## 4 Results and Discussion
### Measurements  
Among other things, the designed test stand permitted a number of fluid pumping tests to be carried out in a model system with an artificial heart using ventricular and valve balloons. The pressure in the hydraulic system was varied by increasing the flow resistance using the hydraulic valve setting, which enabled the simulation of different operating states of the circulatory system through the fluid (blood) pressure in the system. Tests were carried out with two hydraulic valve settings (valve (7) in Figure 5):  
Without reduction of the flow cross-sectional area (minimum pressure in the aorta about 30 mm Hg (Figures 9 and 10)); and  
With a reduced cross-sectional area of the adjustable valve (minimum aortic pressure approximately 45 mm Hg (Figures 11 and 12)). For the case of additional resistance, hydraulic system tests were performed using the operation of a helium-fed ventricular balloon by an external system with an operating frequency of 60, 80, and 100 cycles per minute in order to pump the fluid. The inflation \(t_{i}\) and deflation \(t_{d}\) times of the ventricular and valve balloons were also varied at a ratio of \(\tau\),  
\[\tau=\frac{t_{i}}{t_{i}+t_{d}}, \tag{1}\]  
in the range from 0.25 to 0.5. Below are examples of test results for the values of \(\tau=0.3\), 0.4, and 0.5. Depending on the ratio \(\tau\) of the inflation and deflation times of the ventricular and valve balloons (systolic to diastolic time of the heart), the maximum aortic pressure changes. For \(\tau=0.3\) at a heart rate of 60 cycles/minute, the maximum aortic pressure is 101 mm Hg (Figure 9a), while for \(\tau=0.4\) it is about 87 mm Hg (Figure 9c), and for \(\tau=0.5\) it is about 75 mm Hg (Figure 9e). In contrast, the minimum aortic pressure hardly changes and is at about 30 mm Hg (Figure 9a,c,e). The maximum pressure in the heart chambers with \(\tau\) being 0.3, 0.4, 0.5 is, respectively, 127 mm Hg (Figure 9a), 109 mm Hg (Figure 9c) and 94 mm Hg (Figure 9e). The gas pressure in the pneumatic supply line close to the balloons varies between \(-163\) and \(+248\) mm Hg (Figure 9b,d,f). If the operating frequency is increased in the system, this increases the fluid pressure in the aorta and ventricle, as well as the balloon gas pressure. The fluid pressure in the aorta for a heart rate of 80 cycles/min and \(\tau=0.3\) was 117 mm Hg (Figure 10c), and for a  
Figure 8: Operation of the ventricular and valve balloons. heart rate of 100 cycles/min, it was about 128 mm Hg (Figure 10e). The maximum fluid pressure in the chambers was also higher at 147 mm Hg for 80 cycles/min (Figure 10c) and 163 mm Hg for 100 cycles/min (Figure 10e), respectively. Depending on the frequency of the external system, the flow rate varied from about 1.5 kg/min for 60 cycles per minute, to about 2 kg/min for 80 cycles per minute, to about 2.5 kg/min for 100 cycles per minute. For the case of a hydraulic system with additional flow resistance, hydraulic system tests were also performed using the operation of a helium-fed ventricular balloon by a portable external system to pump the fluid, applying additional flow resistance by reducing the flow cross-sectional area of the hydraulic valve (7) in Figure 5). Tests on the heart model were performed for operating frequencies of 60, 80, and 100 cycles per minute. The test results for \(\tau\) values of 0.3, 0.4, and 0.5 are shown below. Figure 9: Experimental results—aortic, atrial, ventricular, and balloon pressure distributions (no resistance). Depending on the value of \(\tau\), the maximum aortic pressure varies. For \(\tau=0.3\) at a heart rate of 60 cycles/minute, the maximum aortic pressure is 110 mm Hg (Figure 10(a)); for \(\tau=0.4\), about 100 mm Hg (Figure 10(c)); and for \(\tau=0.5\), about 87 mm Hg (Figure 10(e)). In contrast, the minimum aortic pressure again hardly changes and is at about 45 mm Hg (Figure 10(a),c,e).