<qa>  What is the effect of Variable Ventilation 1 in patients with ARDS?In this interventional study we want to evaluate the effect of Variable ventilation 1 in patients with ARDS. Patients will be mechanically ventilated with Variable Ventilation 3 step with increased level of variability and traditional CMV, for a total of 4 step. Each step lasts in 60 minutes. During this period we will record the gas exchange parameters and the mechanics of the respiratory system. 