In addition, rescue sedation will be provided by a patient-controlled sedation using propofol/ alfentanil in both groups. Patient-controlled sedation (PCS) has been shown to provide safe and effective sedation for the individual patient by allowing the patient to decide whether he or she requires sedation and to determine the amount required. PCS has been used for various procedures performed under local and regional anesthesia. PCS with propofol and alfentanil has been used for sedation of patients during endoscopy.