## 4 Discussion  
Since it was shown that patients with end-stage heart failure can recover cardiac function with a combination of LVAD and medical therapies [4; 6; 8; 9; 20; 25; 28; 29], an additional noninvasive tool for outpatient hemodynamic monitoring appears to be necessary. If a certain degree of myocardial function following LVAD implantation is assessed in clinics during the post-operative period, the ability to evaluate myocardial function noninvasively during the outpatient period is essential and should be addressed. Despite potential complications attributable to various discrepancies that may vary from mechanical aspects to electric (windings losses, rotor losses, friction loss, etc. ), finding reliable correlations between parameters of different devices and hemodynamic parameters will be necessary. However, the precise estimation may be a limiting factor in this case. Nevertheless, provided that these estimations were performed in a correct and meticulous manner, it is possible to evaluate the correlation between the pulsatility index of pump electric current and myocardial function. The use of the Sputnik LVAD was in the focus of represented experiments. The major goal was to determine the possibility of assessing the energetic parameters of the heart using the recorded pump electric parameters [20]. The assumption about the correlation between investigated parameters (\(CO\) vs. \(PI\) and \(CPO\) vs. \(PI\)) was confirmed by means of represented correlation and regression analyses. According to presented statistical data, quadratic models of both types are better fits for \(CO\) estimation using the proposed \(PI\) than linear ones. In terms of statistical significance, the best fits for \(CO\) in the given ranges of \(PI\) can be determined as quadratic models with the included linear term. Regression analysis of \(CPO\) models revealed almost the same trends in statistical significance. However, concern on the physical nature of the investigated the relationships should be addressed to assume their expected form and compare it to the obtained models. At first, a monotonically increasing function for these relationships can be assumed as an expected form in the physiological range. It can be discussed separately whether this function should be strictly increasing or just nondecreasing. Since the coverage of the physiological range of contractility measures is out of doubt in this study, a further assumption is to suppose this function in a simple form. For this, presented forms of regression models were chosen. Secondly, if zero pulsatility of the pump electric current corresponds to zero pump support, it must be obvious that a certain positive level of contractility should be observed at this point. This positive contractility level can be determined as an actual contractility level of the unsupported left ventricle. For linear models, there is only one (linear \(CO\) model at 59 bpm) that estimates contractility level at zero pulsatility of the pump current with a positive value. Other linear models for both _CO_ and _CPO_ estimate this level with a negative value. Furthermore, with HR increasing, this negative estimate level significantly decreases. On the contrary, quadratic models with linear term significantly overestimate this contractility level, making it grow with increasing HR. As a mean, quadratic models with excluded linear term address this concern in a more or less appropriate manner for both _CO_ and _CPO_ responses, except for the only model of _CPO_ at 86 bpm where this level is estimated as negative. Thus, quadratic models with the excluded linear terms can be suggested as the most appropriate for _CO_ estimation using _PI_ given their overall statistical significance and adequacy for measured data sets along with better conformity to the physical nature of the relationship. Although the same considerations may be attributed to _CPO_ estimation using _PI_, it should be noted that measured data sets were significantly corrected to eliminate autocorrelation of regression residuals.