How sad for you and your family.
I have no personal experience with your situation but I do have a DD with a chronic illness. As you mention your DD's age is a difficult one and it can be a very fine line between saying something and nothing. Coming to terms with your illness is no doubt a process you are working through yourself and something your daughter will also be experiencing. It sounds as if the diagnosis is fairly recent and with a diagnosis generally comes a prognosis which can be both a relief and a shock because of it's finality. You have reached a point where you have obvious signs, your heart monitor and oxygen, and I may be wrong but deep down she will know you are seriously ill. At 12 or any other age for that matter I think denial is a very normal part of that process. At this point there are probably no questions because she is scared of the answers but they will come in time or it may be that the questions wont come but you will do the talking and she will do the listening.
I would not be surprised with your DD being a giftie that she isn't already researching your condition herself. Do you think this is possible?
eta: are you familiar with Elisabeth Kubler Ross? The stages of grieving are also applicable to those dealing with chronic illness and their loved ones.
http://www.ekrfoundation.org/about-grief
