Initial fluid resuscitation remains the first treatment step for most children experiencing circulatory failure and/or systemic hypotension. Only one-half of these patients respond to fluid administration by a significant increase in cardiac output. A positive fluid balance is a poor prognostic factor that increases mortality. There are few markers validated in children to assess volume reactivity by dynamic ultrasound parameters mainly based on heart-lung interaction.

In this work, the investigators propose to investigate whether dynamic parameters validated in adults, such as the superior vena caval collapsibility and the variability of cardiac output during an end-expiratory and end-inspiratory occlusion, are also reliable indicators of volume responsiveness in sedated children under controlled-mode ventilation.

After standardized volume expansion, patients who present an increase of cardiac index measured by transthoracic echocardiography greater than or equal to 15% of baseline measurement will be considered responders .

A previous similar study using respiratory variations in aortic blood flow to predict fluid responsiveness in ventilated children reported a ROC curve area 0.85, while the prevalence rate of circulatory failure was 50%. Given these assumptions, a sample size of 38 subjects per group (Responders / No responders) was estimated to provide a AUC equivalent. A total of 76 subjects should be included.

Inclusion Criteria :

• Patient < 18 years (child and neonate)
• Sedated and mechanically ventilated under controlled-mode ventilation
• In whom fluid administration was planned by the attending physicians

Exclusion Criteria :

• High-frequency oscillatory ventilation
• Cardiac arrhythmia
• Congenital heart defect