Question:
If a person is not feeling well on Yom Kippur, may he check his blood pressure? If the reading is abnormal, does this indicate that he should stop fasting?
Answer:
Yes.
If a person is feeling unwell on Yom Kippur, he may check his blood pressure. If he has access to a non-digital blood pressure machine, then it may certainly be used.
If he only has access to a digital blood pressure machine, then the ruling depends on his condition. If he is merely curious or mildly uncomfortable, he should not use it. However, if he feels weak throughout his entire body, feels genuinely ill, or is bedridden or close to being bedridden due to his condition, then he may check his blood pressure even with a digital device. Since pressing buttons on a digital monitor involves Rabbinic concerns, he should activate the machine with a shinui, such as pressing the buttons with the back of his finger, knuckle, or in some other unusual fashion. Alternatively, he should ask a gentile to turn it on. This follows the general rule that one who is bedridden or weakened in his entire body may perform Rabbinically prohibited acts with a shinui, or ask a gentile, when needed for his illness.
Regarding whether he should continue fasting, this is primarily a medical rather than a halachic determination. The blood pressure reading itself does not automatically dictate whether one must break his fast. Rather, the information should be evaluated by a competent physician who can determine whether the situation constitutes a danger or possible danger. The halachic ruling is then based on that medical assessment.
Nevertheless, as a matter of general medical guidance, a systolic blood pressure above 180 is commonly considered severe hypertension and may indicate a potentially dangerous situation. Likewise, blood pressure below approximately 90 systolic may indicate significant hypotension. If such readings are accompanied by symptoms severe enough that the individual already feels ill, weak throughout his body, dizzy, faint, or bedridden, this may certainly be a sign that medical assessment is needed and that continuing the fast could be unsafe. In such a case, a physician should be contacted immediately.
Can Fasting Affect Blood Pressure?
Yes. Fasting can affect blood pressure. Medical literature has shown that fasting may lower blood pressure in some individuals and may also contribute to dehydration, which can further impact blood pressure and circulation. Furthermore, both eating and drinking can influence blood pressure levels, and therefore an abnormal blood pressure reading on Yom Kippur may indeed be related to the fast. Accordingly, when an individual develops symptoms together with abnormal blood pressure readings during a fast, those findings should be taken seriously and evaluated medically.
Conclusion:
A person who is not feeling well on Yom Kippur may check his blood pressure. A non-digital monitor may be used freely, while a digital monitor may be used with a shinui if the person is weakened in his entire body or bedridden. Whether to continue fasting depends on medical assessment and not on the blood pressure reading alone. Nevertheless, markedly high or low readings together with significant symptoms may indicate a potentially dangerous situation and require immediate medical guidance.
Sources:
Admur 328:19 regarding Rabbinic prohibitions performed with a shinui for one who is bedridden or weakened throughout his body. All the following Poskim regarding measuring for illness: Admur 306:18; Michaber 306:7; Tur 306; Ravayah 202; Maharam Merothenberg 512; Hagahos Maimanis Shabbos 24:4; Elya Raba 306:21; SSH”K 40:2; Or Letziyon 2:36 footnote 13; Yabia Omer 9 O.C. 108 156; Yalkut Yosef Shabbos 5 p. 405See Kaf Hachaim 306:65 See regarding Sakana of high blood pressure: Piskeiy Teshuvos 328:8; 21, 26 footnote 179;