Inclusion Criteria:  Aneurysmal subarachnoid hemorrhage, after craniotomy or clipping Arterial blood gas satisfy with PaCO2<35mmHg and pH>7.45 Presence of an endotracheal tube Assisted ventilation mode,CPAP/PSV ICP monitoring  Exclusion Criteria:  Age <18 years Pregnancy Chronic obstructive pulmonary disease Allergy to opioids Clinically relevant hepatic or renal failure Hemodynamic instability TCD windows cannot detect cerebral blood flow Refuse to participate the study