In an earlier edition of Daily Halacha, we noted that if a competent and reliable doctor instructed a patient that he must eat on Yom Kippur, he should eat in intervals of at least ten minutes, consuming no more than 30 grams at a time. This way, he receives the nutrition he needs while avoiding the severe prohibition that would render one liable to Karet.
A similar Halacha applies in the case of a patient who is instructed by a competent and reliable physician to drink on Yom Kippur. Pregnant women, for example, often need to keep themselves hydrated and are unable to go an entire day without drinking. Such patients must likewise drink in small installments. The quantity of liquid that one must drink on Yom Kippur in order to be liable to Karet is "Melo Lugmav," or approximately 1.5 oz. Accordingly, several Halachic authorities, including Hacham Ovadia Yosef and Hacham Bension Abba Shaul, rule that a patient who needs to drink on Yom Kippur should sip 1.4 oz. and then wait ten minutes before drinking another 1.4 oz. (Strictly speaking, the patient may drink in intervals of nine minutes, but as a safeguard the Poskim advise waiting ten minutes.)
However, if the patient needs to drink more frequently, then he may drink in smaller intervals. The Shulhan Aruch ruled that one is liable to Karet only if he drinks a "Melo Lugmav" within the period of time it normally takes to drink a Rebi’it, or 3.2 ounces. This, of course, is a very short period of time, no more than ten seconds. Therefore, if the patient is unable to wait ten minutes after drinking 1.4 oz., he may wait just ten seconds after every ounce. Although it is preferable to wait ten minutes, patients who are medically unable to do so may and should rely on the Shulhan Aruch’s ruling and drink in ten-second intervals.
An interesting question arises in the case of an ill patient who needs to eat or drink on Yom Kippur but has the option of receiving his needed nutrients via intravenous. Feeding via intravenous does not violate the prohibition against eating on Yom Kippur, and thus, seemingly, this option should be utilized whenever possible to enable patients to observe the Yom Kippur fast without endangering their health. However, Rav Shlomo Zalman Auerbach (Israel, 1910-1995) ruled that to the contrary, an ill patient who needs to eat on Yom Kippur should eat normally, and not be fed via intravenous. Rav Shlomo Zalman noted that if we require a patient to pursue the option of intravenous, he or other people may mistakenly conclude that eating is forbidden on Yom Kippur even for ill patients, except via intravenous. As a result, in situations where intravenous is not an option, they will refuse to eat and thereby endanger their health. It is vitally important for people to realize that eating is permissible on Yom Kippur when it is medically necessary, as determined by a competent and reliable healthcare professional, and not to give the impression that even seriously ill patients must fast. Therefore, Rav Shlomo Zalman ruled, ill patients should not be given intravenous instead of food on Yom Kippur. This ruling is cited approvingly by Hacham Ovadia Yosef. As such, one should not try to "outsmart" Halacha by using intravenous instead of food on Yom Kippur; in situations where Halacha allows eating on Yom Kippur, the patient should eat, and not utilize an intravenous.
(Based on Yalkut Yosef – Yamim Noraim, p. 371)
Summary: One who is medically required to drink on Yom Kippur should preferably drink no more than 1.4 oz. in ten-minute intervals. If this does not suffice, the patient may drink this amount in intervals of ten seconds or so. One who is medically required to eat or drink on Yom Kippur should do so even if the option of intravenous is available.