Oscillating viscometer--evaluation of a new bedside test

A viscometer for bedside blood measurements was developed, consisting of an oscillating resonator probe mounted directly into a disposable vacutainer tube for blood withdrawal. It was tested in vitro on blood samples with variable hematocrits (20-60%), increasing fibrinogen concentrations (0-20 g/l), increasing concentrations of an admixed radiographic contrast medium and erythrocyte suspensions in dextran 40 and dextran 70. Results were compared with those obtained with a conventional Couette viscometer. Oscillating viscometry yielded generally higher values than Couette viscometry, and had a good sensitivity for changes in hematocrit with a good correlation between the two methods (r=0.96, p<0.0001). Oscillating viscosity depended on the resonator frequency, it was higher at 3900 Hz than at 215 Hz, suggesting a viscoelastic behavior of blood. Erythrocyte aggregation, induced by increasing fibrinogen concentrations or dextran 70, affected oscillating viscometry. At a high frequency, i.e. a smaller penetration depth of the shear wave, oscillating viscosity tended to decrease, which suggests a depletion of the boundary layer from erythrocytes when they aggregate. At low frequency with a deeper shear wave penetration (about 50 microm), erythrocyte aggregation increased oscillating viscosity. Bedside tests in 17 patients with coronary heart disease and 10 controls confirmed the easy practicability of the test and showed lower oscillating viscosity in these patients despite higher fibrinogen concentrations presumably due to increased erythrocyte aggregation. We conclude that oscillating viscometry is an interesting bedside test, which is capable of providing new information on the biorheology of the erythrocyte-poor boundary layer near the vessel wall.