 After ensuring a steady-state period of remifentanil infusion of at least 5 minutes, the standardized painful stimuli consisting of the intracutaneous pain model and tetanic stimulation for the time period of 30 s with 80 milliampere, 50 Hz are applied. All stimulations are accompanied by the measurement of the analgesic monitoring-indices. Moreover the investigators measure and inspect changes in heart rate and blood pressure as well as occurrence of defensive movements of the patients. 