MATHEMATICAL-ANALYSIS OF ISOVOLEMIC HEMODILUTION INDICATES THAT IT CAN DECREASE THE NEED FOR ALLOGENEIC BLOOD-TRANSFUSION

Background: The implementation of acute isovolemic hemodilution prior to surgical blood loss is a strategy used in an attempt to diminish th e need for or obviate allogeneic transfusion and to avert the potentia l, attendant complications. Studies examining the efficacy of this tec hnique have produced conflicting results. Study Design and Methods: Th e present mathematical analysis was undertaken to resolve these confli cts by determining the efficacy of hemodilution and examining the infl uence of the variables affecting the outcome. Efficacy was defined as the volume of additional blood loss permitted and the volume and numbe r of units of allogeneic blood saved from transfusion. A mathematical analysis evaluated the impact of circulating blood volume and initial and target hematocrits on the efficacy of isovolemic hemodilution. it was assumed that 1) hemodilution was completed before surgical blood l oss; 2) transfusion of removed blood was begun when the target hematoc rit was reached and last Surgical blood was replaced at a rate that ma intained the target hematocrit; 3) allogeneic transfusion was begun af ter all autologous blood drawn was transfused 4) normovolemia was main tained; and 5) a unit of allogeneic blood contains 175 mL of red cells . Results: The analysis showed that isovolemic hemodilution can result in substantial additional allowable surgical blood loss that can dimi nish the need for or obviate allogeneic transfusion of red cells. Larg er circulating blood volume, higher initial hematocrits and lower targ et hematocrits increase the efficacy of hemodilution. Removal and isov olemic replacement of 1 to 2 units of blood provide minimal potential savings, as does hemodilution to a circulating (target) hematocrit of 30 percent. The extension of hemodilution to a hematocrit of (or below ) 20 percent allows a disproportionately greater surgical blood loss a nd diminishes the need for allogeneic transfusion. It allows, for exam ple, an additional 4.5 L of surgical blood loss, which represents a sa vings of 4 units of allogeneic blood when a patient with an initial bl ood volume of 5.0 L and a hematocrit of 45 percent undergoes isovolemi c hemodilution to a hematocrit of 15 percent. Conclusion: Isovolemic h emodilution can diminish or in some circumstances eliminate the need f or allogeneic transfusion.