Blood-Brain Barrier Permeability

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.