# A Portable Device for the Measurement of Venous Pulse Wave Velocity
## 5 Discussion  
For the first time, a prototype for the measurement of pulse wave velocity in the venous compartment was presented and validated. The device integrates all necessary electrical and pneumatic features, while remaining compact, battery powered, and with floating ground. This features make it electrically safe and suitable to be used on patients in hospital wards. The validation test demonstrated that the quality of the measurement is comparable with previous reports based on a personal computer and laboratory instrumentation [2,3], achieving good reliability (CoV < 4%) and sensitivity. In fact, in all 4 tested subjects, vPWV in the arm was significantly affected by a simulated fluid challenge: the PLR. This manoeuvre, performed from the supine position, is considered to displace about 150 ml of blood from the legs to the upper body, with an ensuing increase in peripheral venous pressure of less than 1 mmHg [3]. Over the last two decades, technological advancement and lower costs have led to the widespread use of single board computer platforms, such as Raspberry Pi or Jetson Nano, in a variety of fields. In particular, Raspberry Pi platform was employed in several applications in the field of biological and physiological research [8], e.g., for impedance cardiography measurement [9] and for monitoring the anaesthetic status of a patient [10]. Indeed, Raspberry Pi offers a wide range of board connectivity, custom plug-ins and interfaces that allow to develop low-cost solutions. However, a limitation of the RPi is the lack of analogue-to-digital converters. In fact, all devices that integrate RPi with the purpose of signal acquisition need an A/D converter and some circuits for signal conditioning and amplification. The A/D converter can be either on a system on a chip (SoC) [9], as in our case, or directly embedded in a microcontroller [10]. The add-on A/D board that we adopted had problems in simultaneously acquiring from multiple channels, and we observed the appearance of sampling noise at the highest sampling frequencies. We could, however, achieve satisfactory performance by sampling one analogue channel at the time and by limiting the sampling rate to 7.5 kHz. Therefore, we decided not to simultaneously acquire all signals as in the PC-based system, but to switch from one channel to another with the correct timing. Switching channel time delays were negligible and do not affect the measurement. More significant delays occur during the handling of two simultaneous processes: the output of the cuff inflation digital signal and the acquisition of the Doppler shift signal. Those delays are probably responsible for the small difference observed in vPWV values estimated by RPi and the PC-based system. Passing digital data from A/D converter device to RPi can use different communication protocols; the most common are SPI and Inter Integrated Circuit (I\({}^{2}\)C). Both are serial protocols based on a master-slave architecture. I\({}^{2}\)C can handle more than one master at the same time and offers the advantage of easy addition of any new slaves [9,10]. SPI, on the other hand, presents a more restrictive architecture, but offers higher speed of communi  
Figure 6: Time course of vPWV during the experimental session in one subject. Values yielded by RPi-based prototype (blue) compared with those returned by original PC-based system (green). cation [9]. In the same project, both protocols can be used for different purposes. In this approach, it is possible to exploit the advantages of both and adapt to the communication protocol offered by the A/D converter that best suits the needs [9]. In our case, the A/D plug-in board was equipped with an SPI bus. In addition, RPi provides convenient features concerning the possibility of autonomous data processing as well as wireless communication with a remote PC for input of working settings and data display and storage. In our case, this functionality was used to manage the measurement process via GUI from a wirelessly connected computer.