Estimation of Arterial Diameter Using Bioimpedance Spectroscopy on a Human Wrist Phantom

Yu, Yang
Lowe, Andrew
Anand, Gautam
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Doctor of Philosophy
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Auckland University of Technology

Background: Blood pressure measurement (BPM) is a well-known clinical method to monitor cardiovascular function, and it is also a reliable predictor of death and cardiovascular disease. Since the beginning of this century, a type of cuff-less continuous BPM has been investigated based on the strong relationship between pulse wave propagation (i.e., pulse wave velocity, pulse transit time, and pulse arrival time) and blood pressure (BP). A comprehensive review was undertaken to explore the limitations of the existing pulse wave propagation method (PWPM), including the techniques, mathematical models, and clinical protocols. It was found that the lack of absolute arterial diameter information in previous studies might limit the performance of existing PWPM because the arterial diameter change has been proved to be one significant factor in BP change from both theoretical and experimental perspectives. Hemodynamic monitoring is concerned with the dynamic blood flow within the human circulatory system. Bio-impedance measurement (BIM) can sense the physical and electrochemical processes in human tissues and hence can monitor various physiological variations. In the context of ambulatory hemodynamic monitoring, there has been interest in portable BIM for both diagnostic and research purposes by placing the electrodes on human extremities, such as the wrist. The radial artery is a common location to detect pulsatile blood due to the thin surrounding tissue layers, which has been used for pulse wave propagation determination. When a pulse wave arrives, the amount of blood inside the artery increases and the measured impedance decreases because of the higher conductivity of the blood. Therefore, BIM at the wrist is a promising technique to estimate the arterial diameter, offering an improvement in PWPM for cuffless BPM in the future. Objectives: The main objective of this research is to improve the accuracy of arterial diameter estimation from bio-impedance signals by reaching a consensus between observed impedance values (from computational simulation and phantom experiments) and mathematical modelling. This thesis focuses on the effects of different electrode configurations on current density and electric field (E-field) distribution within the wrist, aiming to achieve a reasonably uniform E-field distribution such that the cross-sectional area changes of the blood can be estimated more accurately. Method: Finite element analysis was performed on a 3D human wrist segment containing fat, muscle, and a blood-filled radial artery. Then, the skin layer, bones and a contralateral blood-filled ulnar artery were stepwise added, helping to understand the dielectric response of multi-tissues and blood flow in the 𝛽 -dispersion band (from 1 kHz to 100 MHz), the current distribution throughout the wrist, and the optimisation of electrode configurations for arterial pulse sensing. Two wrist phantoms were fabricated to verify the simulation results from both one-artery model and two-arteries models. Each wrist phantom contained two components: (1) the surrounding tissue simulant was fabricated by mixing 20 wt.% gelatine power with 0.017 M sodium chloride (NaCl) solution, (2) the conductive blood was simulated using 0.08 M NaCl solution. The blood-filled artery was constricted by a commercial desktop injection pump, and the impedance change was synchronously measured using the multi- frequency impedance analyser. Main results: The simulation results indicated the promising abilities of band electrode method to generate a more uniform current distribution than the traditional spot electrode approach. Both simulation and phantom experimental results demonstrated that a longer spacing between current-carrying (CC) electrodes with shorter spacing between pick-up (PU) electrodes in the middle region can sense a more uniform E-field, engendering a more accurate arterial diameter estimation. For the one-artery model, the arterial diameter could be accurately estimated with an average percent error of less than 1% in both simulation and phantom experiments. For two synchronously pulsatile arteries, the band electrode configuration exhibited a significantly higher accuracy in sensing overall blood volume change throughout the measured region. Conclusion: In summary, this thesis contributes to the accurate quantification of arterial diameter-dependent impedance variance by investigating the effect of electrode configuration. A promising band electrode configuration was developed for more accurate arterial diameter estimation from the numerical simulation and tissue phantom perspectives. More accurate arterial diameter estimation via BIM could further improve the performance of existing PWPM and cuffless BPM in the future.

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