The investigators aim to validate markers of metabolic fluid-responsiveness in children with acute circulatory failure following cardiac surgery. This would allow physicians to identify which patient could benefit the most from fluid expansion, thus avoiding useless and potentially dangerous fluid expansions that could lead to fluid overload. To this end, The investigators will evaluate the diagnostic accuracy of the ratio of central venous to arterial carbon dioxide tension (Pv-aCO2) to arteriovenous oxygen content (CavO2), a simple biological marker of anaerobic metabolism, for the diagnosis of metabolic fluid responsiveness defined as a significant increase in oxygen consumption (VO2) after fluid expansion.