Dialyzer Performance in the HEMO Study:In Vivo K 0 A and True Blood Flow Determined from a Model of Cross-Dialyzer Urea Extraction

Replication of fouling in vitro in hollow fiber dialyzers by albumin immobilization

For designing and evaluating the dialyzer and investigating the optimal therapeutic conditions, in vitro studies bring us many useful findings. In hemodialysis, however, the membrane fouling due to protein molecules reduces solute removal performance. Therefore, we investigated a method for replicating the fouling in dialyzers in aqueous experiments. After the albumin solution was circulated in the test circuit with a dialyzer, a glutaraldehyde solution was pumped into the dialyzer to immobilize albumin on the hollow fiber membrane. Under various immobilization conditions, the permeability of creatinine and vitamin B₁₂ was evaluated by dialysis experiments. The creatinine clearance after immobilization of albumin was decreased, suggesting pore plugging by our fouling replication method. The glutaraldehyde crosslinked albumin molecules that adhered them to the membrane firmly. Moreover, the degree of fouling may be controlled by changing the concentration of albumin solution and the volume of glutaraldehyde solution used for immobilization. Our fouling replication method was applied to three types of polyester polymer alloy (PEPA) dialyzers and one polysulfone (PSf) dialyzer. This method enables to evaluate the permeability of various dialyzers with fouling in vitro that will be of great help in collecting data for designing dialyzers.

Routine Kt/V and Normalized Protein Nitrogen Appearance Rate Determined From Conductivity Access Clearance With Infrequent Postdialysis Serum Urea Nitrogen Measurements

RATIONALE & OBJECTIVES

Conventional monitoring of hemodialysis dose is implemented using urea kinetic modeling based on single-pool Kt/V, which requires both pre- and postdialysis serum urea nitrogen (SUN) measurements. We compared this conventional approach to one in which Kt/V is calculated using conductivity clearance, thereby reducing the need for regular postdialysis SUN measurements.

STUDY DESIGN

Comparative study of 2 diagnostic tests.

SETTING & PARTICIPANTS

Prevalent patients receiving maintenance hemodialysis for at least 2 years for whom both urea reduction ratio (URR) and average conductivity clearance (Kecn) were measured.

TESTS COMPARED

During the initial 8 months (baseline interval), average Kecn and URR were used to calculate a median patient-specific, modeled, calibration solute distribution volume (Vcal). During months 9 to 16 (period 1) and 17 to 24 (period 2), Kt/V was conventionally computed using URR and also by a new method using Vcal and Kecn without postdialysis SUN values. We examined the percentage error between these 2 methods of calculating Kt/V.

OUTCOMES

Concordance between the 2 methods of calculating Kt/V.

RESULTS

Among 1,093 patients, mean individual-level median single-pool Kt/V values derived using the conventional method during the baseline interval, period 1, and period 2 were 1.62±0.24 (SD), 1.66±0.24, and 1.67±0.24, respectively. During periods 1 and 2, patient-level median Kt/V values derived using Kecn were 1.64±0.24 and 1.65±0.24, respectively. Percent differences between patient-level median values of Kt/V (conductivity minus conventional URR methods) were-0.63%±7.7% and-0.75%±8.4% for periods 1 and 2. Normalized protein nitrogen appearance were comparable between the 2 methods.

LIMITATIONS

Data were collected over 2 years. Study was limited to in-center hemodialysis patients dialyzed 3 times per week. Dialysis session length was not adjusted for treatment interruptions.

CONCLUSIONS

A new method of calculating Kt/V based on Kecn that requires fewer postdialysis SUN measurements provided diagnostic data comparable to those from conventional use of URR and has the potential to avoid errors related to postdialysis blood sampling and measurement.

Changes in Total Protein Concentration Due to Fluid Removal During and Shortly after Hemodialysis

BACKGROUND

Changes in plasma volume during hemodialysis are complex and have been shown to depend on the rate of fluid removal and the degree of fluid overload. We examined changes in total protein concentration during and shortly after a dialysis treatment in archived data from the HEMO study.

METHODS

During follow-up months 4 and 36 of the HEMO study, additional blood samples were obtained during a typical dialysis session at 30 and 60 min after dialysis. In 315 studies from 282 patients where complete data were available, we calculated the concentration change in total protein and compared it to the modeled change in both total body water and extracellular fluid space as derived from 2-pool urea kinetic modeling.

RESULTS

The mean postdialysis modeled urea volume (V) was 31.1 ± 6.18 L. Mean fluid removal was 2.76 ± 1.27 kg, over a session length of 207 ± 28 min. The ratio of predialysis V to postdialysis V averaged 1.090 ± 0.040. The mean TP ratios (post/pre) at 0, 30, and 60 min postdialysis averaged 1.121 ± 0.070 (SD), 1.091 ± 0.090, and 1.091 ± 0.086. The dialysate to serum sodium gradient, studied in a different group of treatments where this information was available, had no impact on these findings, nor did the length of the interdialytic interval.

CONCLUSIONS

On average, after equilibration, the change in plasma volume due to fluid removal is similar to the modeled change in total body water (urea space), irrespective of dialysate to serum sodium gradient. This supports previous observations that during dialysis with ultrafiltration, plasma volume contracts to a lesser degree than the interstitial volume and that some fluid may be removed from spaces other than the extracellular fluid.

Creatinine generation from kinetic modeling with or without postdialysis serum creatinine measurement: results from the HEMO study

Background

A convenient method to estimate the creatinine generation rate and measures of creatinine clearance in hemodialysis patients using formal kinetic modeling and standard pre- and postdialysis blood samples has not been described.

Methods

We used data from 366 dialysis sessions characterized during follow-up month 4 of the HEMO study, during which cross-dialyzer clearances for both urea and creatinine were available. Blood samples taken at 1 h into dialysis and 30 min and 60 min after dialysis were used to determine how well a two-pool kinetic model could predict creatinine concentrations and other kinetic parameters, including the creatinine generation rate. An extrarenal creatinine clearance of 0.038 l/kg/24 h was included in the model.

Results

Diffusive cross-dialyzer clearances of urea [230 (SD 37 mL/min] correlated well (R2 = 0.78) with creatinine clearances [164 (SD 30) mL/min]. When the effective diffusion volume flow rate was set at 0.791 times the blood flow rate for the cross-dialyzer clearance measurements at 1 h into dialysis, the mean calculated volume of creatinine distribution averaged 29.6 (SD 7.2) L], compared with 31.6 (SD 7.0) L for urea (P < 0.01). The modeled creatinine generation rate [1183 (SD 463) mg/day] averaged 100.1 % (SD 29; median 99.3) of that predicted in nondialysis patients by an anthropometric equation. A simplified method for modeling the creatinine generation rate using the urea distribution volume and urea dialyzer clearance without use of the postdialysis serum creatinine measurement gave results for creatinine generation rate [1187 (SD 475) mg/day; that closely matched the value calculated using the formally modeled value, R2 = 0.971].

Conclusions

Our analysis confirms previous findings of similar distribution volumes for creatinine and urea. After taking extra-renal clearance into consideration, the creatinine generation rate in dialysis patients is similar to that in nondialysis patients. A simplified method based on urea clearance and urea distribution volume not requiring a postdialysis serum creatinine measurement can be used to yield creatinine generation rates that closely match those determined from standard modeling.

Kt/Vurea and Nonurea Small Solute Levels in the Hemodialysis Study

The Hemodialysis (HEMO) Study showed that high-dose hemodialysis providing a single-pool Kt/V_urea of 1.71 provided no benefit over a standard treatment providing a single-pool Kt/V_urea of 1.32. Here, we assessed whether the high-dose treatment used lowered plasma levels of small uremic solutes other than urea. Measurements made ≥3 months after randomization in 1281 patients in the HEMO Study showed a range in the effect of high-dose treatment compared with that of standard treatment: from no reduction in the level of p-cresol sulfate or asymmetric dimethylarginine to significant reductions in the levels of trimethylamine oxide (-9%; 95% confidence interval [95% CI], -2% to -15%), indoxyl sulfate (-11%; 95% CI, -6% to -15%), and methylguanidine (-22%; 95% CI, -18% to -27%). Levels of three other small solutes also decreased slightly; the level of urea decreased 9%. All-cause mortality did not significantly relate to the level of any of the solutes measured. Modeling indicated that the intermittency of treatment along with the presence of nondialytic clearance and/or increased solute production accounted for the limited reduction in solute levels with the higher Kt/V_urea In conclusion, failure to achieve greater reductions in solute levels may explain the failure of high Kt/V_urea treatment to improve outcomes in the HEMO Study. Furthermore, levels of the nonurea solutes varied widely among patients in the HEMO Study, and achieved Kt/V_urea accounted for very little of this variation. These results further suggest that an index only on the basis of urea does not provide a sufficient measure of dialysis adequacy.

Determinants of phosphorus mobilization during hemodialysis

Our recent work proposed a pseudo one-compartment model for describing intradialysis and postdialysis rebound kinetics of phosphorus. In this model, phosphorus is removed directly from a central distribution volume with the rate of phosphorus mobilization from a second, very large compartment proportional to the phosphorus mobilization clearance. Here, we evaluated factors of phosphorus mobilization clearance and postdialysis central distribution volume from 774 patients in the HEMO Study. Phosphorus mobilization clearance and postdialysis central distribution volume were 87 (65, 116) ml/min, median (interquartile range), and 9.4 (7.2, 12.0) liter, respectively. The phosphorus mobilization clearance was significantly higher for male patients than for female patients. Both the phosphorus mobilization clearance and the postdialysis central distribution volume were significantly associated with postdialysis body weight but negatively with the predialysis serum phosphorus concentration. The postdialysis central distribution volume was also significantly associated with age. Overall, the postdialysis central distribution volume was 13.6% of the postdialysis body weight. Thus, the phosphorus mobilization clearance during hemodialysis is higher when predialysis serum phosphorus concentration is low and higher in male patients than in female patients. The central distribution volume of phosphorus is a space approximating the extracellular fluid volume.

A model to predict optimal dialysate flow

Diffusive clearance depends on blood (Qb) and dialysate flow (Qd) rates and the overall mass transfer area coefficient (KoA) of the dialyzer. In this article we describe a model to predict an appropriated AutoFlow (AF) factor (AF factor = Ratio Qd/Qb), that is able to provide adequate Kt/V for hemodialysis patients (HDP), while consuming lower amounts of dialysate, water and energy during the treatment. We studied in vivo the effects of three various Qd on the delivered dose of dialysis in 33 stable HDP. Hemodialysis was performed at Qd of 700 mL/mn, 500 mL/mn, and with AF, whereas specific dialysis prescriptions (treatment time, blood flow rate [Qb], and type and size of dialyzer) were kept constant. The results showed that increasing the dialysate flow rate more than the model of AF predicted had a small effect on the delivered dose of dialysis. The Kt/V (mean ± SD) was 1.52 ± 0.16 at Qd 700, 1.50 ± 0.16 at Qd 500, and 1.49 ± 0.15 with AF. The use of the AF function leads to a significant saving of dialysate fluid. The model predicts the appropriate AF factor that automatically adjusts the dialysate flow rate according to the effective blood flow rate of the patient to achieve an appreciable increase in dialysis dose at the lowest additional cost.

Dialysis time: does it matter? A reappraisal of existing literature

PURPOSE OF REVIEW

The length of time (Td) required for adequate maintenance hemodialysis therapy is perceived as a substantial patient burden. Technological advancements have allowed shortening Td over the past three decades. However, failure to detect improved outcomes with higher dialysis dose has prompted renewed interest in the potential impact of longer Td.

RECENT FINDINGS

Ongoing trials are focused on increasing the frequency of treatments, although the feasibility of having most patients agreeing to more than five treatments per week remain doubtful. Furthermore, survival was better in short daily hemodialysis with Td of 180 vs. 90 min. Within thrice weekly dialysis, several recent epidemiological studies have shown improved survival associated with Td more than 4 h. Improved outcomes for long in-center nocturnal hemodialysis (6-8 h, 3×/week), similar to what has been performed in Tassin for the last 30 years, have also been reported.

SUMMARY

Compelling rationale and recent outcome data support use of longer Td. Improved management of salt and water may be the cause for the dissociation of dialysis time and small molecule clearance. In most industrialized countries, hemodialysis care systems in place have the capacity to accommodate it. Until such time that results from prospective randomized trials are available, we believe that physicians should prescribe and exert all efforts to convince thrice weekly hemodialysis patients to accept 4 h as minimum Td.

Modeled urea distribution volume and mortality in the HEMO Study

BACKGROUND AND OBJECTIVES

In the Hemodialysis (HEMO) Study, observed small decreases in achieved equilibrated Kt/V(urea) were noncausally associated with markedly increased mortality. Here we examine the association of mortality with modeled volume (V(m)), the denominator of equilibrated Kt/V(urea).

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Parameters derived from modeled urea kinetics (including V(m)) and blood pressure (BP) were obtained monthly in 1846 patients. Case mix-adjusted time-dependent Cox regressions were used to relate the relative mortality hazard at each time point to V(m) and to the change in V(m) over the preceding 6 months. Mixed effects models were used to relate V(m) to changes in intradialytic systolic BP and to other factors at each follow-up visit.

RESULTS

Mortality was associated with V(m) and change in V(m) over the preceding 6 months. The association between change in V(m) and mortality was independent of vascular access complications. In contrast, mortality was inversely associated with V calculated from anthropometric measurements (V(ant)). In case mix-adjusted analysis using V(m) as a time-dependent covariate, the association of mortality with V(m) strengthened after statistical adjustment for V(ant). After adjustment for V(ant), higher V(m) was associated with slightly smaller reductions in intradialytic systolic BP and with risk factors for mortality including recent hospitalization and reductions in serum albumin concentration and body weight.

CONCLUSIONS

An increase in V(m) is a marker for illness and mortality risk in hemodialysis patients.

Do clinical outcomes in chronic hemodialysis depend on the choice of a dialyzer?

Nephrologists are presented with a range of choices when selecting a dialyzer for chronic hemodialysis. Dialyzers differ in the material, structure, permeability and surface area of their membrane, and how the dialyzer is sterilized. Opinions vary regarding the impact of dialyzer characteristics on patient outcomes and which, if any, of these properties to take into account when choosing a dialyzer can be confusing. In the general dialysis population, there is no compelling evidence that the choice of a membrane material from among those materials currently in clinical use has a significant impact on morbidity or mortality (although there are rare patients who will react adversely to a given dialysis membrane). Similarly, most dialyzers are capable of adequately removing small solutes, such as urea, provided they are used with an appropriate blood flow rate and treatment time to ensure delivery of a single-pool Kt/V(urea) of at least 1.25 for men and 1.65 for women. However, in some dialysis patient subpopulations, the results of randomized clinical trials suggest that use of dialyzer containing high-flux membranes confers an outcome advantage. The extent to which this advantage is realized might also depend on how the dialyzer is used, with application in convective therapies such as hemodiafiltration being superior to diffusive therapies such as hemodialysis. This possibility is currently the subject of several large clinical trials.

Effect of increasing dialysate flow rate on diffusive mass transfer of urea, phosphate and beta2-microglobulin during clinical haemodialysis

Background. Diffusive clearance depends on blood and dialysate flow rates and the overall mass transfer area coefficient (K_oA) of the dialyzer. Although K_oA should be constant for a given dialyzer, urea K_oA has been reported to vary with dialysate flow rate possibly because of improvements in flow distribution. This study examined the dependence of K_oA for urea, phosphate and β₂-microglobulin on dialysate flow rate in dialyzers containing undulating fibers to promote flow distribution and two different fiber packing densities.

Methods. Twelve stable haemodialysis patients underwent dialysis with four different dialyzers, each used with a blood flow rate of 400 mL/min and dialysate flow rates of 350, 500 and 800 mL/min. Clearances of urea, phosphate and β₂-microglobulin were measured and K_oA values calculated.

Results. Clearances of urea and phosphate, but not β₂-microglobulin, increased significantly with increasing dialysate flow rate. However, increasing dialysate flow rate had no significant effect on K_oA or K_o for any of the three solutes examined, although K_o for urea and phosphate increased significantly as the average flow velocity in the dialysate compartment increased.

Conclusions. For dialyzers with features that promote good dialysate flow distribution, increasing dialysate flow rate beyond 600 mL/min at a blood flow rate of 400 mL/min is likely to have only a modest impact on dialyzer performance, limited to the theoretical increase predicted for a constant K_oA.

Combining dialysate and blood recirculation to boost uremic toxin removal: theory and simulation study

Emerging concepts in renal replacement therapies such as daily hemodialysis (HD) and wearable artificial kidney (WAK) meet the challenges derived from the aging of renal disease population. The successful results of blood recirculation to improve the dialysis clearance of medium-high uremic toxins suggest that this technique could be used both in daily HD and WAK. However, blood recirculation induces a reduction of small solutes clearance. This work analyzes the ability of a nonregenerated recirculating dialysate technique to avoid that reduction. The study uses in silico simulations with in vitro data from several commercial dialyzers and very compact dialyzer models derived from them. The resulting combined optimization system (recirculating blood-dialysate system [RBDS]) reached low solute dialysance improvements above 100% in different scenarios. Our outcomes show the RBDS performs best with very compact dialyzers, in agreement with the requirements of daily HD and WAK.

Downloadable computer models for renal replacement therapy

Mathematical models can predict solute clearances and solute concentrations during renal replacement therapy. At present, however, most nephrologists cannot use these models because they require mathematical software. In this report, we describe models of solute transport by convection and diffusion adapted to run on the commonly available software program Excel for Macintosh computers and PCs running Windows. Two programs have been created that can be downloaded from http://www.stanford.edu/~twmeyer/ or http://dev.satellitehealth.com/research/journal.asp. The first, called 'Dr Addis Clearance Calculator', calculates clearance values from inputs including the blood flow Q(b), the hematocrit, the ultrafiltration rate Q(f), the dialysate flow rate Q(d), the reflection coefficient sigma and the mass transfer area coefficient K(o)A for the solute of interest, and the free fraction f if the solute is protein bound. Solute concentration profiles along the length of the artificial kidney are displayed graphically. The second program, called 'Dr Coplon Dialysis Simulator', calculates plasma solute concentrations from the clearance values obtained by the first program and from additional input values including the number of treatments per week, the duration of the treatments, and the solute's production rate and volumes of distribution. The program calculates the time-averaged solute concentration and provides a graphic display of the solute concentration profile through a week-long interval.

Serum beta-2 microglobulin levels predict mortality in dialysis patients: results of the HEMO study

In the randomized Hemodialysis (HEMO) Study, chronic high-flux dialysis, as defined by higher beta-2 microglobulin (beta(2)M) clearance, compared with low-flux dialysis did not significantly alter all-cause mortality in the entire cohort but was associated with lower mortality in long-term dialysis patients. This analysis examined the determinants of serum beta(2)M levels and the associations of serum beta(2)M levels or dialyzer beta(2)M clearance with mortality. In a multivariable regression model that examined 1704 patients, baseline residual kidney urea clearance and dialyzer beta(2)M clearance were strong predictors of predialysis serum beta(2)M levels at 1 mo of follow-up, with regression coefficients of -7.21 (+/-0.69 SE) mg/L per ml/min per 35 L urea volume (P < 0.0001) and -1.94 (+/-0.30) mg/L per ml/min (P < 0.0001),respectively. In addition, black race and baseline years on dialysis correlated positively whereas age, diabetes, serum albumin, and body mass index correlated negatively with serum beta(2)M levels (P < 0.05). In time-dependent Cox regression models, mean cumulative predialysis serum beta(2)M levels but not dialyzer beta(2)M clearance were associated with all-cause mortality (relative risk = 1.11 per 10-mg/L increase in beta(2)M level; 95% confidence interval 1.05 to 1.19; P = 0.001), after adjustment for residual kidney urea clearance and number of prestudy years on dialysis. This association is supportive of the potential value of beta(2)M as a marker to guide chronic hemodialysis therapy.

Removal of P-cresol sulfate by hemodialysis

Protein-bound solutes are poorly cleared by dialysis. Among the most extensively studied of these solutes is p-cresol, which has been shown to be toxic in vitro. This study examined the form in which p-cresol circulates and quantified its removal by hemodialysis. HPLC analysis of plasma from hemodialysis patients contained a peak whose mobility corresponded to synthetic p-cresol sulfate (PCS) but no detectable unconjugated p-cresol. Treatment with sulfatase resulted in recovery of this peak as p-cresol, confirming its identity. Subsequent studies compared the removal of PCS and another protein-bound solute, indican, to the removal of urea during clinical hemodialysis treatments. PCS and indican were 94 +/- 1% and 93 +/- 2% bound to plasma protein, respectively. Protein-binding caused a predictable decrease in measured dialytic clearance, which averaged 20 +/- 4 ml/min for PCS and 25 +/- 5 ml/min for indican as compared with 260 +/- 20 ml/min for urea. Volumes of distribution for the protein-bound solutes were greater than the plasma volume, averaging 15 +/- 7 L for PCS and 14 +/- 3 L for indican as compared with 37 +/- 7 for urea. Solute reduction ratios were 20 +/- 9% for PCS, 30 +/- 7% for indican, and 69 +/- 5% for urea. We conclude that p-cresol circulates in the form of its sulfate conjugate, PCS. PCS is poorly removed by hemodialysis because its clearance is limited by protein binding and the ratio of its volume of distribution to its clearance is high.

Effects of high-flux hemodialysis on clinical outcomes: results of the HEMO study

Among the 1846 patients in the HEMO Study, chronic high-flux dialysis did not significantly affect the primary outcome of the all-cause mortality (ACM) rate or the main secondary composite outcomes, including the rates of first cardiac hospitalization or ACM, first infectious hospitalization or ACM, first 15% decrease in serum albumin levels or ACM, or all non-vascular access-related hospitalizations. The high-flux intervention, however, seemed to be associated with reduced risks of specific cardiac-related events. The relative risks (RR) for the high-flux arm, compared with the low-flux arm, were 0.80 [95% confidence interval (CI), 0.65 to 0.99] for cardiac death and 0.87 (95% CI, 0.76 to 1.00) for the composite of first cardiac hospitalization or cardiac death. Also, the effect of high-flux dialysis on ACM seemed to vary, depending on the duration of prior dialysis. This report presents secondary analyses to further explore the relationship between the flux intervention and the duration of dialysis with respect to various outcomes. The patients were stratified into a short-duration group and a long-duration group, on the basis of the mean duration of dialysis of 3.7 yr before randomization. In the subgroup that had been on dialysis for >3.7 yr, randomization to high-flux dialysis was associated with lower risks of ACM (RR, 0.68; 95% CI, 0.53 to 0.86; P = 0.001), the composite of first albumin level decrease or ACM (RR, 0.74; 95% CI, 0.60 to 0.91; P = 0.005), and cardiac deaths (RR, 0.63; 95% CI, 0.43 to 0.92; P = 0.016), compared with low-flux dialysis. No significant differences were observed in outcomes related to infection for either duration subgroup, however, and the trends for beneficial effects of high-flux dialysis on ACM rates were considerably weakened when the years of dialysis during the follow-up phase were combined with the prestudy years of dialysis in the analysis. For the subgroup of patients with <3.7 yr of dialysis before the study, assignment to high-flux dialysis had no significant effect on any of the examined clinical outcomes. These data suggest that high-flux dialysis might have a beneficial effect on cardiac outcomes. Because these results are derived from multiple statistical comparisons, however, they must be interpreted with caution. The subgroup results that demonstrate that patients with different durations of dialysis are affected differently by high-flux dialysis are interesting and require further study for confirmation.