Blood viscosity and optimal hematocrit in preterm and full-term neonates in 50-to 500-µm tubes

Abstract

Blood viscosity is an important determinant of blood flow resistance. Because a substantial part of flow resistance arises in small arteries and arterioles with diameters of 100 ixm and less, rheologic properties of blood from preterm infants (24 to 36 wk of gestation), full-term neonates, and adults were measured in glass tubes with diameters of 50, 100, and 500 for a wide range of adjusted feed hematocrits (0.15-0.70). At each of the feed hematocrits, blood viscosity decreased when going from a 500-jum tube to a 50-/tm tube. The viscosity reduction increased with increasing hematocrit. Moreover, the viscosity reduction was more pronounced in the neonates than in the adults. At a hematocrit of 0.70, the viscosity reduction averaged 56% in preterm infants, 50% in full-term neonates, and 39% in adults (p < 0.005). However, the viscosity reductions at a hematocrit of 0.30 were only 35, 29, and 19%, respectively (p < 0.05). In all four groups, blood viscosity increased exponentially with increasing hematocrit. The steepness of the hematocrit-viscosity curves decreased with decreasing tube diameter and with decreasing maturity of the infants. Erythrocyte transport efficiency (hematocrit/blood viscosity) was calculated to estimate the optimal hematocrit (i.e. hematocrit with maximum erythrocyte transport). In 500-Aim tubes, the optimal hematocrit was about 0.40 in all of the groups. In 100-jum tubes, the optimal hematocrit was 0.44 ± 0.05 in the adults and 0.52 ± 0.04 in the neonates (p < 0.05). In 50-/tm tubes, the optimal hematocrit was 0.51 ± 0.04 in adults and 0.60 ± 0.05 in the neonates. There was no significant difference in the optimal hematocrit among preterm and full-term infants. Our results suggest that the strong viscosity reduction at high hematocrits may help to maintain oxygen transport in polycythemic neonates. © 1992 International Pediatric Research Foundation, Inc.