Definitions. 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 is defined as a central venous hematocrit >65%. The clinical significance of this value results from the curvilinear relationship between the circulating RBC volume (hematocrit) and whole blood viscosity. Above a hematocrit of 65%, blood viscosity, as measured in vitro, rises exponentially.
Hyperviscosity is defined as a viscosity >14 cP at a shear rate of 11.5/s measured by a viscometer. (Note: Normal serum viscosity units are reported as centipoises [cP]). Hyperviscosity is the cause of clinical symptoms in infants presumed to be symptomatic from polycythemia. Many polycythemic infants are also hyperviscous, but this is not invariably the case. The terms polycythemia and hyperviscosity are not interchangeable.
Polycythemia. Polycythemia occurs in 2–4% 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.
Hyerviscosity. Hyperviscosity without polycythemia occurs in 1% of normal (nonpolycythemic) newborns. In infants with a hematocrit of 60–64%, a fourth have hyperviscosity.
Pathophysiology. Clinical signs attributed to hyperviscosity may result from the regional effects of hyperviscosity, including tissue hypoxia, acidosis, and hypoglycemia, and from the formation of microthrombi within the microcirculation. An important caveat, however, is that the same clinical signs may result from coexisting perinatal circumstances in the presence or absence of hyperviscosity. Potentially affected organs include the central nervous system, the kidneys and adrenal glands, the cardiopulmonary system, and the gastrointestinal tract. Blood viscosity depends on the interaction of frictional forces in whole blood. These forces are defined as shear stress (refers to frictional forces within a fluid) and shear rate (a measure of blood flow velocity). The shear rate in the aorta is 230/s and only 11.5/s in the small arterioles and venules. As the viscosity increases, such as in the microcirculation, blood with a high hematocrit may virtually cease flowing. 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. Under normal conditions, low plasma fibrinogen levels and, correspondingly, low plasma viscosity actually may protect the microcirculation of the neonate by facilitating perfusion and contributing to low whole blood viscosity.
RBC aggregation. Aggregation of erythrocytes occurs only in areas of ...
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