Low tidal volume ventilation (LTV) has been proposed and widely used in patients with acute respiratory distress syndrome (ARDS) to prevent ventilator-induced lung injury (VILI) and mitigate its effects. The LTV strategy is intended to protect the "baby lung" from overdistension while simultaneously allowing acutely injured tissue to continually collapse. Airway pressure release ventilation (APRV) is a highly effective strategy improving lung recruitment and oxygenation in clinical studies, but its effects on lung injury and mortality is debatable. Animal studies revealed that APRV could normalize post-injury heterogeneity and reduce the risk of VILI. Our objective was to investigate the impact of APRV and LTV on regional ventilation and perfusion distribution in ARDS patients by electrical impedance tomography (EIT).

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.