A lack of practical instrumentation has historically been the major impediment to wider utilization of blood viscometry by clinical researchers and physicians.
The vast majority of prior clinical research involving blood viscosity has been performed with rotating viscometers such as cone-and-plate or Couette viscometers. Cone-and-plate viscometry involves a cone-shaped disc which rotates at a specific angular velocity and a second circular-shaped plate that remains stationary. A blood sample is placed between the cone and plate, and as the rotation of the cone drags upon layers of fluid, shear forces (or tangential frictional forces) are exerted by the fluid on the stationary plate. The circular plate turns at an angle depending on fluid viscosity. A precision spring that is linked to the plate is used to measure the corresponding torque, which is thereby used to determine the viscosity of blood [1].
Revolutionary when it was first introduced in the 1950’s, this technique was initially developed to assess the non-Newtonian viscosity of industrial fluids such as paints and polymer solutions. The system became commercially available as the Wells Brookfield viscometer, and over decades, many thousands of peer-reviewed clinical studies have been published, reporting a wide range of insights generated using this type of instrument. However, these studies yielded incomplete results because rotating viscometers were unable to measure viscosity at more than one shear rate at a time. Each shear rate value for viscosity would have to be manually “dialed” into the machine, and the instruments had limited ranges of shear rates for which they could give accurate measurements. For example, the cone-and-plate viscometer cannot measure viscosity at shear rates less than 25 reciprocal seconds due to inaccuracies in torque measurement. Importantly, because viscosity tests were performed and reported by different researchers at different shear rate values, standardization of methods was impossible, and meta-analyses were technically difficult.
The 1960’s to 1980’s marked three decades of great scientific interest in clinical viscometry. During this first wave of hemorheology research, the Wells Brookfield and Contraves couette-type viscometers attained broad utilization in academia, with U.S. researchers focused primarily on the theoretical implications of hemorheology as a science and the center of clinical viscometry seated in Europe. In 1986, a method for measuring red blood cell aggregation using ultrasound backscattering was developed [2], and in 1994, a laser diffraction technology (LORCA) method was invented for determining red blood cell deformability and aggregation [3].
Building upon basic research conducted at NASA during the 1980’s, a team of scientists based in Philadelphia, Pennsylvania began to investigate ways to overcome the inherent limitations of rotating viscometry. Returning to much earlier principles of capillary blood flow and viscometry, which were pioneered by French biophysicist Jean Léonard Marie Poiseuille in the 1840’s and Swedish pathologist Robin Fåhræus in the 1920’s, Drexel University fluid dynamics researcher Young I. Cho created a simple disposable U-shaped capillary tube designed specifically for assessing a complete non-Newtonian shear-viscosity profile of human blood in 2002. The geometry of the capillary tube was fixed to enable the measurement of blood viscosity in a range of shear rates relevant to pulsatile blood flows within the vasculature. The U-shaped, gravity-driven design enabled a sweeping, decelerating flow profile to be captured as blood equilibrated from one column of the tube through the horizontal capillary to the second column such that the blood viscosity can be measured from a high shear rate of 300 reciprocal seconds to low shear rates of 1 or 5 reciprocal seconds in a single automated scan. This computerized scanning capillary viscometer (SCV) is the most advanced clinical viscometer in the world and has been used in a range of validation tests and clinical studies reported in over 30 peer-reviewed publications.
- Wells RE Jr, Denton R, Merrill EW. Measurement of viscosity of biologic fluids by cone plate viscometer. J Lab Clin Med 1961; 57:646-56.
- Boynard M, Lelievre JC, Guillet R. Aggregation of red blood cells studied by ultrasound backscattering. Biorheology 1986; 24(5), 451-461.
- Hardeman MR, Goedhart PT, Dobbe JG, Lettinga KP. Laser-assisted optical rotational cell analyser (LORCA): A new instrument for measurement of various structural hemorheological parameters. Clinical Hemorheol Microcirc 1994; 14, 605-18.
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