Inclusion Criteria:  Age ≥ 18 years; Admitted to ICU because of aSAH. Predicted to stay in ICU ≥ 24 hours Provide written informed consent.  Exclusion Criteria:  Unable to perform NIRS monitoring because of anatomic factors; Patients with severe organ failure; In addition to aneurysmal subarachnoid hemorrhage, there are other serious intracranial diseases (intracranial infection, cerebral hemorrhage, cerebral infarction, brain trauma, other patients after intracranial surgery); Patients with carotid plaque or thrombus; Patients with severe intracranial pneumatosis after craniotomy.