Office of Naval Research Contract N00014-88-c-0118 Technical Report 90-01 the Disposable Porous Bed Viscometer; a New Method for Measuring Blood Viscosity

A disposable porous bed viscometer (PBV) that has rheologic properties related to that of the microvasculature has recently been developed for the measurement of whole blood viscosity (WBV) under conditions of low shear and low stress. The measured endpoint of apparent viscosity with the PBV is the time for a given volume of whole blood to flow a calibrated distance through the porous bed. The various conditions of sample collection, sample storage and temperature required to obtain accurate and reproducible results with the PBV are described. The mean WBV of 242 normal persons was 22.7 ± 5.3 seconds corresponding to an apparent viscosity of 5.7 ± 1.3 centipoise, a value in general agreement with previous studies obtained using rotational viscometers. There was, as expected, a significant difference in the WBV of normal men and women related to their different packed cell volumes. Platelets and granulocytes also influenced WBV, but in proportion to their contribution to the total packed cell volume. In normal subjects, fibrinogen level did not significantly influence WBV. The PBV is a rapid and reproducible means to measure WBV in normal populations which should have clinical application in disorders in which ease of performance and replication is an important consideration. INTRODUCTION Human blood has been recognized as a non-Newtonian liquid for several decades (1-4). Guyton (5) has emphasized the relation between whole blood viscosity (WBV) and packed cell volume (PCV) at all flow rates. In addition he noted that as the velocity of flow decreases the viscosity of blood increases dramatically (5). This nonNewtonian behavior has been the subject of much research and speculation. Research on the flow properties of blood have led to both macrorheological and microrheological concepts (2,3), according to which either red cell to red cell interactions, individual red cell deformability or both are considered to be the cause of the nonNewtonian properties of whole blood. This non-Newtonian behavior makes it difficult to predict the viscosity of blood in various parts of the vascular tree. A direct measurement is needed when an estimate of viscosity is required. Studies on normal human and animal blood with a coaxial cylinder electromagnetic viscometer operating at low rates of shear stress which are relevant to microcirculatory flow have shown the existence of yield shear stress (YSS), a value below which flow of whole blood totally ceases (6,7). These studies (6) have also shown that the non-Newtonian range extends from 0 up to about 10-50 inverse seconds of shear rate. A limitation of these studies was that they were performed under conditions possibly not relevant to the microcirculation which consists of branching, interlaced capillaries rather than one channel of fixed and constant width. The purpose of the present report is to describe a disposable porous bed viscometric (PBV) device. The PBV was designed to evaluate the apparent viscosity of blood as it flows through the communicating random channels of a porous bed comparable in pore diameter to the lumens of arterioles and venules (8,9). A previous study in dogs has shown that the flow rates in the PBV are comparable to those observed in microcirculatory vessels (8), e.g. approximately 40 u mean pore diameter at low stress and low shear conditions (8). Since this yields measurements within 3-4 minutes after venipuncture, it allows for use of anticoagulated or non-anticoagulated