A person with a heart condition should consult with his doctor to determine whether he may fast on Yom Kippur. If a person is weak from a heart attack, he should eat, b’shiurim, i.e., a small amount of food (i.e., about a kezayit) in under nine minutes.
Hacham Ben Zion Abba Shaul was once told, after he suffered a heart attack, that he cannot fast. He was very distraught by this. In turn, he consulted with another doctor who permitted him to fast, as long as he doesn’t stand during the day. The halacha tells us that if two doctors argue, one may follow the doctor permits him to fast, as long as the sick person feels that he can fast. In this case, he followed the opinion of the second doctor.
Similarly, regarding diabetes, if a person’s sugar level rises, he is told to drink b’shiurim, less than a m’lo lugmav (a cheek full) every nine minutes. Sometimes, even if his sugar is low, he will be affected. Therefore, a person with diabetes must consult with his doctor before Yom Kippur regarding when he must eat or drink.
One who suffers from a kidney condition must drink a lot of water, even more than the shiurim. Although some doctors are lenient, there may still be long-term damage to the kidneys, and therefore a number of Poskim write that those with a kidney condition should drink regularly on Yom Kippur.
Finally, regarding one who recently had an operation: Within seven days of the operation, if the person says he must eat, he may eat regularly, similar to a woman who has recently given birth. If he underwent a major operation, some claim that for the entire thirty days after the operation he may eat b’shiurim, in small amounts.
Summary: Although we offered rough guidelines for those with heart and kidney conditions, diabetes, and those who recently underwent an operation, each person should consult with his doctor to receive specific guidelines for how to behave on Yom Kippur.