Net fluid absorption under membrane transport models of peritoneal dialysis

Recommended Clinical Practices for Maximizing Peritoneal Dialysis Clearances

Data from the Canada-U.S.A. (CANUSA) Study have recently confirmed a long-suspected linkage between total clearance and patient survival in peritoneal dialysis (PD). Recognizing that what we have historically accepted as adequate PD simply is not, the Ad Hoc Committee on Peritoneal Dialysis Adequacy met in January, 1996. This committee of invited experts was convened by Baxter Healthcare Corporation to prepare a consensus statement that provides clinical recommendations for achieving clearance guidelines for peritoneal dialysis. Through an analysis of 806 PD patients, the group concluded that adequate clearance delivered with PD can be achieved in almost all patients if the prescription is individualized according to the patient's body surface area, amount of residual renal function, and peritoneal membrane transport characteristics. Use of 2.5 L to 3.0 L fill volumes, the addition of an extra exchange, and giving automated peritoneal dialysis patients a “wet” day are all options to consider when increasing weekly creatinine clearance and KTN. Rather than specify a single clearance or KTN target, the recommended clinical practice is to provide the most dialysis that can be delivered to the individual patient, within the constraints of social and clinical circumstances, quality of life, life-style, and cost. The challenge to PD practitioners is to make prescription management an integral part of everyday patient management. This includes assessment of peritoneal membrane permeability, measurement of dialysis and residual renal clearance, and adjustment of the dialysis prescription when indicated.

Clinical and Physiological Effects of a New, Less Toxic and Less Acidic Fluid for Peritoneal Dialysis

Objective

To report our first clinical experience with a new continuous ambulatory peritoneal dialysis (CAPD)fiuid (PD-Bio), which is nearly devoid of glucose degradation products and has a higher pH (6.3) than conventional peritoneal dialysis (PD) solutions, and to discuss in general terms some acute and long-term effects of conventional acidic solutions containing glucose degradation products.

Design

1) Pilot study on 4 patients investigated using a modified peritoneal equilibration test (PET) and cytobiology parameters. 2) Computer simulation study, assuming that conventional acidic solutions cause vasodilatation and recruitment of capillary surface area initially (during 0–60 minutes) in a PD dwell.

Patients

Four stable CAPD patients were chosen in an open cross-over study. After a period of three months using conventional PD fluid, the patients were switched to three months on the new PD fluid.

Results

Cancer antigen 125 increased significantly, and patients with discomfort/infusion pain during the control period improved during the period with the new fluid. No significant changes were observed in mass-transfer coefficients or drained volumes with the new solution. PH in the effluent dialysis was, however, higher for PD-Bio at all times during a two-hour dwell. In the computer simulation study, a less acidic solution caused an initially lower rate of glucose dissipation and improved ultrafiltration (UF) after a four -hour dwell, as compared to a conventional PD solution.

Conclusion

A new, differently produced, less toxic and less acidic PD fluid (PD-Bio) seems to be better tolerated than a conventional acidic solution with respect to discomfort/infusion pain. Theoretically, neutralized solutions should show slightly improved UF profiles over conventional acidic solutions, according to the computer simulation analysis. Furthermore, it is speculated that a neutral, less acidic, less toxic fluid would cause less interstitial-mesothelial alterations and less impairment of UF capacity than conventional solutions during longterm CAPD.

The Peritoneal Permeability and Surface Area Index

The value of the peritoneal equilibration test (PET) derived dialysate-to-plasma creatinine ratio (D/Pcr) as a measure of the peritoneal membrane's diffusive characteristics is obscured by the major role of convection in determining this value, particularly for low transporters. A simple index, derived from PET data, may permit a more accurate clinical measure of peritoneal diffusion.

Theoretical Analysis of Osmotic Agents in Peritoneal Dialysis. What Size is An Ideal Osmotic Agent?

In this article the difference between osmotic fluid flow (ultrafiltration) as driven by osmotic pressure and diffusion through thin leaky membranes is discussed. It is pointed out that water transport induced by osmosis is fundamentally different from the process of water diffusion. Applying modern hydrodynamic pore theory, the molar solute concentration and the solute concentration in grams per 100 mL, exerting the same initial transmembrane osmotic pressure as a 1% glucose solution, was investigated as a function of solute molecular weight (MW). It was then assumed, based on experimental data, that the major pathway responsible for the peritoneal osmotic barrier characteristics is represented by pores of radius ~47 å. With increasing solute radius, the osmotic reflection coefficient (σ) and, hence, the osmotic efficiency per mole of solute will increase. However, simultaneously, the molar concentration per unit solute weight will decrease. The balance point between these two events apparently occurs at a solute MW of approximately 1 kCa. An additional advantage of using solutes of high MW as osmotic agents during peritoneal dialysis (PC), rather than increased osmotic efficiency per se, lies in the fact that large solutes, due to their low peritoneal diffusion capacity, will maintain a sustained rate of ultrafiltration (osmosis) over a prolonged period. To illustrate this, we have performed computer simulations of peritoneal fluid transport according to the three-pore model of peritoneal permselectivity. According to these simulations, 4% of an 800 Ca polymer solution (+50 mmol/ L above isotonicity) will produce the same cumulative amount of intraperitoneal fluid volume ultrafiltered (UF) during 360 -400 minutes as 4% of a 2 kCa polymer solution (+20 mmol/L) or 6.5% of a 10 kCa polymer solution (+6.5 mmol/L) having the same electrolyte concentration as dialysis solutions conventionally used for PC. Similar cumulative UF volumes (during 400 minutes) can be obtained by a 2.5% glycerol (+272 mmol/L) or a 3.2% glucose-containing dialysis solution (+177 mmol/L) with conventional electrolyte composition.

Peritoneal Dialysis Kinetic Modeling: Validation in a Multicenter Clinical Study

Objective

To clinically validate the use of a computer-based kinetic model for peritoneal dialysis (PD) by assessing the level of agreement between measured and modeled values of urea and creatinine clearances and ultrafiltration (UF).

Design

An open multicenter observational study.

Patients

There were 111 adult continuous ambulatory peritoneal dialysis (CAPD) patients (47 female, 64 male) in four centers. All patients underwent a four-hour peritoneal equilibration test (PET) using 2.5% dextrose but with variable fill volumes (range: 1 -3 L). Patients with a residual renal function greater than 10 mL/min were excluded.

Main Outcome Measures

Correlations and limits of agreement between measured and modeled values of total weekly urea KTN, total weekly normalized creatinine clearance (L/week/1.73 m2), daily drain volume (L), net ultrafiltration (L), daily peritoneal urea clearance (L/day), and daily peritoneal creatinine clearance (L/day). Measured values were obtained from 24-hour urine and dialysate collections while modeled values were based on results from the PET in combination with the PD ADEQUEST® kinetic program.

Results

The results show there is excellent agreement between measured and modeled urea KTN and creatinine clearances, with concordance correlations of 0.94 and 0.92, respectively. Given the excessive variation and limited range in ultrafiltration values, the concordance correlation between measured and modeled UF was only 0.50. In terms of daily peritoneal clearances and ultrafiltration, the level of precision (i.e., standard deviation) in the differences between modeled and measured values is ±1.05 L/ day for urea clearance, ±1.03 L/day for creatinine clearance, and ±0.919 L/day for ultrafiltration. By contrast, the level of precision (i.e., standard deviation) in the differences between two measured values is estimated to be ±0.979 L/day for urea clearance, ±0.802 L/day for creatinine clearance, and ±0.707 L/day for ultrafiltration. Defining the limits of clinical agreement to be ±2 standard deviations of the differences between two clinically measured 24-hour clearances (or ultrafiltration), we find that 94% of the modeled urea clearances, 87% of the modeled creatinine clearances, and 86% of the modeled ultrafiltration values fall within the limits of clinical agreement.

Conclusion

Data from a carefully performed PET and overnight exchange can, in combination with a scientifically validated kinetic model, provide clinicians with a powerful mathematical tool for use in CAPD dialysis prescription management. Although not intended to replace actual measurements, kinetic modeling can prove useful as a means for predicting clearances for various alternative prescriptions and perhaps also as a means for checking certain types of noncompliance.

A Multinational Clinical Validation Study of Pd Adequest 2.0

Objective

To clinically validate the use of the newly released kinetic modeling program, PD ADEQUEST 2.0 for Windows (Baxter Healthcare Corporation, Deerfield, IL, U.S.A.), by assessing the level of agreement between measured and modeled values of urea and creatinine clearances (CCr), glucose absorption, total drain volumes, and net ultrafiltration for all forms of peritoneal dialysis.

Design

A nonrandomized, multinational, prospective longitudinal study.

Patients

The study involved 104 adult patients [41 on continuous ambulatory peritoneal dialysis (CAPD), 63 on automated peritoneal dialysis (APD)] from 16 centers in 7 countries. All patients underwent a 4-hour peritoneal equilibration test (PET) but with varying percentage dextrose concentrations (1.5% or 2.5% dextrose) and varying fill volumes (range 1.5 – 2.5 L). Patients with a residual renal function greater than 10 mL/min were excluded, as were patients who had peritonitis within 6 weeks prior to baseline.

Main Outcome Measures

Correlation coefficients and Bland–Altman limits of agreement were used to assess the level of agreement between measured and modeled values of weekly peritoneal urea Kt/V (pKt/V) and total Kt/V, weekly peritoneal creatinine clearance (pCCr, L/week/ 1.73 m2) and total CCr (L/week/1.73 m2), daily drain volume (L/day), net ultrafiltration (UF, L/day), daily peritoneal urea and creatinine mass removal (g/day), and daily peritoneal glucose absorption (g/day). Measured values were obtained from three repeat 24-hour urine and dialysate collections per patient, while modeled values were calculated using the Baxter PD ADEQUEST 2.0 program in conjunction with kinetic parameters estimated from a 4-hour PET and long-dwell exchange independent of the 24-hour collections.

Results

The results show there is excellent agreement between measured and modeled urea Kt/V and CCr with concordance correlation coefficients ranging from 0.83 to 0.97 among CAPD and APD patients. There was also excellent agreement between measured and modeled values of glucose absorption and total drain volumes (concordance correlations of 0.90 and 0.98, respectively). This level of agreement was further supported by a Bland– Altman analysis of individual differences, including differences between measured and modeled net UF (coefficient of clinical agreement ranged from 0.66 to 0.92).

Conclusions

Data from a carefully performed PET and overnight exchange can, in combination with a scientifically and clinically validated kinetic model, provide clinicians with a powerful mathematical tool for use in CAPD and APD prescription management. Although not intended to replace actual measurements, kinetic modeling can prove useful as a means for quickly estimating approximate levels of clearance for a wide variety of alternative prescriptions. This, in turn, should speed up the process by which a physician can optimize the dose of dialysis suitable for a given patient and his/her lifestyle.

Modeling of Icodextrin in PD Adequest® 2.0

Objective

To validate the use of a modified three-pore model for predicting fluid transport during long dwell exchanges that use a 7.5% icodextrin solution.

Design

A nonrandomized, single group, repeated measures study.

Patients

Ten peritoneal dialysis patients underwent a single 8-hour exchange of a 7.5% icodextrin solution. All patients were naïve to icodextrin.

Main Outcome Measures

A modified three-pore model was used to model solute and fluid transport during each 8-hour exchange. Concordance correlation coefficients were used to estimate the level of agreement between modeled and measured values of net ultrafiltration (UF) and intraperitoneal volume.

Methods

Each patient underwent a modified 8-hour standard peritoneal permeability analysis using a 2-L 7.5% icodextrin exchange. Dextran 70 was added to the icodextrin solution as volume marker to estimate fluid transport kinetics. Transcapillary UF, fluid absorption, and intraperitoneal volumes were assessed via the volume marker at 0, 5, 15, 30, 60, 120, 240, 300, 360, 420, and 480 minutes.

Results

There was strong agreement (concordance correlation = 0.9856) between net UF as measured by the volume marker data and net UF as modeled using the modified three-pore model implemented in PD Adequest (Baxter Healthcare, Deerfield, Illinois, USA).

Conclusions

Net UF and intraperitoneal volumes for long dwell exchanges using a 7.5% icodextrin solution can be accurately modeled with a modified three-pore model. Steady state icodextrin plasma levels are needed to accurately predict net UF for chronic users of icodextrin.

Physical Processes in Polymeric Filters Used for Dialysis

The key physical processes in polymeric filters used for the blood purification include transport across the capillary wall and the interaction of blood cells with the polymer membrane surface. Theoretical modeling of membrane transport is an important tool which provides researchers with a quantification of the complex phenomena involved in dialysis. In the paper, we present a dense review of the most successful theoretical approaches to the description of transport across the polymeric membrane wall as well as the cell⁻polymer surface interaction, and refer to the corresponding experimental methods while studying these phenomena in dialyzing filters.

Optimizing Automated Peritoneal Dialysis Using an Extended 3-Pore Model

Introduction

In the current study, an extended 3-pore model (TPM) is presented and applied to the problem of optimizing automated peritoneal dialysis (APD) with regard to osmotic water transport (UF), small/middle-molecule clearance, and glucose absorption.

Methods

Simulations were performed for either intermittent APD (IPD) or tidal APD (TPD). IPD was simulated for fill and drain volumes of 2 L, whereas TPD was simulated using a tidal volume of 0.5 L, 1 L, or 1.5 L with full drains and subsequent fills (2 L) occurring after every fifth dwell. A total of 25 cycles for a large number of different dialysate flow rates (DFR) were simulated using 3 different glucose concentrations (1.36%, 2.27%, and 3.86%) and 3 different peritoneal transport types: slow (peritoneal equilibrium test D/P_crea < 0.6), fast (peritoneal equilibrium test D/P_crea > 0.8), and average. Solute clearance and UF were simulated to occur during the entire dwell, including both fill and drain periods.

Results

It is demonstrated that DFRs exceeding ∼ 3 L/h are of little benefit both for UF and small-solute transport, whereas middle-molecule clearance is enhanced at higher DFRs. The simulations predict that large reductions (> 20%) in glucose absorption are possible by using moderately higher DFRs than a standard 6 × 2 L prescription and by using shorter optimized “bi-modal” APD regimens that alternate between a glucose-free solution and a glucose-containing solution.

Discussion

Reductions in glucose absorption appear to be significant with the proposed regimens for APD; however, further research is needed to assess the feasibility and safety of these regimens.

Automated peritoneal dialysis prescriptions for enhancing sodium and fluid removal: a predictive analysis of optimized, patient-specific dwell times for the day period

BACKGROUND

Remaining edema-free is a challenge for many automated peritoneal dialysis (APD) patients, especially those with fast ("high") transport characteristics. Although increased use of peritoneal dialysis (PD) solutions with high glucose concentrations may improve volume control, frequent use of such solutions is undesirable.

METHODS

We used the 3-pore kinetic model to evaluate 4 alternative therapy prescriptions for the APD day exchange in anuric patients with high, high-average, and low-average transport characteristics. Four prescriptions were modeled: Therapy 1: Optimal, individualized dwell times with a dry period. Therapy 2: Use of a midday exchange. Therapy 3: Use of an icodextrin-containing dialysate during a 14-hour dwell. Therapy 4: Use of optimal, individualized dwell times, followed by an icodextrin dwell to complete the daytime period. The alternative therapies were compared with a reference standard therapy using glucose solution during a 14-hour dwell. The nighttime prescription was identical in all cases (10 L over 10 hours), and all glucose solutions contained 2.27% glucose. Net ultrafiltration (UF), sodium removal (NaR), total carbohydrate (CHO) absorption, and weekly urea Kt/V for a 24-hour period were computed and compared.

RESULTS

The UF and NaR were substantially higher with therapy 1 than with standard therapy (1034 mL vs 621 mL and 96 mmol vs 51 mmol respectively), without significant changes in CHO absorption or urea Kt/V. However, therapy 1 resulted in reduced β2-microglobulin clearance (0.74 mL/min vs 0.89 mL/min with standard therapy). Compared with therapy 1, therapy 2 improved UF and NaR (1062 mL vs 1034 mL and 99 mmol vs 96 mmol); however, that improvement is likely not clinically significant. Therapy 2 also resulted in a higher Kt/V (2.07 vs 1.72), but at the expense of higher glucose absorption (difference: 42 g). The UF and NaR were highest with a long icodextrin-containing daytime dwell either preceded by a short optimized dwell (1426 mL and 155 mmol) or without such a dwell (1327 mL and 148 mmol).

CONCLUSIONS

The 3-pore model predictions revealed that patient-specific optimal dwell times and regimens with a longer day dwell might provide improved UF and NaR options in APD patients with a variety of peritoneal membrane transport characteristics. In patients without access to icodextrin, therapy 1 might enhance UF and NaR and provide a short-term option to increase fluid removal. Although that approach may offer clinicians a therapeutic option for the overhydrated patient who requires increased UF in the short term, APD prescriptions including icodextrin provide a means to augment sodium and fluid removal. Data from clinical trials are needed to confirm the predictions from this study.

Three-pore model predictions of 24-hour automated peritoneal dialysis therapy using bimodal solutions

BACKGROUND

Recently, bimodal peritoneal dialysis (PD) solutions containing low concentrations of Na have been shown to increase 24-hour ultrafiltration (UF) or UF efficiency (UF volume per gram of carbohydrate or CHO absorbed) and Na removal in high ("fast") transport patients during automated PD therapy. We used computer simulations to compare UF efficiency and Na removal at equivalent 24-hour UF volumes using either a generic bimodal solution (2.27% glucose + 7.5% icodextrin) during the long dwell or an increase in the glucose concentration during the short dwells, with all solutions containing Na at the conventional concentration (132 mEq/L).

METHODS

The 3-pore model has been shown to accurately predict peritoneal transport for PD solutions containing glucose or icodextrin, or both. Here, we used that model to calculate 24-hour UF volume, CHO absorption, and Na removal for high (H), high-average (HA), and low-average (LA) transport patients on automated PD. Nighttime therapy consisted of 1.36% or 2.27% glucose solution (or both), and daytime therapy consisted of either Extraneal (Baxter Healthcare Corporation, Deerfield, IL, USA) or a bimodal solution.

RESULTS

As expected, addition of glucose to either the long dwell or the short dwells resulted in increased UF volume and glucose absorption. The increase in UF was a function of patient transport type (bimodal range: 288 - 490 mL; short-dwell range: 323 - 350 mL), and the increase in CHO absorption was smaller with glucose added to short dwells than with bimodal solution (range: 18 - 30 g vs. 34 - 39 g). The 24-hour UF efficiency was higher when high glucose concentrations were used during short-dwell exchanges than when a bimodal PD solution was used for the long dwell (0.6 to 1.2 mL/g vs. -0.1 to 0.5 mL/g). By contrast, Na removal was lower with the short-dwell exchanges (28.3 - 30.7 mmol vs. 36.2 - 53.3 mmol), likely because of more pronounced Na sieving.

CONCLUSIONS

Our modeling studies predict that generic bimodal PD solutions will provide higher Na removal but not higher 24-hour UF efficiency compared with current automated PD prescriptions using Extraneal for the long dwell and glucose-containing solutions for the short dwells. The modeling predictions from this study require clinical validation.

Kinetic Analysis of Peritoneal Fluid and Solute Transport with Combination of Glucose and Icodextrin as Osmotic Agents

Background

Controlling extracellular volume and plasma sodium concentration are two crucial objectives of dialysis therapy, as inadequate sodium and fluid removal by dialysis may result in extracellular volume overload, hypertension, and increased cardiovascular morbidity and mortality in end-stage renal disease patients. A new concept to enhance sodium and fluid removal during peritoneal dialysis (PD) is the use of dialysis solutions with two different osmotic agents.

Aim

To investigate and compare, with the help of mathematical modeling and computer simulations, fluid and solute transport during PD with conventional dialysis fluids (3.86% glucose and 7.5% icodextrin; both with standard sodium concentration) and a new combination fluid with both icodextrin and glucose (CIG; 2.6% glucose/6.8% icodextrin; low sodium concentration). In particular, this paper is devoted to improving mathematical modeling based on critical appraisal of the ability of the original three-pore model to reproduce clinical data and check its validity across different types of osmotic agents.

Methods

Theoretical investigations of possible causes of the improved fluid and sodium removal during PD with the combination solution (CIG) were carried out using the three-pore model. The results of computer simulations were compared with clinical data from dwell studies in 7 PD patients. To fit the model to the low net ultrafiltration (366 ± 234 mL) obtained after a 4-hour dwell with 3.86% glucose, some of the original parameters proposed in the three-pore model (Rippe & Levin. Kidney Int 2000; 57:2546-56) had to be modified. In particular, the aquaporin-mediated fractional contribution to hydraulic permeability was decreased by 25% and small pore radius increased by 18%.

Results

The simulations described well clinical data that showed a dramatic increase in ultrafiltration and sodium removal with the CIG fluid in comparison with the two other dialysis fluids. However, to adapt the three-pore model to the selected group of PD patients (fast transporters with small ultrafiltration capacity on average), the peritoneal pore structure had to be modified. As the mathematical model was capable of reproducing the clinical data, this shows that the enhanced ultrafiltration with the combination fluid is caused by the additive effect of the two different osmotic agents and not by a specific impact of the new dialysis fluid on the transport characteristics of the peritoneum.

Development of a computer-aided diagnosis system for continuous peritoneal dialysis: an availability of the simultaneous numerical optimization technique for kinetic parameters in the peritoneal dialysis model

We developed a novel diagnostic system for continuous peritoneal dialysis, which evaluates the peritoneal permeability and hydraulic conductance (peritoneal function) by applying the kinetic model and a clinical test with minimum nursing time. Kinetic parameters for the peritoneal function were determined by the simultaneous numerical optimization techniques (SNOT). Furthermore dialysis outcome and ultrafiltration volume were estimated and predicted by using the kinetic model with a set of optimal kinetic parameters, which were in agreement with measured data (r(2)>0.82). The clinical implementation of the SNOT is very useful to explore the better prescriptions for each patient's clinical condition.

Simulations of osmotic ultrafiltration failure in CAPD using a serial three-pore membrane/fiber matrix model

Ultrafiltration failure (UFF) is a common complication of long-term peritoneal dialysis (PD). Functionally UFF is in most cases characterized by an enhanced peritoneal mass transfer area coefficient for glucose (PSg) combined with a largely unchanged peritoneal glucose osmotic conductance (LpSσg). Morphologically, marked UFF occurs with fibrosis of the submesothelial zone in the peritoneum, combined with vasculopathy and vascular proliferation in deeper tissues. To computer simulate UFF, changes both in the vasculature and in the interstitium have to be taken into account. For that purpose, we used a three-pore membrane/fiber matrix serial barrier model, applying the three-pore model to the capillaries and the fiber-matrix model to the interstitium. The parameters of the three-pore model have been published previously. The interstitial fiber density was set at 0.5% (vol/vol) and the fiber radius ( rf) at 6 Å during control. If the interstitial fiber density was increased from 0.5 to 3%, and rfto 7.5 Å (cf. collagen) while the capillary surface area was increased by 40% from control, then PSgincreased from 9.3 to 11.5 ml/min, while the UF coefficient (LpS) was largely unchanged. Further increases in vascular surface area combined with further increases in fiber density caused further increments in PSg, whereas LpS remained unchanged. It is concluded that a matrix of fibers coupled in series with a three-pore membrane may be used for simulating the pathophysiological alterations occurring in the peritoneum in UFF, explaining the commonly observed “uncoupling” of small solute transport (PS) from the peritoneal UF coefficient (LpS) in this condition.

Mass transfer of calcium across the peritoneum at three different peritoneal dialysis fluid Ca2+ and glucose concentrations

BACKGROUND

In peritoneal dialysis, the rate of ultrafiltration has been predicted to be a major determinant of peritoneal calcium (Ca2+) removal. Hence, dialysis fluid glucose concentration should be an important factor governing the transperitoneal Ca2+ balance. The aim of this study was to test the effect of various dialysate glucose levels and selected dialysate Ca2+ levels on Ca2+ removal in peritoneal dialysis patients.

METHODS

Patients (N = 8) received, during a 7-week period, 2 L of lactate (30 mmol/L)/bicarbonate (10 mmol/L)-buffered peritoneal dialysis solutions containing either 1.5% glucose and 1.0 mmol/L Ca2+ or 2.5% glucose and 1.6 mmol/L Ca2+, or 4% glucose and 2.5 mmol/L Ca2+, respectively, provided in a three-compartment bag (trio system). Patients underwent standardized (4-hour) dwells, one for each of the three dialysates to assess permeability-surface area product (PS) or mass transfer area coefficients (MTAC) for ionized and "freely diffusible" Ca2+, lactate, glucose, bicarbonate, phosphate, creatinine, and urea.

RESULTS

There was a clear-cut dependence of peritoneal Ca2+ removal on the rate of ultrafiltration. For large peritoneal to dialysate Ca2+ gradients (2.5 mmol/L Ca2+ in 4% glucose) a close fit of measured to simulated data was predicted by the three-pore model using nonelectrolyte equations. For low transperitoneal Ca2+ concentration gradients, however, directly measured Ca2+ data agreed with the simulated ones only when the peritoneal Ca2+ PS was set lower than predicted from pore theory (6 mL/min).

CONCLUSION

There was a marked ultrafiltration dependence of transperitoneal Ca2+ transport. Nonelectrolyte equations could be used to simulate peritoneal ion (Ca2+) transport provided that the transperitoneal ion concentration gradients were large. Based on our data 1.38 mmol/L Ca2+ in the dialysis fluid would have created zero net Ca2+ gain during a 4-hour dwell for 1.5% glucose, whereas 1.7 and 2.2 mmol/L Ca2+ would have been needed to produce zero Ca2+ gain for 2.5% glucose and 3.9% glucose, respectively.

Peritoneal Function and Adequacy Calculations: Current Programs versus PD Adequest 2.0

Objective

Our current programs (CPs) were compared to PD Adequest 2.0 (PD-A) for calculations of peritoneal membrane transport and dialysis adequacy.

Design

Thirty peritoneal equilibration tests (PETs) and 24-hour balances (24hBs) were conducted and calculated using our CPs and PD-A.

Patients and Methods

Thirty hospital-controlled peritoneal dialysis (PD) patients were studied. The inclusion of correction factors (for glucose or plasmatic water) and of residual volume, and the use of 3 or 6 peritoneal samples were analyzed to discover the differences between programs. The main outcome measures were peritoneal permeability and adequacy parameters, evaluated by Student t-test (mean and paired comparisons) and linear regression for correlation.

Results

No significant differences were found in D/P values for small solutes. At the first step, mass transfer area coefficient (MTAC) urea and MTAC creatinine were significantly higher in DP-A than in CP, but MTAC glucose did not differ. The causes of differences were: ( 1 ) inclusion of a correction factor for aqueous plasmatic concentration of small solutes in CP; ( 2 ) lack of inclusion of residual volume in peritoneal volumes in CP; and ( 3 ) use of 6 peritoneal samples in CP versus 3 in PD-A. At the second step, when the input data were made equivalent for both programs, the differences disappeared for MTAC urea, creatinine, and glucose (mean comparison), but creatinine and glucose remained different by paired comparison. Similar results were obtained when a correction for plasmatic aqueous concentration was applied to the data in both programs [MTAC urea: 22.60 ± 4.27 mL/min (CP) vs 22.43 ± 4.61 mL/min (PD-A), nonsignificant, r = 0.97; MTAC creatinine: 9.76 ± 3.83 mL/min (CP) vs 10.61 ± 3.07 mL/min (PD-A), nonsignificant, r = 0.98; MTAC glucose: 13.30 ± 3.12 mL/min (CP) vs 11.87 ± 3.41 mL/min (PD-A), nonsignificant, r = 0.92]. Creatinine and glucose were different by paired t-test. No significant differences were found in Kt/V and urea generation rate. Weekly creatinine clearance [WCCr: 70.71±16.71 L (CP) versus 79.33±18.73 L (PD-A), p < 0.001] and creatinine generation rate [CrGR: 0.56 ± 0.18 mg/min (CP) versus 0.61±0.19 mg/min (PD-A), p < 0.001) were significantly higher in PD-A than in CP owing to the lack of creatinine correction according to glucose concentration in the PD-A adequacy program. Finally, normalized protein nitrogen appearance according to Bergström [1.09 ± 0.20 g/kg/d (CP) versus 1.03 ± 0.21 g/kg/d (PD-A), p = 0.01] was different owing to the different algorithms and normalization method: standardized body weight in CP and actual body weight in PD-A.

Conclusions

Provided that equivalent data are used, PD-A and CP yield similar results. The PD-A program needs external correction of data input: ( 1 ) for plasmatic water concentration in MTAC calculations, and ( 2 ) for peritoneal glucose interference with creatinine analysis (Jaffé method) in WCCr and CrGR calculations; otherwise, it may give falsely optimistic results.

Computer simulations of ultrafiltration profiles for an icodextrin-based peritoneal fluid in CAPD

BACKGROUND

The three-pore model of peritoneal transport has the ability to predict ultrafiltration (UF) profiles rather accurately, even when high molecular weight (MW) solutes are employed as osmotic agents in continuous ambulatory peritoneal dialysis (CAPD). In the present simulations, we wanted to assess, for various theoretical perturbations, the UF properties of a peritoneal dialysis (PD) solution with an osmotic agent having an average MW of 20 kD and a "number average MW" of 6.2 kD, which is similar to that of icodextrin (ICO).

METHODS

For a PD solution containing a completely monodispersed 20 kD MW osmotic agent, the degree of UF modeled is much higher than that reported for ICO. Hence, to model the behavior of ICO, we subdivided the ICO molecules into eight or more different MW size fractions. For simulations using six or eight subfractions, we obtained an excellent fit of simulated to reported UF data. More dispersed solutions produced UF profiles similar to that with eight fractions.

RESULTS

A 2.05 L 7.5% ICO PD solution, despite being slightly hypotonic, yielded a UF volume of nearly 600 mL in 12 hours, modeled for patients not previously exposed for ICO. After nine hours, the UF volume exceeded that produced by 3.86% glucose. The UF rate and volumes increased in proportion to (1) the ICO concentration, (2) the peritoneal surface area, and (3) the peritoneal UF coefficient, but was almost insensitive to increases in the instilled fluid volume. Simulated for patients previously exposed to ICO, having steady-state plasma concentrations of ICO degradation products, the predicted UF volume at 12 hours was reduced to approximately 400 mL.

CONCLUSION

Employing the three-pore model of peritoneal transport and taking into account the polydispersed nature of ICO, it was possible to accurately computer simulate the UF profiles of ICO in accordance with reported data. The simulations suggest an advantage of using ICO in patients with type I UF failure, where UF with a high-MW osmotic agent will exceed that seen in patients not showing UF failure who are on glucose-based PD solutions.

Computerized Kinetic Modeling: A New Tool in the Quest for Adequacy in Peritoneal Dialysis

Until recently, kinetic modeling of peritoneal dialysis (PD) was performed by engineers, scientists, or nephrologists at major teaching institutions. Now there are several “user-friendly” computer programs which permit the practicing nephrologist and dialysis staff to monitor adequacy of the individual PD patient and to optimize the dialysis prescription. In this brief article, the capabilities, methods, and data requirements of three programs are reviewed, and specific recommendations for the selection of a particular program are discussed.

Transperitoneal transport of sodium during hypertonic peritoneal dialysis

The mechanisms of transperitoneal sodium transport during hypertonic peritoneal dialysis were evaluated by kinetic modelling. A total of six nested mathematical models were designed to elucidate the presence or absence of diffusive, non-lymphatic convective and lymphatic convective solute transport. Experimental results were obtained from 26 non-diabetic patients undergoing peritoneal dialysis. The model validation procedure demonstrated that only diffusive and non-lymphatic convective transport mechanisms were identifiable in the transperitoneal transport of sodium. Non-lymphatic convective sodium transport was the most important quantitative transport mechanism during the first 90 min of the dwell. Significant sodium sieving was demonstrated and explains the observation of hypernatremia in dialysis with hypertonic dialysis fluid.

Ultrafiltration and solute kinetics using low sodium peritoneal dialysate

Low sodium peritoneal dialysate has been reported to enhance sodium loss and alleviate signs of fluid overload in continuous ambulatory peritoneal dialysis patients. To elucidate the mechanisms involved, we compared ultrafiltration and solute kinetics using low sodium dialysate (LNaD; 105 mEq/liter sodium, 2.5% glucose, 348 mOsm/liter), conventional dialysate with equal osmolality (CD1.5; 132 mEq/liter sodium, 1.5% glucose, 348 mOsm/liter) and conventional dialysate with equal glucose concentration (CD2.5; 132 mEq/liter sodium, 2.5% glucose, 403 mOsm/liter). A 2 liter, six hour exchange of each dialysate was performed on separate days in 10 chronic peritoneal dialysis patients. Transperitoneal solute diffusion was assessed by calculating the permeability-area product (PA) of the peritoneal membrane from the dependence of plasma and dialysate solute concentrations on tie. Net fluid removed using LNaD of 190 +/- 90 (SEM) ml was similar to that using CD2.5 (250 +/- 90 ml) but higher (P < 0.01) than that using CD1.5 (-200 +/- 60 ml). Sodium loss was higher using LNaD (72 +/- 11 mEq, P < 0.01) and CD2.5 (41 +/- 12 mEq, P < 0.05) than using CD1.5 (-18 +/- 8 mEq). Changes in plasma sodium concentration were small during each dwell and were not different among the study dialysates. PA values for urea (23.4 +/- 1.6 ml/min), creatinine (10.0 +/- 1.0 ml/min), and glucose (10.3 +/- 1.3 ml/min) were similar when determined in each dialysate. The PA value for sodium (7.6 +/- 1.5 ml/min) could only be accurately determined in LNaD. We conclude that: (1) net fluid removed is greater using LNaD than CD1.5 despite similar osmolalities because LNaD has a higher glucose concentration and glucose is a more effective osmotic solute than sodium; (2) sodium loss when using LNaD is enhanced by both diffusion and convection; and (3) sodium diffuses across the peritoneum slower than urea, creatinine and glucose. These data suggest that LNaD alleviates signs of fluid overload by increasing net fluid removal and enhancing sodium loss.

Assessing the peritoneal dialysis capacities of individual patients

A method for measuring the peritoneal dialysis capacity (PDC) of the individual patient has been developed as an aid to treatment of patients with renal failure and peritoneal dialysis. The patient collects the data him or herself during an almost normal CAPD day using a carefully designed protocol whereby the nursing time is kept to a minimum. The three-pore model is used to describe the PDC with three physiological parameters: (1.) the 'Area' parameter (A0/delta X), which determines the diffusion of small solutes and the hydraulic conductance of the membrane (LpS); (2.) the final reabsorption rate of fluid from the abdominal cavity to blood (JVAR) when the glucose gradient has dissipated; and (3.) the large pore fluid flux (of plasma, JVL), which determines the loss of protein to the PD fluid. In the adult PD population (age 60, N = 97) the normal 'Area' parameter was 23,600 cm/1.73 m2, with an SEM of 650. The JVAR was 1.49 ml/min/1.73 m2 and JVL was 0.078 ml/min/1.73 m2. The PDC parameters were reproducible and could adequately predict the concentrations of the test solutes as well as that of beta 2-microglobulin. The results in terms of clearance, 'UF volume' and nutritional consequences were presented on easily understandable graphs, whereby patient compliance was improved. These physiological parameters are highly dynamic, as evidenced by the marked increases observed during peritonitis. It seems safe to conclude that PDC is a useful tool to achieve adequate dialysis and to enhance the understanding of PD exchange.