Volume expansion is frequently used to treat critically ill patients with acute circulatory failure. The goal of volume expansion is to increase left ventricular stroke volume and consequently cardiac output. However, about 50% of patients with acute circulatory failure will respond to fluid challenge (preload- dependent patients). Therefore, the ability to predict fluid responsiveness in critically ill patients is crucial, particularly for ARDS patients because of increased alveolar-capillary membrane permeability, and avoiding unnecessary fluid loading has been shown to have a positive effect on patient outcome.Among the dynamic parameters used at the bedside to identify fluid responsiveness, pulse pressure variation (ΔRESPPP) is one of the most accurate in patients with acute circulatory failure receiving invasive mechanical ventilation. However, most studies evaluated patients ventilated with large tidal volumes (≥ 8 ml/kg). Therefore, the validity of ΔRESPPP to identify fluid responsiveness is still debated when lower tidal volumes are used.