Remifentanil Effect on Burst Suppression Ratio

Subjects will enter the ward, after which an 18 G intravenous cannula will be installed and will be monitored under the routinely applied ASA standard. Frontal EEG activity will be recorded using an anesthetic depth monitor and a baseline EEG will be recorded for 120 seconds with eyes closed. Subjects will then be preoxygenated. Once the oxygen concentration at the end of expiration reaches 90%, the administration of remifentanil will begin, under the Minto pharmacological model, to reach an effect site concentration of 7.5 ng/mL. Once the remifentanil concentration is reached, the infusion of propofol will begin at a rate of 15 mg/kg/h using Base Primea Orchestra, until loss of consciousness (LOC) is achieved. LOC will be defined as loss of response to verbal and tactile stimulation, which will be corroborated every 30 seconds. After this, the propofol infusion will be maintained at the concentration at which the LOC occurred, based on Marsh's pharmacological model, and rocuronium 0.6 mg/kg will be administered to facilitate orotracheal intubation.

Once the patient has been intubated, the remifentanil will be lowered to a concentration of 4 ng/mL and the surgery will continue. After 5 min after the LOC occurred, according to randomization, the concentration of remifentanil will remain at 4 ng/mL or increase to 7.5 ng/mL, and 10 min after the LOC occurred the propofol infusion rate will be changed to 15 mg/kg/h for 10 min. After this, the propofol concentration at which the LOC occurred will be returned and remifentanil will be maintained at 4 ng/mL. After 5 min after completing the previous step, remifentanil will be increased to 7.5 ng/mL or maintained at 4 ng/mL in the opposite way to what happened previously. After 10 min after completing the previous step, propofol will be dosed again at a fixed rate of 15 mg/kg/h for 10 min. Finally, after this time has elapsed, the concentration of propofol at which the LOC occurred will be returned and the remifentanil will be dosed according to the surgical requirements. Throughout the protocol, hemodynamics will be maintained with the usual management of drugs such as ephedrine in boluses or norepinephrine in continuous infusion.

• Propofol estimated concentration by Marsh pharmacological model, which generates loss of consciousness [Time frame: Minutes, during anesthesia induction is performed]
• Mean arterial pressure during burst suppression [Time frame: During 10 minutes when propofol dosification is increased to 15 mg/kg/h]
• Total ephedrine dose [Time frame: During all protocol, 40 min]
• Maximal dose of norepinephrine [Time frame: During all protocol, 40 min]
• Bispectral index at burst suppression [Time frame: During 10 minutes when propofol dosification is increased to 15 mg/kg/h]