Effects of ketamine on dose of propofol when general anesthesia maintaining by propofol and remifentanyl

INTERVENTION: group 1:propofol+ketamine 0mg/kg/h+remifentanyl0.1ug/kg/h for maintence of general anesthesia;group 2:propofol+ketamine 0.5mg/kg/h+remifentanyl0.1ug/kg/h for maintence of general anesthesia;group 3:propofol+ketamine 1mg/kg/h+remifentanyl0.1ug/kg/h for maintence of general anesthesia;group 4:propofol+ketamine 1.5mg/kg/h+remifentanyl0.1ug/kg/h for maintence of general anesthesia;group 5:propofol+ketamine 2mg/kg/h+remifentanyl0.1ug/kg/h for maintence of general anesthesia; CONDITION: the use of drugs in the maintence of general anesthesia PRIMARY OUTCOME: dosage of propofol; SECONDARY OUTCOME: heart rate;blood pressure;the time of respiratory recovery;recovery time;Steward Score;the time in PACU;Intraoperative awareness;Fall asleep again;Delirium/Restless;nystagmus;Dizzy;dreams;Visual change;the acceptance of the patient;the propofol concentration in plasma when operation;the ketamine concentration in plasma when operation; INCLUSION CRITERIA: 1. Age between 18 to 60 years; 2. Elective operation; 3. The time of operation is about 1 to 3 hours; 4. ASA grades between I to II; 5. BMI between 18 to 24.