[Calculation of the blood volume to be removed for intentional normovolemic hemodilution].
Intentional haemodilution is more and more frequently used. Although it is a simple technique, it can only be carried out with a maximum of safety if the operator has an idea of the amount of blood that needs to be removed to obtain the required haematocrit value. Several methods have been suggested, using more or less complex calculations, or nomogrammes. A new simple rule is presented here which gives an estimation of the amount of blood to be removed in three steps: 1) the theoretical total blood volume (VS), according to weight, height and sex of the patient, 2) the ratio (R) between the initial and required final haematocrit, and 3) the volume of blood to be removed (Vp) according to VS and R. A mono-compartment model with identical inflow and outflow was used. A multicentre trial with 229 haemodilutions showed that the volume removed (mean = 1,325 ml; SD = 642 ml) was 15% less than the calculated volume (mean = 1,526 ml; SD = 561 ml). No one factor was found to explain this difference. The haematocrit values obtained by microcentrifuge were compared with those obtained by a laboratory automatic counter. The mean of the differences was 0.002 (SD = 0.029). Because of the possible errors involved in estimating the total blood volume and the haematocrit before dilution, it is essential that the haematocrit be checked at least once during the haemodilution.