## 3 Results  
According to the results obtained with the Shapiro-Wilk test, the two mean R-R series are normally distributed (\(p>0.05\)). Figure 2 shows the mean R-R intervals plotted as a function of time for each group of patients. The first mean value obtained from the R-R intervals is placed at 10 min, and the last mean value is at 184 min. Adjacent points are separated by 6 min according to the maximum overlap described in the _HRV processing_ section to maximize the number of analysis points. The mean and standard deviation obtained were \(814.38\pm 20.40\) ms for the group of hypotensive patients and \(829.23\pm 30.05\) ms for the group of non-hypotensive patients. The \(t\)-test showed significant differences between the groups (\(p=0.029\)). The regression models obtained were y = \(838.36-0.24t-0.0005t^{2}\) (ms) (\(p\ll 0.05\), \(R=0.65\), and \(R^{2}=0.42\)) for the hypotensive group and y = \(757.56+2.18t-0.012t^{2}\) (ms) (\(p\ll 0.05\), \(R=0.95\), and \(R^{2}=0.90\)) for the non-hypotensive group. The lowercase \(t\) in the models represents time. From the time-frequency analysis, the spectrograms obtained show clear differences in the power distribution for the hypotensive group compared to the non-hypotensive group. Figure 3 shows the spectrograms obtained from two patients--one from each group--for the visualization purposes. Although the differences are spread over the time-frequency plane, a greater contrast was observed in the HF band, where the power was smaller for the hypotensive patients compared to the non-hypotensive patients. Figure 4 shows the mean spectral power obtained from the VLF, LF, and HF bands for the two studied groups of patients, along with their corresponding regression models and confidence intervals. The VLF power is lower for the hypotensive group compared to the non-hypotensive group (Figure 4, panel a). The obtained regression models were y = \(557.04\) + \(10.6t\) - \(0.050t^{2}\) (ms\({}^{2}\)/Hz) (\(p\ll 0.05\), \(R\) = 0.81, and \(R^{2}\) = 0.66) and y = \(514.88\) + \(20.87t\) - \(0.092t^{2}\) (ms\({}^{2}\)/Hz) (\(p\) = 0.001, \(R\) = 0.53, and \(R^{2}\) = 0.28), respectively. The models reached their maximum at 107 min (hypotensive group) and 113 min (non-hypotensive group). For the LF power, the models obtained were y = \(475.13\) + \(7.88t\) - \(0.027t^{2}\) (ms\({}^{2}\)/Hz) (\(p\ll 0.05\), \(R\) = 0.77, and \(R^{2}\) = 0.59) for the group of non-hypotensive patients and y = \(556.29\) + \(7.27t\) - \(0.052t^{2}\) (ms\({}^{2}\)/Hz) (\(p\ll 0.05\), \(R\) = 0.68, and \(R^{2}\) = 0.46) for the group of hypotensive patients (Figure 4, panel b). According to these models, the maximum is reached at 145 min for the first group and at 70 min for the second group. This causes the LF power to decrease first in the second group and reach a smaller value at the end of the HD therapy compared to the first group. The results for the HF power and the two groups of patients are shown in Figure 4 (panel c). The model corresponding to the non-hypotensive group is y = \(153.67\) + \(6.76t\) - \(0.030t^{2}\) (ms\({}^{2}\)/Hz) (\(p\ll 0.05\), \(R\) = 0.87, and \(R^{2}\) = 0.76), and the one corresponding to the hypotensive group is y = \(157.17\) + \(0.84t\) - \(0.009t^{2}\) (ms\({}^{2}\)/Hz) (\(p\ll 0.05\), \(R\) = 0.69, and \(R^{2}\) = 0.48). The maximums of the models were reached at 105 min for the first group and at 48 min for the second group. All times when the maximums were reached were calculated using the corresponding quadratic model. The results of the sympathovagal ratio for the two groups, as well as the respective regression models and confidence intervals, are plotted in Figure 5. The model for the group of non-hypotensive patients is y = \(3.84\) - \(0.03t\) + \(0.0001t^{2}\) (\(p\ll 0.05\), \(R\) = 0.69, and \(R^{2}\) = 0.48), while that for the group of hypotensive patients is y = \(6.33\) + \(0.04t\) - \(0.0002t^{2}\) (\(p\ll 0.05\), \(R\) = 0.31, and \(R^{2}\) = 0.10). Figure 2: Mean R–R intervals for hypotensive and non-hypotensive patients.