Abstract-Analytic techniques that have been successfully employed in materials science, and to a lesser extent in the study of biologic systems, have potential for improving the application of bioelectric impedance provided that both real and imaginary impedance components can be measured with sufficient accuracy over a given frequency range. Since biologic tissue, particularly animal tissue, is typically highly conductive, phase angles are small, making accurate measurements difficult. A practical four-terminal system employing commercial lock-in amplifiers is described and error sources and corrective techniques are discussed.
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.
This investigation examined the feasibility of applying the conductance catheter technique for measurement of absolute aortic segmental volume. Aortic segment volume was estimated simultaneously in vitro by using the conductance catheter technique and sonomicrometer crystals. Experiments were performed in five isolated canine aortas. Vessel diameter and pressure were altered, as were the conductive properties of the surrounding medium. In addition, a three-dimensional finite-element model of the vessel and apparatus was developed to examine the electric field and parallel conductance volume under different experimental conditions. The results indicated that in the absence of parallel conductance volume, the conductance catheter technique predicted absolute changes in segmental volumes and segmental pressure-volume relationships that agreed closely with those determined by sonomicrometry. The introduction of parallel conductance volume added a significant offset error to measurements of volume made with the conductance catheter that were nonlinearly related to the conductive properties of the surrounding medium. The finite-element model was able to predict measured resistance and parallel conductance volume, which correlated strongly with those measured in vitro. The results imply that absolute segmental volume and distensibility may be determined only if the parallel conductance volume is known. If the offset volume is not known precisely, the conductance catheter technique may still be applied to measure absolute changes in aortic segmental volume and compliance.
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