## 2 Materials and Methods  
### Experimental Setup and Device Description  
The experimental study was carried out using the ViVitro Pulse Duplicator System (PDS, SD2001-1; ViVitro Inc., Victoria, BC, Canada) to stimulate various hemodynamic conditions for adults and pediatric patients. (Figure 1, with the Sputnik LVAD, an axial-flow blood pump with a nonpulsatile flow of up to 10 L/min, connected). The PDS consists of a hydraulic circuit based on the ViVitro Heart Model, the SuperPump pulsatile pump, an ultrasonic flow sensor (ME-11 PXL Clamp-on Tubing Flow sensors; Transonic Systems, Ithaca, NY, USA), and the ViViTest data acquisition system. The Sputnik LVAD features technical performance parameters which are comparable to the HeartMate II (Abbott Laboratories, Abbott Park, II, USA) and the HeartAssist5 (Reliant Heart, Houston, TX, USA) VADs [21]. The SuperPump pulsatile pump is a digitally controlled hydraulic piston pump generating physiological cardiac flows. Digital control allows the operator to set the stroke volume (SV) of the ViVitro Model Heart, which generates the physiological cardiac output in the hydraulic circuit at a pre-set heart rate. The ViVitro Model Heart is an acrylic chamber divided into two parts (piston pump and hydraulic circuit) by the silicon membrane and is driven by the piston pump (SuperPump) to reproduce left ventricular contraction. The hydraulic circuit of the circulatory mock loop consists of an aortic unit, including the aorta and aortic root, which are adjustable compliance chambers; a systemic vascular unit reproducing the physiological resistance of the peripheral circulation; and an atrial unit, which represents an open chamber with a fluid level corresponding to the atrial pressure. Aortic and mitral valves are represented by mechanical disk valves supplied by ViVitro in a standard package. The ultrasonic flow sensor is connected to a signal converter to validate the flow measurements. The signal from the system, along with signals from three pressure sensors connected to the ViVitro Model Heart, aortic and atrial units, are all transmitted to the ViViTest data acquisition system. In combination with the ViViTest software, the system allows the operator to record and analyze the data of physiological flows and pressures in the hydraulic circuit, and also to adjust the control signal of the SuperPump. The waveform of the control signal can be chosen from the list of typical waveforms: FDA (Food and Drug Administration) waveform, triangle waveform, sine waveforms of different systole duration. Furthermore, there is an option to customize waveform according to certain applications. In the presented research, the FDA waveform for the SuperPump control signal was used. There are several operation processes in the ViViTest software that acquire signals from sensors for 10 consecutive cardiac cycles, review and analyze acquired data, compare data of a series of trials, and calibrate the sensors. Data analysis proceeds sequentially from cycle 1 to 10. Pressure and flow waveforms are displayed along with crossover markers defining critical intervals during the cycle. Automatic algorithms calculate the start and end of the aortic and mitral valve pressure and flow cycles. Data dashboard includes maximum, minimum, mean, and back valve pressures; closing, leakage, and total regurgitation volumes for each valve and relative regurgitation in the percentage of total valve volume. A glycerol-water mixture with a dynamic viscosity of 3.5 mPa\(\cdot\)s at 22.0 \({}^{\circ}\)C was used in the hydraulic circuit to reproduce the blood viscosity of a patient with Sputnik LVAD implanted. In this study, we investigated different heart rates (HR), i.e., 59, 73, and 86 ppm, and pump speeds ranging from 7600 to 8400 rpm with 200 rpm-increment steps. This  
Figure 1: Image (**A**) of ViVitro Pulse Duplicator System SD2001-1 with the Sputnik LVAD connected (**B**) and the block diagram. particular speed range was considered because it allows the pump blood flow rate to be maintained in the 1-7 L/min range.