Just several years ago I was shocked to learn that you do CPR if someone doesn't have a pulse, not a defibrillator.

It's a very, very common misconception.

Since then those portable defibrillator units have shown up in public places, which has led me to morbidly wonder two things:

Firstly, how often are they used incorrectly?

And secondly, how do you know when you're supposed to use it? I suspect the answer to this one is "the EMT on the phone will tell you to" but... IDK it would seem unlikely that most people could do something like that in an emergency.