The effects of fat emulsions on adult and neonatal blood viscosity were investigated under controlled conditions in vitro over a wide range of shear rates from 0.35 sec−1 to 1500 sec−1. A 20% fat emulsion (Liposyn), currently used for parenteral nutrition, was added to anticoagulated blood samples at a concentration approximating the highest blood levels expected clinically (0.01 ml 20% Liposyn/ml blood) and at a concentration four times greater. Apparent viscosities of these and control samples without added emulsion were measured at constant hematocrit and plasma protein concentration at 37°C in order to determine the types of interactions that occurred between blood constituents and the lipids. There was no difference found in plasma viscosity, or in whole blood apparent viscosity, between control samples and blood containing the lower concentration of lipid emulsion. Blood containing the higher concentration (0.04 ml 20% Liposyn/ml) of fat emulsion showed a 2–25% increase in whole blood apparent viscosity, with the magnitude of the increase greatest at low shear rates, and also greater in neonatal blood compared to adult blood. In addition, plasma viscosity was also increased at the high lipid dose by an average of 9% and 17% in adult and neonatal blood, respectively, independent of shear rate. Mechanistic analyses of the observed increases suggested that the majority of the whole blood viscosity increase was due to the plasma viscosity increase rather than to large changes in red blood cell aggregation. These viscosity results indicate that although very high doses of lipid emulsions are capable of altering plasma and whole blood viscosity in adults and neonates, the effect within the range of physiologic shear rates is small, and the usual doses of lipid emulsions used clinically are not likely to cause significant alterations in blood viscosity under conditions of normal flow.
