Determination of the Association of Intraoperative Nociception Electroencephalographic Biomarkers and Postoperative Pain

This is a prospective minimal-risk observational study to collect and analyze EEG data during intraepidermal electrical stimulation in patients receiving general anesthesia (GA) for abdominal surgery. To collect nociception-related EEG signal during GA the investigators designed a standardized Intraepidermal Electrical Stimulation (IES) Protocol. The intensity of the electrical stimulus will be calibrated to each participant perception pre-surgery, with 3 different intensities: sensitivity threshold, mild-pain threshold, and moderate-pain threshold. The IES will be administered several times throughout the surgical procedure: a) At baseline, prior to induction of GA, b) Three minutes after loss of consciousness, c) Five minutes after fentanyl administration and orotracheal intubation, d) Five minutes after first incision and every 20 minutes until the end of the surgery, and f) After surgery ends and before extubation. To standardize the anesthesia management as much as possible, all patients will be attended by a reduced number of anesthesiologists. The hypnosis will be achieved with propofol, while analgesia will be mainly achieved with either fentanyl or remifentanil. EEG will be recorded with a 32-channel high standard equipment. Once the patient arrives to the Post-Anesthesia Care Unit (PACU), the investigators will record the pain reported by the patient using the Numeric Pain Rating Scale every 15 minutes during the first hour, and every 30 minutes until patient is discharged from the PACU. Also, the investigators will both, record opioid administration through the surgery and estimate their effect-site concentration with pharmacokinetic/pharmacodynamic models. Besides, the investigators will measure opioid plasmatic concentrations.