Difficulties in demonstrating efficacy of blood substitutes

Predictions of capillary oxygen transport in the presence of fluorocarbon additives

A mathematical model of capillary oxygen transport was formulated to determine the effect of increasing plasma solubility, e.g., by the addition of an intravascular fluorocarbon emulsion. The effect of increased plasma solubility is studied for two distributions of fluorocarbon, when the fluorocarbon droplets are uniformly distributed throughout the plasma and when the fluorocarbon droplets are concentrated in a layer adjacent to the endothelium. The model was applied to working hamster retractor muscle at normal and lowered hematocrit. The intracapillary mass transfer coefficient was found to increase by 18% as the solubility was increased by a factor of 1.7 at a hematocrit of 43%. An additional increase of 6% was predicted when the solubility increase was concentrated in the layer adjacent to the endothelium. At a hematocrit of 25%, the intracapillary mass transfer coefficient increased 14% when the solubility was increased by a factor of 1.7.

Current status of injectable oxygen carriers

In this review the current status of what commonly are termed "blood substitutes" is discussed. The term blood substitute is a misnomer because the formulations under development at this time transport respiratory gases but do not perform the metabolic, regulatory, and protective functions of blood. Either hemoglobin or a perfluorochemical form the base to transport oxygen; the advantages and disadvantages of each base are discussed. The availability of a blood substitute in the U.S. will require approval by the Food and Drug Administration (FDA) and, by law, both its efficacy and safety must be demonstrated prior to approval. Showing efficacy of any blood substitute is complicated by the oxygen reserve and the compensatory mechanisms to acute blood loss in man. The challenge is to prove that the administration of these formulations offer clinical advantages compared with replacement of volume alone. Several efficacy models, the most attractive among them being perioperative hemodilution, should provide data that would bring these formulations into clinical practice. When hemoglobin is not within the favorable environment of the red cell, whether the hemoglobin is derived from expression vectors developed through recombinant biotechnology or from lysed human red cells, it acquires a left-shifted oxygen disassociation curve. Further, because the tetramer disassociates when injected intravenously and the resulting dimers are cleared rapidly from the circulation by the kidneys, intravascular dwell time is brief. Hemoglobins have been modified chemically and linked intramolecularly, intermolecularly, and to macromolecules to correct these problems. While these manipulations have normalized the p50 and extended the dwell time significantly, some toxicity problems remain unresolved. The binding of nitric oxide to hemoglobin preparations and the presumably resultant systemic and pulmonary hypertension observed in animals may be the most difficult to overcome, although the implications of these reactions in man is poorly understood. Perfluorochemicals (PFC) provide a fundamentally different and simpler approach to oxygen transport than hemoglobin formulations. Typically, the PFCs used are liquids composed of 8 to 10 carbon atoms that dissolve oxygen and obey Henry's law. Thus, the recipient's inspired oxygen and cardiac output assume importance. Because they are insoluble in water, PFCs are administered as emulsions, that is, as small droplets about 0.1 to 0.2 microns in diameter. In this respect, they are very similar to the lipid emulsions widely used for parenteral nutrition. Egg yolk phospholipid and poloxamers are most commonly used as emulsifiers. PFCs are not metabolized and are excreted unchanged by the lungs, following temporary storage by the monocyte-macrophage system (MMS).(ABSTRACT TRUNCATED AT 400 WORDS)