I had a training session with a paramedic, who showed me how to assess someone suspected of having had a heart attack, and how to use a defibrillator. It was a sort of public awareness day, not booked, they were in the local square and I was walking past. He was amazing. Answered my every question, most of which started with, "yeah but what if?" The bottom line is they would rather you attempt, than do nothing. The most important thing to remember is: call 999, because no matter what you do, if you haven't called for help, no one's coming.
If I ever needed that skill to save a life, all credit would be given to that paramedic who gave me the most fantastic instruction in a very short space of time.