## 1 Introduction
The spectral analysis of HRV has been used to identify negative events, such as hypotension and bradycardia, in high-risk cardiovascular patients [24]. It has also been applied to ESRD patients before and after they undergo HD therapy. The results of these works showed higher SNS activity compared to PNS activity at the end of each HD session [25; 26; 27; 28; 29; 30]. Similarly, [PERSON] et al. performed a spectral analysis of HRV over segments acquired at the beginning, middle, and end of HD therapy [31]. The analysis included some time-domain parameters, such as the mean R-R, the standard deviation ofall normal to normal (NN) intervals (SDNN), and the root mean square of successive NN differences (RMSSD), as well as blood tests and clinical parameters. Even though including a third point in the spectral analysis improved the assessment of the SNS and PNS during therapy, the approach did not give information on the gradual adaptation of the ANS to the hypovolemic state induced by HD. Importantly, [PERSON]'s work showed that HRV can be useful in predicting hypotensive events. In a study published by [PERSON] et al., the authors searched for the association between HRV and the presence of major adverse events related to mortality in patients receiving HD therapy [32]. Their results showed that a decrease in VLF power is related to an increased risk of major adverse events. This work shows that the evaluation of cardiac function through a spectral analysis of HRV has fundamental prognostic importance for these patients. The hypothesis in this work is that the SNS decreases over time during HD therapy in patients experiencing hypotension events, and that patients who do not show any adverse signs maintain or increase their SNS activity over the duration of HD therapy. To test this hypothesis, we estimated the spectrotemporal power variations of the VLF, LF and HF bands during HD therapy in hypotensive and non-hypotensive patients. We applied the STFT with an epoch twice the size of the standard recommended by the Task Force for HRV Analysis for short-term recordings [13]. The changes in the LF and HF power are related to the adaptive response of the SNS and PNS induced by HD therapy in the CV system [8]. These variations could serve as markers to identify patients in risk of hypotension during HD [31]. Considering the dynamic characteristics of the HRV, it becomes necessary to employ time-frequency analysis tools to track the changes in the spectral power over the intradialysis time. In this work, we employed the STFT because it is a suitable simple-to-use tool that allows to locate the changes in the spectral power over time [33; 34]. Unlike other jobs, we included the VLF band in our analysis, since it is considered that the VLF power reflects the activity of the RAAS [16], which participates in the long-term regulation of blood pressure through angiotensin, whose level varies during HD [30]. Previous studies have shown that using 10 min epochs improves the estimation of the VLF spectral power [21; 35]. Here, we applied the STFT using this modification in the analysis epoch to study the changes in the power spectral distribution along the intradialysis time, comparing hypotensive and non-hypotensive patients. Along with the spectrotemporal analysis, we also analyzed the mean R-R intervals.