Last Saturday night I participated in a phone conversation that dealt with directions about various medical treatment choices that may or may not be needed for someone who is not capable of making those choices themselves.
I found this article by Kathryn Butler at Desiring God covered a lot of the considerations that were being put before us, and approaches how those options can be thought of from a biblical perspective.
It is more positive to have given some thought to this beforehand, and to have even discussed them with those for whom you may end up making choices (or who may be making them on your behalf).
Butler is/was a trauma and critical care surgeon, her reflections do not come from a place of pure theory.
Well worth a read:

Making life-or-death decisions for loved ones cripples many with feelings of guilt and doubt that persist for years, and which can progress to depression, complicated grief, chronic anxiety, and even post-traumatic stress disorder.
So how do we make compassionate, Christ-honoring decisions about our loved ones’ care when the unthinkable happens? How do we discern the right path when time to reflect is nonexistent, and when the mind balks at the ramifications of our choices?

Read the whole post at Desiring God.