Generally, you will be required, as far as possible, to make the same medical decisions that the patient would have made. To do this you might need to examine any specific statements that the patient made (either orally or in writing, such as in a living will), as well as consider the patient's beliefs and values.
If you have no information about what the patient would want, you must act in what you believe would be in the patient's best interest, using your own judgment. To arrive at that decision, you might ask the patient's doctors what kind of benefits and burdens might result from the treatment; you can draw on knowledge that others have about the patient and on their opinions; or you can ask others what they would want if they were in such a situation. However, the more you and the patient have talked, the less likely you will be in the dark about what the patient would want.
Normally the agent becomes the decision maker after the attending physician determines that the patient either temporarily or permanently lacks the ability to make health care decisions. Many states also require that a second physician confirm the patient's incapacity if the decision involves withholding or withdrawing life support treatments.
What do I need to know to make decisions?
You need to gather as much information as possible about the patient's condition. What is wrong with the patient (the diagnosis)? What is likely to happen to the patient because of the disease or medical condition (the prognosis)? If the doctors are unsure of the diagnosis or prognosis, you need to know when they will know more and what they are doing to get more information.
Sometimes the process of obtaining information involves invasive and uncomfortable testing, and you will need to decide if the process should go forward. For example, you may know that the patient would not have wanted invasive testing, or you may decide that the burdens of testing and or treatment outweigh any likely benefits.