Tonight (after the last exam of the first block) I had lots of time, so I looked up advanced directives. Three seconds of research tells me this: "Many Catholic bishops and moralists consider this [living will without healthcare proxy] an unsatisfactory approach, as it does not provide for unforeseen circumstances" (NCBC, ETWN; this page goes into good detail about artificial nutrition and hydration and "extraordinary means").
For reference, here is is a purely Catholic and legally sound healthcare proxy form (designating decision-making authority to another person in the case that you are incapacitated and expressing desires in those cases). It explains very carefully that
the statutory definitions are not always consistent with Catholic moral teaching. For example, the definition of “life-sustaining treatment” under Texas law conflicts with Catholic teaching because the Church considers food and water, even “artificial nutrition and hydration,” as ordinary care, not a “life-sustaining treatment.”It makes general provisions in accord with Catholic teaching before offering the typical curt "I do wish" or "I do not wish" option.
Please note: these differ according to state. Search for "Catholic advance directive" and your state.