I was just reading an educational blog by 
Years ago I looked after a lady on a High Dependency Unit who had survived a resuscitation. She was an older lady who had developed chest pain one morning and thought she would go to the GP about it, but first she took the dog for a walk (you never know how long the trips out are going to take).
By the time she got to the GP, her pain was a lot worse and they took her in for an ECG and called an ambulance - but she collapsed and went into full cardiorespiratory arrest. She was lucky though that they had an AED and the staff were comfortable to put it on and follow the prompts - it took her out of Ventricular Tachycardia and basically saved her life.
Each country has it's own Resuscitation Guidelines - for the teamaustralia members I will direct you to the ANZCOR resuscitation guidelines (Jan,2016) that is my reference. AEDs are now available in many general public areas - shops, airports, casinos and general facilities and venues. That is the same right around the world. It is important to ensure that you make a note where these facilities are, that there is not just one but a number of staff around who are trained to use it and that the machine is maintained.
If a person collapses and there is no one trained to use the machine - don't hold off. Phone the emergency number in your country (Australia is 000 and America 911) and the operator will guide you to get the machine started. Once it's on, it has its own prompts to guide you. Every minute it takes till someone is brought out of a ventricular arrhythmia reduces their chances of a safe and successful resuscitation. The effort you put in now will be worth it in the future...
Reference :
ANZCOR Guideline 7 (January 2016) Automated External Defibrillation in Basic Life Support