A prospective physiologic study, in participants with COVID-19 related or non-COVID-19 related acute respiratory distress syndrome (ARDS) requiring mechanical ventilation less than 48 hours. The investigators assessed the effect of different tidal volume guided by different levels of driving pressure on ventilation inhomogeneity and ventilation/perfusion mismatch by electrical impedance tomography (EIT) in supine and/or prone position.

Once enrolled, an EIT dedicated belt containing 16 electrodes was placed around the participant's chest at the fifth or sixth intercostal space and connected it to an EIT monitor (PulmoVista 500; Dräger Medical GmbH, Lübeck, Germany).

Data were collected in the enrolled day (Day 1),Day 3 and Day 7 including demographic and anthropometric data, a baseline arterial blood gas measurement, and ventilation parameters including type of supplemental oxygen, respiratory rate, fractional concentration of oxygen in inspired air (FiO2).

Volume control mode without spontaneous breathing, PEEP will be set according to best respiratory compliance method, and tidal volume (VT) will be set to reach three levels of driving pressure (High 14-15cmH2O, moderate 10-11 cmH2O, and low 7-8 cmH2O). Each level of VT was maintained 10 minutes in supine and/or prone position. In the last minutes of each phase，the participants received instructions of end expiratory occlusion lasting at least 10 seconds and, 1 seconds after the start, a bolus of 10 mL of 10% NaCl solution was injected via the central venous catheter. Clinical data and outcome will be collected.

Inclusion Criteria:

1. aged ≥ 18 years old
2. admitted to intensive care unit with a moderate to severe ARDS
3. invasive mechanical ventilation

Exclusion Criteria:

1. contraindications for prone position ventilation
2. Past chronic respiratory diseases (long-term family oxygen therapy for chronic respiratory diseases such as pulmonary fibrosis or COPD)
3. New York Heart Association class above II
4. gave written or witnessed verbal informed consent.
5. Contraindications to the use of EIT (e.g., presence of pacemaker or chest surgical wounds dressing) or prone position (as decided by the attending physician)
6. Impending ECMO (on the basis of clinical judgment, including clinical and physiological parameters).

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