Acute respiratory distress syndrome (ARDS) is a heterogeneous syndrome with similar clinicopathological feathers caused by different etiologies. Respiratory supportive strategies is the main ARDS management, and the guidelines recommend low tidal volume to improve clinical outcomes. To be note, overdistension can still occur even if using a tidal volume as low as 6 ml/kg, given the heterogeneous nature of the syndrome. Therefore, adjusting tidal volume level to less than 6ml/kg may reduce ventilator-induced lung injury (VILI) and thus improve outcomes, especially in patients with severe lung injury. Prone position is also an important management in severe ARDS. Prone position can improve ventilation-perfusion (V/Q) matching and reduce the risk of VILI by recruiting dorsal collapsed alveoli. Meanwhile, prone position has also been shown to improve hemodynamics. Recent studies have showed that prone position did not reduce duration of venovenous extracorporeal membrane oxygenation (VV-ECMO) and 90-day mortality in patients with ARDS who receive VV-ECMO with ultra-low tidal volume ventilation. Therefore, the effect of PP on ventilation and lung blood flow in ARDS patients treated with VV-ECMO wiht ultra-low tidal volume ventilation remains unclear.

Inclusion Criteria:

• Adult patients were enrolled if they were undergoing invasive mechanical ventilation and were supported by VV-ECMO for less than 48 hours

Exclusion Criteria:

• younger than 18 years old
• contraindications for prone position ventilation
• past chronic respiratory diseases (long-term family oxygen therapy for chronic
• respiratory diseases such as pulmonary fibrosis or COPD)
• New York Heart Association class above II
• 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)
• severe hemodynamic instability
• gave written or witnessed verbal informed consent

Adult patients with ARDS admitted to the Department of Critical Care Medicine, Zhongda Hospital, Southeast University and receiving VV-ECMO treatment