The presence of significant amounts of heparin in blood samples during cardiopulmonary bypass induces artifactual errors when thromboplastin is used as a reagent. For this reason, whole blood coagulation monitoring with ROTEM® has not been feasible during cardiopulmonary bypass with heparin anticoagulation. Recently recombinant thromboplastin has come available. It's stability is guaranteed by the manufacturer for plasmatic heparin concentrations up to 5UI/l .