title: Mathematical analysis of isovolemic hemodilution indicates that it can decrease the need for allogeneic blood transfusion.
abstract: BACKGROUND
  The implementation of acute isovolemic hemodilution prior to surgical blood loss is a strategy used in an attempt to diminish the need for or obviate allogeneic transfusion and to avert the potential, attendant complications. Studies examining the efficacy of this technique have produced conflicting results.
  
  
  METHODS
  The present mathematical analysis was undertaken to resolve these conflicts by determining the efficacy of hemodilution and examining the influence of the variables affecting the outcome. Efficacy was defined as the volume of additional blood loss permitted and the volume and number 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 loss; 2) transfusion of removed blood was begun when the target hematocrit was reached and lost surgical blood was replaced at a rate that maintained the target hematocrit; 3) allogeneic transfusion was begun after all autologous blood drawn was transfused; 4) normovolemia was maintained; 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 diminish the need for or obviate allogeneic transfusion of red cells. Larger circulating blood volume, higher initial hematocrits, and lower target hematocrits increase the efficacy of hemodilution. Removal and isovolemic 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 and diminishes the need for allogeneic transfusion. It allows, for example, an additional 4.5 L of surgical blood loss, which represents a savings of 4 units of allogeneic blood when a patient with an initial blood volume of 5.0 L and a hematocrit of 45 percent undergoes isovolemic hemodilution to a hematocrit of 15 percent.
  
  
  CONCLUSIONS
  Isovolemic hemodilution can diminish or in some circumstances eliminate the need for allogeneic transfusion.
authors.affiliation: Department of Anesthesia, University of California, San Francisco.
authors.forename: R B
authors.initials: RB
authors.lastname: Weiskopf
mesh_terms: D001803:Blood Transfusion; D006301:Health Services Needs and Demand; D006400:Hematocrit; D006438:Hemodilution / Q000379:methods*; D006801:Humans; D008433:Mathematics; D014184:Transplantation, Homologous