Specific impedance of canine blood.

The specific impedance of canine erythrocytes suspended in plasma was measured in the frequency range from 5 kHz to 1 MHz in samples from three animals in the hematocrit range from zero to packed cells at a temperature of 39 degrees C; measurements were made with a conductivity cell using four electrodes and a current density of 21 microA/cm2. With the use of impedance spectroscopy, data were fitted to an equivalent circuit model; model parameters in turn were fitted as functions of hematocrit. The resultant model can be used to predict specific impedance (real and reactive components) as a function of hematocrit and frequency over a frequency range from 5 kHz to 1 MHz and a hematocrit range from 0 to 80. Over a normal range of hematocrits and at frequencies less than 100 kHz, the current is almost exclusively confined to the plasma, and the specific impedance is nearly equal to the real component; however, at higher frequencies, the complex nature of specific impedance becomes important.

Methods of complex impedance measurements in biologic tissue.

Bioelectric impedance measurements have been used to monitor a variety of physiologic events. While important insights have been gained and useful techniques developed, there are a number of limitations to the methods usually employed. Among these are the inability to define current pathways in complex systems and the inability to distinguish between volumetric changes and materials property changes. Methods that have been used successfully in materials science can be used to address these limitations: these methods involve measurements of both real and reactive components over a wide frequency range coupled with various plotting and analytic techniques. Accurate measurement of the reactive component is inherently difficult since biologic systems are highly conductive. In addition, safety considerations have generally limited bioelectric impedance measurements in humans to frequencies above 20 kHz. For these reasons the techniques have not been widely applied in vivo; however, the techniques have been used in studies of cell suspensions and biologic tissue. This paper reviews these applications, summarizes the theory from a materials science viewpoint, discusses the instrumentation considerations for extension of the techniques to other studies, and presents more recent applications.