Maryland Code, Health-General § 5-604

(a)(1) Except as provided in paragraph (2) of this subsection, an advance directive may be revoked at any time by a declarant by a signed and dated written or electronic document, by physical cancellation or destruction, by an oral statement to a health care practitioner or by the execution of a subsequent directive.

(2) A declarant, knowingly and voluntarily, may elect in an advance directive to waive the right under paragraph (1) of this subsection to revoke any part or all of the advance directive, including the appointment of an agent, during a period in which the declarant has been certified incapable of making an informed decision under § 5-602(e) of this subtitle.