Acute normovolemic hemodilution does not reduce antithrombin concentration for cardiac surgery

Background: Acute normovolemic hemodilution (ANH) is used to reduce allogeneic blood transfusion with cardiac surgery. This procedure involves pre-operatively removing and storing a volume of whole blood and replacing the volume with crystalloid. The stored blood is then available for transfusion, if required. Hemodilution associated with ANH may reduce the effectiveness of heparin anticoagulation due to dilution of antithrombin. The aim of this study was to determine if antithrombin concentrations are reduced in patients who undergo one unit of ANH during cardiac surgery. Methods: Patients scheduled for cardiac surgery (n = 71) were grouped according to whether they did or did not undergo ANH pre-operatively. Antithrombin concentrations were measured before and after ANH. This study had 80% power to detect a difference in reduction of antithrombin concentration of 6% between groups following ANH with an alpha error of <0.05. The effect of one unit ANH was expected to cause a difference of 12% or greater. Results: No significant difference in the concentration of antithrombin between ANH patients and those that did not have ANH, nor was there a difference in the decrease in antithrombin between groups. Conclusions: The results indicate that one unit of ANH does not significantly reduce the concentration of antithrombin prior to cardiac surgery. Thus patients who undergo one unit of ANH are not at increased risk due to dilution of antithrombin.