After approval by the ethics committee, every patient signed a consent form to participate in this observational prospective open-label randomized study. We identified potential participants in the anesthesia pre-admission unit who presented to be seen by an anesthesiologist before their intervention. Details of eligibility criteria described later.

According to the international 10-20 reference system, 4 silver cup electrodes were placed on the left prefrontal (Fp1), parietal (P7), temporal (T7), and occipital (O1) points with the vertex (Cz) as a common reference after an adequate preparation of each scalp skin location using a micro-abrasive paste and degreasing using ethyl ether. The cup electrodes were placed and fixed on the skin with adhesive paste . After a similar skin preparation, an adult adhesive EKG surface electrode was secured to the patient's forehead as a cephalic ground electrode. Thus, by using 6 electrodes, we obtain a 4 channels set-up (prefrontal Fp1-CZ, parietal P7-Cz, temporal T7-Cz, and occipital O1-Cz) with a common vertex reference.

Each of the six electrodes of the montage are respectively connected to one special Y adapter including a double connection to be attached at the same time to the EEG amplifier, for the QEEG recording, and to the A1000™ Aspect Medical System™ amplifier, for the BIS recording. After the initial test of the electrode impedance (\<5000 Ω), this last one was automatically checked all along the investigation to maintain the electrodes impedance below the same threshold. Regarding the QEEG, the low- and the high-frequency filters were set at 0.5 and 30 Hertz (Hz), respectively. EEG five seconds long epochs were sampled every 30 seconds for further analysis in the three respective domains (frequency, time, and power) in each EEG channel. Concerning the BIS monitoring, the EEG low- and high-frequency filters were set similarly to the QEEG whereas EEG sampling corresponds to 5-second segments of traces without smoothing, every 30 seconds. Finally, the BIS and its ancillary parameters simultaneously calculated in each channel, were extracted every 30 seconds. The selection of parameters for the QEEG analysis included Spectral Edge Frequency (SEF), Median EEG Frequency (MEF), Burst Suppression Ratio (BSR) and Total Spectral Power (TSP). Regarding the bispectral EEG analysis, the only parameter was the BIS itself. Thus, each parameter value is obtained for each channel during EEG and BIS sampling. Finally, for clinical monitoring, heart rate (HR), mean arterial pressure (MAP) and pulse oximetry (Sa02) were considered.

After the standard clinical monitoring placement (ECG, non-invasive arterial pressure cuff and ear/finger pulse oximetry device) and a peripheral venous catheter insertion connected to an infusion line, the non-premedicated patients were preoxygenated with a facial mask with 100% oxygen. Capnography via nasal cannula were initially applied and monitored prior to the anesthetic induction and replaced by the manual ventilation system when needed during the general anesthesia (GA) effect occurrence. First, every patient received an intravenous (iv) bolus of 0,2 μg.kg-1 sufentanil. After that, in the group 1 the propofol (P) infusion was started using target-controlled infusion (TCI) with a Total IntraVenous Anesthesia (TIVA) pump (Schnider's pharmacokinetic/pharmacodynamic data set); targeting the effect-concentration of 3µg.ml-1. From then on, the effect concentration was gradually increased by 1µg.ml-1 every two to three minutes until loss of consciousness (LOC as loss of verbal contact, spontaneous ventilation, corneal reflex, and ciliary reflex) occurred. At this point the effect concentration level of propofol upon loss of consciousness (PROPLOC) is recorded. At the same time, in group 2, sevoflurane (S) is started at one minimal alveolar concentration (2% in 50% oxygen) during mask assisted ventilation. Then, in a similar manner to group 1, the S concentration is gradually incremented by 2% until the LOC when the mask ventilation became fully assisted. The corresponding S effect-concentration (SEVOLOC) was also noted. Subsequently, the LOC confirmed, after an interval of 1 to 2 minutes, a first laryngoscopy was performed to test the possibility of intubation of the patients. In case of adequate intubation conditions (no physical or hemodynamical reaction to laryngoscopy and/or normal airway compliance), 0.1 mg.kg-1 of iv cisatracurium was given to prepare definitive intubation. When the patients did not encounter correct intubation conditions (first laryngoscopy failure), the P effect-concentration and the S end-tidal concentration were respectively increased of 1µg.ml-1 and 1% until the test laryngoscope was successful. After intravenous cisatracurium administration, manually assisted ventilation was conducted for 3 minutes in a way to have the partial pressure of end-tidal carbon dioxide (etCO2) between 32-35 mmHg. Intubation followed and intermittent positive pressure ventilation was used maintaining the same values of etCO2. The corresponding drug concentrations of propofol at intubation (PROPIntub) and sevoflurane at intubation (SEVOIntub) were recorded. From the moment the patient is mechanically ventilated, the effect concentration of propofol and the end-tidal concentration of sevoflurane are reduced to 3 to 4 µg.ml-1 and one minimal alveolar concentration (MAC) respectively while awaiting the surgical incision. Then, during the following 15-minute period of data collection no external stimuli were permitted on or around the patient. This pharmacological steady-state period was considered for the comparative topographic analysis between the different electrode categories regarding the general anesthesia effect on the brain. Therefore, the intraoperative period extends from the surgical incision to the end of the surgery. During this period, it is left to the discretion of the anesthetist to add either an iv bolus of sufentanil (0.1 μg.kg-1) and/or an iv bolus of cisatracurium (0.1 mg.kg-1) only if necessary, in the clinical judgment of the practitioner. To note, the parametric values of the QEEG and BIS were not considered to guide the anesthesia throughout the investigation. The postoperative period begins after the end of surgery when the anesthesia is stopped and lasts until the patient is awake and calm, ready to leave the operating room after extubation.