Our previous observations indicate that vasospasm and abnormal THRT occur in 64% and 90% SAH of patients respectively. To demonstrate a 50% reduction in duration of vasospasm or abnormal autoregulation with a power of 80% and a significance level of 5% we will require examinations from 80 patients. This number of patients also has an 83% chance of demonstrating a 50% reduction in the incidence of clinical DID in the first 2 weeks following the ictus. The study is not powered to show a reduction in clinical endpoints although trends will be sought to identify any potential adverse effects.