In vitro assessment of “covertly abnormal” blood rheology;critical appraisal of presently available microrheological methodology. A review focusing on diabetic retinopathy as a possible consequence of rheological occlusion

Abstract

Red cell “flexibility”, generally accepted as a pivotal determinant of blood flow in the microvessels, represents the micromechanical “behavior” of a complex structure consisting in a viscoelastic membrane shell and a cytosolic fluid. It is impossible to describe its overall “material properties” in the sense of physical “constants”; only its behavior can be tested in fluid dynamic conditions kept as constant and well controlled as possible. Under such conditions, an overt abnormal flow behavior can easily be diagnosed. Earlier claims of “covertly abnormal red cell flow behavior” in diabetes mellitus, which were based on measurements of “more rigid” than normal cells have recently been contradicted, the earlier results being attributed to technological or to contamination errors (by the presence of white cells). New data obtained by techniques inherently independent of leukocyte artefacts are reviewed: various types of the “single erythrocyte rigidometer” (SER), polymicroviscometry (PMV) of packed red cells, MyNiPore microsieve conductometry (MMC) and whole blood viscometry. All data reviewed clearly confirm the presence, in certain diabetic patients, of microrheologically abnormal erythrocytes. The reason for the failure of conventional techniques to detect these abnormalities is discussed.