I'm not sure. The problem is that we execute documents without making our wishes known to those around us. Sign the paper, but let your kids know, let your pastor or Rabbi know your wishes, in writing. Involve them. In the rare instances this comes up, you can't speak for yourself.
The problem is that 80% or more of persons who execute "advance directives" revoke them when they are sick (i.e., they change their minds). If people under 40 all made advance directives, this number would be 95%+.
Therefore, respecting someone's "wishes" without regard to the clinical scenario is incompetent at best, if not negligent.
It is not obvious that brain-injured persons (who cannot "change their mind") should be held to the letter of their "prior wishes", since those prior wishes in expressive patients have almost no substance.