Determination of EC50 of Anesthetics During Microelectric Recording in Patients Receiving Deep Brain Stimulation Under Anesthesia

General anesthesia is sometimes required for DBS (deep brain stimulation) surgery in those with excessive fear, anxiety, reduced cooperation, or severe movements due to ' off drug' state. Dexmedetomidine and propofol are both used individually in the sedation for DBS surgery, and were reported to affect the MER (microelectrode recording) to some extent, especially in the higher zone of the dose range. In this study, the investigators plan to use the combination of dexmedetomidine and propofol for the maintenance of general anesthesia in patients with Parkinson's disease. In this study, dexmedetomidine is given with a loading dose of 0.5 mcg/Kg, and continuous infusion with the dose of 0.4 mcg/Kg/hr. Propofol is given within 0.8~1.8 mcg/Kg during the MER recording (based on institutional guideline). MER recording will be observed and correlated wtih the dose of propofol. The Dixon "up-and-down" sequential allocation method was used to determine the EC50 of the value of Cet of propofol. The study aims to determine the dose that is adequate for maintaining general anesthesia with endotracheal intubation and least interference with the MER recording.