Measures to Enhance Life. These keep you comfortable without prolonging life. Examples are pain medications and hospice care. Generally, it is a person who knows you and your values well and is in a good position to represent your wishes to your health care provider. Your condition does not have to be terminal or irreversible to have someone speak on your behalf.
Most states have their own laws on advance directives.
Talk to your health provider, too. Give him or her a copy to put with your medical records.