30,000 people each year in the UK have an out-of-hospital cardiac arrest where the ambulance services attempt resuscitation - and less than 10% of those people will survive. Yet, if a bystander starts CPR they could double a person's chances of survival.

Defibrillation within 3–5 min of collapse can produce survival rates as high as 50–70%.This can be achieved through public access defibrillation, when a bystander uses a nearby AED to deliver the first shock.Each minute of delay to defibrillation reduces the probability of survival to hospital discharge by 10%. In the UK, fewer than 2% of victims have an AED deployed before the ambulance arrives.