Sodium toxicity in peritoneal dialysis: mechanisms and "solutions"

Quantifying Ultrafiltration in Peritoneal Dialysis Using the Sodium Dip

Key Points

Ultrafiltration (UF) is a key component of clinical peritoneal dialysis prescription, but the traditional method to assess UF is hampered by large inaccuracies. Here we propose a novel method, based on a computational model and on a single dialysate sodium measurement, to accurately estimate UF and osmotic conductance to glucose in patients on peritoneal dialysis.

Background

Volume overload is highly prevalent among patients treated with peritoneal dialysis (PD), contributes to hypertension, and is associated with an increased risk of cardiovascular events and death in this population. As a result, optimizing peritoneal ultrafiltration (UF) is a key component of high-quality dialysis prescription. Osmotic conductance to glucose (OCG) reflects the water transport properties of the peritoneum, but measuring it requires an accurate quantification of UF, which is often difficult to obtain because of variability in catheter patency and peritoneal residual volume.

Methods

In this study, we derived a new mathematical model for estimating UF during PD, on the basis of sodium sieving, using a single measure of dialysate sodium concentration. The model was validated experimentally in a rat model of PD, using dialysis fluid with two different sodium concentrations (125 and 134 mmol/L) and three glucose strengths (1.5%, 2.3%, and 4.25%). Then, the same model was tested in a cohort of PD patients to predict UF.

Results

In experimental and clinical conditions, the sodium-based estimation of UF rate correlated with UF rate measurements on the basis of volumetry and albumin dilution, with a R 2 =0.35 and R 2 =0.76, respectively. UF on the basis of sodium sieving was also successfully used to calculate OCG in the clinical cohort, with a Pearson r of 0.77.

Conclusions

Using the novel mathematical models in this study, the sodium dip can be used to accurately estimate OCG, and therefore, it is a promising measurement method for future clinical use.

The role of intra- and interdialytic sodium balance and restriction in dialysis therapies

The relationship between sodium, blood pressure and extracellular volume could not be more pronounced or complex than in a dialysis patient. We review the patients' sources of sodium exposure in the form of dietary salt intake, medication administration, and the dialysis treatment itself. In addition, the roles dialysis modalities, hemodialysis types, and dialysis fluid sodium concentration have on blood pressure, intradialytic symptoms, and interdialytic weight gain affect patient outcomes are discussed. We review whether sodium restriction (reduced salt intake), alteration in dialysis fluid sodium concentration and the different dialysis types have any impact on blood pressure, intradialytic symptoms, and interdialytic weight gain.

Effects of Steady Glucose Concentration Peritoneal Dialysis on Ultrafiltration Volume and Sodium Removal: A Pilot Crossover Trial

BACKGROUND

Volume overload is common in patients treated with peritoneal dialysis (PD) and is associated with poor clinical outcome. Steady concentration PD is where a continuous glucose infusion maintains the intraperitoneal glucose concentration and as a result provides continuous ultrafiltration throughout the dwell. The primary objective of this study was to investigate the ultrafiltration rate and glucose ultrafiltration efficiency for steady concentration PD in comparison with a standard continuous ambulatory PD (CAPD) dwell, using the novel Carry Life UF device.

METHODS

Eight stable patients treated with PD (six fast and two fast average transporters) were investigated four times: a standard 4-hour CAPD dwell with 2 L of 2.5% dextrose solution as control and three 5-hour steady concentration PD treatments (glucose dose 11, 14, 20 g/h, initial fill 1.5 L of 1.5% dextrose solution). All investigations were preceded by an overnight 2 L 7.5% icodextrin dwell.

RESULTS

Intraperitoneal glucose concentration increased during the first 1-2 hours of the steady concentration PD treatments and remained stable thereafter. Ultrafiltration rates were significantly higher with steady concentration PD treatments (124±49, 146±63, and 168±78 mL/h with 11, 14, and 20 g/h, respectively, versus 40±60 mL/h with the control dwell). Sodium removal and glucose ultrafiltration efficiency (ultrafiltration volume/gram glucose uptake) were significantly higher with steady concentration PD treatments versus the control dwell, where the 11 g/h glucose dose was most efficient.

CONCLUSIONS

Steady concentration PD performed with the Carry Life UF device resulted in higher ultrafiltration rates, more efficient use of glucose (increased ultrafiltration volume/gram glucose absorbed), and greater sodium removal compared with a standard 2.5% dextrose CAPD dwell.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

A Performance Analysis of the Peritoneal Ultrafiltration (PUF) Achieved With the Carry Life ® UF, NCT03724682 .

Volume-Independent Sodium Toxicity in Peritoneal Dialysis: New Insights from Bench to Bed

Sodium overload is common in end-stage kidney disease (ESKD) and is associated with increased cardiovascular mortality that is traditionally considered a result of extracellular volume expansion. Recently, sodium storage was detected by Na23 magnetic resonance imaging in the interstitial tissue of the skin and other tissues. This amount of sodium is osmotically active, regulated by immune cells and the lymphatic system, escapes renal control, and, more importantly, is associated with salt-sensitive hypertension. In chronic kidney disease, the interstitial sodium storage increases as the glomerular filtration rate declines and is related to cardiovascular damage, regardless of the fluid overload. This sodium accumulation in the interstitial tissues becomes more significant in ESKD, especially in older and African American patients. The possible negative effects of interstitial sodium are still under study, though a higher sodium intake might induce abnormal structural and functional changes in the peritoneal wall. Interestingly, sodium stored in the interstial tissue is not unmodifiable, since it is removable by dialysis. Nevertheless, the sodium removal by peritoneal dialysis (PD) remains challenging, and new PD solutions are desirable. In this narrative review, we carried out an update on the pathophysiological mechanisms of volume-independent sodium toxicity and possible future strategies to improve sodium removal by PD.

Mild sodium reduction in peritoneal dialysis solution improves hypertension in end stage kidney disease: a case-report study

Introduction

Blood Pressure (BP) control is largely unsatisfied in End Stage Kidney Disease (ESKD) principally due to sodium retention. Peritoneal Dialysis (PD) is the most common type of home dialysis, using a peritoneal membrane to remove sodium, though sodium removal remains challenging.

Methods

This is a case-study reporting two consecutive ESKD patients treated by a novel peritoneal PD solution with a mildly reduced sodium content (130 mmol/L) to treat hypertension.

Results

In the first case, a 78-year-old woman treated by Continuous Ambulatory PD (CAPD) with standard solution (three 4 h-dwells per day 1.36% glucose 132 mmol/L) showed resistant hypertension confirmed by ambulatory blood pressure monitoring (ABPM), reporting 24 h-BP: 152/81 mmHg, day-BP:151/83 mmHg and night-ABP: 153/75 mmHg, with inversion of the circadian systolic BP rhythm (1.01), despite use of three anti-hypertensives and a diuretic at adequate doses. No sign of hypervolemia was evident. We then switched from standard PD to low-sodium solution in all daily dwells. A six-months low-sodium CAPD enabled us to reduce diurnal (134/75 mmHg) and nocturnal BP (122/67 mmHg), restoring the circadian BP rhythm, with no change in ultrafiltration or residual diuresis. Diet and drug prescription were unmodified too.

The second case was a 61-year-old woman in standard CAPD (three 5 h-dwells per day) suffering from hypertension confirmed by ABPM (mean 24 h-ABP: 139/84 mmHg; mean day-ABP:144/88 mmHg and mean night-ABP:124/70 mmHg). She was switched from 132-Na CAPD to 130-Na CAPD, not changing dialysis schedule. No fluid expansion was evident. During low-sodium CAPD, antihypertensive therapy (amlodipine 10 mg and Olmesartan 20 mg) has been reduced until complete suspension. After 6 months, we repeated ABPM showing a substantial reduction in mean 24 h-ABP (117/69 mmHg), mean diurnal ABP (119/75 mmHg) and mean nocturnal ABP (111/70 mmHg). Ultrafiltration and residual diuresis remained unmodified. No side effects were reported in either cases.

Conclusions

This case-report study suggests that mild low-sodium CAPD might reduce BP in hypertensive ESKD patients.

The osmo-metabolic approach: a novel and tantalizing glucose-sparing strategy in peritoneal dialysis

Peritoneal dialysis (PD) is a viable but under-prescribed treatment for uremic patients. Concerns about its use include the bio-incompatibility of PD fluids, due to their potential for altering the functional and anatomical integrity of the peritoneal membrane. Many of these effects are thought to be due to the high glucose content of these solutions, with attendant issues of products generated during heat treatment of glucose-containing solutions. Moreover, excessive intraperitoneal absorption of glucose from the dialysate has many potential systemic metabolic effects. This article reviews the efforts to develop alternative PD solutions that obviate some of these side effects, through the replacement of part of their glucose content with other osmolytes which are at least as efficient in removing fluids as glucose, but less impactful on patient metabolism. In particular, we will summarize clinical studies on the use of alternative osmotic ingredients that are commercially available (icodextrin and amino acids) and preclinical studies on alternative solutions under development (taurine, polyglycerol, carnitine and xylitol). In addition to the expected benefit of a glucose-sparing approach, we describe an ‘osmo-metabolic’ approach in formulating novel PD solutions, in which there is the possibility of exploiting the pharmaco-metabolic properties of some of the osmolytes to attenuate the systemic side effects due to glucose. This approach has the potential to ameliorate pre-existing co-morbidities, including insulin resistance and type-2 diabetes, which have a high prevalence in the dialysis population, including in PD patients.

Sodium Intake and Chronic Kidney Disease

In Chronic Kidney Disease (CKD) patients, elevated blood pressure (BP) is a frequent finding and is traditionally considered a direct consequence of their sodium sensitivity. Indeed, sodium and fluid retention, causing hypervolemia, leads to the development of hypertension in CKD. On the other hand, in non-dialysis CKD patients, salt restriction reduces BP levels and enhances anti-proteinuric effect of renin–angiotensin–aldosterone system inhibitors in non-dialysis CKD patients. However, studies on the long-term effect of low salt diet (LSD) on cardio-renal prognosis showed controversial findings. The negative results might be the consequence of measurement bias (spot urine and/or single measurement), reverse epidemiology, as well as poor adherence to diet. In end-stage kidney disease (ESKD), dialysis remains the only effective means to remove dietary sodium intake. The mismatch between intake and removal of sodium leads to fluid overload, hypertension and left ventricular hypertrophy, therefore worsening the prognosis of ESKD patients. This imposes the implementation of a LSD in these patients, irrespective of the lack of trials proving the efficacy of this measure in these patients. LSD is, therefore, a rational and basic tool to correct fluid overload and hypertension in all CKD stages. The implementation of LSD should be personalized, similarly to diuretic treatment, keeping into account the volume status and true burden of hypertension evaluated by ambulatory BP monitoring.

Characterization of sodium removal to ultrafiltration volume in a peritoneal dialysis outpatient cohort

Background

Failure to control volume is the second most common cause of peritoneal dialysis (PD) technique failure. Sodium is primarily removed by convection, but according to the three-pore model, water and sodium movements are not necessarily concordant. We wished to determine factors increasing sodium to water clearance in clinical practice.

Methods

We reviewed 24-h peritoneal dialytic sodium removal (DSR) and ultrafiltration (UF) volume in consecutive PD patients attending for routine assessment of peritoneal membrane function and adequacy testing. We used a regression model with the DSR/UF ratio as the dependent variable. A second model with DSR as the dependent variable and interaction testing for UF was used as sensitivity analysis.

Results

We included 718 adult PD patients. Mean values were 51.8 ± 64.6 mmol/day and 512 ± 517 mL/day for DSR and UF, respectively. In multivariable analysis, DSR/UF ratio was positively associated with transport type (fast versus slow, P < 0.001), serum sodium (P < 0.001) and diabetes (P = 0.026), and negatively associated with PD mode [automated PD versus continuous ambulatory PD (CAPD), P < 0.001] and the use of 2.27% glucose dialysate (P < 0.001). Sensitivity analysis showed positive interaction with UF for transport type (P < 0.001) and serum sodium (P = 0.032) and negative interaction for PD mode (P < 0.001) and cycles number (P < 0.001).

Conclusions

CAPD, fast transport and high serum sodium allow relatively more sodium to be removed compared with water. Icodextrin has no effect on sodium removal once confounders have been accounted for. Although widely used in the assessment of PD patients, UF should not be considered as a surrogate for DSR in clinical practice.