Ethical approval was gained and consent was obtained from the patient's parents. An observational design was adopted for the present study. Cardiopulmonary bypass was conducted as is routine at the Freeman Hospital. The pump prime had an average volume of 900 ml. This consisted of one unit of fresh frozen plasma, packed cell to target haematocrit of 25%, sodium bicarbonate, heparin, and a balanced crystalloid solution. All patients were subjected to extreme hypothermia (17–21°C), received heparin (3 mg/kg) and protamine reversal, and received modified ultrafiltration after separation from bypass. Blood samples were taken before bypass, on full flow, at minimum temperature, after rewarming, after modified ultrafiltration and on return to intensive care. The samples were analyzed for haemoglobin, platelets, standard coagulation tests and colloid osmotic pressure. The samples were stored for analysis of factors II, V, VII, VIII, IX and X, antithrombin III, and markers of activation (TAT, PTf 1 + 2, D-dimers).