GroupⅠreceived propofol 2 mg/kg intravenously by Fresenius pump at the speed of 250 mg/min, GroupⅡreceived propofol intravenously by Fresenius pump at the speed of 1 mg/kg/min, and observe's assessment of alertness and sedation(OAA/S) scale of the patients was titrated to 1, then changed to a maintenance dose