The goal of this observational study is to determine whether macrophage/monocyte mediated inflammatory signaling contributes to reduced anesthetic requirements in older adults undergoing major abdominal surgery. The main questions it aims to answer are:

• Is there a difference in anesthetic dosing requirements (minimum effective dose) between young and older patients undergoing major abdominal surgery?
• How do electroencephalographic (EEG) signatures under anesthesia correlate with age and systemic inflammatory markers?
• Is there an association between age, levels of circulating inflammatory cytokines, and monocyte/macrophage phenotypes with anesthetic requirements?

If there is a comparison group: Researchers will compare older adult patients undergoing major abdominal surgery to younger adult patients undergoing major abdominal surgery to see if macrophage/monocyte-mediated inflammatory signaling influences anesthetic sensitivity and the risk of postoperative neurocognitive complications in the older population.

Participants will:

• Receive general anesthesia for major abdominal surgery, with continuous recording of anesthetic dose requirements.
• Undergo electroencephalographic (EEG) monitoring during the anesthetic period.
• Provide blood samples for the measurement of circulating inflammatory cytokines and the assessment of monocyte phenotypes.
• Provide peritoneal tissue samples (collected during surgery) to evaluate tissue macrophage populations.
• Provide cerebrospinal fluid (CSF) samples to assess biomarkers of blood-brain barrier permeability.

Inclusion Criteria:

• Age between 18-30 years or over 60 years old
• Scheduled for abdominal surgery
• General anesthesia planned with or without spinal anesthesia
• Able and willing to provide written informed consent.

Exclusion Criteria:

• Chemotherapy or systemic corticosteroid use within the past 2 weeks.
• Pregnancy.
• Active sepsis or systemic infection.
• Emergency surgery.
• Requirement of ketamine or dexmedetomidine during the first hour of anesthesia.
• History or diagnosis of: stroke with persistent neurological deficit, dementia, Alzheimer's disease, Huntington's disease, amyotrophic lateral sclerosis, central nervous system autoimmune disorders, systemic autoimmune diseases, or infectious diseases of the central nervous system.
• Current treatment with immunomodulatory medications.
• Psychiatric disorders with psychotic features.
• Use of psychoactive or psychotropic recreational drugs within the week prior to surgery.

Adult patients from Hospital Clínico Universidad de Chile, in Santiago de Chile, that require elective abdominal surgery and are willing to participate in the study, meeting inclusion criteria and signing the informed consent.