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NeoReviews Vol.5 No.10 2004 e406
© 2004 American Academy of Pediatrics

Blood Viscosity of the Neonate

^* Professor of Pediatrics; Director, Division of Neonatology, Department of Pediatrics, University of Heidelberg, Heidelberg, Germany

The first 300 words of the full text of this article appear below.

	Objectives

 
After completing this article, readers should be able to:

1. List the determinants of blood viscosity in the neonate.
   
2. List the causes of polycythemia and hyperviscosity in the neonate.
   
3. List the signs, symptoms, and diagnostic criteria for polycythemia and hyperviscosity.
   
4. Describe steps for prevention and treatment of polycythemia and hyperviscosity in the neonate.
   

	Introduction

 
Blood viscosity is defined as resistance to the movement of blood. In a circular vessel or tube, the resistance (R) increases with increasing viscosity (V) of the moving fluid and with the resistance resulting from the vessel geometry (Z): R=Z · V. Thus, blood viscosity describes the contribution of blood rheologic factors to blood flow resistance. However, blood viscosity depends on several factors, and the importance of these factors differs among various vessels. The major determinants of blood viscosity are the hematocrit, plasma viscosity, red blood cell (RBC) aggregation, RBC deformability, leukocyte properties, vessel diameter, and the shear forces acting on the RBC. (1)

	Determinants of Blood Viscosity in the Neonate

 
The hematocrit generally is assumed to be the most important determinant of blood viscosity. Blood viscosity rises exponentially with increasing hematocrit (Fig. 1). A marked rise in blood viscosity is seen when the hematocrit exceeds 0.65 L/L, which typically is defined as the threshold for hyperviscosity of the blood. However, in narrow arteries and arterioles and in capillaries, the plasma viscosity becomes the dominant determinant of blood viscosity. The fetal hematocrit increases from 0.33 L/L in the 12th week of gestation to 0.45 L/L in the 30th week and 0.50 L/L in the 40th week at term. At birth, the hematocrit may rise markedly as a result of "blood transfusion" from the placenta to the neonate. (2) A postnatal hematocrit of 0.45 to 0.65 L/L is considered normal in the healthy term neonate.

View larger version (18K): [in this window] [in a new window]   Figure 1. Blood viscosity in tubes that have . . . [Full Text of this Article]

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