In this study, the investigators aim to validate a non-invasive marker of fluid-responsiveness in children with acute circulatory failure based on standardized abdominal compression. This would allow physicians to identify which patient could benefit from a fluid expansion, thus avoiding a potentially useless or even dangerous fluid expansion, leading to fluid overload. To this end, the investigators will evaluate the diagnostic accuracy (sensitivity and specificity) of stroke volume variation induced by standardized abdominal compression for the diagnosis of fluid responsiveness (based on the gold-standard test: significant increase in cardiac index after fluid expansion).

Fluid expansion is the cornerstone of acute circulatory failure treatment in children Although this therapy drastically reduced mortality, several studies in recent years have highlighted the adverse effects of excessive fluid expansion (leading to fluid overload). Currently, the search for indicators to predict fluid responsiveness is a major issue in intensive care. These indicators are based on Franck-Starling's law: if small changes in preload lead to an increase in cardiac output, then fluid responsiveness can be expected. However, in children, the only validated indicator (respiratory variability of peak aortic velocity) can only be used in the absence of any spontaneous respiratory movement, a rare situation in practice. Recently, two pediatric studies investigated a simple clinical test: hepatic or abdominal compression. This clinical maneuver, by increasing venous return via mobilization of the hepato-splanchnic reserve, induces a transient and reversible preload increase. The evaluation of the hemodynamic effects of this "endogenous fluid expansion" allowed, according to the authors, to accurately predict fluid responsiveness. However, several factors reduce the applicability of these results: the small number of studies on this subject, the smaller volume of fluid used than in clinical practice, and the population studied, composed almost exclusively of children in postoperative of cardiac surgery.

In this study, the investigators will evaluate the diagnostic accuracy of abdominal compression for the diagnosis of fluid responsiveness in children with acute circulatory failure, hospitalized in pediatric intensive care unit (PICU) for a medical or a non-cardiac surgical condition, for whom a fluid expansion was prescribed by the physician in charge. The index test will be the stroke volume variation following a standardized abdominal compression (before fluid expansion). The gold standard test will be the variation of cardiac output between baseline and after fluid expansion, a variation > 15% defining fluid responsiveness.

In this non interventional study of diagnostic accuracy, patients will undergo an extra echocardiographic assessment, but no supplemental blood test or invasive parameters will be collected. Simple clinical parameters will be collected within 4 hours after the fluid expansion. Patients will be follow-up until PICU discharge.

Inclusion Criteria:

• Age lower than 15 years old
• Prescription of a 10 to 20ml/kg crystalloid fluid expansion by the physician in charge
• Acute circulatory failure defined by the association of an A criteria (increase of heart rate more than two standard deviation for the age, decrease of systolic or mean blood pressure more than two standard deviation for ) and a B criteria (lactate more than 2 mmol/L, oliguria less than 1mL/kg/h, a capillary filling time more than 3 seconds, mottling)
• Patient hospitalized in PICU

Exclusion Criteria:

• Prematurity (term below 37 gestational week).
• Acute cardiogenic pulmonary edema
• Hemodynamic instability making the delay necessary for abdominal compression and ultrasonography dangerous for the patient
• Non corrected congenital cardiopathy, or inferior to 15 days postoperative.
• Intra-abdominal hypertension or painful abdominal palpation
• Abdominal surgery in the last 15 days
• Supine position contraindicated or deleterious
• No investigator available to assess ultrasonographic measures
• Impairment of echocardiographic acoustic window or restless patient making ultrasonography impossible

At least 24 fluid expansion are needed, in 10 to 24 patients (maximum of 3 fluid expansion per patient). The "number needed to treat" was calculated with Obuchowski method, with an aphla risk of 0.05, a statistical power of 80%, a ratio "responder/nonresponder" of 1:1, and a predicted area under the receiver operating characteristic curve of 0.78 (based on previous studies).

In order to be clinically relevant, since a large majority of patients admitted to our PICU are under 2 years of age, the investigators plan to include 24 fluid expansions administrated to patients under 2 years old. Therefore, the investigators might include more than 24 fluid expansions if some are administrated to patients older than 2 years old. Without this correction, our study might be underpowered if age is found to be a bias in fluid responsiveness.