Polycythemia is an increased total red blood cell (RBC) mass. Polycythemic hyperviscosity is an increased viscosity of the blood resulting from, or associated with, increased numbers of RBCs.
Polycythemia of the newborn. Defined empirically as a central venous hematocrit >65%.
Hyperviscosity. Defined as a viscosity >14 centipoises (cP) at a shear rate of 11.5/s and is measured by a viscometer. Viscosity is the measure of a fluid’s resistance to being deformed. It is defined as the shear stress (refers to frictional forces within a fluid) divided by the shear rate (a measure of flow velocity). In neonates, the major determinant of total blood viscosity is hematocrit, with plasma viscosity playing a minor role. The relationship between hematocrit and blood viscosity is exponential. Hyperviscosity is the cause of clinical symptoms in infants presumed to be symptomatic from polycythemia.
Polycythemia occurs in 1% to 4% of the general newborn population. Half of these patients are symptomatic, although it is not at all certain whether their symptoms are caused by polycythemia.
Hyperviscosity without polycythemia occurs in 1% of normal (nonpolycythemic) newborns. In infants with a hematocrit of 60% to 64%, a fourth will have hyperviscosity.
Polycythemia and hyperviscosity are associated with presumed alteration in organ blood flow. In general, there is a decrease in organ blood flow due to changes in red cell mass, arterial oxygen content, and/or viscosity. The decrease in cerebral blood flow and cardiac output may represent a physiologic response to an increase in arterial oxygen content rather than ischemia. Many infants with polycythemia have a reduced plasma volume, which may lead to hypoglycemia (12%–40% of the infants) as glucose is present in the plasma fraction of the blood. Whole blood glucose concentration might be significantly reduced even when the plasma concentration is normal. Clinical signs attributed to polycythemia may result from coexisting perinatal circumstances in the presence or absence of hyperviscosity.
The frictional forces identified within whole blood and their relative contributions to hyperviscosity in the newborn include the following:
Hematocrit. An increase in the hematocrit is the most important single factor contributing to hyperviscosity in the neonate. An increased hematocrit results from either an absolute increase in circulating RBC volume or a decrease in plasma volume.
Plasma viscosity. A direct linear relationship exists between plasma viscosity and the concentration of plasma proteins, particularly those of high molecular weight, such as fibrinogen. Term infants and, to a greater degree, preterm infants have low plasma fibrinogen levels compared with adults. Consequently, except for the rare case of primary hyperfibrinogenemia, plasma viscosity does not contribute to an increased whole-blood viscosity in the neonate.
Red blood cell aggregation. Occurs only in areas of low blood flow and is usually limited to the venous microcirculation. Because fibrinogen levels are typically low in term and preterm infants, RBC ...