## 4 Discussion
In conclusion, our study showed that PPG-based pulse wave analysis performed in this study operates reliably with acceptable measurement errors and is capable of monitoring subtle alterations in cardiovascular functioning. Although the reliability of PPG-based pulse contour analysis is debated, no data are available on the repeatability of the systems which are used for analysis in research studies. However, as these systems are complex and comprise several sources of measurement errors, it is impossible to validly interpret biological data obtained by PPG-based systems if information concerning repeatability is not available. Therefore, we firstly checked the repeatability of the pulse contour measurements of our telemonitoring system. This was assessed by calculating the variability of the DVP parameters obtained from successive measurements of stable artificial pulse signals, which simulated healthy pulse waves and were generated by a pulse oximeter simulator device. Such variability can be caused by measurement errors of the pulse oximeter instrument and also by the automated algorithm analyzing the detected pulse wave. The combined effect of these two factors on measurement variability was investigated by testing the agreement among the results of five successive measurements performed by the same randomly chosen pulse oximeter device. The variation was smaller than the predefined 2% criterion of acceptance for each parameter (Table 1; Normal condition). Afterward, we extended the investigation to four additional instruments with which we performed the same measurements. We pooled the 5 x 5 measurements and calculated the overall CVs, which then reflected the combined variation caused by measurement error of a single pulse oximeter, analysis by the algorithm, and also the \"interinstrumental\" variability of several pulse oximeters of the same product. The CVs calculated in this way were also below the limit of acceptance (Table 1; Normal condition), showing that measurements are highly repeatable even if different pulse oximeters are used. Testing of pulse oximeter reliability was relevant in this setting because the applied devices had only been tested for the reliability of oxygen saturation and heart rate calculations by the manufacturers, but it was unknown whether they accurately track a continuous pulse wave for minutes. After proving that our system reliably tracks stable signals, we addressed our main question of determining whether PPG-based monitoring and analysis of the DVP are reliable. We aimed to resolve the controversy in which it is often doubted that PPG-based methods can be used as diagnostic tools [21; 33; 34] because they are highly sensitive to errors, causing pulse contour parameters to fluctuate even if there is no alteration in cardiovascular functioning. However, we postulate that these alterations reflect real changes in cardiovascular condition. To show this, we measured test-retest variability under standardized measurement conditions. Measurements were performed in a quiet room at room temperature; in the morning hours, preferably at least two hours after the last meal and coffee; in a sitting, resting position, with hands held calm on a table. Speaking, moving, and mental activity were avoided during data collection [24; 26; 35; 36]. Naturally, this standardization does not remove variability completely. However, the output contour parameters of our telemedical system showed minimal test-retest variability for most of the parameters, namely for b/a, left ventricular ejection time index, mean interbeat interval, stiffness index, and mean heart rate (CVs lower than 10%; Table 2). This indicates that these parameters are suitable for patient follow-up and may well support clinical decision, as the deviation of a measurement from the standard individual value most probably indicates real, physiological, or pathological alterations in cardiovascular function.