Abstract
This investigation is concerned with the unique property of blood, that its electrical impedance varies with flow. A review of past work on this topic shows that the factors upon which this variation depends have not been properly investigated, and for any clinical application, further knowledge of these factors is required. To this end, conductivity cells for use with flowing liquids were designed, and their electrode characteristics determined using saline solution, plasma and blood. This resulted in a new electrical circuit model being proposed. The impedances of samples of human blood were found as functions of a number of parameters. In the majority of tests results were obtained over a wide frequency range (from about 10 kHz to 8 MHz), enabling electrode polarization to be accounted for. These fairly high frequencies, made possible by the use of a transformer ratio-arm bridge, reduced electrode effects to reasonable values. The change in electrical impedance of blood with flow rate was examined as a function of flow tube bore, electrode disposition, temperature and erythrocyte concentration in addition to its dependence on frequency. Major findings concerned the dependence of this change on both bore of flow tube and electrode disposition. A critical examination of hypotheses previously suggested to explain the mechanism of this change, and also of the rheological properties of blood, together with experimental results described here, led to the proposition that viscosity dependence on shear rate is linked to electrical impedance change with flow rate. Thus it would be logical to regard blood impedance as a function of shear rate, as part of a new theory. Some clinical applications were suggested, and a small diameter catheter for detection and monitoring of flow in narrow arteries was investigated. It was shown that such a catheter would also enable measurements of bore of vessels to be obtained.