Samaritans, when given access to automated defibrillators in potentially fatal emergencies, save lives,' Dr Myron Weisfeldt, a cardiologist at Johns Hopkins University in Baltimore, who worked on the study, said in a statement.

The devices are designed to treat sudden cardiac arrest, a potentially deadly event in which the heart stops contracting and fails to pump blood properly.

The laptop-sized, portable defibrillators come with a full set of instructions, guiding even untrained bystanders through the rescue process. Once electrodes are placed on the victim's chest, the machine analyses the person's heart rhythm and delivers a shock if needed.

The study, conducted in 11 cities in the United States and Canada, involved an analysis of patient records from more than 10,600 incidents of cardiac arrest called into 911 emergency telephone lines.

Bystanders administered CPR in nearly 30 per cent of the cases and offered CPR plus an automated defibrillator in 2.4 per cent of the cases, Dr Weisfeldt told an American Heart Association meeting in Orlando, Florida.

'Only 259 patients had an AED applied by a bystander. Their survival is very good. If they needed a shock and the device shocked them, they had a 36 per cent survival rate. That compares to the overall survival rate of 7 per cent,' Dr Weisfeldt said in a telephone interview.

Because the automated defibrillators are in public places where people tend to get fast emergency care, the researchers figure survival rates with a defibrillator are about 2.5 times better than with CPR alone.

'If the same thing is going on in the rest of the United States, there may be as many as 522 lives being saved by AEDs,' Dr Weisfeldt said.

He said the study makes the case for wider use of the devices.

'If you've got a building with 1,000 people and the cost of putting in the device is US$3,000 (S$4,350), it means US$3 per person is the cost of having it, which is less than the cost of the latte I had this morning,' he said.

About 300,000 Americans die from sudden cardiac death each year. -- REUTERS