While current spouses are the legal next of kin and almost always have power of attorney, this doesn't mean a family member won't contest their decisions. But having a formal health care power of attorney document makes their wishes a firmer extension of your own. Your doctor or hospital can give you forms specific to your state. State laws vary, so make sure that the form you're using complies with your state's requirements. Note that some states require that a witness also sign the document, so flubbing that will make it invalid.
Find out more about this book:YOU: The Smart Patient: An Insider's Handbook for Getting the Best Treatment