Informed consent is the biggest part of this. Informed consent means that the person providing instructions to a medical practitioner understands as many of the scenarios, risks and problems as can be anticipated at the time. If I'm in a situation where I'm coming to a doctor and making an advance request, I understand that I might be in a position where I don't have the capacity, physically or mentally, to change my mind or convey a different message at the time.
Isn't that informed consent? I'm giving you consent, being fully informed of the risks that are in place at the time.