Preparing to see the patient

Can we all agree that a Hct of 69.3 is very high?!

What should be going through your head as you prepare to see the patient?

Can we all agree that a Hct of 69.3 is very high?!

What should be going through your head as you prepare to see the patient?

  • Is the patient experiencing symptoms of hyperviscosity?
  • Is the patient at risk for thrombosis and/or bleeding?
  • What is causing such a high Hct?
  • What diagnostic tests are needed?
  • What are the treatment options?

Let’s begin with some physiological principles.

In the following slides we will:

  • Define viscosity.
  • Consider the determinants of blood viscosity (hematocrit being the most important).
  • Assess how blood viscosity affects blood flow.

Definition of viscosity

Viscosity can be defined as the resistance of fluids against flow, which is related to the internal friction of adjacent fluid layers sliding past one another. Fluids in the center of the tube (e.g., blood vessel) move at a higher velocity than fluids that abut the edge of the tube (e.g., endothelium) where drag occurs.

(Think of viscosity as the “thickness” or “stickiness” of a fluid.)

Learn more here.

Parameters that determine blood viscosity

  • Concentration and deformability of cells:
    • Red blood cells (RBCs):
      • Important parameters include:
        • Hematocrit (Hct) :
          • The most important determinant of blood viscosity under normal conditions.
          • Impact on blood viscosity is much higher at low shear rates (e.g., in veins) than at high shear rate (e.g., in arteries).
        • RBC deformability:
          • Depends on:
            • Internal (cytosolic) viscosity, mainly determined by the mean cell hemoglobin concentration.
            • Membrane viscoelasticity, which is dependent on cytoskeleton proteins and lipid bilayer properties.
            • Surface-area-to-volume ratio.
    • White cells
    • Platelets
  • Concentration of plasma proteins:
    • Fibrinogen
    • α1-globulins
    • α2-globulins
    • β-globulins
    • γ-globulins
Of all the parameters above, the Hct is the most important determinant of blood viscosity.

Blood viscosity as a function of the hematocrit:

Note that the increase is non-linear; increased hematocrit causes a disproportionate increase in relative viscosity. At a normal hematocrit of 40%, the relative viscosity of blood is about 4. As the hematocrit exceeds 50%, there is a rapid increase in viscosity. The viscosity increases threefold between a hematocrit of 50% and 80%. Patients with an abnormal elevation in red cell hematocrit (polycythemia) have much higher blood viscosities. In fact, increasing the hematocrit from 40 to 60% (a 50% increase) increases the relative viscosity from 4 to 8 (a 100% increase). Source.

How blood viscosity affects blood flow

Poiseuille’s Law predicts that any increase in blood viscosity should cause a rise in vascular resistance.

Flow velocity in a given tube depends on pressure and flow resistance. According to Poiseuille’s Law, flow resistance depends on the geometry of the tube [length (l) and radius of the tube (r)] and the fluid’s viscosity (η). Flow velocity is calculated using the following formula:

Q, flow; P, pressure difference; η, viscosity; r, radius of tube; l, length of tube.

When applying Poiseuille’s Law to the cardiovascular system, one must consider the mean arterial pressure, the radius and the length of the vessels, and the viscosity of the blood.


  • Whole blood is a non-Newtonian fluid, which means that its viscosity depends on shear rate. At low shear, blood cells aggregate, which induces a sharp increase in viscosity, whereas at higher shear blood cells disaggregate, deform and align in the direction of flow.
  • The highest viscosity is in the postvenule system. Viscosity is actually lower in the capillary due to central streaming of deformable red blood cells.

Let’s rearrange Poiseuille’s Law into a formula that is more familiar to you and more relevant at the bedside.

We will make the following changes:

  1. Replace blood flow (Q) with cardiac output
  2. Replace pressure (P) with the mean arterial pressure
  3. Replace what’s left of the right hand side of the formula (πr4/8ηl) with 1/total peripheral resistance

Now we can rewrite the equation:

This way, we can appreciate that an increase in blood viscosity results in increased total peripheral resistance and decreased cardiac output.

In summary, an increase in Hct results in increased blood viscosity, which in turn increases total peripheral resistance and decreases cardiac output.

What are other causes of hyperviscosity?

Hyperleukocytosis and leukostasis
Waldenstrom’s macroglobulinemia

Hypergammaglobulinemia is the most common cause of hyperviscosity syndrome. This is an example of elevated plasma viscosity.

Sickle cell disease
Sickle cells have decreased deformability, thereby contributing to blood viscosity.,
Profound anemia
Anemia leads to decreased blood viscosity.
Essential thrombocythemia
Because of their small size, platelets contribute little to blood viscosity, even at high numbers.

Sometimes, the term plasma hyperviscosity is used to describe those cases attributed to increased concentration of plasma macromolecules, especially in monoclonal and polyclonal disorders associated with elevation of immunoglobulins. 

Syndromes of high viscosity:

Mucosal hemorrhage:

  • Epistaxis
  • Gingival
  • Gastrointestinal
  • Retinal

Visual Disturbances:

  • Retinal hemorrhage or thrombosis
  • Papilledema
  • Blurring


  • Somnolence or coma
  • Cerebral hemorrhage
  • Seizure
  • Ataxia


  • High-output heart failure

Now you are armed with important background information as you prepare to see the patient.

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