My friend was in a coma for 6 weeks and had brain damage because of the lack of oxygen.
On one hand, this was distressing because she lost mathematical ability. This prevented her working in her previous field as a chemistry researcher.
On the other hand, she switched to psychology via a one year Masters degree and is now working on a fascinating project on hacking the mind's compassion responses to activate the parasympathetic nervous system and damp down anxiety (sympathetic nervous system) triggered by cardiac damage.
Just saying that although brain damage sounds terrifying, people can and do cope with it.
I had a teacher in grade school and one day she didn't come back to class, she had had a gigantic stroke. 6 months later she rolled into class in a wheelchair, a changed person, she could not speak or do basic tasks for herself.
Over the next two years she relearned most of her abilities, walking, talking, thinking, speaking, teaching and was back to "normal" again. Then she had another stroke.
She did not give up, and did it all over again.
She was always a cruel teacher in my eyes (I was very young) but seeing her persist throughout what I would assume would permanently put you out to pasture was an amazing experience and I always had enormous respect for her.
She continued teaching and retired ~20 years later.
My only point regarding
> The average rule is that if heart has not restarted after 6 minutes, brain damage is irreversible
is that it doesn't mean it's worthless to continue CPR. Different people have different preferences about how grievous an injury they would want to survive. Whether to continue comes down to that preference. Have an advanced directive, and make sure your loved ones know what you want to have happen if you are every in an emergency!