Multi-frequency Bioimpedance Variations in a Simulated Human Forearm
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Bioimpedance analysis (BIA) is a popular technique used in the monitoring of various physiological parameters like arterial oscillation, blood volume flow rate and cardiac output. BIA is based on measuring the impedance of the tissue under test which reflects the dielectric behavior of the tissue along with the associated dynamics. This technique generally finds applications as single frequency BIA (SF-BIA) in the form of impedance cardiography (ICG) and impedance plethysmography (IPG), or multi-frequency BIA (MF-BIA) in the form of impedance spectroscopy and tomography. Existing methods of hemodynamic monitoring employ SF-BIA (such as ICG) where a single frequency current is introduced into the tissue, and the obtained output is processed to estimate parameters like stroke volume, cardiac output, and pulse wave velocity (PWV). SF-BIA provides an approximate response of the volume changes and is unable to distinguish the impedance contributions of a single tissue domain from the overall measurements. This research aims at investigating the effect of blood flow-induced changes in the radial artery cross-section in the human forearm through MF-BIA. This offers a novel approach to analyze the multi-frequency impedance response related to blood flow in the peripheral arteries and relate the impedance changes to estimate the changes in the diameter. The thesis presents a simulation model of the fat, muscle and artery tissue layers in a section of human forearm. The model, although assuming isotropic dielectric properties for each tissue, aims at simulating the dielectric response of the tissue layers within the major portion of β dispersion frequency range – 1 kHz to 2 MHz. The main aim of this analysis was to understand the effect of pulsatile blood flow on the MF-BIA response, which was realized by simulating impedance measurements at three radial arterial diameters – 2.3 mm, 2.35 mm, and 2.4 mm. The results indicated a non-linearly decreasing behavior of the impedance spectra with increasing artery diameters, and a Cole-type response. Moreover, a human forearm phantom was developed, to mimic the dielectric properties of human tissues, with the same tissue layers as the simulation model. A coaxial cylindrical sensor was developed and calibrated to estimate the dielectric properties of liquid mixtures and the research identified blood can be simulated using 80% propylene glycol and 20% 4 M NaCl solution, muscle using 3.77% agar and 1.88% gelatine suspended in 0.3% NaCl solution with 18.8% propylene glycol and fat using a suspension of 5% agar in 0.05% NaCl solution. The phantom was tested for the impedance response at the three arterial diameters within the same frequency range and agreed with the simulation response. Analytical modeling was undertaken to investigate, parametrically, the behavior of the system. Two approaches were undertaken – a parametric Debye-type modeling to estimate the impedance contribution from different layers and a more realistic Cole model to fit the response in terms of Cole parameters. Moreover, a modified two dispersion Cole model was proposed to explain the contribution of the artery diameter to the impedance spectra. All models fitted the simulation and experimental data reasonably well and explained observed behavior with artery diameter changes. Finally, a pilot study was performed to measure impedance from three human subjects and estimate the radial artery diameter changes from the measurements. The methodology was validated by comparing the results against ultrasound measurements, performed concurrently on the subjects along with the impedance measurements. The impedance derived diameters measured between 2.2 – 2.4 mm for the 10 frequencies between 3 kHz – 127 kHz with peak-to-peak changes of 0.05 – 0.15 mm. This was found to be in proportion with the ultrasound measurements which yielded diameters between 2.1 – 3 mm for the three subjects with 0.15 – 0.35 mm of peak-to-peak changes. The method exhibited expected behavior and showed promise for further development. In summary, this research aims at investigating the potential of employing MF-BIA to target SF-BIA applications, one of which is hemodynamic monitoring in the human forearm. The objective is to investigate the potential of utilizing MF-BIA approach to overcome the drawbacks of SF-BIA, which is more of an approximated approach. This study has focussed on analyzing the contributions of forearm tissue composition and blood flow in the radial artery, validating the utility of multi-frequency impedance assessment of tissues for more accurate prediction of physiological changes.