Novel technique for Pulse Wave Detection: Electrical Bio-Impedance Plethysmography

Cardiovascular diseases (CVDs) are the leading cause of death worldwide, taking more than 17 million lives according to the World Health Organization (WHO) and costing approximately €210 billion annually to the European Union. Each disease has its risk factors, and one of the most common denominators is the development of atherosclerosis, which is characterized by the thickening or stiffening of the arteries. Arterial stiffness is due to the loss of the artery walls’ distensibility, causing increased blood pressure and speed with which the pulse wave propagates in the arterial tree. Relative to the latter, the Pulse Wave Velocity (PWV) measurement has demonstrated to be one of the best non-intrusive ways to estimate arterial stiffening. Currently, the most reliable methods to measure the PWV, are based Pulse Wave (PW) acquisition at two points using tonometers, usually the carotid and femoral arteries, computing the velocity between them. These devices are operatordependent because of their functional principle: the sensor must be held above the artery with adequate pressure during the entire PW recording time, which makes the signal very susceptible to management. Therefore, nowadays, PWV acquisition techniques present several limits for developing wearable devices for continuous monitoring. As the first step to measure the PWV is to acquire the PW, this thesis project proposes an innovative sensing technology for the PW measurement that would not require expert operators and even be a wearable device. Impedance plethysmography (IPG) was considered because it relates the changes in impedance to blood volume in the artery. For this purpose, the scope of this thesis is to design and implement a system to acquire the PW measuring the IPG. In this project, the evaluation board eval-ADuCM355 from Analog Devices was used, designed explicitly for impedance measurements, has been selected. The board injects a fixed alternating voltage between two electrodes and uses the Discrete Fourier Transform (DFT) to calculate the impedance of the zone of interest. Three ways to use the microcontroller were tested. The first method estimated the impedance using its internal components and signal processing; the second one used the ADuCM355 to inject the fixed voltage while the acquisition and calculations were performed externally; the last one used the internal components to inject and acquire the signal, but the impedance estimation was done in post-processing after the signal was sent via Serial Peripheral Interface (SPI). The third method was the most promising to evaluate the impedance. Therefore, different acquisition sites, electrode placements, and system settings were tested, seeking to find the most suitable configuration to acquire the pulse wave. The parameters evaluated in each configuration were the inter-electrode distance, the relative angle between electrodes and the artery, and the amplitude of the excitation voltage. Finally, with the selected configurations, it was possible to acquire the pulse wave signal, which, in later studies, may be helpful to develop a wearable device for PW detection and PWV monitoring