Impedance Plethysmography: a novel technique for Pulse Wave Detection

Cardiovascular diseases (CVDs) remain the leading cause of death globally, accounting for approximately 30% of annual fatalities. Addressing this persistent public health challenge requires the development of effective strategies for prevention, early diagnosis, and management. A critical marker of cardiovascular health is arterial stiffness, typically assessed through Pulse Wave Velocity (PWV). Current PWV measurement methods involve acquiring Pulse Wave (PW) signals from two anatomical sites, commonly the carotid and femoral arteries, using tonometers and measuring the pulse arrival time to compute the velocity. While these devices are widely used in clinical practice, they are hindered by several inherent limitations, including an inability to support continuous monitoring and a reliance on operator skill to maintain proper sensor placement over the artery during measurement. Impedance Plethysmography (IPG) shows great potential as an alternative technology for developing wearable devices that enable continuous, non-invasive monitoring of arterial stiffness. The aim of this thesis is to demonstrate the feasibility of creating a novel IPG-based device for measuring PWV. By injecting a fixed alternating current into a tissue region crossed by an artery and measuring the resulting voltage drop, it is possible to infer the variation in the electrical impedance of the vessel, related to changes in blood flow. The core of the impedance measurement system is the Howland Current Pump (HCP), which is known for providing a stable output alternating current when a voltage signal is applied at the input. Signal acquisition was carried out using the NiDAQ USB-6259 acquisition board, which provides a high sampling rate essential for accurately demodulating the signal. Prior to in-vivo validation, the hardware’s performance was tested to ensure a stable and safe output current. A Matlab® App Designer-based Graphical User Interface (GUI) was developed for real-time raw signal processing and impedance visualization, improving electrode placement efficiency. It allows the user to adjust both the sampling frequency and the characteristics of the HCP input waveform. The GUI also includes a toolbox for saving data. The integration of each component into the system was carried out with the objective of reducing noise in the acquired signal. To this end, shorter cables were utilized for the connection of the HCP hardware to the electrodes, thereby minimizing the distance to the skin. Furthermore, a voltage follower was placed between the electrodes and the analog input of the acquisition board, to prevent signal dispersion along the cables. The in-vivo validation test was performed by acquiring the subject’s ECG signal in order to confirm the relation between the pulse wave and heart contraction activity. While the system supports signal acquisition from two arterial sites for the calculation of pulse wave velocity, the test focused solely on the carotid artery due to the difficulty in accessing the femoral artery. The results demonstrate the effectiveness of applying the IPG principle to Pulse Wave detection, offering a non-invasive, real-time method for monitoring cardiovascular health. Despite these promising outcomes, electrode placement and subject variability remain challenges for future development, serving as a starting point for adopting a multi-electrode approach that will reduce the need for precise placement and mitigate the effects of variability.