Tidal Volume Challenge and Lung Recruitment Maneuver (TIDALREC): the Reliability of Pulse Pression Variation or Stroke Volume Variation on Fluid Responsiveness.

In order to predict fluid responsiveness in the operating room and therefore benefit of performing fluid administration to improve patient's hemodynamic status, it will test two ventilation strategies : the Tidal Volume Challenge (VtC) and the Lung Recruitment Maneuver (LRM). The objective is to determine whether the variation of 2 parameters such as pulse pression variation (PPV) and stroke volume variation (SVV) during these 2 strategies, allows to predict fluid responsiveness in the operating room for any heavy surgery. All patients will benefit from the 2 ventilation strategies then a fluid administration, called " fluid challenge ", will be performed to discriminate the true responders and others. The order of the ventilation strategies will be determined by randomization.