Background: After surgery with general anesthesia, it is common for parts of the lungs to collapse, a condition called atelectasis. This can lead to low blood oxygen levels and other lung complications. Doctors use a setting on the breathing machine called PEEP (Positive End-Expiratory Pressure) to help keep the lungs open, but the best level to use is still debated.

Purpose of the Study: The goal of this research is to find a PEEP level that minimizes the risk of lung collapse and low oxygen levels after surgery. The investigators will use lung ultrasound, a safe and non-invasive imaging method, to check the health of the lungs at the patient's bedside.

The investigators will not assign treatments; they will observe the outcomes based on the PEEP level chosen by the patient's anesthesiologist during routine care. A simplified ultrasound scan will be used to score the amount of lung collapse before and after surgery. The main outcomes will be the frequency of lung collapse and the frequency of low oxygen levels (defined as SpO₂ of 90% or less).

Background and Rationale Postoperative pulmonary complications (PPCs) are a significant cause of morbidity and mortality, associated with a nearly 20% increase in lethality and prolonged hospital stays. The most common PPC is atelectasis, which can trigger more severe complications and develops in up to 90% of patients following the induction of general anesthesia. While computed tomography (CT) is the gold standard for diagnosing atelectasis, lung ultrasound (LUS) has emerged as a rapid, reliable, and validated bedside tool that is superior to standard chest radiography, with a high sensitivity (87.7%) and specificity (92.1%) compared to CT.

A key strategy for preventing atelectasis is the application of positive end-expiratory pressure (PEEP). While ventilation with zero PEEP is considered harmful, the optimal level remains controversial. Large randomized trials (e.g., PROVHILO, PROBESE) have not shown a benefit for universally high PEEP strategies (e.g., 12 cm H₂O) and have noted an increased risk of hemodynamic instability. This contrasts with other studies suggesting benefits from moderate or individualized PEEP levels. This study designed to address this gap by analyzing data to identify a PEEP threshold associated with minimal atelectasis and desaturation, using a simplified LUS monitoring protocol.

Study Objectives The primary objective of this study is to evaluate the effectiveness of a simplified LUS protocol for monitoring postoperative atelectasis and to determine a PEEP level associated with the minimum frequency and severity of atelectasis.

Specific study tasks include:

• To assess the frequency and severity of postoperative atelectasis in patients with different airway management strategies and PEEP levels.
• To determine an optimal PEEP threshold for minimizing the risk of atelectasis and desaturation using ROC analysis.
• To compare the rates of atelectasis and desaturation (SpO₂ ≤90%) between patient subgroups.
• To conduct a multifactorial analysis of risk factors for developing postoperative atelectasis and desaturation.
• To evaluate the prognostic value of the LUS score for predicting the risk of postoperative desaturation.

Study Design and Methodology This is a single-center, retrospective cohort study conducted at the National Medical and Surgical Center n.a. N.I. Pirogov. The level of PEEP not assigned by the protocol but determined by the attending anesthesiologist as part of routine clinical practice.

A simplified 2-zone LUS protocol will be used, focusing on the posterior-basal lung regions most susceptible to collapse. Lung aeration will be quantified using a 4-point scoring system (0-3), with 0 indicating normal aeration and 3 indicating major consolidation. Scans will be performed by one of three competent investigators before anesthesia and within the first hours after surgery, once the patient is fully awake.

Inclusion Criteria:

• Patients undergoing surgical intervention under general anesthesia with mechanical ventilation.
• Airway protection managed with either an endotracheal tube or a laryngeal mask.
• A normal baseline ultrasound of the posterior-basal lung regions, corresponding to a score of 0 on the lung ultrasound scale.

Exclusion Criteria:

• Patients undergoing cardiac or thoracic surgery.
• Presence of a perioperatively identified pneumothorax.
• Inability to obtain adequate ultrasound visualization of the target lung zones.
• Presence of hydrothorax.
• Confirmed perioperative pulmonary aspiration.
• Presence of any pathological findings on the initial baseline ultrasound of the posterior-basal lung regions.
• Requirement for massive blood transfusion during the operation.
• Patients undergoing surgery on the diaphragm.
• Prolonged residual sedation lasting more than 2 hours post-operation

The study population consists of 450 adult patients who underwent surgical procedures under general anesthesia with mechanical ventilation. Data will be collected from patients treated at the Department of Anesthesiology and Resuscitation of the National Medical and Surgical Center n.a. N.I. Pirogov.

The cohort will be formed using a time-stratified randomization method to ensure a representative sample over the study period. All included patients will have a normal lung ultrasound before their procedure.

The population will be divided based on the method of airway protection:

• patients managed with an endotracheal tube (ETT).
• patients managed with a laryngeal mask (LM).

2 months after completion of the study

upon the request