I don't know. I'm a medical student in Tunisia. Here, in order to claim that a patient is dead you need to show its' EEG graph. Well, what's happening is something else.
An EEG is probably necessary for official reasons, before a "certificate of death" can be issued.
But there is a point where the emergency rescuers have to give up, and it probably takes hours before the EEG can be done.
Maybe when the person is already on life support, and you're trying to make a decision about whether the person is "dead" even if most of their organs are alive.