Notarizing any wishes against some medical procedures in case a sudden accident ruins your ability to dissent prevents doctors from being forced to keep your body alive as long as possible.
That doesn't apply to Alzheimer's disease directly though. If you don't want to live when your conscious life is limited to short flashes of awareness among a deeply terrifying melange of visions of the past and hallucinations, DNR laws don't in any way force or even allow doctors to euthanize you. You can persist in this state for many years without ever triggering a DNR check.
