Principle of operation

Introduction

Tissue is provided with oxygen, nutrients etc. by virtue of the blood circulation, which also transports metabolic waste products from these tissues to e.g. the kidney and lungs. The amount of oxygen transported through the capillaries is directly proportional to the amount of oxygenated hemoglobin present in the blood and inversely proportional to the flowing capacity i.e. the viscosity of the blood. In the normal situation the latter is mainly determined by the concentration of red blood cells (RBCs) (hematocrit), their property to deform in order to pass through capillaries with diameters smaller than that of the RBCs and their potential to form aggregates (rouleaux) in low flow regions, which could disturb the flow even more. Disturbances of either one or both of these RBC properties have been described in a wide variety of diseases (References.2-4,7,8,14,21,26,32,42,43,61,63,64)

In order to evaluate the flowing capacity of blood in different parts of the body it is therefore important to have a technique available for the accurate measurement of these RBC properties. The LORCA is therefore an indispensable instrument for hemorheological laboratories in research institutes, clinical departments of hematology, intensive care units etc. It uses a Couette geometry with a static BOB and a rotating cylinder (CUP) to create a simple shear flow. The mean geometric shear rate for small gaps, can be estimated as:(References 20 Dognon A., Loeper J., Housset E., Etude optique de l'aggregation reversible des hematies, Comptes Rendus Soc. Biol., vol. 143, pp. 769-771, 1949., 62 White F.M., Flow between long concentric cylinders, in: Fluid mechanics, McGraw-Hill, Fourth edition, pp. 261-263, 1999 (ISBN: 0-07-069716-7).)

(Equation 1)

With:

Red blood cells cause the blood to be non-Newtonian, which means that the viscosity changes with the applied shear rate. (References 12 Chien S., Sung L.A., Physicochemical basis and clinical implications of red cell aggregation, Clin. Hemorheol., vol. 7, pp. 71-91, 1987., 13 Chien S., Usami S., Dellenback R.J., Gregersen M.I., Nanninga L.B., Guest M.M., Blood viscosity: Influence of erytrocyte aggregation, Science, vol. 157, pp. 829-831, 1967.) During the deformability measurements, blood is diluted in a high viscous medium (dilution 1:200) causing the suspension to behave nearly Newtonian. The shear stress is calculated as:

(Equation 2)

With: