The purpose of this study is to determine whether there is a significant decrease in cerebral oxygen saturation in hypertensive patients undergoing major abdominal surgery and their correlation with standard monitoring parameters.

Inclusion Criteria:

• patients have hypertension more than 3 year,whether well-controlled or uncontrolled,
• scheduled for major abdominal surgery for at least 2 h
• under general anesthesia
• American Society of Anesthesiologists（ASA）physical status : II ~ III
• Age > 30

Exclusion Criteria:

• pre-existing cerebral pathology for example episodes of cerebral ischemia or stroke;
• ASA physical status >IV
• hepatic failure
• renal failure
• preoperative Mini-Mental State Examination (MMSE) score less than 24
• a history of cardiovascular surgery or craniotomy
• cardiac failure
• respiratory failure

patients scheduled for major abdominal surgery such as gastrectomy, colon resection ,liver resection and duodenocephalo-pancreasectomy