Two thirds of people in cardiac arrest in London were given CPR by a passer-by or a relative last year.
An audit by London Ambulance Service has found that members of the public gave life-saving cardio-pulmonary resuscitation (CPR) to nearly 2,500 people in the capital.
London has one of the best cardiac arrest survival rates in the world.
Cardiac arrest happens when the heart stops pumping blood round the body.
London Ambulance Service typically reach patients in seven minutes but bystander early intervention will give the person the best chance of life.
Chest compressions will help to keep blood pumping to the brain, while an early shock from a defibrillator will restore normal heart rhythm.