12. a. If the patient has executed an instruction directive but has not designated a health care representative, or if neither the designated health care representative or any alternate designee is able or available to serve, the instruction directive shall be legally operative. If the instruction directive provides clear and unambiguous guidance under the circumstances, it shall be honored in accordance with its specific terms by a legally appointed guardian, if any, family members, the physicians, nurses, other health care professionals, health care institutions, and others acting on the patient's behalf.
b. If the instruction directive is, in the exercise of reasonable judgment, not specific to the patient's medical condition and the treatment alternatives, the attending physician, in consultation with a legally appointed guardian, if any, family members, or others acting on the patient's behalf, shall exercise reasonable judgment to effectuate the wishes of the patient, giving full weight to the terms, intent, and spirit of the instruction directive. Departure from the specific terms and provisions of the instruction directive shall be based upon clearly articulable factors not foreseen or contemplated by the instruction directive, including, but not limited to, the circumstances of the patient's medical condition.
c. Nothing in this act shall be construed to impair the legal force and effect of an instruction directive executed prior to the effective date of this act.