Brief Summary (Kısa Özet)The purpose of this study is to evaluate the safety and efficacy of a cerebral oximetry-guided low fraction of inspired oxygen (FiO_2) ventilation strategy compared to a standard high FiO_2 strategy in patients undergoing elective thoracic surgery requiring one-lung ventilation (OLV).Oxygen toxicity and related postoperative pulmonary complications remain significant risks during thoracic anesthesia. While standard protocols often rely on high inspired oxygen concentrations to maintain peripheral oxygen saturation (SpO_2), this approach may induce hyperoxia. Conversely, lowering FiO_2 arbitrarily can increase the risk of hypoxemia or compromise tissue oxygenation. This prospective, parallel-group, 1:1 randomized, single-blind clinical trial aims to safely reduce intraoperative oxygen exposure by using regional cerebral oxygen saturation (rSO_2) monitoring as a physiological guide.A total of 60 participants aged 18 and older, classified as ASA I-III and scheduled for elective thoracic surgery, will be randomly assigned to one of two arms:Experimental Group (Guided Low FiO_2 Strategy): Ventilation will be initiated at a low FiO_2 of 0.5. The FiO_2 level will not be increased unless peripheral saturation (SpO_2) falls below 90% or cerebral oxygenation (rSO_2) drops below 80% of the patient's baseline value.Active Comparator Group (Standard Strategy): Patients will receive conventional anesthesia management, maintaining standard high FiO_2 levels to keep SpO_2 above 90% without cerebral oximetry guidance.The primary outcomes to be measured are the incidence of cerebral desaturation during the perioperative period and the incidence of postoperative delirium assessed via the 3D-CAM tool until hospital discharge. Secondary outcomes include intraoperative hemodynamic stability, arterial blood gas parameters (PaO_2, PaCO_2, pH, lactate), incidence of hypoxemia, total one-lung ventilation duration, postoperative cognitive dysfunction (POCD), and lengths of stay in both the intensive care unit (ICU) and the hospital.By utilizing real-time non-invasive cerebral oximetry, the study intends to demonstrate a safe ventilation protocol that minimizes oxygen toxicity without compromising cerebral oxygenation or clinical recovery outcomes.

Inclusion Criteria:

• Patients aged 18 years or older. Scheduled for elective thoracic surgery requiring one-lung ventilation (OLV). American Society of Anesthesiologists (ASA) Physical Status classification I-III.

Exclusion Criteria:

• History of cerebrovascular disease. Left ventricular ejection fraction Pulmonary hypertension. Interstitial lung disease.

Renal failure. Neuromuscular disease. Pregnancy. Expected OLV duration less than 30 minutes (< 30 min)