Mo. Rev. Stat. § 459.040

A physician, licensed health care professional, medical care facility or employee thereof or other person who, in good faith and pursuant to usual and customary medical standards, causes or participates in the withholding or withdrawal of death-prolonging procedures, which acts are not otherwise unlawful, from a patient pursuant to a declaration made in accordance with sections 459.010 to 459.055 shall not, as a result thereof, be subject to criminal or civil liability or be found to have committed an act of unprofessional conduct.