## 4 Discussion
Anyway, to enhance the precision of pulse contour analysis, we need to advise the users of PPG-based telemedical systems to perform their daily measurements preferably under standard conditions. This standardization does not require any particular cooperation from users; the recommendations are as simple as those for blood pressure measurement and are confined to those conditions which have been reported to influence pulse contour parameters [24; 26; 35; 36]. In our study, we also provided preliminary data on the interpersonal variability of the studied contour parameters (Figure 4). Based on our observations, we can conclude that interpersonal and intrapersonal variabilities of the studied parameters are in the same range for healthy individuals when measurements are performed under standard conditions. This indicates that normal rangescan be identified for these parameters and that deviations from these ranges may reflect DVP--and hence cardiovascular--abnormalities both at individual (when compared to other results of the same patient) and at population levels (when data are compared to values of healthy individuals). However, larger studies should be conducted to define the normal reference ranges for the contour parameters computed by telemedicine systems and to determine which alterations can be considered clinically relevant. Indeed, reference ranges for these parameters are scant in the literature, and they have only limited validity for larger populations [6; 14; 25; 27; 28; 37; 38; 39; 40]. As a satellite question, we also tested in this study how different anatomical disposition of the fingers affects the results of pulse contour analysis. Without question, we recommend the use of the same finger for each measurement. However, for some reason, the occasional use of another finger may occur, which may limit the valid remote interpretation of the recordings. Therefore, we need to be aware of whether this error causes significant alterations in the output results. In healthy individuals, we could observe that there was no relevant difference in pulse contour parameters when measured in parallel on the index and ring fingers of the two hands (Table 3). The calculated ICCs showed that the effect of using different fingers on the variability of the outcomes is much less than the effect of interpersonal differences. Therefore, changing to different fingers does not constitute a relevant measurement error. However, we need to keep in mind that pathological alterations and diseases of the supplying arterial tree may have an impact on the blood flow of the digital arteries. For this reason, at the first patient visit, it is recommended to record pulse signals on several fingers on both sides and analyze whether there are differences in the output parameters. Finally, we evaluated the reliability of our proprietary analysis engine to detect and analyze distinguished deflections of the second derivative PPG curve. Pulse wave analysis was originally extended to the second derivative of the DVP by [PERSON] et al. [14]. They defined notable points of the curve which facilitate understanding of the pressure wave. Since then, several research groups have related the height of the b, c, and d waves to the a-wave to create measures that can index vascular pathologies (vascular aging, hypertension, arterial stiffness) and predict cardiovascular endpoints [14; 28; 41; 42]. Among these points, c and d points are particularly valuable, as they are supposed to hold information about wave reflection [14], and the parameters derived from them provide information about arterial stiffening [14; 35]. However, detection of c and d inflections has reportedly become a challenge for automated algorithms as their position and amplitude change along with pathophysiological alterations of the PPG [8; 22; 43]. In this study, we analyzed the success of c and d point identification by our algorithm and variability of parameters (namely d/a and aging index) derived from these points.