The management of septic shock patients includes the infusion of fluids, however fluids may be deleterious if the patient does not respond by increasing cardiac output. By consequence, it is now recommended to predict the fluid response (fluid or preload responsiveness) before infusing them. In this protocol, the investigators will include critically ill patients spontaneously breathing, for whom the physician in charge has decided to test preload responsiveness. The investigators will collect from the continuous monitoring of arterial pressure of the patient the pulse pressure(PP) which is the difference between systolic arterial pressure and diastolic arterial pressure and the pulse pressure variation (PPV) automatically displayed by the monitor in addition to other clinical (hemodynamic, respiratory) parameters. After one minute of passive leg raising manoeuvre (PLR) the investigators collect the same parameters and the investigators will compare the changes of these parameters in patients who are preload responsive to patients who are not. Preload responsiveness will de defined by echocardiographic parameters before and during PLR. More exactly, a surrogate of cardiac output measured by echocardiography wich is VTI of the sub-aorti flow; an increase of more than 12% defines apreload responsive patient.