You can set the course of your future health care with a Durable Power of Attorney for Health Care and a Living Will Declaration. The latter is more commonly known as a Living Will; in it you specify the conditions under which you approve or disapprove a range of medical procedures for yourself. Through the former, you appoint an individual, again known as an Attorney-In-Fact, to make all the other decisions for your health care.
Your physician is excluded from your possible choices for this job, as are most people professionally involved in your health care, unless they are related to you. Nebraska has a stipulation that anyone who is already the Attorney-In-Fact for ten or more people cannot serve as yours unless he or she is related to you. Whoever you choose must share your values in regards to health care, and be prepared to forcefully advocate your position in the face of other who may disagree.