never mind---I got the answer. I guess hyperventilation can actually induce ischemia to the brain (so it wouldn't prevent hypoxia to the brain as the one choice said)
you are correct that hyperventilating the patient will reduce the PCO2 (the pt is breathing off CO2 more quickly and therefore it is decreased). Blood vessels of the brain are extremely responseive to the level of CO2, and when levels are low, they will constrict as a metabolic response to the decreased levels of CO2 needed to be eliminated (b/c you're already getting rid of it by hyperventilation). When vasoconstriction occurs, this in itself could possibly lead to cerebral ischemia/hypoxia because of decreased blood flow because of the vasoconstriction.
On a side note- it is possible that there can be reduced O2 delivery to the brain during exercise for the similar reasons. When one begins exercise, the stimultion of the sympathetics cause an increase in respiration - which will lead to quasi-hyperventilation/aka breathing off more CO2 than is necessary, and b/c of this the cerebral vessels may vasoconstrict.
Hope this helps!
