With the recent concern on patient's outcome following general anesthesia, automatic control of drugs has been a field of interest. The Bispectral Index (BIS) is an EEG based hypnosis monitor, in current use at the operating theatre as a guiding tool for the anesthesiologist to titrate drugs, and prevent awareness. When trying to model a certain process, it's very important to obtain information of the system behavior under steady-state conditions. In this study the hypnotic and analgesic drugs' effect on the hypnosis index BIS was analyzed, in order to obtain steady-state information of the system response (inputs-output), and in the future model the drugs combined effect. A steady-state index was obtained using a wavelet analysis technique for trend detection. This tool may be used in the future to model the drug's combined effect on the hypnosis indices, and also to bring some insight on disturbances not related to drug changes.