This study aims to evaluate whether guidance by the Oxygen Reserve Index (ORi) improves inspiratory oxygen fraction (FiO₂) titration compared with conventional pulse oximetry (SpO₂)-guided oxygen administration in adult patients undergoing surgery in the beach chair position.

Oxygen therapy is routinely used during general anesthesia to prevent hypoxemia; however, excessive oxygen administration may result in hyperoxia, which has been associated with adverse cardiovascular and pulmonary effects. Standard pulse oximetry may not adequately detect hyperoxia when oxygen saturation values are high.

In this prospective comparative study, patients will receive oxygen titration guided either by SpO₂ alone or by combined ORi and SpO₂ monitoring. The primary outcome is the incidence of intraoperative hyperoxemia, assessed by arterial blood gas analysis. Secondary outcomes include intraoperative oxygenation parameters and hemodynamic variables.

This prospective study will be conducted in adult patients (ASA physical status I-III) undergoing elective orthopedic surgery in the beach chair position under general anesthesia. Following approval by the institutional ethics committee and acquisition of written informed consent, eligible patients will be included in the study.

Standard monitoring will include electrocardiography, noninvasive blood pressure, peripheral oxygen saturation (SpO₂), bispectral index (BIS), perfusion index (PI), pleth variability index (PVI), near-infrared spectroscopy (NIRS), and multiwavelength pulse co-oximetry for Oxygen Reserve Index (ORi) monitoring. Invasive arterial blood pressure monitoring will be established via radial arterial cannulation to allow continuous hemodynamic monitoring and arterial blood gas sampling.

General anesthesia will be induced with intravenous propofol, fentanyl, and rocuronium, followed by orotracheal intubation. Mechanical ventilation will be applied in volume-controlled mode with lung-protective settings. Anesthesia will be maintained using sevoflurane and remifentanil infusion, targeting bispectral index values between 40 and 60.

From anesthesia induction onward, inspiratory oxygen fraction (FiO₂) will be titrated either according to SpO₂ values alone or using combined ORi and SpO₂ guidance. In the ORi-guided group, FiO₂ adjustments will aim to maintain ORi values between 0.2 and 0.5, with reassessment at regular intervals. In the SpO₂-guided group, FiO₂ will be adjusted to maintain SpO₂ values at or above 98%, in accordance with routine clinical practice.

Hemodynamic parameters, oxygenation variables, ORi values, and cerebral oxygenation assessed by NIRS will be recorded at predefined time points during surgery and recovery. Arterial blood gas analysis will be performed to determine PaO₂ and arterial oxygen saturation.

Hyperoxemia will be defined as a PaO₂ value of 100 mmHg or greater and classified as mild or severe. The primary outcome of the study is the incidence of intraoperative severe hyperoxemia. Secondary outcomes include overall oxygenation profiles and intraoperative hemodynamic stability.

The results of this study are expected to provide evidence regarding the utility of ORi-guided oxygen titration in reducing unnecessary hyperoxia during surgery performed in the beach chair position.

Inclusion Criteria:

• Adult patients aged 18 to 65 years
• American Society of Anesthesiologists (ASA) physical status I-III
• Scheduled for elective orthopedic surgery in the beach chair position
• Body mass index (BMI) <35 kg/m²
• Ability to provide written informed consent

Exclusion Criteria:

• Age <18 or ≥65 years
• Finger deformities preventing the use of Oxygen Reserve Index (ORi) monitoring
• Severe anemia (hemoglobin <8 g/dL)
• Pregnancy
• Refusal or inability to provide informed consent
• History of stroke or significant vascular pathology
• Severe cardiac or respiratory disease