Sometimes, sure. However a person can chose not to eat, can refuse meds or needed treatment if they are free, sane and communicative. If they are a prisoner, or insane, or incompetent, or child, or not communicative hey should be force fed. And the choice must be made at the current time -- not by some prior contract.
And a person can volunteer for a especially dangerous service -- Although not without dread warnings against doing so.
So if I'm put into a coma tomorrow, the Living Will/DNR request I signed yesterday should be invalidated?