Advance Directives on Withdrawing Food & Water

A reader had the following comment on Paul Wojda's question about advance directives about withdrawing food & water in PVS:

Under what circumstances would the burdens of ANH (artificial nutrition & hydration) justify its discontinuation for patients in PVS? [Obviously, if such an intervention were futile, it would not be morally obligatory to pursue (or morally blameworthy to discontinue).]  Given the nature of PVS, it wouldn't be right to say that the burdens come in the form of physical or emotional suffering.  The Explanatory document issued by the Vatican seemed to suggest that the expense of ANH is not usually prohibitively burdensome.  Indeed, the general "exceptions" to the norm of continued ANH sketched out by the document seem quite narrow:
“When stating that the administration of food and water is morally obligatory in principle, the Congregation for the Doctrine of the Faith does not exclude the possibility that, in very remote places or in situations of extreme poverty, the artificial provision of food and water may be physically impossible,”
“Nor is the possibility excluded that, due to emerging complications, a patient may be unable to assimilate food and liquids, so that their provision becomes altogether useless. Finally, the possibility is not absolutely excluded that, in some rare cases, artificial nourishment and hydration may be excessively burdensome for the patient or may cause significant physical discomfort, for example resulting from complications in the use of the means employed.”
“These exceptional cases, however, take nothing away from the general ethical criterion, according to which the provision of water and food, even by artificial means, always represents a natural means for preserving life, and is not a therapeutic treatment. Its use should therefore be considered ordinary and proportionate, even when the “vegetative state” is prolonged.”

So it is difficult (though not impossible) to imagine a case in which discontinuation of ANH would be morally sound.  It is doubly difficult to imagine how one could anticipate with certainty (and describe with legal clarity) these cases such that he or she could accordingly formulate his or her advance directive.

While the CDF document and explanatory text do not squarely address the question raised by Paul, they do strongly indicate that the circumstances in which a PVS patient would be burdened by ANH are exceedingly rare.