Hypervolemic hemodilution: An alternative to acute normovolemic hemodilution? A mathematical analysis

Background. Hypervolemic hemodilution has been proposed as an alternative to normovolemic hemodilution to reduce homologous blood transfusions. So far, convincing data supporting this concept are unknown. Materials and methods. We therefore present a mathematical model calculating the efficacy of hypervolemic, normovolemic, and 'no' hemodilution. Hypervolemic hemodilution constituted volume expansion (20% of estimated blood volume) maintained throughout surgery. Normovolemic hemodilution contained isovolemic exchange of blood (40% of estimated blood volume) vs colloid as well as retransfusing blood plus colloid to maintain minimal acceptable hematocrit, e.g., transfusion trigger. To determine the efficacy of each technique maximal allowable blood loss and final postoperative hematocrit were calculated. Maximal allowable blood loss referred to the amount of blood lost during surgery after which homologous blood transfusion became necessary. Results. Recalculating published clinical data strongly validated the formulas used for our model. Hypervolemic hemodilution always revealed lowest maximal allowable blood losses. Normovolemic hemodilution constantly ensured highest maximal allowable blood losses. For blood losses <40% of blood volume, hypervolemic and normovolemic hemodilution provided almost identical final postoperative hematocrits. But in contrast to normovolemic hemodilution, hypervolemic hemodilution did not carry the risk of severe transient, retransfusion-induced hypervolemia. 'No' hemodilution always gave lowest final postoperative hematocrits. Conclusions. Thus, hypervolemic hemodilution cannot replace normovolemic hemodilution to reduce homologous transfusions, but for blood losses <40% of blood volume hypervolemic hemodilution appears to be superior.