Effect of APRV and LTV on Lung Ventilation and Perfusion in Patients With Moderate-to-severe ARDS

Inclusion Criteria: 1. Patients aged ≥18 and ≤80 years; 2. Moderate-to-severe ARDS patients according to the Berlin definition; 3. Endotracheal mechanical ventilation ≤48 h before enrollment; 4. Expected to require continuous invasive mechanical ventilation ≥72 h. Exclusion Criteria: 1. Severe chronic obstructive pulmonary disease, severe asthma, pulmonary bulla, subcutaneous emphysema, mediastinal emphysema, etc; 2. Contraindications to the use of electrical impedance tomography (e.g., chest surgical wounds dressing or presence of pacemaker)； 3. Pulmonary interstitial lesions； 4. End-stage of chronic disease, with an expected survival period of \<6 months； 5. Body mass index \>35 kg/m2; 6. Refractory shock； 7. Intracranial hypertension； 8. Pregnant and parturient woman； 9. Intra-abdominal pressure persisted \> 20 mmHg and could not be relieved within 24 hours； 10. Severe thoracic deformity； 11. Severe cardiac dysfunction； 12. Atrial fibrillation and other malignant arrhythmias that seriously affect cardiac output； 13. Pulmonary embolism； 14. Extracorporeal membrane oxygenation is needed； 15. Prone positioning was performed before randomization； 16. Patients who have participated in other clinical trials within 30 days； 17. Patients who have not signed informed consent.