Methods: This cross-sectional, observational study included 37 critically ill patients with acute circulatory failure requiring fluid challenge. They were sedated and mechanically ventilated with Tidal Volume (VT) 6-7 ml/kg IBW (ideal body weight), monitored by pulmonary artery catheter and arterial line. Mechanical ventilation and hemodynamic parameters, including ΔRESPPP, were measured before and after fluid challenge with 1,000 ml crystalloids or 500 ml colloids. Fluid responsiveness was defined as an increase of at least 15% in cardiac index.