Patients will be aposteriori sorted in two groups: Fluid responders and Fluid non-responders. Fluid responders are defined as patients that show an increase greater than 15 % in stroke volume. The diagnosis ability of the Stroke volume variation after an abdominal compression to predict fluid responsiveness will be investigate and receiving operative characteristic (ROC) curve will be built. The correlation between the variation of stroke volume during abdominal compression and during the fluid load will be studied. Other parameters such as arterial pressure and heart rate will also be investigated.