## 3 Results  
### In-Vitro Setup Validation  
Figure 3 shows the constructed rig to simulate part of the upper-body circulatory system. As explained, this system included a commercial pulsatile pump that replicates the heart activity. This pump can follow different patterns for generating each cardiac cycle. In this study, the built-in pattern was used, which changes the stroke volume according to the SR and TF. Greater stroke volumes could be obtained by decreasing SR and increasing TF.  
Figure 3: Constructed rig for the in-vitro experiment. This consisted of a commercial pump that replicates heart activity, tubing that mimics arteries and veins from the upper-body circulatory system, and a fluid that has similar optical properties as blood. Pressure sensors and photoplethysmographic (PPG) probes were used to measure the pressure in the tubes and the optical pulsatile signals from two developed tissue phantoms, that model the skin. PPG signals were acquired and processed using a custom-made system (ZenPPG) while pressure information was acquired directly from a data acquisition card.  
In order to acquire optical signals such as the PPG, it was necessary to develop a fluid that replicates the optical behaviour of blood, which is referred to as artificial blood in Figure 3. Figure 11 in the Appendix A shows the absorbance spectra obtained when the developed fluid was tested using the spectrophotometer. It can be observed that the artificial blood had a peak absorbance at around 610 nm, replicating part of the optical behaviour of real blood. Since the maximum absorbance was observed at this wavelength, the subsequent analysis with PPG signals and PRV information was performed using only this wavelength.  
A validation of this setup was performed to assess if the measured pressure was affected by changes in SR and TF, and if the correct SR was measured from the acquired optical signals. Figure 12 in the Appendix A exemplifies the signals acquired from the rig using an SR of 60 bpm and a TF of 3 L/min, while Figures 4 and 5 show the behaviour of pressure measurements and pulse rate measurement, respectively, as SR and TF were modified. As can be observed, the measured pressure signals feature good quality and follow the expected behaviour, i.e., the pressure increased as the target flow increased at the same stroke rate, while it decreased when the stroke rate was increased. Also, it can be observed that the pressure measured in the aorta was greater than in the rest of the locations and became lower in the subclavian segment. Then, it started increasing slightly due to the smaller area and increased resistance in the subsequent arterial segments. From the PPG signals, it can be observed that the pulse rate can be reliably extracted from the signals, although the measurement became less reliable with lower stroke volumes, i.e. higher stroke rates and lower target flows.  
Figure 4: Behaviour of mean pressure measured from the aorta, subclavian, brachial, radial and deep palmar arch (DPA) locations while changing stroke rate and target flow.  
Figure 5: Behaviour of pulse rate measured from the radial (**left**) and deep palmar arch (**right**) phantoms using photoplethysmographic sensors while changing stroke rate and target flow.