Hemodilution - A blood conservation measure: Normovolemic, hypervolemic or none? - When and how?

Background: Reports in the literature claim acute hypervolemic hemodilution (HHD) a simple alternative to acute normovolemic hemodilution (ANH). However, this is to be answered definitely, only, by mathematical modeling (MM) of ANH versus HHD. Methods: MM of HHD and ANH is based on an exponential decline of the hematocrit (hct) during isovolemically replaced blood loss (BL) (1). The model analyzes a 'standard patient' (70 kg of body weight (b.w.), presumed blood volume 70 ml/kg b.w., initial hct 45% and minimal hct 18%) with an initial preload (iPL) of 200 ml prior to ANH (4 units exchanged, each 500 ml) as well as for 'no ANH' and maintaining hct_min during ongoing BL; and of 1000 ml iPL for HHD and maintaining hypervolemia during BL. Results: Concerning maximal allowable BL (MABL) it is greatest with ANH (6443 ml) and close for HHD (4310 ml) and 'no ANH' (4469 ml). Considering final hct resulting after BL of 2000 ml and with normovolemia reestablished hct-values are close for HHD (32.1%) and for ANH (35.1%); however, ANH is associated either with wasting of ANH-blood (up to 2000 ml at maximum) if ANH-blood is not retransfused or with severe hypervolemia (up to 2000 ml) if ANH-blood is retransfused. Conclusion: For assumed BL of <3 ltr (i.e. approx. 50% of PBV) HHD is a simple / reasonable alternative ' supplement to ANH, while the latter is effectively administered if assumed BL is >3 Itr and low hct-values are tolerated.