 Acute kidney injury AKI complicates over 50 of cardiac surgical procedures in children where it increases morbidity and the use of healthcare resources. The pathogenesis of AKI is poorly understood, current diagnostic tests lack specificity and sensitivity, and there is no effective treatment. Improving outcomes in patients at risk of AKI has recently been defined as a National Health Service priority. The investigators are currently undertaking a program of work that is evaluating the role of plasma-derived microvesicles MV and MV associated microRNAs miRNA as diagnostic biomarkers or therapeutic targets in cardiac surgery patients at risk of developing AKI. Preliminary results indicate that these biomarkers may have clinical utility in adults. An important consideration is whether these biomarkers also have utility in children undergoing cardiac surgery. Measurement of MV at serial time points in children presents ethical challenges related to conducting clinical research in critically ill subjects. It also presents technical challenges related to the very small volumes of blood that may be sampled safely from babies and infants undergoing surgery. The aim of the study is to provide estimates of the perioperative variance of MV concentrations in 24 children undergoing cardiac surgery, as well as the frequency of AKI and other adverse events, protocol adherence and recruitment rates. This will assist with the design of a subsequent prospective observational study that will consider the role of MVmiRNA in children undergoing cardiac surgery. 