Patients with ARDS often suffer a gravity-dependent alveolar collapse, resulting in a reduction of tidal volume, residual alveolar excessive distension, and ventilator-related lung injury(VILI) induced by unreasonable ventilator setting.Prone ventilation (PPV) improves the gravity-dependent alveolar ventilation and promotes lung recruitment in the gravity-dependent area and improves lung compliance. Previous studies showed that prolonged PPV combined with low tidal volume(LTV) lung protected ventilation can significantly reduce the mortality of patients with moderate to severe ARDS.Although more than 60% of patients with moderate to severe ARDS due to COVID-19 has been widely implemented PPV,studies showed an improvement in oxygenation in patients with ARDS(the P/F radio improved by more than 20% before and after PPV) was 9-77%, that is, That is, some patients are unresponsive to PPV. In addition, some patients showed CO2 responsiveness after PPV(ventilation rate (VR) decreased significantly after PPV).The tools for monitoring the effects of PPV on ventilation and blood flow at bedside are still lacking, Electrical impedance tomography (EIT) is a non-invasive, non-radiative, real-time bedside lung imaging technique that can monitor local lung ventilation distribution. This study intends to use EIT to evaluate pulmonary ventilation, blood flow distribution and local V/Q ratio before and after PPV, as well as to monitor the changes in pulmonary physiology before and after PPV, explore the mechanism of PPV improving oxygenation by combined with the changes in oxygenation, and explore the factors that predict and affect PPV responsiveness.

Acute respiratory distress syndrome (ARDS) is presented as acute hypoxemia and pulmonary edema due to the increased permeability of alveolar capillaries. Endothelial damage injury and swelling, microthrombosis, and hypoxic pulmonary vasoconstriction can lead to low pulmonary blood vessels perfusion and even occlusion, while patients with ARDS often suffer a gravity-dependent alveolar collapse, resulting in a reduction of tidal volume, residual alveolar excessive distension, and ventilator-related lung injury(VILI) induced by unreasonable ventilator setting.Prone ventilation (PPV) improves the gravity-dependent alveolar ventilation and promotes lung recruitment in the gravity-dependent area and improves lung compliance. Besides, pulmonary blood perfusion is less affected by gravity distribution, thus the improvement of gravity-dependent alveolar ventilation can significantly reduce shunt, and lung heterogeneity and improve V/Q radio. Previous studies showed that prolonged PPV combined with low tidal volume lung protected ventilation can significantly reduce the mortality of patients with moderate to severe ARDS.Although more than 60% of patients with moderate to severe ARDS due to COVID-19 has been widely implemented PPV,studies showed an improvement in oxygenation in patients with ARDS(the P/F radio improved by more than 20% before and after PPV) was 9-77%, that is, That is, some patients are unresponsive to PPV. In addition, some patients showed CO2 responsiveness after PPV (ventilation rate (VR) decreased significantly after PPV).The tools for monitoring the effects of PPV on ventilation and blood flow at bedside are still lacking, Electrical impedance tomography (EIT) is a non-invasive, non-radiative, real-time bedside lung imaging technique that can monitor local lung ventilation distribution. By injecting hypertonic saline through a central vein catheter, we can obtain lung perfusion images to indicate local lung blood flow distribution. In addition, combined with lung ventilation images, we can evaluate the pulmonary shunt, dead space, V/Q ratio, to better clarify the physiological and pathological status of lung.This study intends to use EIT to evaluate pulmonary ventilation, blood flow distribution and local V/Q ratio before and after PPV, as well as to monitor the changes in pulmonary physiology before and after PPV, explore the mechanism of PPV improving oxygenation by combined with the changes in oxygenation, and explore the factors that predict and affect PPV responsiveness.

Inclusion Criteria:

• 1. Age ≥18 years. 2. Patients diagnosed with ARDS according to the Berlin definition and need to endotracheal intubated and mechanical ventilated in prone position within 48 hours of endotracheal intubation 3. PaO2/FiO2 < 150 mmHg with positive end-expiratory pressure (PEEP) ≥ 5 cmH2O according to the Berlin definition.

Exclusion Criteria:

• 1. Contraindications of EIT such as chest wound dressing, installation of pacemaker, defibrillator, etc.

  2. Unstable vertebral fracture 3. Within 15 days after severe facial trauma or facial surgery 4 within 15 days after tracheal surgery or sternotomy 5. Hemodynamic instability or recent cardiac arrest 6. Increased intraocular pressure. 7. Unstable femoral or pelvic fractures and pelvic external fixation. 8 He had severe chest wall disease and unstable rib fractures. 9 Recent cardiothoracic surgery. 10. Pneumothorax 11. Chronic lung disease: severe obstructive pulmonary disease, severe asthma, interstitial lung disease.

  12. Maternal 13. Extracorporeal membrane oxygenation(ECMO) had been administered on admission to the ICU.

  14. Intracranial hypertension 15. Pulmonary embolism, acute or chronic right heart failure 16. Severe cardiac dysfunction (New York Heart Association class III or IV, acute coronary syndrome, or sustained ventricular tachyarrhythmia), cardiogenic shock; 17. No informed consent was obtained

Patients who met inclusion exclusion criteria will implement EIT monitoring at the time of before the prone position, prone position for 16 hours and prone position ending 8h.