Just to recap, passive euthanasia is removing any hindrances that prolong death. This would include, but not be limited to, withholding any life-extending procedure. The Shulchan Aruch (Yoreh Deah 339:1) has something to say on the matter, and it's not quite what I would have thought it would have been:
"If there is anything which causes a hindrance to the departure of the soul, such as the presence near the patient's house of a knocking noise, such as wood chopping, or if there is salt on the patient's tongue, and these things hinder the soul's departure, it is permissible to remove them because no act is involve, only the removal of the impediment."
Rabbi Moshe Feinstein, z"l, gave some provisions with regards to the permissibility (Igrot Moshe CM 2:74): "If physicians have no way to cure a dying patient or ease his burden, but they do have the ability to prolong the dying process, they should not intervene."
What this exactly entails has been of debate. Does this mean witholding pain medication? Can one pull the feeding tube? It is even more difficult to ascertain these answers from a Jewish perspective. As R. Moshe Tendler, an Orthodox rabbi, put it:
"Inherent in the Jewish point of view is an expression of confidence that decisions on these great issues do not reside in heaven. We don’t accept an infallible, irrevocable voice from heaven to clarify even these most complex issues. This of course is a disadvantage that we have compared with other religions. Using the biblical ethical principles...may help us all, while we study the problems with integrity and devotion, to surely reach the proper conclusion, one that is pleasing to us and to G-d". (The Mount Sinai Journal of Medicine, 51:54-57, 1984)
These are complicated issues and what we define as passive euthanasia and under what circumstances we would perform it are in need of continuous debate. Dealing with death is as inevitable as it is painful, and at least in this specific topic, there are no clear answers. Whatever ethicals conclusions one comes to, as Tendler states, let they be made in accordance with Jewish values.