> If their efforts are stalled by a lack of oxygen, neurons are rapidly inundated with ions which build to toxic levels, causing irreversible damage.

I mean, we may get into a discussion when does the brain cell die, like what if it still is a living cell, but can't act as a neuron anymore.

But it can be sufficient to keep the brain from functioning other than at an exceptionally basic level. E.g., no voluntary movement, speech, understanding, consciousness.