The authors of a report into defibrillators reviewed all calls made to the South Central Ambulance Service between September 2011 and August 2012 following a heart attack.
During the course of the study, the service received 1,035 calls about confirmed cardiac arrests away from a hospital - the equivalent of one for every 600 members of the public each year.
It can be frightening and confusing when someone collapses in front of you and has a cardiac arrest, but some people will not survive without the aid of a defibrillator.
These machines are fully automated. When you open the lid a recorded message will instruct you how to use it.
All you need to do is attach the pads and press a button. Sensors in the defibrillator will detect if a shock should be delivered, so people should feel confident about stepping in.
For 44 of these incidents (4.25%), in 34 different locations, the caller was able to access an external defibrillator. However, it was successfully retrieved and used in less than half the cases (18 cases) before the arrival of an ambulance.
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