## 3 Methods--Experimental Stand  
In order to study the effect of reduced lumen in blood vessels on the performance of the left ventricular assist device, the experimental setup shown in Figure 2 was constructed. This stand is equipped with a flexible heart model containing a set of balloons, i.e., a ventricular balloon and a valve balloon. The ventricular balloon is used in order to pump fluid (artificial blood) from the left ventricle into the circulatory system, while the valve balloon ensures the occluding of the valve orifice (regurgitant valve opening). To ensure that the operating conditions of the circulatory system closely resemble real-life conditions, the hydraulic system was enhanced with compensation vessels. These vessels simulate the flexibility of the human circulatory system. Additionally, an adjustable hydraulic valve was incorporated to alter the flow resistance within the hydraulic system. A mechanical aortic valve (check valve) was placed in the fluid flow line. In addition, a flexible transparent heart model was printed from resin on a 3D printer. Through the transparent wall of the model, the operation of the ventricular and valve balloons could be observed. More than a dozen sensors were placed in the hydraulic (artificial blood) and pneumatic systems (helium). Pressure sensors (High Precision Transmitter ATM.1 ST STS Sensor Technik Sirnach) allowed pressure to be measured at several locations in the hydraulic system, even in the left ventricle and atrium of the heart model. The aortic pressure sensor was located at the beginning of the aorta (behind the aortic valve) rather than downstream of the valve, as this made it possible to compare the pressure in the aorta with the pressure in the left chamber. Also, the so-called Wiggers diagrams (described in the next paragraph) show pressure plots in this part of the aorta. The pressure of the gas in the pneumatic system close to the balloon assembly was measured using the SPAU-B2R-H sensor from Festo. Moreover, the flow rate in the hydraulic system was also determined using a highly accurate Optibatch 4011 C from Krohne. An electric heater was also placed in the hydraulic system to maintain the temperature of the fluid at around 34-36 \({}^{\circ}\)C. In addition, data, such as liquid temperature and liquid density, were read from the artificial blood flow rate measuring device. Measurements of the circulatory system were taken, i.e., pressure in front of and behind the heart model ((2), (3) in Figure 2), pressure in the atrium and left ventricle of the heart model, and the flow rate (flow meter (6) in Figure 2). Gas pressure was measured in the pneumatic system in front of a set of balloons placed in the heart model ((4) in Figure 2). The testing of the left ventricular assist device was performed at balloon set operating frequencies of 60, 100, 120, 140, and 160 cyc/min. The designed and constructed stand (Figure 2) enables the modeling of the impact of reduced blood vessel lumen on the performance of the LVAD. In this stand, a very important role is played by the control valve (7) in Figure 2, which facilitates the regulation and variation of the flow resistance in the hydraulic (blood) system. Using this valve, three different loads on the hydraulic system were obtained, which correspond to the reduced lumen of the blood vessels, namely  
* Case 0--No reduction in the blood vessel lumen, i.e., fully open valve (7) in Figure 2;
* Case 1--The first stage of reducing the blood vessel lumen. This involves partially closing the adjustable valve (7), thereby reducing the flow cross-sectional area. The valve is gradually closed until there is a noticeable change in the flow rate;
* Case 2--The second stage of reducing the blood vessel lumen. This is achieved by further partial closure of the adjustable valve (7), reducing the flow cross-sectional area more significantly. The valve is closed to the point where a reduction in the flow rate becomes visible.