I believe that this is an intensely personal decision which belongs to the person, his/her spouse, doctor and spiritual advisor. Everyone else should stay out of the decision, especially the government and "do-gooders". Your spiritual advisor (if they are doing their job) can help you understand what your traditions and beliefs have to say.
You should make sure that your advance medical directives/living will is very clear and direct about your wishes. You also need to have a full and frank discussion with the folks who are listed on your medical power of attorney and (well before any decisions have to be made) with your doctor. Ask each one directly if they understand what what you want. If they don't agree, take them off of your list. Don't blame them - no matter what your wishes are, they are a terrible burden on them.
I have been having to make all of my parents' medical decisions for the past year+. It helps (but doesn't take away the tears) that the three of discussed what they wanted before aliens stole their brains. It also helps that my college room mate and I can still call each other and have a good cry when we need it.
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