There are several types of Coma that a person could be in:

One type would be that she is in so much pain. A person could also be in a coma from a blow to the head.

So, I'm thinking either the pain and injuries are so great for Anne Heche or she is in what is called "an induced Coma" which my father in law was put in for 2 to 3 weeks time. At the time he was put into an induced coma it was in the early 2000s but the technology and research regarding induced comas has come a long way since then. 

At that time they gave you before they woke you up a drug to take away all your memories from dreams and hallucinations while you were unconscious for 2 or 3 weeks in an induced coma so the person can remain sane and who they were before the induced coma. So, this could be what they have done to Anne Heche too.

Any way you look at it this is quite serious what she is going through and it is definitely debatable whether she will survive all this or not at this point. Because Burns are one of the hardest things to survive because of potential infection.

The largest single organ in a human body is your skin which is your main barrier to infection. So, one of the hardest things to do is to treat infections before they become too serious to treat when someone is burned very bad like she appears to be. Also, the pain is sort of like being on fire until you get to a different stage of healing than this early in the healing process.