Cerebral Ischemia occurs when cerebral blood flow is less than

*Acute occlusion of an intracranial vessel causes a reduction in blood flow to the brain region it supplies*Cerebral blood flow to zero causes death of brain tissue within 4-10 minutes*<16-18 mL/100 g tissue per minute cause infarction within an hour; * <20 mL/100 g tissue per minute cause ischemia without infarction unless prolonged for several hours or days.*If blood flow is restored prior to a significant amount of cell death, the patient may experience only TIA.*Tissue surrounding the core region of infarction is ischemic but reversibly dysfunctional and is referred to as the ischemic penumbra. *The ischemic penumbra will eventually infarct if no change in flow occurs.(Ref: Harrison's 18/e p3271 )