Pathological Flow Properties of Blood in the Fetus and Neonate

Normal Hemorheology: Plasma proteins, plasma viscosity, and red blood cell (RBC) aggregation are very low in the fetus and the preterm infant, but increase with increasing gestational and postnatal age. Moreover, both the viscosity (Fåhræus-Lindqvist effect) and the hematocrit reductions (Fåhræus effect) in narrow tubes are more pronounced in the fetus and neonate than in adults. As a result of these peculiar hemorheological properties, viscosity of blood (measured in 50 μm diameter tubes) with given hematocrit is markedly lower in the fetus (−50% at 25 wk gestation) and neonate (−25%) than in adults. Blood viscosity in 50-μm tubes is similar in neonates with a hematocrit of 0.70 l/l as in adults with a hematocrit of 0.50 l/l. This may explain why polycythemia does not impair systemic, cerebral and intestinal RBC transport in neonates as long as the hematocrit does not exceed 0.70 l/l and why polycythemic neonates are less susceptible to serious complications than polycythemic children and adults.

Pathological Hemorheology: Since high flow conditions in the fetus and neonate are strongly dependent on the favorable rheologic properties of their blood, hemorheolocal abnormalities may contribute to the high risk of the fetus and neonate to ischemic damage of vital organs (e.g. brain) in a variety of disorders: 1) Perinatal asphyxia may be associated with impaired RBC deformability, increased RBC aggregation and leukocyte count, thereby aggravating tissue hypoxia. 2) In growth-retarded fetuses, increased hematocrit, RBC aggregation and leukocyte count may contribute to the high risk of ischemic damage of vital organs (e.g. brain). 3) Fetal and neonatal cells are less protected from radicals than adults, thereby predisposing them to oxidative damage. 4) Neonatal septic shock may be associated with markedly decreased RBC defo rm ability , increase in the count of rigid, immature leukocytes, RBC aggregation and plasma viscosity. 5) Infants of poorly-controlled insulin-dependent diabetic mothers show markedly increased hematocrit and decreased RBC deformability. This may increase their risk of thromboses.