Brain ischemia is ischemia of the brain. Presumably, you have already noodled this out for yourself, so we can get a little more fussy about it and define brain ischemia as any pathophysiological state in which cerebral blood flow to all or any part of the brain is insufficient to meet the metabolic demands of brain tissue. Brain ischemia, unless corrected very quickly or treated with an effective and efficacious neuroprotective strategy (which, so far, doesn't exist) results in loss of brain tissue. And we hate that.

Brain ischemia is a component of many clinical scenarios important to emergency physicians, including acute ischemic stroke, acute hemorrhagic stroke, head trauma, prolonged severe hypotension and, most importantly for our purposes, cardiac arrest.