The sedimentation of dextranated human red blood cells in the presence of controlled shear

The object of this work is to investigate the sedimentation behaviour of human erythrocytes at controlled shear rates, with a view to establishing a clinical test for the Erythrocyte Sedimentation Rate (ESR). An instrument (the Perspex Device) is designed and constructed for this purpose, consisting of a transparent concentric cylinder viscometer in which the inner cylinder (with a range of diameters) is rotated to produce shear rates down to 0.12 inverse seconds. The settling erythrocytes are monitored with a cathetometer. Simple solutions of various dextrans in physiological saline are used as the suspending media, and the experiments are performed at a temperature of 25.0 degrees C with a standardised haematocrit of 40.0 %. Settling at controlled shear rates is found to be reproducible compared with that found in vertical Westergren tubes, due to the disruption of three-dimensional aggregate networks, which cause low reproducibility in the tubes where the shear rate varies from a maximum at the tube wall to a minimum at the tube axis. Several blood and instrument parameters are examined for their influence upon the sedimentation behaviour of erythrocytes; these include the effect of instrument gap, the variation of which produces a reproducible wall effect. This wall effect gives rise to higher values of critical shear rate (required for a maximum sedimentation rate) for decreasing values of instrument gap. Viscosity measurements are made to allow corrected sedimentation rates to be obtained, and to emphasise the importance of good temperature control. The protective effect of dextrans against red cells undergoing haemolysis is studied, and it is indicated that the phenomenon is not due to a raising of the osmotic pressure of these solutes. Insignificant levels of haemolysis are produced when erythrocytes are sheared at 5410 inverse seconds for up to 150 minutes in a viscometer. Recommendations are made to eliminate the wall effect in the Perspex device and allow the instrument to be used as a clinical test for the sedimentation behaviour of blood which is more reproducible than the tests currently used.