## 1 Introduction  
Chronic kidney disease (CKD) progresses with renal dysfunction and systemic complications, including cardiorenal syndrome (CRS), which is a bidirectional heart-kidney interaction in which mutual dysfunction drives cardiovascular risk and disrupts the autonomic nervous system (ANS) balance, which is critical for physiological regulation and adaptive capacity [1, 2]. This autonomic imbalance heightens susceptibility to arrhythmias, particularly under physiological stress, creating a vicious cycle of cardiorenal deterioration [3, 4, 5]. It can be understood as an out-of-balance dynamic control system in which the sympathetic nervous system acts as a \"stuck accelerator\" and the parasympathetic nervous system functions as a \"weak brake\", leading to inefficient operation and the progressive wearing of the system. In advanced CKD (stage 5), hemodialysis--performed thrice weekly (3-5 h/session)--corrects fluid/chemical imbalances but triggers triphasic autonomic dysfunction: predialysis sympathetic hyperactivity, intradialytic fluid shift-induced sympathetic surges, and post-dialysis sympathetic predominance with parasympathetic suppression [6; 7; 8; 9]. HRV is a useful tool for assessing the activity and balance between the sympathetic and parasympathetic branches of the ANS [10]. Heart rate variability (HRV) is assessed via multidomain metrics: time-domain indices (SDNN for total variability, RMSSD for parasympathetic activity), frequency-domain ratios (LF/HF reflecting sympathetic-parasympathetic balance), and non-linear Poincare parameters (SD1 for parasympathetic tone, SD2 for autonomic interplay), collectively quantifying ANS dynamics [11; 12]. The HRV measurement duration is tailored to study either short-term (2-5 min) records that capture baseline autonomic activity under controlled conditions or long-term 24-to-48 h recordings that reveal circadian rhythms. A dynamic windowed analysis tracks transient ANS triggered via the interactions with physiological stressors [13; 14]. A high HRV reflects a flexible and balanced ANS associated with better cardiovascular health, lower stress, and greater capacity for adaptation and recovery [15]. On the other hand, a reduced HRV usually indicates autonomic dysfunction, the predominance of sympathetic tone, and less adaptability, factors related to chronic stress, cardiovascular diseases, and a higher risk of complications [16]. HRV in CKD patients undergoing hemodialysis is analyzed to understand the status of the ANS and its possible relationship with cardiovascular complications [17]. Hemodialysis patients exhibit chronic autonomic imbalance (reduced HRV indices), marked by pre-dialysis sympathetic dominance (reduced RMSSD/HF, elevated LF/HF), intradialytic parasympathetic suppression (reduced SDNN), and partial post-dialysis recovery (elevated HF/RMSSD, reduced LF/HF), yet persistent dysregulation elevates arrhythmia risk fourfold during the post-session critical period [18; 19; 20; 21]. Arrhythmias associated with hemodialysis represent one of the main causes of mortality, with a particularly high incidence of premature ventricular contractions (PVCs) observed in almost all patients [22; 23]. These alterations of the normal heart rhythm not only reflect a state of electrical instability but also impact HRV indices. Patients with frequent PVCs exhibit reduced SDNN/RMSSD and elevated LF/HF ratios, reflecting heightened sympathetic dominance and parasympathetic suppression--a dysregulation that synergizes with hemodialysis-induced autonomic dysfunction to amplify arrhyththmogenic risk [24]. This study aimed to characterize the 48 h dynamics of HRV indices among 51 end-stage renal disease (ESRD) patients undergoing hemodialysis, with a focus on autonomic perturbations triggered via the dialysis cycle. By analyzing HRV fluctuations before, during, and after hemodialysis--critical phases of autonomic stress--we evaluated how arrhyththmogenic triggers, particularly PVCs, correlate with transient declines in HRV metrics (e.g., SDNN, RMSSD, LF/HF ratio).