BLOOD-VISCOSITY AND OPTIMAL HEMATOCRIT IN PRETERM AND FULL-TERM NEONATES IN 50-MU-M TO 500-MU-M TUBES

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-mu-m 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-mu-m 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-mu-m tube to a 50-mu-m 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-mu-m tubes, the optimal hematocrit was about 0.40 in all of the groups. In 100-mu-m 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-mu-m 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.