Great query. I am going to give you a really poor answer, frankly because there aren’t any good ones.
The entire way that we in the intensive care unit (ICU) classify a sick person’s condition is almost totally subjective. So, someone who has been really ill and is now better and ready to leave the ICU for the floor is”Stable, Improved”. Easy.
Someone who is still really sick but not deteriorating could be called “Stable but critical”. Acceptable.
Someone who requires minute to minute management to prevent loss of life or limb could / would be called “Critical, Unstable”. Acceptable.
To my mind these are all pretty useless. What really matters is that we clinicians leave the ICU, go to the waiting room, and talk to the family of the sick person. Telling them in language they can understand – instead of DoctorSpeak – how the person they love is doing.
My experience over some 30 years doing this is that the families will ask the same questions over and over, until they understand. Part of our job is to answer them as often as they need to hear.
This seems at least as important as the very subjective classification system we use.
As to what precisely “Critical but Grave” means…….I / we wouldn’t use that classification term, but to me it means the person is in a world of hurt, and may die.
I hope this helps a bit.