Arterial Pressure Derived Dynamic Parameters to Detect Preload Responsiveness in Mechanically Ventilated Patients Under Spontaneous Mode

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 mechanically ventilated patients under a spontaneous mode, 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 be 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.

\*Objectives: Primary : to assess the diagnostic performance of the decrease in Pulse Pressure Variation (PPV) during a Passive Leg Raising test (PLR) to predict preload responsiveness in mechanically ventilated patients under spontaneous mode hospitalized in intensive care. Secondary: * Evaluate the diagnostic performance of the increase in Pulse Pressure (PP) during a PLR test to predict preload responsiveness in this same population. * Compare before/after PLR measurements other hemodynamic data (blood pressure, cardiac output, heart rate). * Experimental scheme: it is a diagnostic and monocentric study. * Population / patients: Inclusion criteria: * Patient over 18 years old. * Hospitalized in intensive care. * For which the clinician in charge of the patient will need to predict the response to fluids and decide on a possible infusion of fluids, because of: * Acute circulatory failure with mean arterial pressure \< 65mmHg or \< 30mmHg of its baseline value for hypertensive patients. * And/or other signs of hemodynamic instability (tachycardia, mottling, oliguria, hyperlactatemia). * In mechanically ventilated patients under spontaneous mode. * Previously equipped with an arterial catheter. * Affiliated to a social security scheme. Non-inclusion criteria: * Patient with arrythmia (PPV is not applicable). * Patient with intra-abdominal hypertension (PLR test is not valid). * Patient with a contraindication/impossibility to the PLR maneuver (lower limb amputation, respiratory intolerance). * Patient with poor echogenicity noted previously. * Patient protected by law. * Investigation Plan: Patients meeting the inclusion criteria and not presenting any non-inclusion criteria may be included. The patient will be informed, or his/her relatives if he/she is unable to express their will, of the objectives and the progress of the study. If the patient, or his/her relatives, does not object to participate to the study, the course of the study will be as follows: 1. The following measurements will be performed before and after the PLR test in spontaneously breathing patients (SB) to evaluate the preload dependency: * Hemodynamic data: systolic (SAP), diastolic (DAP), mean (MAP), Pulse Pressure (PP), Heart Rate (HR), value of the Pulse Pressure Variation (PPV), Cardiac Index (CI) if cardiac output monitoring is already present. * Echographic data including: time-velocity integral of sub-aortic flow VTI Sub AO); aortic outflow chamber diameter (Diam outflow tract); data from the transmitral pulsed Doppler as well as the data from the tissue Doppler at the mitral annulus (E wave velocity; A wave; E' wave); left ventricular end-diastolic surface. 2. Collection of demographic parameters (age, sex, comorbidities), acute pathology and severity scores (SAPSII, APACHE II, SOFA) in the patient file. 3. Collection of oxygenation parameters: mode, Fio2 level od pressure support and of PEEP, in addition to the tidal volume and RR.