New transfusion strategies: Red cell substitutes

Red cell substitutes are solutions that can potentially be used in emergenc ies or during surgery when rapid expansion of the blood volume with an oxyg en carrier is needed. The three main types of products in development are b ased on cell-free hemoglobin, perfluorocarbon emulsions, or liposome-encaps ulated hemoglobin. None is currently approved for clinical use, but several are in advanced clinical trials. Outside the red blood cell, hemoglobin is subject to degradation and heme loss. It readily diffuses in the plasma sp ace and effectively scavenges nitric oxide. These properties must be unders tood and controlled if hemoglobin-based products are to fulfill their promi se. The development of red cell substitutes affords us a deeper insight int o how oxygen is delivered to tissues in the microcirculation and how blood- flow distribution is regulated within and between organs. As red cell subst itutes become available to clinicians and scientists, clinical applications are expected to expand.