High inspired oxygen concentrations (FiO₂) are commonly administered during general anesthesia to prevent hypoxemia. However, excessive oxygen administration may result in hyperoxemia, which has been associated with impaired hypoxic pulmonary vasoconstriction, increased oxidative stress, ventilation-perfusion mismatch, and postoperative pulmonary complications.
During one-lung ventilation (OLV) for thoracic surgery, higher FiO₂ is often required because of increased intrapulmonary shunting, making hyperoxemia a frequent occurrence. The Oxygen Reserve Index (ORI) is a noninvasive pulse oximetry-derived parameter that reflects moderate hyperoxemia (PaO₂ approximately 100-200 mmHg) and may facilitate more precise oxygen titration than pulse oximetry alone.