## 4 Discussion  
In this study, the cardiac function of a child's cardiovascular system under continuous-flow VAD support and the haemodynamic performance of HVAD, HM2 and HM3 heart pumps have been evaluated. All the devices generated similar pump outputs and mean aortic pressures for the selected operating speeds. The left-ventricular end-diastolic volumes and diameters were also similar under the support of HVAD, HM2 and HM3. The most remarkable difference was in the aortic pulse pressure under the support of each device. Aortic pulse pressure was relatively low under the support of the HM3 device, which has the steepest pump H-Q curve among the simulated continuous-flow VADs [18]. Steep pump H-Q characteristics result in reduced arterial pulsatility during continuous-flow VAD support [26; 27]. One of the effects of reduced pulsatility in the patients with continuous-flow VADs is endothelial dysfunction [28], which may cause further adverse events in patients [29]. Therefore, continuous-flow VADs with flat H-Q characteristics that provide better pulsatility may additionally be more beneficial in improving the endothelial function in paediatric patients. It should be noted that although the constant operating speed of HM3 reduced the arterial pulse pressure, the artificial-pulse operating mode increased it remarkably. Therefore, varying-speed operating modes, such as in HM3, may help to overcome issues related to reduced pulsatility. The HVAD Lavare cycle also increased the arterial pulse pressure; however, the increased pulse pressure was somewhat similar to the arterial pulse pressure in the cardiovascular system model with HM2. Moreover, HM3's artificial-pulse mode is repeated every 2 s whilst the HVAD Lavare cycle lasts only 3 s in a 60 s period. Therefore, the HM3 artificial pulse may be more beneficial to children in comparison with the HVAD Lavare cycle. Thrombosis rates are reportedly lower with this device when compared to its predecessor HM2 [30]. Thrombus formation on the rotating component is caused by heat generated in the pump [31]. Centrifugal pumps can generate flow rates similar to those of axial pumps, at lower operating speeds, which also generate less heat in the device. Therefore, these third-generation devices also have their advantages, such as lower complication rates in patients with continuous-flow VADs. Moreover, the HM2 device was suitable for the patients above 1.3 m\({}^{2}\) body surface area [7]. HM3 can be implanted to patients with a body surface area lower than 0.8 m\({}^{2}\), as reported in [9], and HVAD was suitable for the patients above 0.6 m\({}^{2}\) body surface area [10]. Again, patients with a relatively small body surface area can be implanted with HM3 or HVAD. Figure 6: Static (st) and dynamic (dy) pressure head and flow rate relations of HVAD, HM2 and HM3 devices at 2600 rpm, 8700 rpm and 5200 rpm operating speeds, respectively. Left-ventricular end-diastolic diameter may be one of the predictors of myocardial recovery in children. A smaller left-ventricular end-diastolic diameter during VAD support may result in myocardial recovery, whereas insufficient change in left-ventricular end-diastolic diameter may be a predictor of mortality under VAD support [32]. HM3 achieved slightly better unloading at the end of diastole in comparison to HVAD and HM2. The mechanism of myocardial recovery under VAD support remains unclear and may depend on different factors. However, the simulation results show that HM3 can achieve better end-diastolic unloading for the same pump output and mean arterial pressure. Baroreflex regulation of systemic arteriolar resistance was adjusted manually in this study. It was assumed that systemic arteriolar resistance in the cardiovascular system model was maintained at the healthy levels with the restored mean arterial pressure under continuous-flow VAD support. Any changes in arterial blood pressure are detected by baroreceptors in the walls of the large systemic arteries and are then transmitted to the central nervous system.