The purpose of this study is to observe the changes of physiological indicators such as ventilation and oxygenation in the prone position of ARDS patients, to observe the change trend of each patient's indicators, and to observe whether different trends of indicators predict different prognosis of patients.

PP has been shown to effectively improve the prognosis of patients with severe ARDS, but not all patients with ARDS can benefit from PP, and the implementation of the PP has a certain clinical risks and side effects, we lack a convenient predictors to guide doctors to determine whether patients should implement the PP. Clinical usually observe the P/F improved as whether PP treatment effective reference, but there are literature points out that the rising P/F after PP can not predict the better prognosis. one of the mechanisms for ARDS patients benefit from PP is proved to be improve V/Q, the change of the V/Q may determine whether the PP treatment effectively is an important indicator. PaO2:FiO2 ratio was usually used to evaluate the effect of PP, but some studies did not support this conclusion. PP has been shown to change ventilation-perfusion ratio, dead space may be better physiological indicators, simplify dead space ratio and Ventilatory ratio can be easily accessed at bedside and is associated with mortality of patients with ARDS. The purpose of this study is to observe the predictive value of P/F, VD-etCO2/VT, and VR on the treatment effect of ARDS undergoing PP.

Inclusion Criteria:

1. Intubated patients
2. Undergoing mechanical ventilation
3. Moderate to severe ARDS
4. Undergoing prone positioning
5. Age≥18 years old

Exclusion Criteria:

1. Pregnant
2. Cardiac structural shunt
3. Extracorporeal membrane oxygenation performed
4. Undergo extracorporeal CO2 removal
5. Pulmonary embolism
6. Other situation that may affect the accuracy of PetCO2 monitoring
7. Prone positioning session less than 10 hours

We conducted a prospective cohort study in a intensive care unit in West China Hospital, Sichuan University, China. Adult patients (age≥18years) who were diagnosed with moderate to severe ARDS according to the Berlin definition, received mechanical ventilation and prone positioning were included.

Any data collected during this study can be acquired from the correspondingauthor upon a reasonable request.

Half a year after the publication of the research paper.

Any data collected during this study can be acquired from the correspondingauthor upon a reasonable request.