Spurious estimations of sodium removal during CAPD when [Na](+) is measured by Na electrode methodology

Relationship between sodium removal, hydration and outcomes in peritoneal dialysis patients

BACKGROUND

Fluid overload (FO) in peritoneal dialysis (PD) patients is associated with mortality. We explore if low daily sodium removal is an independent risk factor for mortality. We examined severely FO PD patients established for >1 year in expectation that PD prescription would have been optimized for solute clearance and ultrafiltration. We also wish to determine the relationship between kt/v and sodium removal.

METHODS

Retrospective analysis of 231 PD patients with FO ≥2.0 L and compared with 218 PD patients who were euvolaemic throughout their PD treatment. Patients were followed up until death censored for transplantation.

RESULTS

Mean daily sodium removal in overhydrated patients was only 75 mmoles (=1.7 g). CAPD usage was more common in patients with the highest sodium removal. Achievement of UK guidelines for solute clearance and daily fluid removal were not independent predictors of mortality. Markers of sarcopenia (low serum albumin and high CRP) were associated with increased mortality, but these parameters were not independent predictors in a model that included functional assessment (Karnofsky score). Daily sodium removal was not predictive of mortality but the imprecision of clinically used sodium assay should be noted. The correlation between Na and kt/v is statistically significant but R² was weak at .07.

CONCLUSION

While diabetic males were more likely to become overhydrated, these factors did not increase mortality further. Traditional targets of 'dialysis adequacy' did not predict survival. Kt/v is not a good indicator of sodium removal which can be surprisingly low. Measuring sodium clearance may help clinicians optimize PD modality (CAPD vs. APD).

Validation by Computer Simulation of Two Indirect Methods for Quantification of Free Water Transport in Peritoneal Dialysis

Background

In peritoneal dialysis, approximately 40% of the total osmotic ultrafiltration (UF) induced by glucose can be predicted to be due to “free” water transport across aquaporin-1 (APQ-1). Theoretically, it would be possible to assess the fraction of free water transport in the early phase of a hypertonic dwell, when UF rate is high and the relative contribution of Na+ diffusion is low. La Milia et al. [La Milia V. et al. Fast-fast peritoneal equilibration test (FAST-FAST-PET): a simple method for peritoneal hydraulic permeability study [Abstract]. Nephrol Dial Transplant 2002; 17 (Suppl 1):17–18] suggested a technique to assess sodium-associated water transport based on sodium removal (Na+R) divided by the plasma Na+ concentration during a “fast-fast” (60 minute) peritoneal equilibration test (PET) for 3.86% glucose, yielding an estimate of the UF passing through the small pores (UFSP). Free water transport (UF through ultrasmall pores; UFUSP) was obtained by subtracting UFSP from total UF. Although peritoneal Na+ transport is almost totally convective, this technique will slightly overestimate small-pore UF due to the presence of some small-pore Na+ diffusion from the circulation during the dwell. A way of dealing with this problem was presented recently by Smit (Smit W. et al. Quantification of free water transport in peritoneal dialysis. Kidney Int 2004; 66:849–854).

Methods

In the present study we used the three-pore model of peritoneal transport to predict the degree of overestimation of UFSP for the technique presented by La Milia et al., and any potential deviations from theory for the technique presented by Smit et al. Simulations were performed under ordinary conditions and during simulated UF failure for 3.86% glucose. The fractional UF coefficient accounted for by APQ-1 was set at 2%.

Results

Estimating the UFSP from the sodium-associated water transport according to the method by La Milia et al. consistently overestimated UFSP and underestimated UFUSP. These errors were, however, minimal for dwells lasting between 30 and 80 minutes. The technique by Smit et al. to calculate aquaporin-mediated water flow (UFUSP), using an elaborate correction for Na+ diffusion from the circulation during the dwell, seemed accurate in most situations but, in general, tended to moderately overestimate UFUSP at early dwell times (<30 minutes) and underestimate UFUSP at long dwell times (4 hours).

Conclusions

The technique presented by La Milia et al. to calculate free water transport during a fast-fast PET was found to be surprisingly accurate, although the procedure would further improve by the introduction of a correction algorithm. The technique by Smit is even more accurate for dwells up to 4 hours’ duration. However, since the Smit technique is elaborate, it is less practical for routine determinations of aquaporin-mediated water transport in peritoneal dialysis.

Fluid and Electrolyte Transport across the Peritoneal Membrane during CAPD According to the Three-pore Model

In the present review, we summarize the principles governing the transport of fluid and electrolytes across the peritoneum during continuous ambulatory peritoneal dialysis (CAPD) in “average” patients and during ultrafiltration failure (UFF), according to the three-pore model of peritoneal transport. The UF volume curves as a function of dwell time [V( t)] are determined in their early phase by the glucose osmotic conductance [product of the UF coefficient (LpS) and the glucose reflection coefficient (σg)] of the peritoneum; in their middle portion by intraperitoneal volume and glucose diffusivity; and in their late portion by the LpS, Starling forces, and lymph flow. The most common cause of UFF is increased transport of small solutes (glucose) across the peritoneum, whereas the LpS is only moderately affected. Concerning peritoneal ion transport, ions that are already more or less fully equilibrated across the membrane at the start of the dwell, such as Na+(Cl–), Ca2+, and Mg2+, have a convection-dominated transport. The removal of these ions is proportional to UF volume (approximately 10 mmol/L Na+and 0.12 mmol/L Ca2+removed per deciliter UF in 4 hours).

The present article examines the impact on fluid and solute transport of varying concentrations of Ca2+and Na+in peritoneal dialysis solutions. Particularly, the effect of “ultralow” sodium solutions on transport and UF is simulated and discussed. Ions with high initial concentration gradients across the peritoneum, such as K+, phosphate, and bicarbonate, display a diffusion-dominated transport. The transport of these ions can be adequately described by non-electrolyte equations. However, for ions that are in (or near) their diffusion equilibrium over the peritoneum (Na+, Ca2+, Mg2+), more complex ion transport equations need to be used. Due to the complexity of these equations, however, non-electrolyte transport formalism is commonly employed, which leads to a marked underestimation of mass transfer area coefficients (PS). This can be avoided by determining the PS when transperitoneal ion concentration gradients are steep.

Peritoneal Equilibration Test Reference Values Using A 3.86% Glucose Solution during the First Year of Peritoneal Dialysis: Results of a Multicenter Study of a Large Patient Population

Background

The original peritoneal equilibration test (PET) was used to classify peritoneal dialysis (PD) patients using a 2.27% glucose solution. It has since been suggested that a 3.86% glucose solution be used because this provides better information about ultrafiltration (UF) capacity and the sodium (Na) sieving of the peritoneal membrane.

Objective

The aim of this study was to determine reference values for a PET using a 3.86% glucose solution (PET-3.86%).

Methods

We evaluated the PET-3.86% in a large population of incident PD patients attending 27 Italian dialysis centers.

Results

We evaluated the results of 758 PET-3.86% in 758 incident PD patients (1 test per patient). The mean duration of PD was 5 ± 3 months. The ratio of the concentrations of creatinine in dialysate/plasma (D/PCreat) was 0.73 ± 0.1 (median 0.74). The ratio between the concentrations of glucose at the end/beginning of the test (D/D0) was 0.25 ± 0.08 (median 0.24). Ultrafiltration uncorrected and corrected for bag overfill was respectively 776 ± 295 mL (median 781 mL) and 675 ± 308 mL (median 689 mL). Sodium sieving was 8.4 ± 3.8 mmol/L (median 8.0 mmol/L).

Conclusion

The results of the study provide PET-3.86% reference values for the beginning of PD that can be used to classify PD patients into transport classes and monitor them over time.

Is Adapted APD Theoretically More Efficient than Conventional APD?

Background

A modified version of automated peritoneal dialysis (APD) using not only variable dwell times but also variable fill volumes has been tested against conventional APD (cAPD) with fixed dwell volumes in a randomized controlled clinical study. The results have indicated that the modified schedule for APD, denoted adapted APD (aAPD), can lead to improved small solute clearances, and, above all, a markedly increased sodium removal (NaR). To theoretically test these results, we have modeled aAPD vs cAPD in computer simulations using the 3-pore model (TPM).

Methods

The TPM, modified by including a transient, initial inflation of small solute mass transfer area coefficients (PS values), was employed. For simulations of osmotic ultrafiltration (UF), the TPM uses a constantly inflated value for PS for glucose and also a reduced value for PS for Na+, setting the peritoneal lymphatic reabsorption term at 0.3 mL/min. The simulations were performed by assuming that increases in intraperitoneal hydrostatic pressure (IPP) are transmitted to the capillary level ( via vein compression) and therefore do not significantly affect the Starling balance. Furthermore, the effective peritoneal surface area (A) was set to be variable as a function of intraperitoneal volume (IPV).

Results

The simulations demonstrated a minor improvement of small solute clearances (∼0.7 – 1.6%) and a very small improvement of UF and NaR in aAPD compared to cAPD.

Conclusions

Due mainly to the increased fill volumes in 3 out of 5 dwells in aAPD, this modality caused minor increases in small solute clearances and marginal effects on UF and NaR. The computer simulations point to a need for accurate sodium determinations in aAPD, considering all the methodological problems and pitfalls relevant to determining dialysate Na+ concentrations and peritoneal sodium mass balance.

Should sodium removal in peritoneal dialysis be estimated from the ultrafiltration volume?

In peritoneal dialysis (PD), ultrafiltration (UF) volume is the sum of solute-free- and solute-coupled-water removal, a dynamic process throughout the entire dwell exerted via aquaporin-1 (AQP1) and small pores, respectively. Determination of sodium sieving is used as a parameter for AQP1 function analysis, while coupled water removal is essential for adequate sodium and water balance and thus blood pressure control. The diffusive capacity of glucose via the small pores determines the dynamic crystalloid osmotic gradient. The osmotic conductance, i.e., milliliter of UF per gram of glucose absorbed, quantifies cooperation between small-pores and AQP1 channels. In continuous ambulatory peritoneal dialysis, with dwell times beyond glucose-induced sodium-sieving effects, approximate dialytic sodium removal (DSR) may be estimated from the UF volume (in average 100 mmol Na/L UF), while DSR is lower, with shorter cycle times, in automated PD (APD); therefore, effluent sodium concentrations should be measured. Applying dialysis mechanics, i.e., varying dwell time and dwell volume-as proposed in adapted APD to the PD prescription-may provide unmatched high DSR relative to UF volume, findings which are not sufficiently explained by the three-pore model of PD. Overall DSR should therefore be measured rather than estimated from UF volume.

Glucose interference in direct ion-sensitive electrode sodium measurements

Circulating sodium concentration is commonly measured by both direct and indirect ion-sensitive electrode (ISE). We describe an unusual case with a high elevation of serum glucose (162 mmol/L) where direct ISE sodium measurement was 9 mmol/L higher than the indirect measurement in the absence of any cause for pseudohyponatraemia. In vitro experiments showed that very high glucose concentrations increased the sodium in direct, but not in indirect ISE measurement. This effect was insufficient to account for the entire difference between the measurements seen in the patient, indicating that other factors, for example pH and bicarbonate concentration, must also be involved. This effect may influence interpretation of sodium status in patients with gross hyperglycaemia.

Effects of Ionized Sodium Concentrations on Ultrafiltration Rate in Peritoneal Dialysis Using Lactate and Lactate/Bicarbonate Solutions

Objective

To investigate the possible effects of different concentrations of ionized sodium (NaI) on peritoneal ultrafiltration (UF) rate using lactate (Lac) and lactate/bicarbonate (Lac/Bic) dialysis solutions.

Design

Two random consecutive (after an interval of 48 hours) peritoneal equilibration tests (PETs) were performed in 13 patients (4 males and 9 females) on regular continuous ambulatory peritoneal dialysis (PD) treatment for at least 3 months. Two different PD solutions containing anhydrous glucose 3.86% were used: a 40 mmol/L Lac solution and a 15/25 mmol/L mixed Lac/Bic solution. Concentrations of total sodium (NaT) and NaI were measured by flame photometer and direct ion-selective electrode respectively.

Results

Dialysate concentrations of NaT were not different during PETs using Lac and Lac/Bic. Dialysate concentrations of NaI in fresh PD solutions were different (133.3 ± 1.7 vs 128.2 ± 3.9 mmol, p < 0.0001); however, these differences disappeared just after the end of the infusion of the fresh solutions. Peritoneal UF rate was not significantly different during PETs using Lac versus Lac/Bic (609 ± 301 mL vs 542 ± 362 mL). The dialysate-to-plasma ratios of sodium concentrations at 60 minutes of the PETs (which are expressions of free water transport) were not different using Lac versus Lac/Bic (0.89 ± 0.04 vs 0.89 ± 0.04 respectively, p = 0.96). All the other classical parameters of the PET were not different between Lac and Lac/Bic.

Conclusions

The higher dialysate concentrations of NaI due to lower dialysate pH and consequently the higher effective osmolality of the fresh Lac PD solutions did not influence peritoneal UF rate, probably because of the fast reduction of NaI concentrations due to rapid correction of dialysate pH at the end of the infusion of Lac solutions into the peritoneal cavity.

Peritoneal transport assessment by peritoneal equilibration test with 3.86% glucose: a long-term prospective evaluation

The peritoneal equilibration test (PET) with 3.86% glucose concentration (3.86%-PET) has been suggested to be more useful than the standard 2.27%-PET in peritoneal dialysis (PD), but no longitudinal data for 3.86%-PET are currently available. A total of 242 3.86%-PETs were performed in 95 incident PD patients, who underwent the first test during the first year of treatment and then once a year. The classical parameters of peritoneal transport, such as peritoneal ultrafiltration (UF), D/D(0), and D/P(Creat), were analyzed. In addition, the absolute dip of dialysate sodium concentration (DeltaD(Na)), as an expression of sodium sieving, was studied. D/D(0) was stable, and a progressive decrease in UF was observed after the second PET, whereas D/P(Creat) firstly increased and then stabilized. DeltaD(Na) was the only parameter showing a progressive decrease over time. On univariate analysis, D/D(0) and DeltaD(Na) were found to be significantly associated with the risk of developing UF failure (risk ratio (RR) 0.987 (0.973-0.999), P=0.04, and RR 0.768 (0.624-0.933), P=0.007, respectively), but on multivariate analysis only DeltaD(Na) showed an independent association with the risk of developing UF failure (RR 0.797 (0.649-0.965), P=0.020). UF, D/D(0), and D/P(Creat) changed only in those patients developing UF failure, reflecting increased membrane permeability, whereas DeltaD(Na) significantly decreased in all patients. The 3.86%-PET allows a more complete study of peritoneal membrane transport than the standard 2.27%-PET. DeltaD(Na) shows a constant and significant reduction over time and is the only factor independently predicting the risk of developing UF failure in PD patients.

Mini-peritoneal equilibration test: A simple and fast method to assess free water and small solute transport across the peritoneal membrane

BACKGROUND

Loss of ultrafiltration (UF) of peritoneal membrane is one of the most important causes of peritoneal dialysis failure. UF is determined by osmotic forces acting mainly across small pores (UFSP) and ultrasmall pores or free water transport. At present, only semiquantitative estimates or complicated computer simulations are available to assess free water transport. The aim of this study was to assess free water transport during a 3.86% peritoneal equilibration test lasting 1 hour. In this condition, sodium transport is mainly due to convection, allowing the estimate of ultrafiltration of small pores and then of free water transport (total UF - UFSP).

METHODS

In 52 peritoneal dialysis patients we performed a 3.86% peritoneal equilibration test (4 hours) and a 3.86% mini-peritoneal equilibration test (1 hour) and compared UF and small solute transports obtained with the two methods.

RESULTS

During the 3.86% mini-peritoneal equilibration test, UFSP and free water transport were 279 +/- 142 mL and 215 +/- 86 mL, respectively; free water transport well correlated to total UF during the 3.86% peritoneal equilibration test (r= 0.67). The groups of peritoneal transporters, categorized according to glucose dialysate ratio (D/D(0)) and to creatinine/plasma ratio (D/P(Creat)), were in good agreement for the two peritoneal equilibration tests (weighted kappa 0.62 and 0.61, respectively).

CONCLUSION

The 3.86% mini-peritoneal equilibration test is a simple and fast method to assess free water transport. It also gives information about total UF and small solute transports and it is in good agreement with the 3.86% peritoneal equilibration test.

A detailed analysis of sodium removal by peritoneal dialysis: comparison with predictions from the three-pore model of membrane function

BACKGROUND

The development of fluid and salt retention is a potential problem for all peritoneal dialysis (PD) patients. Sodium removal by the peritoneum is predominantly determined by convective fluid loss but influenced by diffusion and sieving due to free water transport as predicted by the three-pore model (TPM). The aim of the study was to establish the effect of transport status, dwell length and glucose concentration on observed ultrafiltration (UF), dialysate sodium concentration ([Na(+)](D)) and removal, and compare this with that predicted by a computer program based on the principles of the TPM.

METHODS

This was a cross-sectional study of UF and [Na(+)](D) collected prospectively from dwells classified by length, glucose concentration and membrane transport characteristics. Solute transport, converted to area parameter and UF capacity, was measured on each occasion by the peritoneal equilibration test. These parameters, along with plasma [Na(+)], were entered into the computer model. Fixed values for other parameters, e.g. hydraulic conductance and lymphatic absorption and sump volume, were used.

RESULTS

A total of 1853 dwells from 182 patients [10% were on automated PD (APD)] were analysed. There was a high degree of correlation (r = 0.83-95, P<0.001) between the observed and predicted values for UF, [Na(+)](D) and sodium removal across the full range of dwell categories. The model overpredicted UF as the net volume increased with increasing glucose concentration, independently of solute transport. This bias was not fully explained by the preferential use of hypertonic dialysate by patients with reduced UF capacity. The prediction of [Na(+)](D) described sodium sieving, which was overestimated in a small number of patients with UF failure. There were no discrepancies between continous ambulatory PD (CAPD) and APD patients.

CONCLUSION

This analysis endorses the TPM as a description of membrane function, particularly in relation to sodium sieving and removal. The relationship between dialysate glucose concentration and achieved UF appears to be more complex; even accounting for extended time on treatment and reduction in the osmotic conductance in patients preferentially using hypertonic exchanges, further adjustments may be needed to account for the tendency to overestimate UF.

Clinical consequences of an individualized dialysate sodium prescription in hemodialysis patients

BACKGROUND

Predialysis plasma sodium (Na(+)) concentration is relatively constant in hemodialysis (HD) patients, and a higher dialysate Na(+) concentration can promote an increase in the interdialytic fluid ingestion to achieve an individual's osmolar set point, and individualization of dialysate Na(+) concentration may improve interdialytic weight gain (IDWG), blood pressure (BP), and HD-related symptoms.

METHODS

Twenty-seven nondiabetic, non-hypotension prone HD patients were enrolled in a single-blind crossover study. Subjects underwent nine consecutive HD sessions with the dialysate Na(+) concentration set to 138 mEq/L (standard Na(+) HD), followed by nine sessions wherein the dialysate Na(+) was set to match the patients average pre-HD plasma Na(+) measured three times during the standard Na(+) phase multiplied by 0.95 (individualized dialysate Na(+) HD). Dry weight, dialysis prescription, and medications were not modified during the six weeks of the study.

RESULTS

Pre-HD Na(+) was similar in both periods of the study (standard Na(+) HD, 134.0 +/- 1.4 mEq/L; individualized Na(+) HD, 134.0 +/- 1.5 mEq/L; P= 0.735). There was a significant decrease in interdialytic weight gain (2.91 +/- 0.87 kg vs. 2.29 +/- 0.65 kg; P< 0.001), interdialytic thirst scores, and episodes of intradialytic hypotension in the individualized Na(+) period compared with the standard phase. Pre-HD BP was lower in individualized Na(+) HD in patients with uncontrolled BP at baseline (N= 15), but not in those with controlled BP at baseline (N= 12) (DeltaBP -15.6/-6.5 mm Hg in uncontrolled vs. DeltaBP +6.4/+4.5 mm Hg in controlled, P= <0.001 for systolic BP and P= <0.001 for diastolic BP).

CONCLUSION

An individualized Na(+) dialysate based on predialysis plasma Na(+) levels decreases thirst, IDWG, HD-related symptoms, and pre-HD BP (in patients with uncontrolled BP at baseline).

Sodium removal and sodium concentration during peritoneal dialysis: effects of three methods of sodium measurement

BACKGROUND

Sodium removal (NaR) may have a major impact on the survival of peritoneal dialysis patients. The dialysate/plasma sodium concentration ratio (D/P(Na)) is an indirect index of transcellular water transport by aquaporin channels, and thus of ultrafiltration. Sodium concentration can be assessed by means of flame photometry (F), and direct (D-ISE) or indirect ion-selective electrodes (I-ISE), but these methods have different properties. I-ISE is being used increasingly in clinical laboratories. The aim of this study was to evaluate NaR and D/P(Na) using the three different measurement methods.

METHODS

We performed peritoneal equilibration tests (PETs) in 44 peritoneal dialysis patients and calculated the NaR. We also calculated D/P(Na) during the test; plasma and dialysate sodium concentrations were measured by F, D-ISE and I-ISE.

RESULTS

NaR was lower (P<0.001) with D-ISE (69+/-29 mmol) than with F (81+/-29 mmol) or I-ISE (79+/-28 mmol). D/P(Na) was also lower at baseline (0.92+/-0.02 vs 0.95+/-0.02 and 0.95+/-0.02; P<0.001), after 60 min (0.87+/-0.03 vs 0.90+/-0.03 and 0.90+/-0.03; P<0.001) and at the end of PET (0.88+/-0.04 vs 0.92+/-0.04 and 0.92+/-0.04; P<0.001) when measured by D-ISE in comparison with F and I-ISE, respectively.

CONCLUSIONS

NaR and D/P(Na) were lower when measured by the D-ISE method compared with the F and I-ISE methods. NaR and D/P(Na) were similar when measured by F or I-ISE. I-ISE can be used reliably in the evaluation of NaR and D/P(Na) in everyday clinical practice of peritoneal dialysis.