If you are appointing someone for taking decision in case you have become unconscious, mentally sick, by executing a bond favoring authorization to the agent, it is called medical power of attorney. It is also known as health care POA. Many states law permit you to add your wish in case you would like to receive life sustaining treatment if you become unconscious or brain dead in the document. The agent appointed by you will enable your wishes in case you are seriously ill and not in a position to take your own decision. You are entitled to make changes, additions in the document whenever you like. Even if you have documented a power, still you can make some changes by discussing the issue with your health care agent.
This is not the same as Living Will where you can appoint some agent to take decisions on your behalf and by writing a Living will you can express regarding the issue of life sustaining treatment during the terminal stages of life.
The document of medical power of attorney is also called Advance health care Directives' since you mention the clear instructions as what should be done in case you become mentally ill. Both the document of Living Will and medial POA helps in fulfilling your wishes in case when you are unable to make out any decision in the future. Some states have actually combined the factors of both the documents for the benefit of the bond executor.
The deed is the most needed document in case of medical emergency in future. In case you have not made any medical POA then your family may be forced to go to court for appointing a guardian' for monitoring your medical care. The person appointed by the court may not be the preferred man by you. It is better to prepare the document well in advance to avoid this uncomfortable situation. Your family will not have to seek the court order at the time of most difficult period. Planning well in advance can help your family to tackle the situation properly.
A medical power of attorney becomes effective only you become unconscious or unable to make decision and hence you can still make any changes as long as you are stable.