If you can get an AED to someone who is having a Sudden Cardiac Arrest within the first 3 minutes they have around 70% survival rate and for every minute that one is not available it is thought the survival rate drops by another 10%.
Doing CPR will bide that person some valuable time whilst an AED if one is available to arrive at the scene. So you can see why having an AED makes a big difference to someone who requires it to get the heart back into normal rhythm.
In the UK AEDs (defibrillators) are becoming more commonplace in public areas, these are kept in slightly heated cabinets because if they are in an exposed position to the elements especially during the winter months. This will protect the lithium battery from degrading so quickly because of the cold, cabinets are kept around 5ºC.
These are fitted into lockable cabinets to prevent theft and tampering, public AEDs are on a database and ambulance control will tell the person where the nearest one is and also give the keylock code so it can be retrieved.