CathInfo.com

But are feeding tubes and the like, especially where they do not require any incision (i.e., nasopharyngeal) or other unnatural intervention, ordinary (albeit alternative) means that we are obliged to avail ourselves of, as opposed to extraordinary means that do not thus oblige?

YES.
You can't "pull the plug" on someone who just needs hydration and/or nutrition. That means they will dehydrate and starve to death. That's what happened to Terry Schiavo! No Catholic in good standing can approve of what was done to her. That is a pro-life issue. Not a matter of opinion, not optional. There is no "moral dilemma" there; the case is quite clear-cut.

You must provide hydration, nutrition, oxygen, hygiene, pain relief. Not required are extreme means;  examples, nutrition when the body cannot utilize it, oxygen via intubation, invasive surgeries, experimental or very costly or painful treatments against the patient’s wishes.  It is required if the patient is able to express his wishes for prolonging life by such means.  I think it can differ from person to person.  If a 96 year old has had a stroke, is in a coma, has pneumonia, can not utilize tube or IV nutrition, is on a respirator, and then his heart stops, it’s cruel to do chest compressions or open chest heart massage to bring him back.  To what?  
If however, a young, healthy person is seriously injured in an accident and there is a reasonable chance of recovery, even if partially, then it’s required to use such measures as chest compressions, intubation, respirator, medically induced coma in order to give life the best chance.  
There are cases of people who have awoken from a decade or more of semi-comatose state.  They have said that they were aware during that time, but unable to respond.  People who were supposedly brain dead have revived on their own after several hours.  That’s why donation of essential organs is 
immoral.  The person is alive.  So-called “live” donations, if done willingly, are moral.  One can donate a kidney, a portion of a liver, a bone marrow transplant, plasma, skin…You can live with one kidney, livers, bone marrow, blood, and skin all regenerate.

If however, a young, healthy person is seriously injured in an accident and there is a reasonable chance of recovery, even if partially, then it’s required to use such measures as chest compressions, intubation, respirator, medically induced coma in order to give life the best chance. 

Required ? No it is not. It is desirable but not, strictly speaking, required.

 So-called “live” donations, if done willingly, are moral.  One can donate a kidney, a portion of a liver, a bone marrow transplant, plasma, skin…You can live with one kidney, livers, bone marrow, blood, and skin all regenerate.

Permissible, but dangerous for the donor. See Dr. Byrne on the subject.