A power of attorney for health care and a living will are two separate legal documents, although they operate together and usually are referred to as advance directives. Both documents are governed by state law. Although the main provisions are largely the same, the technical requirements for these documents may vary. Health care personnel are required to treat your advance directives as valid, unless there is substantial evidence to doubt their authenticity. If the legal requirements for an advance directive are not met, the document could be challenged in court and voided.
The person who has drawn up the medical POA and living will has the power to supersede the documents. For example, Texas law gives the patient the authority to override or revoke the medical POA and living will at any time. In California, you can orally designate someone to be your agent to make health care decisions for you by informing the supervising health care provider who is treating you. The oral designation will supersede any previously written directive. Similarly, if there are two advance directives, the most recent one trumps the earlier one.
Health Care Provider
State laws usually give health care professionals a way to supersede a medical POA when the instructions from a patient's agent clash with their conscience or with the policy of a hospital or other medical facility. In California, a doctor may refuse to comply if he believes the agent is requesting care that isn't medically effective or in compliance with generally accepted medical standards. In such cases, the patient and agent must be immediately informed and all reasonable efforts must be made to place the patient in another facility that will honor the advance directive.
Some states go farther than others in carving out exceptions that supersede a medical POA. In Wisconsin, your agent can ask a doctor to remove a feeding tube if so specified in the advance directive, but the doctor can override the advance directive if he believes the removal will be painful or increase the patient's discomfort. In addition, your agent can't ask that orally-ingested food and water be withheld unless it is medically contraindicated.