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To determine âcessation of all functions of the entire brain, including the brain stem,â physicians must determine the presence of unresponsive coma, the absence of brainstem reflexes, and the absence of respiratory drive after a CO 2 challenge.
Rating Newest Oldest. Best Answer: I am a Respiratory Therapist and the answer is 3 minutes. Brain damage is irreversible no matter how long you are without oxygen. Once brain damage begins you can never regain those cells that have died.