Spectral entropy relies on the extent of disorder in both EEG and electromyography (EMG) signals and returns 2 values: State Entropy (SE) and Response Entropy. SE is computed over the frequency range of 0.8-32 Hz and reflects the level of hypnosis while Response Entropy is computed over the frequency range of 0.8-47 Hz and reflects also EMG activity. BIS and SE are dimensionless numbers scaled from 100 to 0 for BIS and from 91 to 0 for SE. We hypothesized that SE can be used as the control variable. In preparation for a large multi-center control trial, we propose a prospective randomized study to evaluate the effectiveness of such a closed-loop anesthesia system. Two groups of patients are compared: one in which propofol and remifentanil are administered by the anesthesiologist using target-controlled infusion systems, and one in which propofol and remifentanil are administered automatically by the combined closed-loop anesthesia system. In both groups, the goal is to maintain SE between 40 and 60, the recommended range during anesthesia by the manufacturer. We expect the combined closed-loop anesthesia system group to do similar or better.