The goal of this study is to explore whether specific intraoperative EEG signals (brain waves or neuronal electrical activity) are associated with the severity of acute 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.

Inclusion Criteria:

• American Society of Anesthesiologist Performance Status I or II
• Scheduled for elective laparoscopic abdominal surgery under General Anesthesia

Exclusion Criteria:

• Body Mass Index > 35 kg/m2
• Past history or suspected difficult airway
• Craniofacial malformations
• Use of regional anesthesia technique during or after the surgery
• Severe arrhythmia or use of a pacemaker device
• Severe neuropsychiatric illness (Mayor depression, bipolar disorder, schizophrenia)
• Regular use of psychoactive drugs
• Any injury in the right hand
• Past history of peripheral neuropathy
• Diabetes Mellitus
• Known lesion in the spinothalamic tract.
• Analgesics used in the past 72 hours
• Known allergy to propofol
• Previous diagnosis of a condition associated with chronic pain (such as fibromyalgia, rheumatoid arthritis, osteoarthritis, migraine)

Adult patients schedule for elective abdominal surgery under general anesthesia.