The value of oxygen-carrying solutions in the operative setting, as determined by mathematical modeling

BACKGROUND: The use of oxygen carriers (red cell RBC] substitutes) in acute trauma and in surgery, with or without the use of acute normovolemic hemodilution (ANH), is being investigated. Mathematical modeling was used to assess the impact of RBC substitutes, with or without ANH, in the elective surgical setting. STUDY DESIGN AND METHODS: Mathematical equations and computer models were developed on the basis of previously described mathematical principles, for better understanding of the potential efficacy of RBC substitutes for blood needs with or without ANH. Savings were calculated for a patient with a blood volume of 5000 mL and an initial hematocrit (Hct) of 45 or 30 percent. RESULTS: Substantial increases in the tolerable blood losses (or reduced allogeneic RBC needs) were most evident when the use of an RBC substitute to achieve severe ANH to a Hct that the patient might not otherwise have been able to tolerate was combined with the use of RBC substitutes as replacement for the surgical blood subsequently lost. However, the benefit was greatly dependent on the patient's initial Hct. For example, for a patient with a blood volume of 5000 mL and an initial Hct of 45 percent, a blood loss of approximately 2500 mL resulted in a final Hct of 28 percent without the use of an RBC substitute or ANH. In contrast, with the combined use of staged ANH with an RBC substitute and the RBC substitute for lost surgical blood, a blood loss of up to 14.5 L could be tolerated. However, in an anemic patient (blood volume 5000 mL, initial Hct 30%), a Hct of 28 percent cannot be sustained without the use of allogeneic RBCs for any of the described strategies, even when blood losses were as low as 1 L. CONCLUSION: The use of RBC substitutes has the potential to result in a substantial reduction in allogeneic RBC exposure. This benefit is essentially limited to the nonanemic patient when the use of an RBC substitute is combined with severe ANH and there is concomitant large perioperative blood loss. Anemic patients can be expected to have only limited benefit, because of an inability to sequester an adequate volume of autologous RBCs via ANH.