Background: It has been recommended that intraoperative acute normovolemic hemodilution (ANH) be considered for patients expected to experience surgical blood loss of 20% or more of their blood volume. Previous mathematical analyses have not evaluated the potential efficacy of ANH in terms of fraction of blood volume lost. Since decrease of oxygen-carrying capacity is a function of erythrocyte loss relative to blood volume, the purpose of this analysis was to provide an assessment of ANH applicable to all blood volumes and to determine whether this recommendation is appropriate.
Methods: Equations were developed to describe the fractional blood volume loss (blood volume loss/blood volume; VRem/VBld) required to reduce hematocrit below a “trigger” hematocrit with maintenance of isovolemia. This is also the minimum fractional blood volume loss required for initial erythrocyte savings by any conservation technique. Equations were also developed to describe the fractional surgical blood volume loss for which ANH will obviate the need for transfusion of erythrocytes from any source other than those removed by ANH, and the fractional surgical blood volume loss required for ANH to save a defined volume of erythrocytes.
Results: Acute normovolemic hemodilution can extend the allowable fractional surgical blood loss before erythrocyte transfusion is required. The VRem/VBld required to initiate erythrocyte savings is approximately 0.5–0.9. The efficacy of ANH in terms of erythrocytes saved cannot be expressed as a function of the fractional blood volume lost alone. To save 1 unit of erythrocytes requires a fractional surgical blood loss of approximately 0.7–1.2 for the usual surgical patient when the transfusion trigger hematocrit is 0.18–0.21.
Conclusions: This analysis suggests that surgical blood loss should be 0.50 or more for ANH to begin to “save” erythrocytes and 0.70 or more of the patient’s blood volume for ANH to save 1 unit erythrocytes, for the usual surgical patient with an initial hematocrit of 0.32–0.36 and a transfusion “trigger” hematocrit (the value at which transfusion is initiated) of 0.18–0.21.