Fasting: Becoming Unconscious After Making the Intention to Fast
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1537
If someone who makes the intention before the time of ṣubḥ prayers to fast the next day but unwillingly becomes unconscious, and during the day he regains consciousness, then based on obligatory precaution, he must complete the fast of that day; and if he does not complete it, he must keep a qaḍāʾ fast for it. If he becomes unconscious willingly (for example, in order to have an operation, he consents to a doctor making him unconscious), the obligatory precaution is that he must complete the fast of that day and keep a qaḍāʾ fast for it as well.