The aim of this study is to compare the accuracy of the changes in CVI in response to a standardized noxious stimulus during various targeted pseudo-steady-state concentrations of remifentanil and various steady state level of BIS as guided by propofol effect compartment controlled TCI (closed-loop).

Patients will be allocated to receive a specific effect-site concentration of remifentanil and propofol. For remifentanil, concentrations will be targeted at 0, 2, 4 and 6 ng/ml. After an equilibration period, anesthesia will be induced in all patients using propofol in order to reach and maintain a BIS value of 70, 50 or 30. After an equilibration period(from start of propofol infusion), OAA/S will be measured in all patient. All patients will receive a standard pain stimulus (30 seconds, 50 mA) in order to observe the influence of the effect-site concentration of propofol and remifentanil on the CVI. A time window of 5 minutes after the stimulus will be respected to observe the changes in both BIS and CVI. After this period, the study will be terminated and classical maintenance of anesthesia will be provided.

Inclusion Criteria:

• ASA class I and II patients requiring general anesthesia for elective surgical procedures

Exclusion Criteria:

• patient refusal
• weight less than 70% or more than 130% of ideal body weight,
• neurological disorder
• recent use of psycho-active medication, including alcohol.