Hi,
I am not sure how high the liver enzymes have to be to be considered critical, but I can tell you about my own experience with cat liver disease. My cat had fatty liver syndrome, a very serious and usually fatal liver disease in cats. She was critical and beyond. However, the liver is the one organ that can regenerate itself if given time to heal. She had to be fed a special diet (and tube fed at the vet and a home with syringe for a bit), but she recovered. She had liver disease when she was about 7-8 years old, she passed away at 14 years old. Once she recovered there was no sign that anything had been wrong - she was back to her usual self. Liver disease is very serious and can be time consuming during the illness, but recovery is possible!