My memory is a bit hazy, but I thought what you are describing is very common with people who flatline and come back? I have vague memories that a new anesthetic drug was developed and used on soldiers undergoing surgery in the Vietnam war, and there was something about it that caused the same kind of reaction in those who were put under. Again, my memory is very hazy on the subject. I should go do some research and update this comment (and I just might).

EDIT I did a little searching. I think it might have been an old report about Ketamine before it became more wide known. Apparently it was used during the Vietnam War.

https://en.wikipedia.org/wiki/Ketamine#Near-death_experience