Viscosity Reduction of Red Blood Cells from Preterm and Full-Term Neonates and Adults in Narrow Tubes (Fahraeus-Lindqvist Effect)

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Abstract
In artificial tubes as well as in blood vessels with diameters less than 500 /µm, blood viscosity decreases with decreasing diameter (Fahraeus-Lindqvist effect). Our study measured viscosity of red blood cells (RBC) from 10 preterm infants, 10 term neonates, and 10 adults by means of a capillary viscometer. RBC were suspended in buffer at hematocrits of 0.20,0.40, and 0.60 1/1 (1.001/1=100%). Tubes with diameters of 50,100, and 500 µm were perfused with these suspensions. Viscosity in the 500-µm tubes was not significantly different, at any hematocrit, among the three groups. Viscosity decreased at each of the adjusted hematocrits in the three groups when going from a 500-µm tube to a 50-µm tube. At a hematocrit of 0.60 1/1, viscosity reduction averaged 48 ± 7% in the preterm infants, 42 ± 8% in the full-term neonates, and 35 ± 5% in the adults, whereas the reductions at a hematocrit of 0.20 1/1 were only 32 ± 6, 27 ± 4, and 24 ± 6%, respectively. For the combined data from the neonates and adults, there was a significant inverse relationship of the viscosity in 50-µm tubes at a hematocrit of 0.60 1/1 to the mean corpuscular volume (r = 0.69). To evaluate whether increased membrane elasticity of neonatal RBC contributes to the stronger viscosity reduction of neonatal RBC in narrow tubes, heated neonatal and adult RBC were also studied. The resulting loss of membrane elasticity caused a marked decrease in the viscosity reduction in 50-µm tubes, particularly in the neonates. These data indicate that both the large cell volume and the increased membrane elasticity of neonatal RBC contribute to the enhanced viscosity reduction in narrow tubes. The stronger viscosity reduction of neonatal RBC in narrow vessels may be an important prerequisite for the low vascular resistance and high flow conditions in neonates. Moreover, the present data suggest that a high hematocrit does not impede blood flow in neonates as much as in adults, unless the neonatal RBC are exchanged for adult RBC.