[Artificial blood substitutes ].

Abstract

The most important function of blood is gas transport. In the attempt to replace this function, two lines of investigation were followed. Gas transport using of hemoglobin involves saturable physicochemical mechanisms. The affinity of human "stroma-free" hemoglobin for oxygen is too high, hampering the release of oxygen in tissue. The binding of oxygen to stroma-free hemoglobin can be improved by coupling of hemoglobin to pyridoxal phosphate. By cross-linking with, for example, dialdehydes (particularly glutaraldehyde) the production of hemoglobin molecules of higher molecular weight and greater stability is possible. However, first trials in humans using these preparations fall short of expectations. The stability of the polymers and of the preparations was not as good as expected. Another possibility is the use of water-insoluble fluorocarbons (perfluorocarbons), which have a high capacity for physical gas transport. The disadvantage of using emulsions of these substances is the necessity for high partial oxygen pressure to attain sufficient gas transport. The complete insolubility in water impedes the use of the metabolic inert fluorocarbons, because they can only be eliminated via the lungs. Despite these problems the fluorocarbons have been extensively used in humans. In the USA the use of some emulsions is allowed for special indications. Extended clinical use of fluorocarbons and of hemoglobin derivates cannot be expected in the near future. Substitution of the blood hydrocolloid albumin has been practiced for many years, synthetic hydrocolloids being used to replace the colloid osmotic pressure of albumin and also to decrease the blood viscosity by hemodilution and hence improve flow. In Germany hydroxyethyl starch (HES) is the most-used hydrocolloid with the least side effects. Anaphylactic reactions restrict the use of dextran, and gelatin derivatives, because of their short half-life, are not of clinical importance in Germany. Hydrocolloids with a half-life of 3-6 h (dextran 40, middle-substituted HES) are preferred, while those with a half-life of about 20-30 h (dextran 60/70, highly substituted HES) are infrequently used.