Get over here.
Normally one is held to use only ordinary means -- according to the circumstances of persons, places, times and culture -- that is to say, means that do not involve any grave burden for oneself or another. A stricter obligation would be too burdensome for most people and would render the attainment of the higher, more important good too difficult. Life, health, all temporal activities are in fact subordinated to spiritual ends.
Pius XII clearly notes here that ordinary means change "according to the circumstances of persons, places, times and culture". I think sinkspur's critique is penetrating: time hasn't stopped in 1958. As for the supposed "grave burden", in Terri Schiavo's case the "burden" was simply that she'd be alive. There's no reasonable way to construe that as a burden. Banez says, on whether a man has an obligation to be amputated to save himself, "Although a man is held to conserve his own life, he is not bound to extraordinary means but to common food and clothing, to common medicines, to a certain common and ordinary pain: not, however, to a certain extraordinary and horrible pain, nor to expenses which are extraordinary in proportion to the status of this man". Tube feeding is now common. Period.
Thus John Paul II:
I should like particularly to underline how the administration of water and food, even when provided by artificial means, always represents a natural means of preserving life, not a medical act. Its use, furthermore, should be considered, in principle, ordinary and proportionate, and as such morally obligatory, insofar as and until it is seen to have attained its proper finality, which in the present case consists in providing nourishment to the patient and alleviation of his suffering.