In this study of diagnostic accuracy, the investigators aim to validate a faster, simpler, and noninvasive test of fluid-responsiveness in critically ill children. This test is based on an assessment of the hemodynamic effects of a standardized abdominal compression, using electrical cardiometry. This would help physicians to identify more easily which patient 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 electrical cardiometry-based stroke volume (SV) variation induced by a standardized abdominal compression to predict fluid responsiveness (define as a 15% increase in echocardiographically measured SV after volume expansion) will be measured.