The investigators conducted a prospective physiological observation study on ARDS population, described their physiological and EIT indexes with various PEEP titration methods, and also compared and analyzed those indexes to determine whether there were differences among various PEEP titration methods.The objective of this study was to explore and uncover the physiological differences in response to optimal PEEP in different ways，and analyzed the correlation of the optimal PEEP in these different ways.

Inclusion Criteria:

• Intubated and mechanical ventilated patients in ICU within 48 hours
• Be over 18 years old
• With moderate-severe ARDS (PaO2 /FIO2 ,200 mm Hg)

Exclusion Criteria:

• Severe chronic obstructive pulmonary diseases
• Pulmonary bulla(>1CM) or pneumothorax or pneumomediastinum or subcutaneous emphysema
• Pregnancy
• BMI≥35kg/m2
• Suspected pulmonary vascular hemorrhage
• Disturbance of consciousness or intracranial hypertension
• Severe cardiac dysfunction (New York Heart Association Class III or IV, acute -Coronary syndrome or persistent ventricular tachyarrhythmia), cardiogenic shock or after major cardiac surgery
• Hemodynamic instability (i.e., mean arterial pressure [MAP]<60 mm Hg despite vasopressor use
• The medical team considering short periods of high pressure to be unsafe
• Contraindications for placement of esophageal pressure catheters (e.g., esophageal varix or fracture of skull base)
• Contraindications for EIT monitoring (including pacemaker, or cutaneous diseases which limiting electrode placement)
• Agonal stage or end-stages of diseases

ntubated and mechanical ventilated patients over 18 years with moderate-severe ARDS (PaO2 /FIO2 ,200 mm Hg) admitted to the ICU within 48 hours were enrolled from October 2022 to July 2024.