This is not true. It is necessary to have a second legal document for decisions about medical treatment. First you choose someone that you trust who knows how you feel about these issues. This person is called an agent. The agent may be anyone over the age of 18, except for the physician who takes care of you.
Most people select a family member or close friend.
The choice is up to you. You should talk to the agent that you have chosen and discuss how you feel about medical decisions, such as being kept alive by machines when you cannot make those decisions. Once you have talked about all of these things and you know that your agent understands your wishes, you can complete and sign the legal document. When this is done, notify your doctor, the hospital and your family of your decision. A copy of this document should be available each time you are hospitalized.