Treatment of acute respiratory distress syndrome (ARDS) relies on invasive mechanical ventilation with supposedly protective settings (low tidal volume ventilation). Mortality of ARDS remains high in observational studies (40 to 50%). Approximately 30% of ARDS patients exhibit tidal hyperinflation despite low tidal volume ventilation, suggesting that personalization of tidal volume is required to improve ARDS prognostic. To date, reliable bedside tools to adjust tidal volume are lacking. Excessive tidal volume can be detected using computed tomography by quantification of tidal hyperinflation, but this technique is reserved to research studies and requires patient transport to imaging facility.

Mechanical ventilation generates cardio-pulmonary interaction, whose magnitude is influenced by tidal volume and respiratory system characteristics. Pulse pressure variation is a bedside tool with potential to quantify cardio-pulmonary interactions. Increasing tidal volume will decrease right ventricular preload and increase right ventricular afterload, hence maximizing cardio-pulmonary interactions.

The investigators hypothesize that pulse pressure variation might help to detect excessive tidal volume during a tidal volume challenge (i.e. stepwise increase in tidal volume)

Inclusion Criteria:

1. Age greater then 18 years old
2. ARDS according to the BERLIN definition with PaO2/FiO2 ratio ≤ 150 mm Hg
3. invasive mechanical ventilation in volume controlled mode with tidal volume set to 6 ml/kg predicted body weight
4. use of sedation and neuromuscular-blocking agents
5. arterial catheter allowing computation of pulse contour cardiac output calibrated with thermodilution
6. central venous catheter implanted in the superior vena cava territory
7. esophageal balloon
8. Computed tomography planned by attending physician

Exclusion Criteria:

1. Previous inclusion in current study
2. Acute cor pulmonale
3. ECMO
4. Arterial pH < 7.21 despite respiratory rate set to a maximum of 35/min
5. Pneumothorax or bronchopleural fistula
6. Decision to withdraw or withhold life sustaining treatment with 24 hours from inclusion
7. Contra-indication of transport to imaging facility
8. Intracranial hypertension
9. Tricuspid or pulmonary mechanical valve
10. Tricuspid or pulmonary infective endocarditis
11. Pace maker with intracardiac leads
12. Right ventricle tumor
13. Complete left bundle block
14. Intrathoracic metallic device
15. COPD
16. Cardiac arrythmia
17. Vesical pressure > 15 mm Hg
18. Lower limb amputation
19. Inferior vena cava thrombosis
20. Patient under an exclusion period relative to participation to another clinical trial
21. Patient under a legal protective measure
22. Patient not affiliated to social security
23. lac of patient/representative consent
24. Pregnancy
25. Breast feeding