My nephew is a casualty of the fentanyl epidemic and experienced something similar. He OD’d and had no pulse when first responders arrived. They worked on him for 30 minutes and he recovered a weak pulse on the way to the ER. It was enough to keep him alive for several more days but ultimately he had to be removed from life support.
Not sure about the physiology of it but something about that period of time without oxygen often leads to swelling at the brain stem and a likely permanent vegetative state. So the family chose to pull his plug. Thank goodness, at the end, most of his organs were salvageable. He is gone but three other lives were saved or improved. (Silver linings and all that...) Not sure if the same is true for those with CO poisoning.
MOO.