Intradialytic hypertension (IDH) is a well-recognized and established complication of hemodialysis that affects an estimated 10-15% of the dialysis population and is associated with an increased risk for cardiovascular adverse events and mortality. The major pathogenic mechanisms include volume and sodium overload, endothelial dysfunction and enhanced vasoconstriction potentially through the renin-angiotensin-aldosterone system (RAAS) and sympathetic nervous system (SNS) activation. Preliminary uncontrolled studies have demonstrated that in order to achieve proper control of blood pressure (BP) in patients with IDH, volume control with achievement of dry weight, as well as the minimization of sodium load through alteration of dialysate sodium may improve BP. To this day, 3 studies have attempted to evaluate the effect of low dialysate sodium on BP levels in patients with IDH; one study that included 16 patients, compared the effect of low (5 milliequivalent/litre (mEq/L) lower than serum sodium) versus high (5 mEq/L higher than serum sodium) dialysate sodium concentration on BP levels only during the dialysis session; another study examined the effect of low (136 mEq/L) compared to standard (140 mEq/L) sodium dialysate, again, only on peridialytic and intradialytic BP; and only one randomized cross-over study used 24h ABPM to assess the effect of individualized isonatremic vs hyponatremic vs standard dialysate sodium. Hence, the aim of this study is to examine the effect of low (137mEq/L) vs standard (140mEq/L) dialysate sodium on 48h ambulatory blood pressure monitoring (ABPM) in patients with IDH, using appropriate design of randomized crossover study. In addition this is the first study examining the effect of low dialysate sodium on ambulatory central BP, arterial stiffness indices and BP variability in patients with IDH.

This is an interventional randomized crossover study performed in the Department of Nephrology, Hippokration Hospital, Thessaloniki, Greece. For the purposes of this study, adult patients (>18 years) with end stage kidney disease (ESKD) being treated with hemodialysis (HD) (on standard thrice-weekly HD treatment) for at least 3 months with intradialytic hypertension, fulfilling the inclusion/exclusion criteria were invited to participate. 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.

Patients will be assessed for eligibility during the selection process (records of peridialytic BP measurements of the previous 2-week period will be assessed). Patients will be instructed to arrive to the Dialysis unit 30 min to 1 hour prior to their scheduled dialysis session, on 3 different dialysis days. Baseline evaluation of participants includes the recording of demographics and anthropometric characteristics, medical history, comorbidities, concomitant medications and dialysis-related parameters, as well as physical examination and venous blood sampling for routine laboratory tests. At baseline evaluation patients' hydration status will be assessed with lung ultrasound, while peridialytic BP and BP over the intradialytic period will be assessed with the Mobil-O-Graph device (IEM, Stolberg, Germany). The study includes two treatment periods (low vs standard dialysate sodium) with washout period of 2 weeks between them. Patients will be randomized to 2 groups, which will receive the intervention in the opposite order. Block randomisation will be used to determine treatment order based on a computer-generated randomization list. The first group (A) will undergo dialysis with low dialysate sodium (137 mEq/L) for 4 sessions starting from a mid-week session (i.e Wednesday or Thursday). Immediately before the beginning of the 4th session patients will be assessed with lung ultrasound and the 48h ambulatory BP monitoring will start using the Mobil-O-Graph device. After a 2-week washout period this group (A) will undergo dialysis with standard dialysate sodium (140 mEq/L) for 4 sessions and at the start of the 4th session, again, patients will be assessed with lung ultrasound and the 48h ABPM will begin. The second group will undergo dialysis first with standard dialysate sodium (140 mEq/L) for 4 sessions and then, after the 2-week washout period, with low dialysate sodium (137 mEq/L) for 4 sessions, with similar evaluations at the end of each intervention. Patients will not be aware of the order in which they will receive the 2 different dialysate sodium concentrations.

Inclusion Criteria:

• Adult ESKD individuals treated with a standard thrice weekly hemodialysis schedule for at least 3 months
• Patients with intradialytic hypertension, defined as SBP rise ≥10 mmHg from pre- to post-dialysis in at least 4 out of 6 consecutive sessions
• Patients that are considered clinically euvolemic
• Ability to provide informed written consent

Exclusion Criteria:

• Post-dialysis SBP <130 mmHg in at least 4 out of 6 consecutive sessions during the 2-week selection period, prior to study entry
• Previous non-functional arteriovenous fistula in the contralateral brachial arm area of the one used for vascular access that could interfere with proper ambulatory BP recording
• Patients with contraindications to receive the intervention (low dialysate sodium), i.e patients with frequent intradialytic hypotension episodes requiring intervention with fluid administration
• Pre-dialysis serum sodium <130 or >142 mEq/L at recruitment
• Modification of dry weight or antihypertensive treatment during one month before study initiation
• History of seizures or disequilibrium syndrome
• Hospitalization for any cause during one month before study initiation
• History of malignancy or any other condition with poor prognosis