Survival rates for Londoners who had a cardiac arrest out of hospital and witnessed by another person have more than doubled in the last six years.
Thomas Doyle had no history of heart problems before his attack
A report from the London Ambulance Service shows 10 out of one hundred Londoners who suffered a cardiac arrest out of hospital last year lived.
That compares to four out of every one hundred Londoners who survived six years ago.
But the figures also showed about nine out of 10 cardiac arrest victims died.
The London Ambulance Service welcomed the latest figures, saying new training for the public was a factor.
Dr Rachael Donohoe, one of the authors of the report said: "A number of factors have contributed to the improved survival rate, one of these being effective bystander cardiopulmonary resuscitation (CPR), which is known to double a person's chance of survival."
LAS REPORT FINDINGS
The average age of a cardiac arrest patient was 68 years
About three quarters of cardiac arrests in London occurred in the home
Of those that occurred in public, over a third took place in the street
Emergency calls for cardiac arrest were highest between the hours of 0800 and 1200
Cardiac arrests occurred most frequently on a Friday
10% of cardiac arrests occurred during November
Money from the Heritage Lottery has also helped, she said.
It funded more than 400 extra defibrillators - machines that deliver a shock to re-start the heart - in public places and trained more than 2,500 people across London to use them.
Thomas Doyle, 55, from Waterloo, who had no history of heart problems, was walking through his local market one afternoon last July when he went into cardiac arrest.
He said: "All I remember is that my right leg went numb and then I was out like a light. Next thing I knew I was coming to in hospital. I can't thank the crew enough for saving my life."