## 4 Results and Discussion  
Figure 6 shows an illustrative Wiggers diagram with the characteristic points highlighted, such as the opening and closure of the mitral and aortic valves. On the \(x\)-axis is the time \(t\), which in this case includes two complete heart cycles, and on the \(y\)-axis are the aortic, atrial and ventricular pressures \(p\). Figure 6 provides a reference point for the experimental plots in Figures 7 and 8. Experimental studies were carried out on the experimental stand in Figure 3 for several hundred different pumping parameters of the so-called artificial blood. These tests involved different versions of the pneumatic line. Parameters such as the density and temperature of the liquid, the frequency of operation of the system (number of balloon inflation and  
Figure 6: Reference Wiggers diagram: mo— mitral valve opening, ao—aortic valve opening, mc— mitral valve closure, ac—aortic valve closure. Figure 7: Experimental results—aortic, atrial and ventricular pressure distributions. deflation cycles), the gas pressure in the balloon, the gas pressure in the cylinder enclosing the heart model, and the pressure in the hydraulic (blood) system were varied. The hydraulic flows used the work of the balloons in the heart model from Figures 4 and 5. Figures 7a,c and 8a,c show pressure plots for two adjacent cycles. Figures 7b,d and 8b,d show the average pressure values for thirty cycles with superimposed standard deviations in the form of error intervals. Figures 7 and 8 shows the two extreme results of the left ventricular assist device performance. Using the control system, the appropriate balloon operation was achieved. First, the valve and ventricular balloons were supplied with gas from the suction-discharge device for half a second, and then the balloons were deflated of gas for another half a second. It follows that the assist device worked at a rate of 60 cycles per minute. The plots in Figure 7a correspond to the case where the patient has low blood pressure (hypertension). The aortic pressure took values in the range of approximately 32 to 78 mm Hg. In this case, the average flow rate of a fluid with a density of 1055 kg m\({}^{-3}\) was 2.4 kg min\({}^{-1}\) (2.27 L min\({}^{-1}\)). It is known that with cardiac assist, it is possible to pump fluid (blood) into the hydraulic system when the pressure of the gas in the balloon is higher than the fluid in the left ventricle. This can be seen in Figures 7a and 8a. The gas pressure in the ventricular balloon during blood pumping from the the ventricle exceeds the fluid pressure by approximately 90 mm Hg, while the fluid pressure in the atrium reaches maximum values of less than 11 mm Hg. Interestingly, the maximum fluid pressure in the ventricle is higher than that in the aorta (Figure 7a). This is related to the pressure drop of the flowing fluid in the section between the sensors measuring the pressure in the ventricle (not visible in Figure 3) and the aorta (4), see Figure 3. This pressure difference was approximately 14 mm Hg. For the data in Figure 7c, the aortic pressure reached 137 mm Hg. This pressure will slightly exceed the blood pressure of a healthy person and is classified as hypertension stage 1. Atrial fluid pressure reaches a maximum of 9 mm Hg. The fluid density during the test was 1050 kg m\({}^{-3}\) at the average flow rate 2.3 kg min\({}^{-1}\) (2.19 L min\({}^{-1}\)). This time, the gas pressure in the ventricular balloon during the pumping of fluid from the heart chamber exceeds the fluid pressure by approximately 100 mm Hg (see Figure 8c). In the results shown in Figure 7c the maximum fluid pressure in the ventricle was approximately 23 mm Hg higher than that of the artificial blood in the aorta (sensor (4) in Figure 3). In both results (Figure 7a,c), there were slight pressure pulsations due to the action of the valves (pneumatic mitral and mechanical aortic) located in the hydraulic system.