Atelectasis is a frequent pulmonary complication after general anesthesia, often triggered by preoxygenation and intraoperative hyperoxia. High inspiratory oxygen fractions (FiO₂) can promote absorption atelectasis, ventilation-perfusion mismatch, hemodynamic alterations, and oxidative injury.

This study evaluates the effect of two intraoperative oxygen management strategies-oxygen reserve index (ORI)-guided FiO₂ titration versus fixed 50% FiO₂-on postoperative atelectasis in patients undergoing thoracolumbar spine surgery under general anesthesia. Atelectasis severity will be assessed using lung ultrasonography (LUS), scored across 12 thoracic regions (0-3 per region, total 0-36), while respiratory function changes will be examined via preoperative and 24-hour postoperative spirometry (FVC, FEV₁, FEV₁/FVC).

Because postoperative spirometry may be influenced by pain, Numeric Rating Scale (NRS) scores will be recorded to help distinguish true restrictive patterns from pain-limited respiratory effort.

The study aims to determine whether ORI-guided FiO₂ titration can reduce postoperative atelectasis and improve respiratory outcomes compared with a fixed FiO₂ approach.

Atelectasis is one of the most common pulmonary complications following general anesthesia, and both preoxygenation and intraoperative hyperoxia are major contributing factors. Hyperoxia can lead to absorption atelectasis, ventilation-perfusion mismatch, cerebral and coronary vasoconstriction, decreased cardiac output, and oxidative tissue injury caused by reactive oxygen species. The use of a high inspiratory oxygen fraction (FiO₂) during general anesthesia, particularly in prolonged surgeries, increases the risk of absorption atelectasis.

This study aims to investigate the impact of different intraoperative oxygen management strategies-oxygen reserve index (ORI)-guided FiO₂ titration versus fixed 50% FiO₂-on the development of postoperative atelectasis in patients undergoing thoracolumbar spine surgery under general anesthesia. The presence and severity of atelectasis will be assessed using lung ultrasonography (LUS), and changes in respiratory function will be evaluated objectively through spirometry measurements.

Lung ultrasonography will be performed by the same experienced and certified anesthesia practitioner for both groups. LUS assessment will be conducted in 12 regions (superior and inferior zones along the anterior, lateral, and posterior lines of each hemithorax). Each region will be scored from 0 to 3 according to the degree of aeration:

0 = normal aeration (A-lines predominance, <2 B-lines);

1. = mild loss of aeration (≥3 well-defined B-lines);
2. = moderate loss of aeration (multiple coalescent B-lines or "white lung");
3. = severe loss of aeration (subpleural or complete consolidation with air bronchograms).

The total LUS score ranges from 0 to 36, with higher scores indicating greater loss of aeration and presence of atelectasis.

Spirometry will be performed preoperatively and at 24 hours postoperatively, with patients in a seated position using a nose clip and a single-use cardboard mouthpiece. After a maximal inspiration, patients will be instructed to perform a forceful and sustained expiration according to device guidance. At least three maneuvers will be recorded, and the best values for FVC, FEV₁, and FEV₁/FVC will be documented.

Postoperative spirometry results may be influenced by pain, as inadequate inspiration or shortened expiration can lead to artificially reduced FVC and FEV₁ values. Therefore, postoperative pain will be assessed using the Numeric Rating Scale (NRS, 0-10), enabling differentiation between true atelectasis-related restrictive patterns and pain-limited respiratory effort.

This study focuses on optimizing oxygenation and preventing postoperative pulmonary atelectasis caused by higher intraoperative oxygen exposure in patients undergoing elective thoracolumbar spine surgery under general anesthesia.

Inclusion Criteria:

1. Adults aged 18 to 80 years
2. Patients with an ASA physical status of I, II, or III
3. Individuals with a body mass index (BMI) between 18 and 35 kg/m²

Exclusion Criteria:

1. History of or active upper or lower respiratory tract infection within the past month
2. Pulmonary emphysematous/bullous disease or COPD
3. Pre-existing significant atelectasis or consolidation (Preoperative LUS showing a score of 3 consolidation or ≥2 regions with a score ≥2)
4. Severe obstructive sleep apnea requiring CPAP therapy
5. NYHA class III-IV heart failure, EF <35%, or severe valvular disease
6. Severe arrhythmia or hemodynamic instability requiring high-dose vasopressors
7. Body mass index (BMI) >35 kg/m²
8. Severe anemia (Hb <9 g/dL) or polycythemia (Hb >18 g/dL)
9. Non-diagnostic or unclear preoperative lung ultrasound images
10. Preoperative SpO₂ ≤ 94%
11. Inability to apply the sensor due to finger deformity, or inadequate signal caused by digital hypoperfusion