The purpose of this observational study is to assess the differences and consistency in evaluating lung recruitment potential between the recruitment-to-inflation ratio (R/I ratio) and three-dimensional electrical impedance tomography (3D-EIT) and two-dimensional EIT (2D-EIT) in mechanically ventilated patients with acute respiratory failure. The main questions it aims to answer is:

Are the lung recruitment potentials predicted by 3D-EIT, 2D-EIT and the R/I ratio consistent? Participants will: Undergo a standardized PEEP titration protocol with synchronized EIT monitoring; Have R/I ratios measured at high and low PEEP levels.

Inclusion Criteria:

• Mechanically ventilated patients under sedation and analgesia with no spontaneous breathing.
• Oxygenation index < 300 mmHg.
• Patients who are clinically judged by the attending doctor to require the PEEP titration.

Exclusion Criteria:

• Aged under 18 years.
• Pregnancy.
• Contraindications for EIT examination (e.g. automatic implantable cardioverter defibrillator, implantable pumps, chest wounds limiting electrode belt placement, severe thoracic deformity, and others).
• No informed consent obtained.
• Hemodynamic instability with intolerance to high PEEP levels determined by the attending doctor.
• High-risk populations with pneumothorax, mediastinal emphysema, and other such conditions determined by the attending doctor.

Patients with acute respiratory failure who are intubated, under sedation and analgesia with controlled ventilation and no spontaneous breathing will be recruited from the Department of Critical Care Medicine, Peking Union Medical College Hospital. Additional inclusion criteria include an oxygenation index of less than 300 mmHg, clinical judgment indicating the need for PEEP titration, and no contraindications to PEEP titration or EIT examination.