One-lung ventilation (OLV) is routinely used during thoracic surgery to facilitate surgical exposure but is associated with an increased risk of intraoperative hypoxemia. To prevent hypoxia, high fractions of inspired oxygen (FiO₂) are commonly administered; however, excessive oxygen delivery may lead to hyperoxia and oxidative tissue injury. The Oxygen Reserve Index (ORi) is a novel, noninvasive monitoring parameter that provides real-time information on moderate hyperoxia and may allow more precise titration of FiO₂ during anesthesia.

The aim of this prospective, randomized clinical trial is to evaluate whether ORi-guided intraoperative oxygen management can reduce oxygen exposure during OLV without increasing hypoxemia and to assess its effects on oxidative stress biomarkers and postoperative outcomes. Adult patients undergoing elective thoracic surgery with OLV are randomized to receive either ORi-guided FiO₂ titration or conventional oxygen management. Oxidative stress biomarkers and postoperative pulmonary complications are compared between groups.

One-lung ventilation is an essential component of thoracic anesthesia but poses significant challenges in maintaining adequate oxygenation. In clinical practice, high FiO₂ levels are often applied to prevent hypoxemia; however, prolonged exposure to hyperoxia has been associated with increased oxidative stress, inflammatory responses, and potential tissue damage. Despite growing awareness of oxygen toxicity, optimal intraoperative oxygen titration strategies during OLV remain poorly defined.

The Oxygen Reserve Index is derived from multiwavelength pulse oximetry and provides continuous, noninvasive information on moderate hyperoxia (corresponding to arterial oxygen tensions above normoxia but below severe hyperoxia). ORi monitoring may enable anesthesiologists to titrate FiO₂ more precisely, thereby avoiding unnecessary oxygen exposure while maintaining adequate oxygenation.

In this prospective, randomized clinical trial, adult patients scheduled for elective thoracic surgery requiring OLV are allocated to either an ORi-monitored group or a control group. In the ORi group, FiO₂ is adjusted intraoperatively according to predefined ORi thresholds, whereas in the control group FiO₂ is managed according to standard clinical practice based on pulse oximetry. Blood and tracheal aspirate samples are collected preoperatively and at predefined postoperative time points to measure interleukin-6, superoxide dismutase, and malondialdehyde levels as markers of inflammation and oxidative stress. Postoperative pulmonary complications and surgical site infections are recorded during intensive care unit and ward follow-up.

This study aims to determine whether ORi-guided oxygen titration during OLV can safely reduce intraoperative oxygen exposure and to clarify its impact on biochemical markers of oxidative stress and clinical outcomes such as postoperative pulmonary complications and surgical site infection.

Inclusion Criteria:

• Patients age between 18-85 years Elective case Thoracic surgeries performed with one-lung ventilation using a double-lumen endotracheal tube Left ventricule ejection fraction(LVEF)>45%

Exclusion Criteria:

• heart failure (LVEF) < 45% severe valvular disease admission as an emergency case history of prior lung resection presence of peripheral arterial disease.

The data generated during this study may be made available for use in future meta-analyses upon reasonable request to the corresponding author.