An estimated 37,500 people in the United States have subarachnoid hemorrhage (SAH) every year. SAH is usually secondary to a brain aneurysm that has burst. In SAH the bleeding accumulates around the lining of the brain. SAH is associated with a 51percent mortality rate, and one third of survivors are left functionally dependent. Cerebral vasospasm, which is a delayed narrowing of the cerebral arteries following SAH, has been identified as the most important reason for neurological deterioration and bad outcome in cases of SAH. Cerebral vasospasm may be caused by multiple mechanisms.