Fluid-responsiveness in Children Ventilated With LOW Tidal Volume - The FLOW Study.

Inclusion Criteria: * Age between 0 days and 15 years old * Patient hospitalized in a pediatric intensive care unit. * Prescription of a 10ml/kg (±20%) fluid expansion by the physician in charge, as a standard care treatment of circulatory failure (precise indication is up to the physician in charge). * Mechanical ventilation * Tidal volume \< 6ml/kg (±15%) * Fluid responsiveness echocardiographic assessment as of routine care Exclusion Criteria: * Prematurity (corrected gestational age bellow 37 weeks) * Arythmia * Hemodynamic instability making dangerous the delay necessary for any measurement. * Prone position * Impairment of echocardiographic acoustic window * Extracorporeal membrane oxygenation * Respiratory rate (measured) \> 60 cycles per minute (including high frequency oscillation) * Significant breathing movements (high work of breathing, clinically assessed) * Restlessness with desynchronization patient/ventilator * Open chest * Cardiogenic pulmonary oedema * Known intracardiac or vascular shunt * Known hemodynamically significant valvopathy * Opposition to participate expressed by the patient or by a parent or legal guardian