There's a poll going round the Fedi at the moment about whether you would be able to use an AED/defibrillator on a woman having a heart attack and a lot of people are commenting they'd let the trained person who brought it use it. In the UK at least you don't need to wait for ambulance staff to bring it, they're in lots of public locations like stations, shopping centres and churches. In an emergency if you're on the phone to the ambulance service they will tell you where the nearest one is, but if you want to familiarise yourself with the locations of nearby ones just to be prepared you can see them here: https://www.defibfinder.uk/