If a person is declared incompetent by a psychiatrist can his power of attorney sign for medical procedures?
I know it sounds unusual, but a psychiatrist does not have the authority under law to "declare a person incompetent." Only a judge can make a finding that a person is "incapacitated." A psychiatrist is a doctor, and is qualified to make a medical diagnosis such as bipolar disorder, or schizophrenia or depression, etc. etc. etc. Whether or not a person diagnosed by a doctor as having one or more mental impairments can manage their own medical (or financial) affairs is a legal determination which can only be made by a judge after a hearing (similar to a trial) in a courtroom.
Hope that helps.