Seven thousand patients suffer SAH each year within the UK with young adults (<55 years) being equally affected. Cerebral vasospasm and related cerebral ischemia are the major causes of delayed morbidity and mortality in patients who survive the initial SAH episode. Previous studies have revealed that immediate low cerebral blood flow (CBF) accompanies SAH, particularly in those in coma. However, for better grade patients the low CBF is often delayed and associated with cerebral vasospasm [Knuckey 1985]. Vasospasm often precedes the onset of DID which commonly occurs in the 2nd week after SAH [Knuckey 1985]. Autoregulation is the most accomplished reflex mechanism and reflects an intrinsic ability of the cerebral vessels to dilate in an attempt to maintain constant CBF in the face of poor cerebral perfusion. Following SAH, impairment of autoregulation affects prognosis [Lam 2000]. Indeed, daily assessment of autoregulation can help to identify patients at high risk of clinical deterioration [Smielewski 1997].