<prompt> Cerebral Ischemia occur when cerebral blood flow is less than10 ml 1OOgminute20 ml 1OOgminute40 ml 1OOgminute50 ml1OOgminute  <response>: the threshold of cerebral blood flow CBF below which neuronal function is impaired and the tissue is at risk of infarction is around 22 ml100 g min Hyperventilation causes decreased PaCO2 which subsequently leads to aerial vasoconstriction thus lowering cerebral blood flow CBF, cerebral blood volume, and ICP. This effect is mediated my pH changes in the extracellular fluid which cause cerebral vasoconstriction or vasodilation depending on the pH. Ref Harrison20th edition pg 2567 