1.  What is blood viscoelasticity?

Blood is a complex fluid whose flow properties are significantly affected by the arrangement, orientation and deformability of red blood cells.

Viscoelasticity is a rheological parameter that describes the flow properties of complex fluids like blood. There are two components to the viscoelasticity, the viscosity and the elasticity. The viscosity is related to the energy dissipated during flow primarily due to sliding and deformation of red blood cells and red blood cell aggregates. The elasticity is related to the energy stored during flow due to orientation and deformation of red blood cells.

2.  Why do oscillatory measurements of viscoelasticity provide a more thorough picture of the physiological flow properties of blood?

Blood flow in the circulation is pulsatile. With each beat, the heart pumps energy into the blood. This energy is dissipated and stored. How the blood will dissipate and store energy is related to both the viscosity and elasticity of the blood. Red blood cells (erythrocytes) play a dominant role in blood viscoelasticity and its response to pulsatile flow.

With oscillatory flow, the BioProfiler and Vilastic-3 can measure the viscosity that is related to energy dissipation and elasticity that is related to energy storage.

3.  Why are steady flow measurements of viscosity insufficient to describe the physiological flow properties of blood?Steady flow conditions do not replicate the pulsatility in the circulation and are blind to the significant parameter of elasticity.
4. Who was the first to demonstrate the viscoelasticity of blood?Professor George B. Thurston, of the University of Texas and President of Vilastic Scientific, first presented the viscoelasticity of blood as a function of shear rate in 1972.
5. How does the structure of the blood change during flow and how does it affect viscoelasticity?Blood is a viscoelastic fluid and its rheological properties, viscosity and elasticity, depend on the rate of flow or shear rate. The changes in viscosity and elasticity are a result of changes in the arrangement, orientation and stretching of the red blood cells. The viscoelastic profile of normal human blood can be divided into three regions: Region A – Low Shear RatesRegion B – Mid-Shear Rates and Region C – High Shear Rates.
Region A – Low Shear RatesIn the quiescent state, normal red blood cells will aggregate in a space efficient manner. In the low shear rate region, the cells are in large aggregates and as the shear rate increases, the size of the aggregates diminish. In this range of shear rates, the viscoelasticity is dominated by the aggregation properties of the red blood cells. Deformability plays a lesser role.

Low Shear Region

 Region B – Mid-Shear Rates, Near Unit StrainIn this region the internal stress due to pressure is sufficient to separate aggregated cells causing breakage of aggregates. Then the cells are progressively disaggregated with increasing shear rate. Increasing shear rate causes the cells to orient in the direction of flow. Above a unit strain, a cell is forced to move past its adjacent neighbor. This is accomplished by orientation and deformation. In this region, the influence of aggregation properties on the viscoelasticity diminish and the influence of red cell deformability begin to increase. 

Mid-Shear Region

Region C – High Shear Rates

In this region the increasing shear rate causes normal red blood cells to stretch or deform and align with the flow. The blood will begin to form layers of stretched and packed red blood cells sliding on layers of plasma. In this region the viscoelasticity of the blood is dominated by the deformability of the red blood cells.

High Shear Region

The consequences of the organization of the red blood cells in each flow region on the viscoelasticity is evident in the shear rate dependent viscoelasticity profile of normal human blood (Hct = 45%) seen in the figure below. Measurements were made at 2 Hz and 22 °C.

Viscoelasticity of Blood

6.  How does red blood cell aggregation affect blood viscoelasticity?As a result of changes in plasma protein concentrations and the influence of certain diseases, the tendency of red blood cells to aggregated can be enhanced or diminished.

The figure at right shows an idealized example of how altered aggregation tendencies can affect viscoelasticity. The viscosity and elasticity of blood with low aggregation tendencies (RED) are below the values for normal blood (BLUE) at low shear rates. The viscosity and elasticity of blood with elevated aggregation tendencies (GREEN) are above those for normal blood at low shear rates. But, in the region of high shear rates, where aggregation effects no longer dominate, the viscosity and elasticity approach the same values in each case.

Aggregation Effects
7.  How does red blood cell deformability affect blood viscoelasticity?Normal red blood cells are deformable but many conditions of disease and trauma can reduce their deformability. Red blood cells with reduced deformability will have difficulty forming layers at high shear rates. In the quiescent state cells with an extreme diminishment of deformability will also have difficulty aggregating.

The idealized example at right shows how the viscoelasticity blood containing cells with low deformability (RED) can differ from the viscoelasticity of blood with normal cells (BLUE).

Deformation Effects

 

8.  What is the relationship between blood viscoelasticity and blood flow in the microcirculation?

The red blood cell is an elastic element that dominates the way blood flows in the microcirculation as well as in larger vessels. The red blood cell is also the primary structural element responsible for blood viscoelasticity. Consequently, the viscoelastic properties of blood also govern the flow through the microcirculation.

9.  Have the viscoelastic properties of blood been correlated with clinical conditions?Variations in blood viscoelasticity among normal individuals is very small. Thus, changes due to disease or surgical intervention can be readily identified, making blood viscoelasticity a useful clinical parameter. Variations in blood viscoelasticity are seen in such conditions as cardiovascular disease, peripheral vascular disease, sickle cell anemia, diabetes, stroke and other conditions.

As an example, the viscoelasticity of an individual’s blood with sickle cell disease is markedly different from the viscoelasticity of normal blood. This is clearly seen at high shear rates where the Patient’s Elasticity (Red) is significantly higher than the Normal Elasticity (BLACK).

Pheresis Example
10. What features must a rheometer have to measure blood viscoelasticity?The measurement of blood viscoelasticity requires oscillatory measurements with high precision and sensitivity. Conventional rotational rheometers cannot meet these requirements. A rheometer must be able to operate at frequencies near the pulse rate. A well designed rheometer should also require small sample volumes, minimally expose the operator to the blood sample and be simple to operate. A rheometer designed for the measurement of blood must meet the unique challenges posed by blood.
11.  Is a rheometer available with the necessary sensitivity for the measurement of blood viscoelasticity?

Yes, the BioProfiler and Vilastic-3 provide the required sensitivity and precision that is unavailable with conventional rotational rheometers. Small sample volumes of 0.5 to 1 ml, simple sample handling procedure and simple system operation have made these instrument   an important part of research at such institutions as:

St. Jude’s Children’s Hospital
University of Texas at Austin
The Cleveland Clinic Foundation
Univeristy of Pittsburgh
McGowan Institute for Regenerative Medicine
Penn State University
SUMMA
University of Kyoto
U.S. Food & Drug Administration
Kansas State University
University of Amsterdam
University of California at Berkley
Lovelace Medical Foundation
University of East Carolina School of Medicine
Universidade Estadual de Campinas(Brazil)
12.  Who can provide expertise in the measurement of the viscoelasticity of blood and other biological fluids?We at Vilastic Scientific can. With over 30 years of experience studying the viscoelastic properties of many biological fluids such as blood, plasma, synovial fluid and as well as in kinetics of coagulation, we can provide expert advice on measurement protocols and consultation on data interpretation.

Where can you get more information?

 

 

 

by phone: +1 512-327-4134

by fax: +1 512-327-0655

by email: rheology@vilastic.com

Please visit our Technical Note: Plasma Viscosity and Blood Viscoelasticity“.

A list of publications on the rheology of blood and other biofluids can be found at,  Blood and Biofluid Bibliography.

www.vilastic.com