Can Changes in Dialysate Sodium Concentration Improve Blood Pressure and Endothelial Function in Chronic HD Patients?

1. Background Patients with end stage renal disease lose the ability to effectively regulate the body's sodium and fluid balance. This leads to chronic sodium and fluid accumulation, contributing to elevated blood pressure, increased risk of cardiovascular complications, and fluid retention in tissues. Hemodialysis is therefore a life sustaining treatment in which waste products, electrolytes, and excess fluid are removed from the blood through a filter into dialysate, with the aim of partially restoring this balance. The dialysate is composed by the dialysis machine, where sterile water is mixed with, among other components, salt (sodium), which plays a central therapeutic role.

Chronic sodium and fluid overload in hemodialysis patients is associated with increased mortality, particularly related to cardiovascular disease. Previous studies have shown that cardiovascular mortality increases with the degree of fluid overload and the need for high ultrafiltration volumes during dialysis. The sodium concentration of the dialysate directly affects blood pressure control and interdialytic fluid accumulation. Several studies have demonstrated that a lower sodium concentration in the dialysate can reduce blood pressure and interdialytic weight gain. However, this strategy is associated with risks such as hypotension, muscle cramps, and dehydration. Conversely, a higher sodium concentration may increase thirst, blood pressure, and make it more difficult for patients to adhere to fluid restrictions. A study comparing low versus standard dialysate sodium found reductions in blood pressure and fluid overload after only four dialysis sessions.

The hemodynamic effects of sodium cannot, however, be explained solely by changes in intravascular volume. The endothelium-the innermost cell layer of blood vessels-plays a central role in regulating vascular tone and is covered by a protective glycocalyx layer. Sodium accumulation in tissues can damage this layer and reduce the production of nitric oxide (NO), which is essential for endothelium dependent vasodilation. Reduced NO bioavailability is associated with increases in blood pressure during dialysis and is a negative prognostic marker in hemodialysis patients. Endothelial function can be assessed by measuring plasma NOx and by the Salt Blood Test, which reflects erythrocytes' ability to bind sodium and thereby the systemic capacity for sodium buffering.

Although some interventional studies have shown improvements in endothelial function and blood pressure with lower dialysate sodium, larger meta analyses have not demonstrated a clear reduction in mortality. This suggests that the effect of sodium reduction is likely heterogeneous, and that we still lack knowledge about which patients benefit most from increased sodium removal and through which mechanisms these effects manifest-both clinically and subjectively.

The aim of this study is therefore to investigate whether removing more sodium during dialysis-achieved by reducing the sodium concentration in the dialysate-can improve blood pressure outside dialysis and enhance endothelial function in the short term, while systematically documenting patient experiences and side effects. By combining clinical measurements with patient involvement and a novel method for assessing fluid overload based on carbon monoxide (CO) rebreathing, the study seeks to provide a more precise and holistic understanding of the role of sodium balance in hemodialysis patients. 2. Objective The objective of the study is to investigate whether a low sodium concentration in the dialysate (133 mmol/L) over a 3 week period, compared with the standard concentration (139 mmol/L), affects blood pressure, endothelial function, fluid status, and patient reported experiences of dialysis treatment in patients undergoing chronic in center hemodialysis. 3. Hypothesis

We hypothesize that a lower sodium concentration in the dialysate-compared with the standard concentration-will remove additional sodium during dialysis and thereby: * Reduce 24 hour blood pressure as an indicator of improved blood pressure control. * Increase plasma NOx levels as a marker of improved endothelial function. * Reduce fluid overload, measured by bioimpedance and the CO rebreathing technique. * Lower the Salt Blood Test value, indicating improved erythrocyte sodium buffering capacity. * Reduce plasma levels of inflammatory cytokines, including IL 17, IFN γ, IL 10, IL 6, IL 1β, IL 17A, and TNF α, indicating a lower inflammatory burden. * Reduce the need for ultrafiltration during dialysis and decrease patient reported thirst. 4. Perspective Dialysis patients have a markedly increased risk of cardiovascular disease and premature death compared with the general population, primarily due to disturbances in the regulation of sodium and fluid, which affect both blood pressure and vascular function. Despite this, limited knowledge exists regarding how variations in dialysate sodium concentration influence both physiological and subjective parameters in chronic hemodialysis patients.

This study addresses a key knowledge gap by combining objective measures (blood pressure, NOx, fluid overload, inflammation) with patient reported symptoms (thirst, dizziness, headache), thereby providing a more comprehensive understanding of sodium balance in this population.

If a lower sodium concentration proves effective, it represents a simple and cost effective improvement to dialysis treatment. Adjusting dialysate sodium is already part of routine clinical practice, but optimal individualization remains unclear. Changing dialysate sodium requires no new equipment and can be widely implemented. Improved blood pressure control and reduced inflammatory burden may reduce medication needs, hospitalizations, and complications, potentially lowering healthcare costs. The study will form the basis for future long term studies examining effects on vascular function and prognosis. 5. Outcome Measures Effects will be measured as the difference between parameters during dialysate sodium concentrations of 133 mmol/L and 139 mmol/L (standard).

Primary outcome:

\- Difference in systolic 24 hour blood pressure.

Secondary outcomes: * Plasma NOx levels * Fluid overload (BCM and CO method) * Volume assessed by the CO method * Dialysis data, including clinically assessed ultrafiltration needs (dry weight) * Pulse wave velocity (arterial stiffness) * Plasma levels of inflammatory cytokines * Side effects (questionnaire based) 6. Study Design The study is a randomized, crossover, single blinded interventional trial in which each participant receives both interventions-dialysis with low and standard sodium concentration-in two consecutive 3 week periods. Participants are blinded, while dialysis nurses and investigators are unblinded.

Interventions: * Low sodium concentration: 133 mmol/L * Standard sodium concentration: 139 mmol/L The sequence is determined by randomization. After the first 3 week period, participants cross over to the opposite intervention following a 2 to 3 week washout period. 7. Participants 7.1 Inclusion Criteria

* ≥18 years * Chronic in center hemodialysis ≥3 months * Hemodialysis 3 times weekly * Plasma sodium \>132 mmol/L at the two most recent tests within 3 months * Expected continuation of dialysis \>3 months 7.2 Exclusion Criteria * AMI or diagnosed heart failure with EF \<50% within the past 6 months * Stroke or TIA within the past 6 months * Bilateral lower limb amputation * DDD pacemaker 7.3 Withdrawal Criteria * Discontinuation of dialysis * Unacceptable side effects * Development of an exclusion criterion * Participant request * Non compliance 7.4 Recruitment Participants will be recruited from the Dialysis Clinic, Regional Hospital Gødstrup.

7.5 Sample Size and Statistical Power With an expected SD of 10 mmHg, 80% power, and a significance level of 5% (p \< 0.05), 24 completed participants are required to detect a mean difference of 6 mmHg in systolic 24 hour blood pressure. Allowing for ≥30% dropout, up to 35 participants may be included. 8. Procedure for Recruitment and Inclusion 8.1 Screening of Potential Participants Before contacting a potential participant, the principal investigator reviews the electronic patient journal (EPJ) to assess whether the patient meets the inclusion criteria (≥18 years, ≥3 months of hemodialysis, dialysis three times weekly, plasma sodium \>132 mmol/L on recent blood tests). Plasma sodium must be above 132 mmol/L on the two most recent measurements taken in the Dialysis Clinic within the past 3 months to be eligible. This can be reassessed after a minimum of 3 months if needed.

8.2 Recruitment and Information Meeting Patients deemed eligible are approached in person by the principal investigator, the consulting physician, or the project nurse during their routine dialysis sessions. They receive a brief introduction to the project, and if they express interest, a time and place for a full information meeting is arranged. Patients may bring a companion to the meeting. During the information meeting, oral information is provided by the project investigator based on the written participant information (Appendix 2), which is handed out and includes details on the study's purpose, procedures, methods, potential risks, and participant rights. Patients are also informed that health related and other confidential information may be shared with individuals responsible for mandatory quality control of the study.

8.3 Reflection Period and Opportunity for Private Discussion All patients are offered a minimum reflection period of one week before deciding on participation. They are given the opportunity to ask questions and to speak with the investigator in private if desired. It is emphasized that participation is voluntary and will not affect their usual treatment.

8.4 Informed Consent If the patient wishes to participate, a written informed consent form (Appendix 3) is signed. The consent covers participation in the intervention, blood sampling, questionnaires, and the transfer of relevant data from EPJ and Therapy Manager to the research database (REDCap). It also includes consent for collection of biological material (blood) for storage in a research biobank for later analyses. Participants may opt out of receiving information about significant health findings discovered during the project. Consent is stored in accordance with applicable legislation.

8.5 Inclusion in the Study After signing consent, inclusion and exclusion criteria are reviewed. If the patient meets all criteria, an intervention sequence is assigned by randomization, and study participation is scheduled. The participant is informed about practical aspects, including measurement time points, questionnaires, and contact persons. 9. Procedure for Study Participation The intervention sequence is assigned by randomization in REDCap. Participants are not informed of the sequence.

Study Periods Each intervention period lasts 3 weeks, and all participants complete both periods in a crossover design, separated by a 2 week washout period (extendable to 3 weeks). Study participation is aligned with each patient's fixed dialysis schedule whenever possible. Patients on evening dialysis may remain on their usual schedule but will be asked to arrive earlier on examination days so that blood sampling and nuclear medicine assessments can be completed before 15:00.

Each study period includes 9 dialysis sessions. Examinations are performed during the 9th dialysis session of each period.

To ensure consistency, all study assessments are conducted during the second dialysis session of the calendar week (typically Wednesday or Thursday). Therefore, each study period begins with the third dialysis session of the week (typically Friday

• Diastolic 24-hour ambulatory blood pressure [Срок оценки: Day 19 in each intervention period]
• Pulse wave velocity (arterial stiffness) [Срок оценки: Day 19 in each intervention period]
• Salt Blood Test [Срок оценки: Day 19 in each intervention period]
• Plasma nitric oxide metabolites (NOx) [Срок оценки: Day 19 of each intervention period]
• Inflammatory cytokines [Срок оценки: Day 19 of each intervention period]
• Total blood volume (CO-rebreathing) [Срок оценки: Day 19 of each intervention period]
• Body composition and fluid overload [Срок оценки: Day 19 of each intervention period]
• Ultrafiltration volume [Срок оценки: Week 3 of each intervention period]
• Dry weight [Срок оценки: Day 19 of each intervention period]
• Patient-reported symptoms and quality of life [Срок оценки: Day 19 of each intervention period]