In this study of diagnostic accuracy, the investigators aim to validate a fluid-responsiveness test in critically ill children mechanically ventilated with low tidal volume. This test is the famous respiratory variability of peak aortic velocity (ΔVPeak), which is based on cardiopulmonary interactions but is only validated in mechanically ventilated children with a tidal volume of at least 8ml/kg, which is uncommon nowadays. This would help physicians to identify children ventilated with low tidal volume and suffering from acute circulatory failure that could benefit from a volume expansion, thus avoiding a potentially useless or even dangerous fluid expansion that could lead to fluid overload.

To this end, the diagnostic accuracy of ΔVPeak to predict fluid responsiveness (define as a 15% increase in echocardiographically measured SV after volume expansion) will be measured in mechanically ventilated children with low tidal volume and requiring fluid bolus.

Volume expansion is the cornerstone of acute circulatory failure treatment in children. However, inappropriate administration can lead to fluid overload, which is associated with poor outcome. Thus, the search for indicators to predict fluid responsiveness is a major issue in pediatric intensive care unit. In such an emergency context, this assessment must be as simple and fast as possible, and ideally non-invasive. In children, ΔVPeak is the most used and studied test and has an excellent diagnostic accuracy. Moreover, this echocardiography-based test is noninvasive and easy to perform. Unfortunately, this test is only validated in children ventilated with tidal volume of at least 8ml/kg, a rare situation in practice. However, some recent data in both adults and preterm neonates suggest that fluid responsiveness tests based on cardiopulmonary interactions could be used even in patients on protective ventilation. Yet, to date, the diagnostic accuracy of ΔVPeak has never been investigated in children on protective ventilation (low tidal volume), although it is now the most common ventilation strategy in pediatric intensive care units. Therefore, the investigators hypothesize that ΔVPeak is reliable to predict fluid responsiveness in children mechanically ventilated with 6ml/kg tidal volume.

Therefore, the investigatorswill evaluate the diagnostic accuracy of ΔVPeak in critically ill children mechanically ventilated with low tidal volume, and for whom the physician in charge prescribed a 10ml/kg volume expansion to treat circulatory failure. The index test will be ΔVPeak. Fluid responsiveness will be defined as a > 15% increase in echocardiographically-measured stroke volume between baseline and within one hour after fluid expansion (gold standard test).

In this non-interventional multicentric prospective study of diagnostic accuracy, children requiring a 10ml/kg volume expansion will be included. Volume expansion will be delayed for a few minutes (<3min) in order to complete the baseline echocardiogram with standardized measurements, but no supplemental blood test or invasive parameters will be collected. Another echocardiographic evaluation will be conducted within one hour after volume expansion. Patients will be follow-up until PICU discharge.

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