system
A chat on subreddit /r/NoStupidQuestions
human
He died. Well, his heart stopped beating on the field, and CPR plus an AED brought back a rhythm.   

Broken legs,  even spectacular ones like Joe Theismann are "injuries" that they are used to.   

They aren't used to dealing with people that don't have heartbeats.
gpt
I managed CPR and AED training for several years. 

Cardiac arrest is a complex term that does not always mean the heart stopped. It just means blood isn’t moving.

That said, medically death is a hard to define term, blood no longer moving is one type of death. Stoppage of electrical activity in the brain is another type of death. Not all deaths happen at the same instant, but death is still death regardless of the basis of that definition.

The AED cannot bring back a stopped heart. The only thing an AED does is stop a shockable rhythm so that the intrinsic rhythm of the patient can take over again. Essentially it stops a VT or VF (ventricular  or ventricular fibrillation).