Recent study has shown that inhalatory sedation is a practicable, effective and not risky method in Intensive Care Unit. Sevoflurane effect on cerebral system have been described in previous studies: it causes an increasing of cerebral blood flow and a decrease of oxygen cerebral consumption. Clinical strategy for Subarachnoid Hemorrhage is orientated to increase cerebral blood flow to limit vasospasm phenomena after SAH. Scope of this study is to evaluate the Cerebral Blood Flow variation associated to Isoflurane sedation versus conventional sedation with propofol .