The prevalence of autonomic nervous system (ANS) dysfunction in patients with end-stage kidney disease (ESKD) is considered to be increased. The uraemic environment, as well as the high incidence of comorbid conditions affecting the ANS function (e.g. diabetes mellitus, autoimmune and degenerative neurological diseases), have been proposed to cause important alterations in ANS function. The vast majority of evidence on the prevalence of ANS dysfunction in ESKD patients is derived from small studies elaborating simple methodology. Noteworthy, with the exception of a study in 27 hemodialysis patients which assessed ANS function before and after dialysis in relation to left ventricular filling pressures, and a 2005 Dutch study in 21 patients whether or not they had hypotension during dialysis, no other study used advanced methods to analyze heart rate or blood pressure variability from beat-to-beat recordings, such as this study. In addition, there is no study so far investigating possible changes in the ANS function per dialysis session. Finally, to the best of our knowledge, this is the first work evaluating possible differences in ANS function in hemodialysis compared with peritoneal dialysis individuals.

This is an observational study performed in the Department of Nephrology, Hippokration Hospital, Thessaloniki, Greece. For the purposes of this study, adult patients (>18 years) with ESKD being treated with hemodialysis (HD) (on standard thrice-weekly HD treatment) or peritoneal dialysis (PD) for at least 3 months, fulfilling the inclusion/exclusion criteria were invited to participate. PD and HD patients will be matched by a blinded member of our team based on age, gender and dialysis vintage (i.e. the duration of time after the point they started renal replacement therapy for ESRD) in a 1:1 ratio. All included patients signed a written informed consent form. The study protocol was approved by the Ethics Committee of the School of Medicine, Aristotle University of Thessaloniki. All procedures and evaluations are performed according to the Declaration of Helsinki 2013 Amendment.

Evaluation of participants includes the recording of demographics and anthropometric characteristics, medical history, concomitant medications and dialysis-related parameters, as well as physical examination and venous blood sampling for routine laboratory tests. Participants are instructed to visit the Department one hour before the programmed follow-up visit at their unit (PD patients); HD patients are instructed to visit the Department on 2 consecutive days (mid-week dialysis day and the corresponding dialysis-off day). All procedures are performed in a room with an ambient temperature of 23-24oC. Firstly, participants are oriented and familiarized with the experimental procedures and are prepared for the hemodynamic and cardiovascular examinations. Continuous beat-by-beat BP and heart rate are monitored using finger photoplethysmography (Finometer PRO, Finapres Medical Systems, Amsterdam, the Netherlands) throughout the protocol. An inflatable cuff is placed on the middle finger of the non-access hand maintained at heart level. After a 5-min rest, all participants underwent a mental task (countdown from 100 to 0 by 7, performed twice), an orthostatic test (5 min with the patient at the supine position followed by 5 min with the patient in upright position), and a mild physical task [3-minute submaximal handgrip exercise test (set of 30 s exercise at 35% MVC with 3 s rest] (K-Force, K-invent). At the completion of the exercise protocol, the Rate of Perceived Exertion (RPE) is assessed using the Borg scale.

For data analysis, beat-by-beat SBP and DBP will be averaged per testing session and exported using the Beatscope software (version 1.a, Finapres Medical Systems, Amsterdam, the Netherlands). From pulse pressure profiles, beat-by-beat heart rate (HR) and stroke volume (SV, ml) will be computed, using the Modelflow method; cardiac output (L/min) and systemic vascular resistance (SVR, mmHg.s/mL), will also be computed using Beatscope. HRV analysis will be performed with the HRV Analysis software Kubios (version 3.3.1, Kubios Oy, 2019) by the same researcher to eliminate inter-observer variability. R-R interval series will be checked for ectopic beats or artifacts. The root mean square of successive differences (RMSSD) between the coupling intervals of adjacent R-R intervals will be used as a parasympathetic activity index. Baroreceptor sensitivity (ms/mmHg) will be assessed by examining the spontaneous fluctuations in the blood pressure as assessed by the cross-correlation method using the Beatscope 1a software).

Inclusion Criteria:

• Age >18 years
• ESKD individuals treated with either hemodialysis (standard thrice weekly schedule) or peritoneal dialysis for at least 3 months
• Provision of informed written consent

Exclusion Criteria:

• Antihypertensive treatment modifcation during one month prior to study enrollment
• Modification of treatment for neurological disorders one month prior to study enrollment
• Active malignant disease or other comorbidity with poor prognosis
• History of neurological disorders (e.g. Parkinson's disease, multiple sclerosis, etc) that cause primary ANS dysfunction
• History of ANS dysfunction secondary to diabetes mellitus, amyloidosis, autoimmune disorders, etc.
• Active infection or relevant inter-current illness.
• History of drug or alcohol abuse or severe mental disorder

ESKD individuals treated with either hemodialysis (standard thrice weekly schedule) or peritoneal dialysis