This study try to use EIT ( Electronic impedance tomography ) to evaluate the alveolar heterogeneity and use trans-pulmonary pressure and stain to evaluate the lung injury . The study use these methods to figure out how PEEP impact the alveolar heterogeneity and VILI.

ARDS(Acute respiratory distress syndrome )is common in patients suffering Sepsis, trauma, burning and other sever condition and characterized by diffuse alveolar damage and alveolar heterogeneity.ARDS patient need MV(mechanical ventilation )to maintain oxygenation and need unique MV tragedy to avoid VILI(Ventilator induced Lung injury ).PEEP(post-end-expiratory pressure) is crucial to maintain oxygenation and avoid VILI according to researches. But there are also some other research different voice. This study try to use EIT ( Electronic impedance tomography ) to evaluate the alveolar heterogeneity and use trans-pulmonary pressure and stain to evaluate the lung injury . The study use these methods to figure out how PEEP impact the alveolar heterogeneity and VILI.

Inclusion Criteria:

1. Age≥18 or<80；
2. Moderate or sever ARDS in 2012 <Berlin definition>;
3. Applied mechanical ventilation;
4. Consent signed.

Exclusion Criteria:

1. Asthma or COPD；
2. Cardiogenic edema；
3. Sever neutropenia(<500/mm3)；
4. Hemodynamic astatic: DOPA or Dobutamine >15 µg/kg/ min; NE > 15ug /min；
5. GCS coma score ≤12；
6. Thoracic injury or surgery patients contradict EIT ；
7. Refuse to join in trail；
8. Included in other clinical trail。

Moderate or sever ARDS in 2012 <Berlin definition>;