DIELECTRIC-PROPERTIES OF HUMAN BLOOD AND ERYTHROCYTES AT RADIO FREQUENCIES (0.2-10-MHZ) - DEPENDENCE ON CELL-VOLUME FRACTION AND MEDIUM COMPOSITION

The dielectric properties of human erythrocytes (red blood cells) suspended in whole blood and in isotonic media at various volume fractions (haematocrits) have been studied in the frequency range 0.2-10 MHz, in which the so-called beta-dispersion due to the Maxwell-Wagner effect is known to occur. The capacitance and conductance at 25 degrees C were measured by an instrument interfaced to a computer. The rectangular sample cavity (1 mi volume) contained four pure gold electrode pins, and the sample could be circulated by a roller pump. The frequency-dependence of the permittivity and conductivity were fitted by non-linear least squares regression. Corrections were applied for non-linearity in the dielectric increment at high haematocrit, and for electrode polarisation when diluting the blood in saline. Data were interpreted in terms of a simple equivalent resistor-capacitor circuit. From the measured haematological values the specific membrane capacitance (C-m) and the conductivities internal and external to the cells (sigma'(i) and sigma'(o) respectively) were estimated. The conductivities behaved in a predictable manner with a mean of 0.458 S.m(-1) (s.d. +/- 0.044) for sigma'(i), whereas the value of C-m (and indeed the actual capacitance of the suspension) was dependent on the amount of plasma present. Hence, in stationary normal (anticoagulated) whole blood samples, C-m was as high as 2.98 mu F.cm(-2) (s.d. +/- 0.40), in contrast to about 0.9 mu F.cm(-2) in blood diluted more than two-fold (to less than 20% hct) in isotonic media. The high value remained when the diluent was plasma. The C-m value returned to a high value when washed erythrocytes were reconstituted with plasma, provided that this was present at above a critical or threshold concentration of about 30 vol% in the medium, irrespective of the haematocrit in the range studied (15-44%). The C-m remained low in serum. When added to washed cells in saline, purified fibrinogen had no effect. However, high C-m values were obtained by fibrinogen supplementation to serum and diluted plasma. Applying moderate flow to whole blood approximately halved its high C-m value in an exponential manner with flow rate, whilst the C-m of washed cells (31-67% hct) slightly increased, and converged to the value for whole blood under flow. We interpret the high apparent C-m value in stationary samples to be a result of rapid cell aggregation in the presence of plasma, where rouleaux formation takes place before visible sedimentation sets in.