What happens in BC? Once you are married or are a 'common law partner' - that is, once you have lived together for at least two years - the law says you can make medical decisions for your partner. But if you have not lived together that long, or if you want your spouse or partner to be able to make the crucial end of life decisions, you have to have a Representation Agreement (which is what a BC medical power of attorney is called).
I have myself encountered this issue in a Vancouver hospital which did not want to list my partner as my (decision-making) 'next of kin'. If that ever happens to you, and you are married or common law, then stand your ground, ask to speak to a supervisor, and tell them they have to recognize you as the decision-maker or they are committing a violation of the Human Rights Code.