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International Journal of Bioelectromagnetism Vol. 4, No. 2, pp. 167-168, 2002. |
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www.ijbem.org |
tHE Impedance MEASUREMENT of human blood In relations to THE Hemorheological determinantsL. H. Deng1,2, S. H.
Karagiannoglou2, W. I. Sakkas2, G. D. O. Lowe3,
J. C. Barbenel2 Abstract: Blood impedance is an important area of bioimpedance research with a wide range of applications in cardiovascular hemodynamics and hemorheology. However, it is not well understood how blood impedance measurement is influenced by blood flow determinants, i.e. the hemorhological determinants such as hematocrit, shear rate and blood cells deformability. In an attempt to investigate the relationships between the blood impedance and hemorheological determinants, we developed a system capable of concurrently measuring electrical and rheological properties of blood. Using this system, the impedance of human blood was studied under rheological conditions against the hemorheological determinants. The results show that the blood impedance was determined by all the hemorheological determinants. In stationary condition, the blood impedance was linearly related to the hematocrit with slight deviations of the linear relation by varying plasma contents. In flowing conditions, the impedance was predominantly determined by the shear rate. However, the hematocrit, plasma fibrinogen level and cell deformability, all can alter significantly the effect of shear rate on the blood impedance. It implies that a more rigorous analysis and calibration are needed when one applies blood impedance to cardiovascular circulation phenomena. INTRODUCTIONBlood electrical impedance and its measurement have a wide range of applications in biomedical field especially in cardiovascular medicine such as non-invasive monitoring of vascular hemodynamics known as impedance phlebography [1]. In general, the impedance of blood is determined by a variety of factors including the electrical properties of the blood cells and plasma, the fractional volume concentrations of the blood cells as well as the shape and orientation of the cells [2]. Each of them is complex in its own right. When blood flows, all these factors may change and each factor may contribute in a different way to the overall impedance of the flowing blood. However, the relationship between the blood impedance and the blood flow determinants are not well studied despite the importance of such knowledge to a full understanding and correct interpreting of blood impedance measurement in terms of cardiovascular circulation. This is partly due to a lack of a system to measure the impedance variables in a well-defined flow regime. Thus a system was developed which is capable of concurrently measuring electrical and rheological properties of blood. Using this system, we investigated the relationship between the electrical impedance and the hemorheological determinants including hematocrit representing the volume fraction of blood cells, fibrinogen level in plasma and erythrocyte deformability. The results of this study shows that the blood impedance is related to all the hemorheological determinants, which implies a more rigorous analysis when blood impedance is applied to cardiovascular circulation phenomena as well as a careful calibration of the impedance measurement technique versus the multiple determinants. METHODSThe concurrent measurement system was developed using a Contraves LS30 low shear rheometer as the base unit. A pair of platinum electrodes in cylindrical ring shape were embedded into the wall of Teflon made replica of the Couette type flow chamber of the rheometer. The Teflon flow chamber isolates the measuring unit electrically from the base unit. The electrodes were connected to an auto-balanced impedance measurement system (Wayne Kerr Bridge B642) via a slip-ring mechanism, which transmits the signal from the rotating electrodes to the impedance measurement system. Figure 1. The schematic representation of the concurrent measurement system Human blood were collected from normal subjects and prepared for samples of various hematocrit and fibrinogen concentration as well as altered blood cell’s deformability. The procedures of sample preparation followed the international standard [3] and protocols given in detail elsewhere [4]. Hematocrit was reconstituted a range of 20 to 80 % using either autologous plasma or a solution made of purified fibrinogen in PBS. The latter has a concentration ranging from 2-10 g/l. The deformability of the blood cells were altered by adding different amounts (0.003-0.015 %w/v) of glutaraldehyde [5]. The prepared sample was placed in the measuring chamber and its impedance was measured either at rest or during shearing at a shear rate ranging from 0.945 to 128.5 s-1. Because the impedance was measured in time domain at one frequency (1591.5Hz), the impedance is primarily presented by its conductance component. RESULTSThe results show that the blood impedance is linearly correlated to the hematocrit of the sample as displayed in Fig.1. The presence of the measuring bob of the rheometer only reduced the magnitude of the measured impedance but the nature of linear correlation was unchanged (both the upper and the lower lines in Fig.2). Changing the plasma contents may alter the intercept and slope of the fitted line but not the linearity. Figure 2. The relationship between the impedance and the hematocrit of stationary blood. Figure 3. The normalized conductance increase of blood versus shear rate, at different hematocrit. Fig.2 shows that when subjected to flow, the blood impedance was predominantly determined by the shear rate. The normalized conductance increase due to flow was below 5 % at 0.945 s-1 and increased to over 35 % at 128.5 s-1. On the other hand, the hematocrit, the plasma fibrinogen level and the blood cell deformability also determined the impedance of flowing blood. The effect of the hematocrit is shown in Fig.3. The higher the hematocrit, the more conductance increase would result from the flow. The effect of plasma content on the impedance is shown in Fig. 4. Figure 4. The effect of fibrinogen level on impedance change DISCUSSION From this study, it appears clear that the blood impedance is strongly dependent on the factors that also determine the blood flow properties. The important implication is that any application using blood impedance measurement should take into account the effect of these factors and make due calibration or correction for them. In addition, the impedance may be used as an indicator of different rheological properties. As this study was limited to one frequency, the implications of the results should not be applied beyond the limit of the frequency. At the present frequency, the blood cells are almost none conductive. When the frequency increases, the blood cells will become more and more conductive, which will drastically change the impedance characteristics of the blood under test. Therefore further studies at frequency domain, especially at the high frequency range is desired. Acknowledgments: We thank Ian Tullis, Anne Rumley for their excellent technical assistance. This work is partly supported by ORS scholarship. REFERENCES[1] J.Nyboer, Electrical Impedance Plethysmography, second edition. Charles C. Thomas, Springfield, Illinois, 1970. [2] H.Fricke, “Mathematical treatment of the electrical conductivity and capacity of disperse system. I. The electrical conductivity of a suspension of homogeneous spheroids,” Phys. Rev., vol. 24, pp. 575-586, 1924. [3] International Committee for Standardisation in Haemotology (ICSH), “Guidelines for measurement of blood viscosity and erythrocyte deformability.” Clinical Hemorheology, vol, pp. 439-453, 1986. [4] L.H.Deng, J.C. Barbenel and G.D.O.Lowe, “Influence of hematocrit on the erythrocyte aggregation kinetics for suspensions of red blood cells in autologous plasma.” Biorheology, vol. 31, pp. 193-205, 1994. [5] Lowe, Clinical Blood Rheology, CRC Press Inc., Boca Roton, Florida, USA, 1986
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