Blood substitutes. What they are and how they might be used.

Three classes of materials have been studied as potential blood substitutes: modified hemoglobin solutions, perfluorocarbon emulsions, and liposome-encapsulated hemoglobin. The first two have reached phase III clinical trials, while the third remains in the preclinical state of testing. Hemoglobin is a highly active molecule; hence, modification has been required to avoid potential deleterious effects. Although there has been considerable progress toward bringing such a product to the clinical setting, its development has challenged our understanding of oxygen delivery and use. The perfluorocarbon emulsions have been studied primarily for roles other than as equivalents of conventional banked units of RBCs for transfusion. The study of these molecules has added to our understanding of basic physiologic processes.