Objective: Primary outcome : To evaluate the effects of CCAC on ventilation/perfusion ratios during moderate to severe ARDS. Secondary outcome : To evaluate the effects of CCAC on hemodynamics : left heart morphology, systolic and diastolic function, cardiac output, right heart morphology, systolic function, pulmonary hypertension, volemia. Method In patient with moderate to severe ARDS, CACC is performed manually and the pressure applied will be maintained between 60 and 80 cmH2O. Electrical impedance tomography of ventilation and perfusion will be used for the measurement of the percentage of areas with normal VA/Q ratios, areas of shunt and areas of dead space effect. Left heart morphology, systolic and diastolic function, cardiac output, right heart morphology, systolic function, pulmonary hypertension, volemia will be evaluated by using echocardiography.