The aim of this study is to prospectively evaluate the effect of surgical position on the development of postoperative atelectasis in patients undergoing surgery in supine, prone, lateral decubitus, and lithotomy positions using lung ultrasonography (LUS), and to investigate whether there is a difference between these positions.

Study Groups: Patients will be divided into 4 groups according to the surgical position to be given during the operation:

• Group 1 (Supine, n=25)
• Group 2 (Prone, n=25)
• Group 3 (Lateral Decubitus, n=25)
• Group 4 (Lithotomy, n=25) All patients will receive standard anesthesia induction and maintenance with 1 mcg/kg fentanyl citrate, 2 mg/kg propofol, 0.6 mg/kg rocuronium intravenously, followed by maintenance with desfluran at a MAC of 0.8-1.2 (if necessary). Patient demographic data (age, gender, height, weight, BMI, ASA score, comorbidities) and intraoperative data (heart rate, SpO2, mean arterial pressure, duration of surgery, surgical position, applied PEEP level) will be recorded.

Lung Ultrasonography (LUS) Protocol: LUS evaluation, Dr. The ultrasound will be performed by Burak Ağar using a Mindray ultrasound device [Device brand/model and probe type, e.g., 3.5-5 MHz convex or 7-12 MHz linear probe]. LUS will be performed using a 12-zone protocol consisting of 6 standard zones (upper and lower quadrants of the anterior, lateral, and posterior regions) for both hemithoraxes. The evaluation will be performed at 3 different time points:

1. Preoperative (T0): Immediately before anesthesia induction, in the supine position (Baseline).

2. Postoperative (T1): Immediately before the patient is awakened, in the supine position.

3. Postoperative (T2): 20 minutes after the patient is transferred to the PACU. MLUS Scoring: Each region will be scored between 0-3 according to lung ventilation (MLUS Score):

   • 0 Points: Normal ventilation (presence of A-line, fewer than 3 B-lines)
   • 1 Point: Minor ventilation loss (≥ 3 B-lines or subpleural consolidations ≤ 1 cm in diameter separated by a normal pleural line)
   • 2 Points: Moderate ventilation loss (Multiple small subpleural consolidations ≤ 1 cm in diameter separated by multiple and converging B-lines or a thickened/irregular pleural line)
   • 3 Points: Severe ventilation loss (At least one consolidation > 1 cm in diameter or subpleural consolidations > 1x2 cm in diameter) The total MLUS score will be obtained by summing the scores of the 12 regions (Minimum 0, Maximum 36). Atelectasis will be defined as "predisposition to atelectasis when each quadrant score is 2 or 3".

Inclusion Criteria:

• Being between 18-65 years of age
• Having ASA I-III status
• Planning elective surgery under general anesthesia
• Agreeing to participate in the study and signing the consent form

Exclusion Criteria:

• Patients requiring emergency surgery
• Patients with ASA IV or higher
• Presence of a known serious preoperative lung disease (COPD, Asthma, Pneumonia, Pleural effusion, Pneumothorax, Interstitial lung disease, Bronchiectasis, etc.)
• History of neuromuscular disease
• Body Mass Index (BMI) > 35 kg/m² (Morbid obesity)
• Presence of chest wall deformity or skin lesions that would obstruct LUS imaging
• Cases with an operative duration of less than 60 minutes or more than 300 minutes
• Patients with a history of thoracotomy, sternotomy, or prior open thoracic surgery
• Patients with alcohol or drug dependence
• Patients with significant anemia
• Patients who are pregnant or lactating
• Patients who develop laryngospasm following extubation
• Refusal to provide consent or inability to cooperate

The study will include a total of 100 patients in 4 groups, aged 18-65 years, with an ASA (American Society of Anesthesiologists) physical status between I and III, who are scheduled to undergo general anesthesia in supine, prone, lateral decubitus, and lithotomy positions.