Determinants of phosphorus mobilization during hemodialysis

Phosphate kinetic modeling as an estimate of daily ingested phosphate in hemodialysis patients with or without residual kidney function

BACKGROUND AND AIM

Daugirdas suggested a 2-pool phosphate kinetic model based on his previously established urea kinetic model. The current study aims to assess the level of agreement between the modeled daily ingested phosphorus (DIP) values and the routine method of dietary recall calculations in hemodialysis patients.

METHOD

The study was conducted on 100 hemodialysis patients; 50 were anuric, and the others had residual kidney function (RKF). The level of correlation and agreement between the dietary calculated and modeled DIP were assessed in both study groups.

RESULTS

A statistically significant positive correlation existed between the calculated and modeled DIP (r = 0.79 for the anuric group, r = 0.84 for the RKF group, p < 0.001). There was a significant level of agreement between calculated and modeled DIP in RKF patients only.

CONCLUSION

These findings suggest that phosphate modeling can estimate phosphate intake in RKF patients and be cost-effective in their management.

Analytical solution of phosphate kinetics for hemodialysis

Chronic kidney diseases imply an ongoing need to remove toxins, with hemodialysis as the preferred treatment modality. We derive analytical expressions for phosphate clearance during dialysis, the single pass (SP) model corresponding to a standard clinical hemodialysis and the multi pass (MP) model, where dialysate is recycled and therefore makes a smaller clinical setting possible such as a transportable dialysis suitcase. For both cases we show that the convective contribution to the dialysate is negligible for the phosphate kinetics and derive simpler expressions. The SP and MP models are calibrated to clinical data of ten patients showing consistency between the models and provide estimates of the kinetic parameters. Immediately after dialysis a rebound effect is observed. We derive a simple formula describing this effect which is valid both posterior to SP or MP dialysis. The analytical formulas provide explanations to observations of previous clinical studies.

Phosphate balance during dialysis and after kidney transplantation in patients with chronic kidney disease

PURPOSE OF REVIEW

In patients with chronic kidney disease (CKD), hyperphosphatemia is associated with several adverse outcomes, including bone fragility and progression of kidney and cardiovascular disease. However, there is a knowledge gap regarding phosphate balance in CKD. This review explores its current state, depending on the stage of CKD, dialysis modalities, and the influence of kidney transplantation.

RECENT FINDINGS

Adequate phosphate control is one of the goals of treatment for CKD-mineral and bone disorder. However, ongoing studies are challenging the benefits of phosphate-lowering treatment. Nevertheless, the current therapy is based on dietary restriction, phosphate binders, and optimal removal by dialysis. In the face of limited adherence, due to the high pill burden, adjuvant options are under investigation. The recent discovery that intestinal absorption of phosphate is mostly paracellular when the intraluminal concentration is adequate might help explain why phosphate is still well absorbed in CKD, despite the lower levels of calcitriol.

SUMMARY

Future studies could confirm the benefits of phosphate control. Greater understanding of the complex distribution of phosphate among the body compartments will help us define a better therapeutic strategy in patients with CKD.

Intracellular Phosphate and ATP Depletion Measured by Magnetic Resonance Spectroscopy in Patients Receiving Maintenance Hemodialysis

BACKGROUND

The precise origin of phosphate that is removed during hemodialysis remains unclear; only a minority comes from the extracellular space. One possibility is that the remaining phosphate originates from the intracellular compartment, but there have been no available data from direct assessment of intracellular phosphate in patients undergoing hemodialysis.

METHODS

We used phosphorus magnetic resonance spectroscopy to quantify intracellular inorganic phosphate (Pi), phosphocreatine (PCr), and βATP. In our pilot, single-center, prospective study, 11 patients with ESKD underwent phosphorus (³¹P) magnetic resonance spectroscopy examination during a 4-hour hemodialysis treatment. Spectra were acquired every 152 seconds during the hemodialysis session. The primary outcome was a change in the PCr-Pi ratio during the session.

RESULTS

During the first hour of hemodialysis, mean phosphatemia decreased significantly (-41%; P<0.001); thereafter, it decreased more slowly until the end of the session. We found a significant increase in the PCr-Pi ratio (+23%; P=0.001) during dialysis, indicating a reduction in intracellular Pi concentration. The PCr-βATP ratio increased significantly (+31%; P=0.001) over a similar time period, indicating a reduction in βATP. The change of the PCr-βATP ratio was significantly correlated to the change of depurated Pi.

CONCLUSIONS

Phosphorus magnetic resonance spectroscopy examination of patients with ESKD during hemodialysis treatment confirmed that depurated Pi originates from the intracellular compartment. This finding raises the possibility that excessive dialytic depuration of phosphate might adversely affect the intracellular availability of high-energy phosphates and ultimately, cellular metabolism. Further studies are needed to investigate the relationship between objective and subjective effects of hemodialysis and decreases of intracellular Pi and βATP content.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

Intracellular Phosphate Concentration Evolution During Hemodialysis by MR Spectroscopy (CIPHEMO), NCT03119818.

Acid-base kinetics during hemodialysis using bicarbonate and lactate as dialysate buffer bases based on the H+ mobilization model

BACKGROUND

The H⁺ mobilization model has been recently reported to accurately describe intradialytic kinetics of plasma bicarbonate concentration; however, the ability of this model to predict changing bicarbonate kinetics after altering the hemodialysis treatment prescription is unclear.

METHODS

We considered the H⁺ mobilization model as a pseudo-one-compartment model and showed theoretically that it can be used to determine the acid generation (or production) rate for hemodialysis patients at steady state. It was then demonstrated how changes in predialytic, intradialytic, and immediate postdialytic plasma bicarbonate (or total carbon dioxide) concentrations can be calculated after altering the hemodialysis treatment prescription.

RESULTS

Example calculations showed that the H⁺ mobilization model when considered as a pseudo-one-compartment model predicted increases or decreases in plasma total carbon dioxide concentrations throughout the entire treatment when the dialysate bicarbonate concentration is increased or decreased, respectively, during conventional thrice weekly hemodialysis treatments. It was further shown that this model allowed prediction of the change in plasma total carbon dioxide concentration after transfer of patients from conventional thrice weekly to daily hemodialysis using both bicarbonate and lactate as dialysate buffer bases.

CONCLUSION

The H⁺ mobilization model can predict changes in plasma bicarbonate or total carbon dioxide concentration during hemodialysis after altering the hemodialysis treatment prescription.

A two-pool kinetic model predicts phosphate concentrations during and shortly following a conventional (three times weekly) hemodialysis session

Background

Previous studies have suggested that a conventional two-pool model cannot be used to predict intradialysis and early postdialysis phosphorus concentrations.

Methods

A conventional two-pool urea model was modified by increasing the distal compartment volume from two-thirds to three times the total body water and by the use of a dynamically variable intercompartmental phosphorus clearance during dialysis. The phosphate solver model parameters were derived from an examination of the results in the literature, and fine-tuned using a training set (F4) of 415 Hemodialysis (HEMO) Study patients studied during a dialysis session where phosphorus was measured at 4 months of follow-up. Validation was done in a group of 380 different HEMO Study patients plus 9 from the original F4 group, who were evaluated at 36 months of follow-up.

Results

The model predicted measured median early (1 h) intradialysis, end-dialysis and 30-min postdialysis serum phosphorus levels in the test and validation datasets with little apparent bias, including the highest and lowest deciles of predialysis serum phosphorus. The model tended to underestimate slightly intradialysis serum phosphorus when predialysis serum phosphorus was <3.0 mg/dL (0.97 mmol/L). There was a large scatter and standard deviation among patients, and whether aberrant values represent a patient-specific phenomenon is unclear.

Conclusions

A modified two-pool model using a slightly expanded distal compartment and a dynamically varying intercompartmental clearance, depending on the intradialysis phosphorus concentration, can be used to predict serum phosphorus level during and shortly after dialysis, in patients following a conventional three times per week dialysis prescription.

A Pseudo-One Compartment Model of Phosphorus Kinetics During Hemodialysis: Further Supporting Evidence

A pseudo-one compartment model has been proposed to describe phosphorus kinetics during hemodialysis and the immediate post-dialysis period. This model assumes that phosphorus mobilization from tissues is proportional to the difference between the pre-dialysis serum concentration (a constant) and the instantaneous serum concentration. The current study is exploratory and evaluated the ability of a pseudo-one compartment model to describe the kinetics of phosphorus during two short hemodialysis treatments separated by a 60-min inter-treatment period without dialysis; the latter is the post-dialysis rebound period for the first short hemodialysis treatment. Serum was collected frequently during both hemodialysis treatments and the inter-treatment period to assess phosphorus kinetics in 21 chronic hemodialysis patients. Phosphorus mobilization clearance and pre-dialysis central distribution volume were previously estimated for each patient during the first hemodialysis treatment and the inter-treatment period. Assuming those kinetic parameters remained constant for each patient, serum phosphorus concentrations during the second treatment were used to estimate the driving force concentration (C_df ) for phosphorus mobilization from tissues during the second treatment. Treatment time (117 ± 14 [mean ± standard deviation] vs. 117 ± 14 min), dialyzer phosphorus clearance (151 ± 25 vs. 140 ± 32 mL/min), and net fluid removal (1.44 ± 0.74 vs. 1.47 ± 0.76 L) were similar during both short hemodialysis treatments. Measured phosphorus concentration at the start of the second hemodialysis treatment (3.3 ± 0.9 mg/dL) was lower (P < 0.001) than at the start of the first treatment or C_pre (5.4 ± 1.9 mg/dL). Calculated C_df was 4.9 ± 2.0 mg/dL, not significantly different from C_pre (P = 0.12). C_df and C_pre were correlated (R = 0.72, P < 0.001). The results from this study demonstrate that the driving force concentration for phosphorus mobilization during hemodialysis is constant and not different from that pre-dialysis, providing further evidence supporting a fundamental assumption of the pseudo-one compartment model.

Impact of using two dialyzers in parallel on phosphate clearance in hemodialysis patients: a randomized trial

Background

Dietary restriction and phosphate binders are the main interventions used to manage hyperphosphatemia in people on hemodialysis, but have limited efficacy. Modifying conventional dialysis regimens to enhance phosphate clearance as an alternative approach remains relatively unstudied.

Methods

This was a 10-week, 2-arm, randomized crossover study. Participants were prevalent dialysis patients ( n = 32) with consecutive serum phosphate levels >1.6 mmol/L and on stable doses of a phosphate binder. Following a 2-week run-in period, participants were randomized to initiate dialysis using two high flux dialyzers in parallel (blood flow ≥350 mL/min, dialysate flow 800 mL/min) or standard dialysis using one high flux dialyzer (blood flow ≥350 mL/min, dialysate flow of 800 mL/min). Each regimen was 3 weeks in duration. After a 2-week washout period, participants received the alternate regimen. The primary outcome was the mean difference in phosphate clearance by dialyzer strategy. Secondary outcomes were phosphate removal and pre-dialysis serum phosphate.

Results

Phosphate clearance for the double dialyzer strategy did not differ significantly from the single dialyzer strategy [mean difference 7.5 mL/min (95% confidence interval, 95% CI, -6.1, 21.0), P = 0.28]. There was no difference in total phosphate removal and pre-dialysis phosphate between the double and single dialyzer strategies [total phosphate removal mean difference -0.2 mmol (95% CI -4.1, 3.7), P = 0.93; pre-dialysis mean difference 0.01 mmol/L (95% CI -0.18, 0.21), P = 0.88]. There was no difference in the proportion of participants who experienced at least one episode of intradialytic hypotension (32 versus 47%, P = 0.13). A limitation of the study was frequent protocol deviations in the dialysis prescription.

Conclusions

In this study, the use of two dialyzers in parallel did not increase phosphate clearance, phosphate removal or pre-dialysis serum phosphorus when compared with a standard dialysis treatment strategy. Future studies should continue to evaluate novel methods of phosphate removal using conventional hemodialysis.

[Modelling of phosphorus transfers during haemodialysis]

Chronic kidney disease causes hyperphosphatemia, which is associated with increased cardiovascular risk and mortality. In patients with end-stage renal disease, haemodialysis allows the control of hyperphosphatemia. During a 4-h haemodialysis session, between 600 and 700mg of phosphate are extracted from the plasma, whereas the latter contains only 90mg of inorganic phosphate. The precise origin of phosphates remains unknown. The modelling of phosphorus transfers allows to predict the outcome after changes in dialysis prescription (duration, frequency) with simple two-compartment models and to describe the transfers between the different body compartments with more complex models. Work using ³¹P nuclear magnetic resonance spectroscopy performed in animals showed an increase in intracellular phosphate concentration and a decrease in intracellular ATP during a haemodialysis session suggesting an intracellular origin of phosphates.

Effect of Treatment Duration and Frequency on Uremic Solute Kinetics, Clearances and Concentrations

The kinetics of uremic solute clearances are discussed based on two categories of uremic solutes, namely those that are and those that are not derived directly from nutrient intake, particularly dietary protein intake. This review highlights dialysis treatments that are more frequent and longer (high-dose hemodialysis) than conventional thrice weekly therapy. It is proposed that the dialysis dose measures based on urea as a marker uremic solute, such as Kt/V and stdKt/V, be referred to as measures of dialysis inadequacy, not dialysis adequacy. For uremic solutes derived directly from nutrient intake, it is suggested that inorganic phosphorus and protein-bound uremic solutes be considered as markers in the development of alternative measures of dialysis dose for high-dose hemodialysis prescriptions. As the current gap in understanding the detailed kinetics of protein-bound uremic solutes, it is proposed that normalization of serum phosphorus concentration with a minimum (or preferably without a) need for oral-phosphorus binders be targeted as a measure of dialysis adequacy in high-dose hemodialysis. For large uremic solutes not derived directly from nutrient intake (middle molecules), use of extracorporeal clearances for β₂ -microglobulin that are higher than currently available during thrice weekly therapy is unlikely to reduce predialysis serum β₂ -microglobulin concentrations. High-dose hemodialysis prescriptions will lead to reductions in predialysis serum β₂ -microglobulin concentrations, but such reductions are also limited by significant residual kidney clearance. Kinetic data regarding middle molecules larger than β₂ -microglobulin are scarce; additional studies on such uremic solutes are of high interest to better understand improved methods for prescribing high-dose hemodialysis prescriptions to improve patient outcomes.

More Dialysis Has Not Proven Much Better

Patients maintained on standard three times weekly hemodialysis have a high mortality rate and a limited quality of life. Some of this illness is due to systemic diseases that have caused kidney failure, and thus may be irreversible. But we presume that imperfect replacement of normal kidney function by dialysis contributes importantly. Patients on hemodialysis are subject to fluctuations in extracellular fluid volume and inorganic ion concentrations and their plasma levels of many organic waste solutes remain very high. It is thus natural to suppose that their health could be improved by increasing the intensity of dialysis treatment. But despite a great deal of work over the past 20 years, evidence that such improvement can be obtained is generally lacking. Specific benefits can indeed be achieved. Patients who cannot control their intradialytic weight gains or plasma phosphate levels with standard therapy can benefit from extending treatment time. But we cannot promise the average patient that longer or more frequent treatment will reduce mortality or improve the quality of life.

Modifications to bicarbonate conductivity: A way to increase phosphate removal during hemodialysis? Proof of concept

Introduction Hyperphosphatemia and cardiovascular mortality are associated particularly with end-stage renal disease. Available therapeutic strategies (i.e., diet restriction, calcium [or not]-based phosphate binders, calcimimetics) are associated with extrarenal blood purification. Compartmentalization of phosphate limits its depuration during hemodialysis. Several studies suggest that plasmatic pH is involved in the mobilization of phosphate from intracellular to extracellular compartments. Consequently, the efficiency of modified bicarbonate conductivity to purify blood phosphate was tested. Methods Ten hemodialysis patients with chronic hyperphosphatemia (>2.1 mmol/L) were included in the two three-sessions-per week periods. Bicarbonate concentration was fixed at 40 mmol/L and 30 mmol/L in the first and second periods, respectively. Phosphate depuration was evaluated by phosphate mobilization clearance (K_M ). Findings Although bicarbonatemia was lower during the second period (21.0 ± 2.7 vs. 24.4 ± 3.1 mmol/L, P < 0.01), no difference was observed in phosphatemia (2.4 ± 0.5 vs. 2.3 ± 0.4 mmol/L, P = NS). The in-session variation of phosphate was lower (-1.45 ± 0.42 vs. -1.58 ± 0.44 mmol/L, P < 0.05) and K_M was higher during the second period (82.94 ± 38.00 vs. 69.74 ± 24.48 mL/min, P < 0.05). Discussion The decrease of in-session phosphate and the increase in K_M reflect phosphate refilling during hemodialysis. Thus, modulation of serum bicarbonate may play a role in controlling the phosphate pool. Even though correcting metabolic acidosis during hemodialysis remains important, alkaline excess can impair phosphate mobilization clearance. Clinical trials are needed to test the efficiency and relevance of a strategy where bicarbonatemia is corrected less at the beginning of sessions.

Removal of Phosphorus by Hemodialysis

The control of serum phosphorus by dialysis is made difficult by the fact that intradialytic blood levels tend to be low, and because phosphorus is removed almost exclusively from the plasma during its passage through the dialyzer. The most practical way to increase phosphorus removal is to extend dialysis time, although attention to dialysis efficiency (surface area, advanced membrane, and higher blood and dialysate flow rates) also plays a role. Benefits of hemodiafiltration in helping control serum phosphorus have been claimed, but not found in all studies. Because serum phosphorus levels tend to plateau during the later parts of a dialysis session, extending weekly dialysis time is of greater benefit for phosphorus removal than for urea removal. Increasing dialysis frequency also probably has a small benefit. It appears that 18-30 hours of dialysis per week are required to obviate the need for phosphorus binders. Several promising models of phosphorus kinetics are under development. These may help predict the change in treatment on serum phosphorus levels, but their ability to do so has not yet been critically assessed.

Evaluation of a pseudo-one-compartment model for phosphorus kinetics by later-phase dialysate collection during blood purification

PURPOSE

Phosphorus removal is a major issue to assess for physicians engaging in hemodialysis. A pseudo-one-compartment model was reported as a novel model for phosphorus kinetics. We aimed to evaluate the adequacy of this model from the standpoint of the total mass of removed phosphorus during prolonged treatment.

METHODS

Dialysate was collected during 6-h hemodialysis and hemodiafiltration treatment in 5 patients. Later-phase (from 4 to 6 h) dialysate was collected separately. Mobilization clearance (K(m)) and dialyzer clearance (K) were calculated by simple arithmetic operations utilizing stable serum phosphorus concentrations in this later phase. Volume of the accessible compartment (V(0)) was estimated by a fitting method. Amounts of removed phosphorus were calculated with these parameters and compared with measured values. The best sampling time points during treatment were also assessed, when the parameters were determined by serial serum phosphorus concentrations alone.

RESULTS

Pearson's correlation coefficient (R) between calculated and measured values of removed phosphorus was 0.991 and the concordance correlation coefficient (ρ) was 0.987. When K(m), K and V(0) were determined by serial serum concentrations alone, including those at 0, 1, 4, and 6 h, the calculated mass of removed phosphorus had high R (0.974-0.975) or ρ (0.966-0.972) with the measured values.

CONCLUSIONS

We confirmed that a pseudo-one-compartment model is useful for the estimation of removed phosphorus mass during prolonged blood purification by collecting dialysate. When the parameters are determined by a fitting method using serial serum concentrations alone, sampling at 0, 1, 4, and 6 h seems to be adequate.

Potassium kinetics during hemodialysis

Hyperkalemia in hemodialysis patients is associated with high mortality, but prescription of low dialysate potassium concentrations to decrease serum potassium levels is associated with a high incidence of sudden cardiac arrest or sudden death. Improved clinical outcomes for these patients may be possible if rapid and substantial intradialysis decreases in serum potassium concentration can be avoided while maintaining adequate potassium removal. Data from kinetic modeling sessions during the HEMO Study of the dependence of serum potassium concentration on time during hemodialysis treatments and 30 minutes postdialysis were evaluated using a pseudo one-compartment model. Kinetic estimates of potassium mobilization clearance (K(M)) and predialysis central distribution volume (V(pre)) were determined in 551 hemodialysis patients. The studied patients were 58.8 ± 14.4 years of age with predialysis body weight of 72.1 ± 15.1 kg; 306 (55.4%) of the patients were female and 337 (61.2%) were black. K(M) and V(pre) for all patients were non-normally distributed with values of 158 (111, 235) (median [interquartile range]) mL/min and 15.6 (11.4, 22.8) L, respectively. K(M) was independent of dialysate potassium concentration (P > 0.2), but V(pre) was lower at higher dialysate potassium concentration (R = -0.188, P < 0.001). For patients with dialysate potassium concentration between 1.6 and 2.5 mEq/L (N = 437), multiple linear regression of K(M) and V(pre) demonstrated positive association with predialysis body weight and negative association with predialysis serum potassium concentration. Potassium kinetics during hemodialysis can be described using a pseudo one-compartment model.