Blood substitutes

Artificial oxygen carriers are promising substances to avoid allogeneic blood transfusions and related side effects. In addition, artificial oxygen carriers may be effective antiischaemic agents in a variety of diseases and conditions that are characterized by compromised tissue oxygenation. Artificial oxygen carriers may be grouped into haemoglobin-based oxygen carriers (HBOCs) and perfluorocarbon emulsions. The biological efficacy of both groups has been documented in a variety of animal experiments [6,7], and phase III trials are ongoing at present to prove their clinical efficacy.In HBOCs (see article by Baron J-F, this issue of Critical Care), the haemoglobin is either human (outdated human blood), bovine or from a genetically engineered source [7]. All haemoglobin solutions tested contain modified haemoglobin to improve oxygen off-loading, by decreasing oxygen affinity, and to reduce side effects. At present, haemoglobin solutions in clinical trials are aqueous solutions, but the production of micro-encapsulated haemoglobin particles (neo red cells) might be a future option [8,9]. Comparison between HBOCs is difficult because there are very few studies that directly compare different products. It appears though that larger haemoglobin molecules, in particular haemoglobin polymers with a small residual haemoglobin monomer fraction, are better tolerated; this is probably due to a reduced penetration of these relatively large molecules into the vessel wall [10].In the group of perfluorocarbon emulsions, only perflubron emulsion is in phase III testing. It has been shown recently that treatment with perflubron emulsion in conjunction with pure oxygen ventilation was more effective than retransfusion of autologous blood in reversing physiologic transfusion triggers [11]. With perflubron emulsion patients thus may tolerate lower haemoglobin levels, and perflubron emulsion may be used to modify the acute normovolemic haemodilution (ANH) into 'augmented ANH' (see arti