## 2 Implantable Left Ventricular Assist Device  
A schematic of the device, which was first proposed in [45; 46], for supporting the left ventricle is shown in Figure 1. The design consists of a set of two flexible balloons, with the larger ventricular balloon (2) placed in the left ventricle (5), and the smaller valve balloon (1) located in the mitral valve (3). Both balloons form expanding and contracting working chambers and can be adjusted to match the size of the individual heart chambers. Between the balloons, there is a narrowing (9) that separates the two working chambers. Inside the balloons is a pneumatic line (7) that supplies gas to both balloons by means of a suction-discharge device. Various designs of this line are possible, which can consist of two separate lines to supply each balloon separately or one common line [45]. This study used a pneumatic line in the form of a single line to supply both balloons. Importantly, this line has one outlet located at the top of the line in the valve balloon area and four smaller orifices in the ventricular balloon area, which are located under the narrowing (9). The position, number, and size of the orifices will ensure that the balloons work properly, ensuring appropriately synchronized inflation and deflation of the balloons. The correct choice of pneumatic line parameters resulted from experimental work, where a number of different solutions and their impact on balloon operation were tested. Importantly, a single pneumatic line does not require additional synchronization with the suction-discharge device control system, making the whole system simpler and more reliable. Figure 1: Principle of operation: (1)—valve balloon, (2)—ventricular balloon, (3)— mitral valve, (4)—aorta, (5)—left ventricle, (6)—left atrium, (7)—pneumatic line, (8)—aortic valve, (9)—narrowing. The most important principle of the device is that the expansion and contraction of the balloons is synchronized with normal heart function. This means that during the systolic phase of the heart, the ventricular and valve balloons are inflated with inert gas (helium), resulting in an increase in stroke volume into the aorta (4) via the aortic valve (8). In addition, the valve balloon occludes or completely replaces the mitral valve. In contrast, during the diastolic phase of the heart, both balloons are deflated, which, among other things, allows blood to flow from the atrium (6) into the left ventricle (5). A suction-discharge (Section 3.2), is used to transfer gas to the balloon set. The purpose of this device is to cyclically increase and reduce the volume of the balloons so that, during the systolic phase, the valve balloon is inflated first (mitral valve occlusion) and then the ventricular balloon (increase in stroke volume). During the diastolic phase, both balloons are deflated. The operation of the suction-discharge device is controlled by a control system (ODROID computer, Section 3.2) which ensures synchronization in line with the cardiac cycle. Signals are read and recognized in real time from ECG (electrocardiogram) measurements. Moreover, the control device also allows the volume of gas in the ventricular balloon to be varied during each cycle, thus ensuring a precise degree of circulatory support. This makes it possible to adjust cardiac output by changing the stroke volume. In addition, the control device allows for continuous monitoring of key cardiac and device parameters. The operating principle of the device is shown in Figure 1. In the first stage of the cycle (start of diastole), the valve and ventricular balloons are deflated. The mitral valve is open, and the aortic valve is closed, causing blood to flow from the atrium into the left ventricle. In the second stage (end of diastole), the valve balloon is inflated with gas (helium), which causes the mitral valve to close and stops the inflow of blood from the atrium, ending the diastolic cycle. In the third stage (start of systole), the ventricular balloon is inflated, and the valve balloon is closed throughout.