"Comparison of Oxygen Reserve Index (ORI), End-Tidal Oxygen, and Time-Based Preoxygenation Targets With Arterial Blood Gas During Preoxygenation

The goal of this observational study is to learn how well different preoxygenation targets reflect the actual amount of oxygen in the blood in adults having planned surgery under general anesthesia. Preoxygenation means breathing pure oxygen through a face mask before going to sleep, so that the body has an oxygen reserve while the breathing tube is placed. Doctors usually decide that preoxygenation is adequate either after 3 minutes of breathing oxygen or when the oxygen level in exhaled air (end-tidal oxygen, EtO2) reaches 90% or more. A newer finger sensor measures the Oxygen Reserve Index (ORI), a noninvasive number that may show oxygen reserve continuously. However, it is not clear how closely these targets match the oxygen level measured directly in arterial blood (PaO2). The main questions this study aims to answer are whether the arterial oxygen level (PaO2) differs when ORI reaches 0.45, when EtO2 reaches 90%, and at the 3rd minute of preoxygenation, and whether an ORI value of 0.45 or higher reflects an adequate arterial oxygen reserve. Participants who already need an arterial line for their surgery will breathe 100% oxygen through a tight-fitting face mask for up to 5 minutes before anesthesia induction. Small blood samples will be taken from the existing arterial line at each target point. No additional invasive procedure will be done for the study.

Inclusion Criteria: * Age 18-75 years * ASA physical status I-III * Scheduled for elective surgery * Planned general anesthesia with endotracheal intubation * Invasive arterial cannulation planned for clinical reasons * Suitable for ORI measurement with an appropriate sensor * Able to provide written informed consent Exclusion Criteria: * Emergency surgery * SpO2 below 94% on room air * Advanced chronic obstructive pulmonary disease (COPD) or severe restrictive lung disease * Hemodynamic instability * Severe peripheral circulatory disorder * Inadequate ORI signal quality * Significant mask leak * Hemoglobin below 8 g/dL * Suspected methemoglobinemia or carboxyhemoglobinemia, or a related hemoglobinopathy * Pregnancy * Failure to obtain data according to the protocol, missing data, or withdrawal of consent