When there is no blood actively being pumped to the brain, the nerve
cells in the cerebral cortex
die in two minutes. The nerve cells in the midbrain
that control unconscious activity such as breathing can last thirty minutes and the nerve cells in the spinal cord
can last an hour without blood flow. The brain has no way of storing oxygen
. Nerve cells require oxygen and glucose
to function. If the heart were not pumping, the oxygen in the stagnant blood would be used up by the cerebral cortex in two minutes. In order to be aware of something the cerebral cortex has to be capable of receiving input. For sound to be interpreted by the auditory cortex there would have to be fresh blood supply. Conceivably at the level of the midbrain there might be some emotional recognition. When people are in a coma, depending how deep, they do react to painful stimuli (that's how they test how deep the coma is) and the patient can sometimes shout out emotional language in response. The terminology "brain dead" is used for people who are in a vegetative state (resuscitated after the oxygen supply was cut off e.g. by drowning). I know this is a very clinical response to a very painful question. You and your family have my sympathy.