Blood and plasma viscosity and optimal hematocrit in the normal newborn infant

Abstract

Rheological properties of neonatal and adult blood, adjusted to fixed hematocrits, were measured using a capillary (100 µm diameter) and a cone-plate viscometer. Compared to adults, the newborn infant’s plasma viscosity was significantly lower (P<0.005). Capillary viscosity of neonatal blood adjusted to hematocrits of 0.15 to 0.70 l/l was also lower, particularly at higher hematocrits. Viscosity of whole blood adjusted to a hematocrit of 0.60 l/l,measured with the cone-plate viscometer, was markedly lower in neonates, while the viscosity of RBC suspended in an albuminated buffer as well as the relative viscosity (i.e. ratio of blood to plasma viscosity) was similar in neonates and adults. Hemoglobin flux through the 100 µm-tubes was higher in newborn infants at any hematocrit with a maximum at a hematocrit of 0.45 to 0.50 l/l compared to a peak flux at 0.40 l/l in adults. These results suggest that the main difference in rheological properties between neonatal and adult blood lies in the lower plasma viscosity of the neonates resulting in decreased blood viscosity and increased hemoglobin flux through tubes.