Anthropometry-based equations overestimate the urea distribution volume in hemodialysis patients

Routine online assessment of dialysis dose: Ionic dialysance or UV-absorbance monitoring?

For three-weekly hemodialysis, a single-pool Kt/V target of at least 1.4 together with a minimal dialysis dose Kt at 45 L for men and 40 L for women per each session is currently recommended. Fully automatic online calculation of Kt and Kt/V from conductivity or UV-absorbance measurements in the dialysate is standardly implemented on some hemodialysis monitors and makes it possible to estimate the dialysis dose without the need for blood or dialysate samples. Monitoring the UV-absorbance of the spent dialysate is the most direct method for estimating Kt/V as it does not require an estimate of V. Calculation of ionic dialysance from conductivity measurements is the most direct method for estimating Kt and BSA-scaled dialysis dose. Both ionic dialysance monitoring and UV-absorbance monitoring may help detect a change in urea clearance occurring during the session, but this change must be interpreted differently depending on the monitoring being considered. An abrupt decrease in urea clearance results in a decrease in ionic dialysance but, paradoxically, a sudden increase in estimated urea clearance provided by dialysate UV-absorbance monitoring. Healthcare teams who monitor both ionic dialysance and UV-absorbance in their hemodialysis units must be clearly informed of this difficulty.

KDOQI Clinical Practice Guideline for Nutrition in CKD: 2020 Update

The National Kidney Foundation's Kidney Disease Outcomes Quality Initiative (KDOQI) has provided evidence-based guidelines for nutrition in kidney diseases since 1999. Since the publication of the first KDOQI nutrition guideline, there has been a great accumulation of new evidence regarding the management of nutritional aspects of kidney disease and sophistication in the guidelines process. The 2020 update to the KDOQI Clinical Practice Guideline for Nutrition in CKD was developed as a joint effort with the Academy of Nutrition and Dietetics (Academy). It provides comprehensive up-to-date information on the understanding and care of patients with chronic kidney disease (CKD), especially in terms of their metabolic and nutritional milieu for the practicing clinician and allied health care workers. The guideline was expanded to include not only patients with end-stage kidney disease or advanced CKD, but also patients with stages 1-5 CKD who are not receiving dialysis and patients with a functional kidney transplant. The updated guideline statements focus on 6 primary areas: nutritional assessment, medical nutrition therapy (MNT), dietary protein and energy intake, nutritional supplementation, micronutrients, and electrolytes. The guidelines primarily cover dietary management rather than all possible nutritional interventions. The evidence data and guideline statements were evaluated using Grading of Recommendations, Assessment, Development and Evaluation (GRADE) criteria. As applicable, each guideline statement is accompanied by rationale/background information, a detailed justification, monitoring and evaluation guidance, implementation considerations, special discussions, and recommendations for future research.

What volume to choose to assess online Kt/V?

INTRODUCTION

Urea distribution volume (V) can be assessed in different ways, among them the anthropometric Watson Volume (V_W). However, many studies have shown that V_W does not coincide with V and that the latter can be more accurately estimated with other methods. The present multicentre study was designed to answer the question: what V to choose to assess online Kt/V?

MATERIALS AND METHODS

Pre- and postdialysis blood urea nitrogen concentrations and the usual input data set for urea kinetic modelling were obtained for a single dialysis session in 201 Caucasian patients treated in 9 Italian dialysis units. Only dialysis machines measuring ionic dialysance (ID) were utilized. ID reflects very accurately the mean effective dialyser urea clearance (Kd). Six different V values were obtained: the first one was V_W; the second one was computed from the equation established by the HEMO Study to predict the single pool-adjusted modelled V from V_W (V_H) (Daugirdas JT et al. KI 64: 1108, 2003); the others were estimated kinetically as: 1. V_ID, in which ID is direct input in the in the double pool variable volume (dpVV) calculation by means of the Solute-solver software; 2. V_Kd, in which the estimated Kd is direct input in the dpVV calculation by means of the Solute-solver software; 3. V_KTV, in which V is calculated by means of the second generation Daugirdas equation; 4. V_SPEEDY, in which ID is direct input in the dpVV calculation by means of the SPEEDY software able to provide results quite similar to those provided by Solute-solver.

RESULTS

Mean± SD of the main data are reported: measured ID was 190.6 ± 29.6 mL/min, estimated Kd was 211.6 ± 29.0 mL/min. The relationship between paired data was poor (R² = 0.34) and their difference at the Bland-Altman plot was large (21 ± 27 mL/min). V_W was 35.3 ± 6.3 L, V_H 29.5 ± 5.5, V_{_}ID 28.99 ± 7.6 L, V_{_}SPEEDY 29.4 ± 7.6 L, V_KTV 29.7 ± 7.0 L. The mean ratio V_W/V_ID was 1.22, (i.e. V_W overestimated V_ID by about 22%). The mean ratio V_H/V_ID was 1.02 (i.e. V_H overestimated V_ID by only 2%). The relationship between paired data of V_ID and V_W was poor (R² = 0.48) and their mean difference at the Bland-Altman plot was very large (-  6.39 ± 5.59 L). The relationship between paired data of V_ID and V_H was poor (R² = 47) and their mean difference was small but with a large SD (- 0.59 ± 5.53 L). The relationship between paired data of V_ID and V_SPEEDY was excellent (R² = 0.993) and their mean difference at the Bland-Altman plot was very small (- 0.54 ± 0.64 L). The relationship between paired data of V_ID and V_KTV was excellent (R² = 0.985) and their mean difference at the Bland-Altman plot was small (- 0.85 ± 1.06 L).

CONCLUSIONS

V_ID can be considered the reference method to estimate the modelled V and then the first choice to assess Kt/V. V_SPEEDY is a valuable alternative to V_ID. V_KTV can be utilized in the daily practice, taking also into account its simple way of calculation. V_W is not advisable because it leads to underestimation of Kt/V by about 20%.

Volume Estimates in Chronic Hemodialysis Patients by the Watson Equation and Bioimpedance Spectroscopy and the Impact on the Kt/Vurea calculation

Background

Accurate assessment of total body water (TBW) is essential for the evaluation of dialysis adequacy (Kt/V_urea). The Watson formula, which is recommended for the calculation of TBW, was derived in healthy volunteers thereby leading to potentially inaccurate TBW estimates in maintenance hemodialysis recipients. Bioimpedance spectroscopy (BIS) may be a robust alternative for the measurement of TBW in hemodialysis recipients.

Objectives

The primary objective of this study was to evaluate the accuracy of Watson formula-derived TBW estimates as compared with TBW measured with BIS. Second, we aimed to identify the anthropometric characteristics that are most likely to generate inaccuracy when using the Watson formula to calculate TBW. Finally, we derived novel anthropometric equations for the more accurate estimation of TBW.

Design and Setting

This was a cross-sectional study of prevalent in-center HD patients at St Michael's Hospital.

Patients

One hundred eighty-four hemodialysis patients (109 men and 75 women) were evaluated in this study.

Measurements

Anthropometric measurements including weight, height, waist circumference, midarm circumference, and 4-site skinfold (biceps, triceps, subscapular, and suprailiac) thickness were measured; fat mass was measured using the formula by Durnin and Womersley. We measured TBW by BIS using the Body Composition Monitor (Fresenius Medical Care, Bad Homburg, Germany).

Methods

We used the Bland-Altman method to calculate the difference between the TBW derived from the Watson method and the BIS. To derive new equations for TBW estimation, Pearson's correlation coefficients between BIS-TBW (the reference test) and other variables were examined. We used the least squares regression analysis to develop parsimonious equations to predict TBW.

Results

TBW values based on the Watson method had a high correlation with BIS-TBW (correlation coefficients = 0.87 and P < .001). Despite the high correlation, the Watson formula overestimated TBW by 5.1 (4.5-5.8) liters and 3.8 (3.0-4.5) liters, in men and women, respectively. Higher fat mass and waist circumference (general and abdominal obesity) were correlated with the greater TBW overestimation by the Watson formula. We created separate equations for men and women based on weight and waist circumference.

Limitations

The main limitation of our study was the lack of an external validation for our novel estimating equation. Furthermore, though BIS has been validated against traditional reference standards, our assumption that it represents the "gold standard" for body compartment assessment may be flawed.

Conclusions

The Watson formula generally overestimates TBW in chronic dialysis recipients, particularly in patients with the highest waist circumference. Widespread reliance on the Watson formula for derivation of TBW may lead to the underestimation of Kt/V_urea..

Ionic Dialysance and Determination of Kt/V in on-line Hemodiafiltration with Simultaneous Pre- and Post-dilution

Purpose

A direct determination of Kt/V using ionic dialysance for estimating K and bio-impedancemetry for estimating V is compared with the usual indirect estimation based on the second generation Daugirdas equation during a new technique of hemodiafiltration with simultaneous pre- and postdilution (mixed-HDF).

Methods

In 31 informed consented patients, the urea distribution volume (V) is estimated by total body water (VBCM) measured by the Body Composition Monitor (BCM; Fresenius Medical Care, Bad Homburg, Germany) based on bio-impedance spectroscopy. The value (KOCM t)/VBCM is calculated during 114 mixed-HDF sessions (duration 4 hours) from the measurement of ionic dialysance KOCM by the OCM module, standard on the 5008 dialysis monitor (Fresenius Medical Care, Germany). The single pool (Kt/V)sp is determined from blood urea concentration measurements using the Daugirdas equation.

Results

Mixed-HDF is a very high-efficiency hemodialysis with a delivered dialysis dose Kt/V near from 2 per 4-hour session. (KOCM t)/VBCM (1.97 ± 0.28) is consistent with (Kt/V)sp (2.01 ± 0.34) with a correlation coefficient at 0.72.

Conclusions

Direct calculation of Kt/V from estimating K by OCM and V by BCM is consistent with the usual indirect estimation by the second generation Daugirdas equation. Therefore, the regular determination of V by BCM allows the estimation of single-pool Kt/V at each session without the need of blood sampling.

Quantification of Hemodialysis Dose: What Kt/V to Choose?

Background

Quantification of hemodialysis became more accurate and easier after the advent of ionic dialysance and the use of methods for estimating urea distribution volume (V). The aim of this study was to compare different methods of hemodialysis dose assessment: Kt/VDau (Daugirdas 2nd generation), Kt/VOCM (Kt by OCM (Online Clearance Monitor) and V by Watson), and Kt/VBCM (Kt by OCM and V by bio-impedance); and to assess the dialysis adequacy, defined by a Kt/V≥1.4.

Design

Prospective, observational study.

Methods

35 hemodialysis sessions were evaluated in 35 chronic hemodialysis patients. During each session, we measured simultaneously, Kt/VOCM, Kt/VBCM and calculated Kt/VDau by performing blood samples before and after each session.

Results

35 patients, gender (M/F: 19/16), mean age of 50.49 years, were evaluated. We noted a difference between the three methods of evaluating Kt/V index: Kt/VDau, Kt/VOCM and Kt/VBCM (1.82 ± 0.29; 1.45 ± 0.23; 1.8 ± 0.33, p<0.001). Comparison of Kt/VOCM with Kt/VDau and Kt/VBCM leads to a significant systematic underestimate of Kt/V by 22% and 20.5% respectively. Better agreement between Kt/VDau and Kt/VBCM was observed. The adequate hemodialysis was achieved, according to three methods: Kt/VDau, Kt/VOCM and Kt/VBCM respectively in 100%, 57,1% and 88.6% of the cases.

Conclusions

The Kt/V index is different depending on the method used for its evaluation. The three methods can be used for quantification of hemodialysis with a better agreement between Kt/VDau and Kt/VBCM. In this study, Kt/VOCM results underestimate hemodialysis efficiency. This difference has to be considered when applying quantification of hemodialysis to clinical practice.

The Kt/V by ionic dialysance: Interpretation limits

The availability of hemodialysis machines equipped with online clearance monitoring (OCM) allows frequent assessment of dialysis efficiency and adequacy without the need for blood samples. Accurate estimation of the urea distribution volume (V) is required for Kt/V calculated from OCM to be consistent with conventional blood sample-based methods. A total of 35 patients were studied. Ionic dialysance was measured by conductivity monitoring. The second-generation Daugirdas formula was used to calculate the Kt/V single-pool (Kt/VD). Values of V to allow comparison between OCM and blood-based Kt/V were determined using Watson formula (VWa), bioimpedance spectroscopy (Vimp), and blood-based kinetic data (Vukm). Comparison of Kt/Vw ocm calculated by the ionic dialysance and Vw (Kt/Vw ocm) with Kt/VD shows that using VW leads to significant systematic underestimation of dialysis dose by 24%. Better agreement between Kt/V ocm and Kt/VD was observed when using Vimp and Vukm. Bio-impedancemetry and the indirect method using the second-generation Daugirdas equation are two methods of clinical interest for estimating V to ensure greater agreement between OCM and blood-based Kt/V.

A non-invasive, on-line deuterium dilution technique for the measurement of total body water in haemodialysis patients

BACKGROUND

Despite its importance, total body water (TBW) is usually estimated rather than measured due to the complexity of isotope dilution methods. The aim of this study was to demonstrate the applicability in haemodialysis (HD) patients of a recently developed on-line breath test, previously validated in healthy subjects, that uses the gold standard deuterium dilution method to measure TBW. In particular we wished to show that a pre-dialysis estimation was as good as a post-dialysis equilibrated measurement in order to avoid patients needing to remain behind after dialysis treatment.

METHODS

The dispersal kinetics of breath HDO, measured using a flowing afterglow mass spectrometer (FA-MS) following ingestion of D(2)O immediately post-dialysis, were determined in 12 haemodialysis patients and used to calculate the absolute TBW(PostHD) after full equilibration. TBW(PreHD) was then determined from breath samples taken immediately prior to the next dialysis. This measurement was adjusted for the interdialytic weight change and urine output (TBW(PreHD-adjusted)) and compared to the TBW(PostHD). The accuracy and precision of FA-MS was also assessed using known concentrations of deuterium-enriched water samples.

RESULTS

Mean TBW(PostHD) was 50.0 +/- 9.3 L and TBW(PreHD-adjusted) was 50.7 +/- 9.0 L. They were highly correlated (R = 0.99, P < 0.001) with a CV of 2.6%. The mean difference was +0.74 L (SEM 0.35, 95% CI -0.03 to 1.51 L, P = 0.059), compatible with a daily insensible loss of 0.37 L. Accuracy and precision of FA-MS were comparable to the previous validation work.

CONCLUSIONS

This non-invasive adaptation of the D isotope dilution method for determining TBW can be applied to haemodialysis patients who show deuterium equilibration kinetics identical to normal subjects; a pre-dialysis estimation may be used to determine TBW, and so avoiding the necessity to remain behind after dialysis making this suitable for application in the clinical setting.

Dialysis dose (Kt/V) and clearance variation sensitivity using measurement of ultraviolet-absorbance (on-line), blood urea, dialysate urea and ionic dialysance

BACKGROUND

An on-line monitoring system for dialysis dose calculations could make it possible to provide an adequate dialysis dose that is consistently given to haemodialysis (HD) patients. The aim of this study was to compare dialysis dose (Kt/V) using four different methods and their sensitiveness to a reduction in clearance.

METHODS

Six patients were monitored on-line with ultraviolet (UV)-absorbance at a wavelength of 297 nm in three consecutive dialysis sessions during 1 week. During the last treatment, the clearance was reduced by approximately 25% by decreasing the blood flow. For the determination of UV-absorbance, a spectrophotometer was connected to the fluid outlet of the dialysis machine with all spent dialysate passing through a flow cuvette. The equilibrated Kt/V (eKt/V) estimated by UV-absorbance was compared with eKt/V from the ionic dialysance method using the on-line clearance monitor (OCM) and the appurtenant software dose-calculation tool DCTool (Fresenius Medical Care, Germany), eKt/V calculated from the dialysate-urea slope and with eKt/V from pre- and post-dialysis blood-urea samples as reference.

RESULTS

The study demonstrates that the sensitiveness to clearance reduction is similar in the four methods compared for eKt/V. When the different methods were compared, the mean eKt/V of UV-absorbance was 1.21 +/- 0.20, blood 1.30 +/- 0.21, dialysate 1.32 +/- 0.21 and OCM (using the DCTool) 1.31 +/- 0.21. The standard deviation was of the same magnitude.

CONCLUSION

The UV-method gives a similar response to clearance reduction compared with the other methods when comparing dialysis dose. The high sampling rate by continuous monitoring of UV-absorbance allows evaluation of the clearance process during dialysis and gives immediate feedback to on-line adjustments.

Resistance to intercompartmental mass transfer limits β2-microglobulin removal by post-dilution hemodiafiltration

Although clearance of beta(2)-microglobulin is greater with hemodiafiltration than with high-flux hemodialysis, beta(2)-microglobulin concentrations after long-term hemodiafiltration are only slightly less than those obtained with high-flux hemodialysis. Resistance to beta(2)-microglobulin transfer between body compartments could explain this observation. beta(2)-Microglobulin kinetics were determined in patients receiving on-line post-dilution hemodiafiltration for 4 h with 18 l of filtration. Plasma beta(2)-microglobulin concentrations were measured during and for 2 h following hemodiafiltration and immediately before the next treatment. The filter clearance of beta(2)-microglobulin was determined from arterial and venous concentrations. The beta(2)-microglobulin generation rate was calculated from the change in the plasma concentration between treatments. The intercompartmental clearance was obtained by fitting the observed concentrations to a two-compartment, variable volume model. The plasma clearance of beta(2)-microglobulin by the filter was 73 +/- 2 ml/min. Plasma beta(2)-microglobulin concentrations decreased by 68 +/- 2% from pre- to post-treatment (27.1 +/- 2.2-8.5 +/- 0.7 mg/l), but rebounded by 32+/-3% over the next 90 min. The generation rate of beta(2)-microglobulin was 0.136 +/- 0.008 mg/min. The model fit yielded an intercompartmental clearance of 82 +/- 7 ml/min and a volume of distribution of 10.2 +/- 0.6 l, corresponding to 14.3 +/- 0.7% of body weight. Hemodiafiltration provides a beta(2)-microglobulin clearance of similar magnitude to the intercompartmental clearance within the body. As a result, intercompartmental mass transfer limits beta(2)-microglobulin removal by hemodiafiltration. This finding suggests that alternative strategies, such as increased treatment times or frequency of treatment, are needed to further reduce plasma beta(2)-microglobulin concentrations.

Double target comparison of blood-side methods for measuring the hemodialysis dose

BACKGROUND

Despite the fact that urea kinetic modelling has been successfully applied to quantify the hemodialysis since the beginning of the 1980s, there is not a consensus yet concerning which is the most proper dialysis dose index and the method for calculating it. In this work, we propose that a combined measurement of the dialysis dose from two complementary perspectives of the removal process should provide a more complete description of dialysis than a measurement alone. This hypothesis is reviewed and the measuring methods are compared.

METHODS

A cross-sectional randomized clinical study over 98 stable ESRD patients submitted to thrice-weekly hemodialysis was carried out with the aim of comparing 16 blood-side methods for measuring the hemodialysis dose from patient and dialyzer perspectives. The availability of urea rebound measurements and computational resources have been taken into account.

RESULTS

The outcomes point to four novel blood-side methods as the most accurate for measuring the effective dialysis system Kt/V (mKt/V) in clinical conditions. Their limits of agreement (mean +/- 2.SD) range from 1.93 +/- 2.09% for a non-iterative method without the urea rebound measurement (BUN3) to -0.08 +/- 0.58% for an iterative method with BUN3. The best non-iterative blood-side method for measuring the equilibrated Kt/V (eKt/V) is the second generation formula of Daugirdas (-2.42 +/- 1.05%) when BUN3 is available and the rate equation of Daugirdas and Schneditz (-1.74 +/- 7.91%) when BUN3 is not available. The difference mKt/V-eKt/V is significant and positive, and increases with the dialysis dose in a personalized manner.

CONCLUSION

We have confirmed the arguments that support the hypothesis of the study. The best blood-side methods for the combined measurement of dialysis dose as a function of the available resources have been determined.

Ionic dialysance allows an adequate estimate of urea distribution volume in hemodialysis patients

BACKGROUND

An adequate estimation of urea distribution volume (V) in hemodialysis patients is useful to monitor protein nutrition. Direct dialysis quantification (DDQ) is the gold standard for determining V, but it is impractical for routine use because it requires equilibrated postdialysis plasma water urea concentration. The single pool variable volume urea kinetic model (SPVV-UKM), recommended as a standard by Kidney Disease Outcomes Quality Initiative (K/DOQI), does not need a delayed postdialysis blood sample but it requires a correct estimate of dialyser urea clearance.

METHODS

Ionic dialysance (ID) may accurately estimate dialyzer urea clearance corrected for total recirculation. Using ID as input to SPVV-UKM, correct V values are expected when end-dialysis plasma water urea concentrations are determined in the end-of-session blood sample taken with the blood pump speed reduced to 50 mL/min for two minutes (U(pwt2')). The aim of this study was to determine whether the V values determined by means of SPVV-UKM, ID, and U(pwt2') (V(ID)) are similar to those determined by the "gold standard" DDQ method (V(DDQ)). Eighty-two anuric hemodialysis patients were studied.

RESULTS

V(DDQ) was 26.3 +/- 5.2 L; V(ID) was 26.5 +/- 4.8 L. The (V(ID)-V(DDQ)) difference was 0.2 +/- 1.6 L, which is not statistically significant (P= 0.242). Anthropometric volume (V(A)) calculated using Watson equations was 33.6 +/- 6.0 L. The (V(A)-V(DDQ)) difference was 7.3 +/- 3.3 L, which is statistically significant (P < 0.001).

CONCLUSION

Anthropometric-based V values overestimate urea distribution volume calculated by DDQ and SPVV-UKM. ID allows adequate V values to be determined, and circumvents the problem of delayed postdialysis blood samples.

Quantifying daily hemodialysis

Nearly all published reports and clinical studies of hemodialysis solute kinetics are confined to thrice-weekly dialysis schedules. Over the past 40 years, clinical experience with dialysis treatments given three times per week has expanded enormously, but it was not until the Hemodialysis (HEMO) study results were revealed that nephrologists became fully aware of the limits of usefulness of infrequent dialysis. In light of continued reports of improved quality of life and survival with daily dialysis, it appears that the limits of thrice-weekly dialysis may be extended when treatments are given more often. Analysis of solute kinetics during and between dialyses supports the notion that a more frequent schedule delivers more efficient dialysis and that methods can be developed to allow a comparison of risks among patients treated 3-7 days per week. One such method, based on the concept of solute seclusion, suggests that at the currently established minimum standard dose, approximately 50% of the improvement in solute control afforded by seven treatments per week is achieved by increasing the frequency to four treatments per week. The same model shows that seven treatments per week afford an improvement in solute control that is approximately 80% as effective as continuous dialysis. These conclusions are similar to those derived from a completely different model based on peak concentration toxicity. Neither of these models has been clinically tested, so caution must be advised when treating individual patients.

Urea space and total body water measurements by stable isotopes in patients with acute renal failure

BACKGROUND

Knowledge of urea volume of distribution (Vurea) in patients with acute renal failure (ARF) is critical in order to prescribe and monitor appropriate dialytic treatment. We have recently shown that in ARF patients, Vurea estimation by urea kinetic modeling is significantly higher than total body water (TBW) by anthropometric estimation. However, these estimates of Vurea and TBW have not been validated by isotopic methods, considered as reference measurement standards.

METHODS

In this study, we measured Vurea by [13C]urea and TBW by deuterium oxide (D2O) in 21 patients with ARF (14 males, 7 females, age 62.0 +/- 10.6 years old, 83% Caucasian, 17% African American) at three different centers. These measurements were compared to TBW estimates from anthropometric and bioelectrical impedance (BIA) measurements.

RESULTS

Our results show that Vurea by [13C]urea (51.0 +/- 11.7 L) is significantly higher than TBW estimated by all other methods (TBW by D2O: 38.3 +/- 9.8 L, P < 0.001; TBW by BIA: 45.7 +/- 15.7 L, P= 0.08; TBW by Watson formula: 38.3 +/- 7.3 L, P < 0.001; TBW by Chertow formula: 39.3 +/- 7.8 L, P= 0.002, all versus Vurea). Despite significant overestimation of the absolute value and considerable variation, Vurea significantly correlated with TBW by BIA (r= 0.66, P < 0.01) and TBW by D2O (r= 0.5, P= 0.04). There was also significant correlation between D2O and BIA determined TBW (r= 0.8, P < 0.001).

CONCLUSION

In terms of useful guidelines to prescribe a specific dose of dialysis in patients with ARF, conventional estimates of TBW as surrogates for Vurea should be used with caution. We propose that these conventional estimates of TBW should be increased by approximately 20% (a factor of 1.2) to avoid significant underdialysis.

Prescribing an equilibrated intermittent hemodialysis dose in intensive care unit acute renal failure

BACKGROUND

Prospective, formal, blood-side, urea kinetic modeling (UKM) has yet to be applied in intermittent hemodialysis for acute renal failure (ARF). Methods for prescribing a target, equilibrated Kt/V (eKt/V) are described for this setting.

METHODS

Serial sessions (N= 108) were studied in 28 intensive care unit ARF patients. eKt/V was derived using delayed posthemodialyis urea samples and formal, double-pool UKM (eKt/Vref), and by applying the Daugirdas-Schneditz venous rate equation to pre- and posthemodialysis samples (eKt/Vrate). Individual components of prescribed and delivered dose were compared. Prescribed eKt/V values were determined using in vivo dialyzer clearance estimates and anthropometric (Watson and adjusted Chertow) and modeled urea volumes.

RESULTS

eKt/Vref (mean +/- SD = 0.91 +/- 0.26) was well-approximated by eKt/Vrate (0.92 +/- 0.25), R= 0.92. Modeled V exceeded Watson V by 25%+/- 29% (P < 0.001) and Adjusted Chertow V by 18%+/- 28% (P < 0.001), although the degree of overestimation diminished over time. This difference was influenced by access recirculation (AR) and use of saline flushes. The median % difference between Vdprate and Watson V was reduced to 1% after adjusting for AR for the 22 sessions with < or =1 saline flush. The median coefficients of variation for serial determinations of Adjusted Chertow V, modeled V, urea generation rate, and eKt/Vref were 2.7%, 12.2%, 30.1%, and 16.4%, respectively. Because of comparatively higher modeled urea Vs, delivered eKt/Vref was lower than prescribed eKt/V, based on Watson V or Adjusted Chertow V, by 0.13 and 0.08 Kt/V units. The median absolute errors of prescribed eKt/V vs. delivered therapy (eKt/Vref) were not large and were similar in prescriptions based on the Adjusted Chertow V (0.127) vs. those based on various double-pool modeled urea volumes (approximately 0.127).

CONCLUSION

Equilibrated Kt/V can be derived using formal, double-pool UKM in intensive care unit ARF patients, with the venous rate equation providing a practical alternative. A target eKt/V can be prescribed to within a median absolute error of less than 0.14 Kt/V units using practical prescription algorithms. The causes of the increased apparent volume of urea distribution appear to be multifactorial and deserve further investigation.

Determination of urea distribution volume for Kt/V assessed by conductivity monitoring

BACKGROUND

Kt/V can be calculated continuously during dialysis without blood samples using the ionic dialysance method. Unlike the usual method using blood samples, a precise value for the patients' urea distribution volume is required. This study compared different methods for the determination of urea distribution volume (V) to evaluate their use in Kt/V measurement, based on conductivity monitoring.

METHODS

Ten patients were studied during 40 dialysis sessions. Total body water and V were determined using bioimpedance spectroscopy (BIS), anthropometric data, and blood-based kinetic data. Ionic dialysance was measured by conductivity monitoring.

RESULTS

Total body water measured by bioimpedance was determined as VBIS= 37.0 +/- 7.1 L or 49.6 +/- 4.4% of body weight. V determined using ionic dialysance as input to urea kinetic modeling (UKM) was found to correlate well with total body water (VKecn= 36.4 +/- 5.2 L). All anthropometric equations overestimated measured V: VWatson= 40.7 +/- 3.9 L, VHume= 41.8 +/- 2.5 L, VChertow= 44.6 +/- 3.3 L, and VChumlea= 43.1 +/- 2.9 L. Single-pool Kt/V obtained by kinetic modeling was used as reference (Kt/V)SPVV= 1.49 +/- 0.15. Using different Vs as the V component in the ionic dialysance Kt/V, we obtained: Kecn*t/VWatson= 1.34 +/- 0.12, Kecn*t/VBIS= 1.51 +/- 0.21 and Kecn*t/VKecn= 1.52 +/- 0.18.

CONCLUSION

The single-pool Kt/V calculated using the ionic dialysance method agreed with the conventional blood sample method provided that V was calculated using BIS or urea kinetics. V by either method was reproducible and varied little in an individual patient. Monthly determination of V allows determination of Kt/V for each dialysis session by ionic dialysance.

Anthropometrically estimated total body water volumes are larger than modeled urea volume in chronic hemodialysis patients: effects of age, race, and gender

BACKGROUND

The modeled volume of urea distribution (Vm) in intermittently hemodialyzed patients is often compared with total body water (TBW) volume predicted from population studies of patient anthropometrics (Vant).

METHODS

Using data from the HEMO Study, we compared Vm determined by both blood-side and dialysate-side urea kinetic models with Vant as calculated by the Watson, Hume-Weyers, and Chertow anthropometric equations.

RESULTS

Median levels of dialysate-based Vm and blood-based Vm agreed (43% and 44% of body weight, respectively). These volumes were lower than anthropometric estimates of TBW, which had median values of 52% to 55% of body weight for the three formulas evaluated. The difference between the Watson equation for TBW and modeled urea volume was greater in Caucasians (19%) than in African Americans (13%). Correlations between Vm and Vant determined by each of the three anthropometric estimation equations were similar; but Vant derived from the Watson formula had a slightly higher correlation with Vm. The difference between Vm and the anthropometric formulas was greatest with the Chertow equation, less with the Hume-Weyers formula, and least with the Watson estimate. The age term in the Watson equation for men that adjusts Vant downward with increasing age reduced an age effect on the difference between Vant and Vm in men.

CONCLUSION

The findings show that kinetically derived values for V from blood-side and dialysate-side modeling are similar, and that these modeled urea volumes are lower by a substantial amount than anthropometric estimates of TBW. The higher values for anthropometry-derived TBW in hemodialyzed patients could be due to measurement errors. However, the possibility exists that TBW space is contracted in patients with end-stage renal disease (ESRD) or that the TBW space and the urea distribution space are not identical.

Kinetics and dosing predictions for daily haemofiltration

BACKGROUND

Thrice-weekly haemofiltration affords excellent outcome when it is used to treat chronic renal failure patients. Daily haemofiltration (DHF) has recently been proposed as a more intensive therapy option, but the total ultrafiltration or exchange volume (replacement volume plus net ultrafiltration volume) requirements for adequate solute clearances during this novel therapy are unknown.

METHODS

We calculated theoretical solute kinetic profiles during six times per week DHF for comparison with those during thrice-weekly haemodialysis using a high-flux dialyser (HFHD) or during continuous ambulatory peritoneal dialysis (CAPD). HFHD and CAPD were chosen for comparison because K/DOQI guidelines have defined adequate treatment doses for these therapies. Steady-state concentrations were calculated using a two-compartment model of an anuric patient with 35 l of total body water for five solutes: urea, creatinine, vitamin B(12), inulin and beta(2)-microglobulin. Solute distribution volumes and generation rates were taken from the literature, and excess fluid (1 l/day) was assumed to accumulate in and be removed from the extracellular fluid compartment. Theoretical predictions of solute clearance were compared for a 15-l exchange volume/session during DHF, urea Kt/V of 3.6/week during HFHD and urea Kt/V of 2.0/week during CAPD as solute-specific values of the equivalent renal clearance (EKR) and standard Kt/V (stdKt/V) recently defined by Gotch. Additional comparisons of solute clearances were performed between DHF and other daily therapies including six times per week short daily haemodialysis (SDHD) and six times per week nocturnal haemodialysis (NHD).

RESULTS

The calculated results predict that: (i) urea clearance during DHF with an exchange volume of 90 l/week (6x15 l) is equivalent to those during HFHD and CAPD based on urea stdKt/V; and (ii) middle molecule clearances during DHF exceed those achieved during HFHD and CAPD based on either EKR or stdKt/V. As expected, DHF therapy was inferior regarding the clearance of urea and other small solutes to SDHD and NHD; however, DHF therapy was superior to SDHD regarding the clearance of larger middle molecules, approaching the clearances achieved by NHD.

CONCLUSIONS

We predict that an exchange volume of approximately 40% of total body water (15/35 l=43%) per session will provide adequate clearance of small solutes and substantial clearance of middle molecules during six times per week DHF therapy. These theoretical predictions require clinical validation.

Techniques for assessing and achieving fluid balance in acute renal failure

Fluid therapy, together with attention to oxygen supply, is the cornerstone of resuscitation in all critically ill patients. Hypovolemia results in inadequate blood flow to meet the metabolic requirements of the tissues and must be treated urgently to avoid the complication of progressive organ failure, including acute renal failure. The kidney plays a critical role in body fluid homeostasis. Renal dysfunction disturbs this homeostasis and requires special attention to issues of fluid balance and fluid overload. In addition, fluid therapy is the only treatment that has been shown to be effective in the prevention of acute renal failure. Special attention to volume status is therefore required in patients at risk for acute renal failure. Hypovolemia is also a major causal factor of morbidity during hemodialysis and may contribute to further renal insults. Although the importance of fluid management is generally recognized, the choice of fluid, the amount, and assessment of fluid status are controversial. As the choice of fluids becomes wider and monitoring devices become more sophisticated, the controversy increases. This article provides an overview of the concept of fluid management in the critically ill patient with acute renal failure.

Computer simulation of small-solute and middle-molecule removal during short daily and long thrice-weekly hemodialysis

BACKGROUND

More intensive hemodialysis (HD) regimens (short daily and long thrice-weekly HD) provide potential opportunities for improved patient outcome. An adequate dialysis dose for these regimens cannot be established from the existing literature.

METHODS

Using computer simulation, we compared conventional HD with short daily HD and long thrice-weekly HD using two dose measures of solute clearance: equivalent renal clearance (EKR) and a generalized standard Kt/V (stdKt/V) for urea, creatinine, vitamin B12, inulin, and beta2-microglobulin. Solute kinetics were simulated using a variable-volume two-compartment mathematical model.

RESULTS

Calculated EKR values were greater during short daily HD compared with those during conventional HD by 16.9%, 15.5%, 16.1%, 5.2%, and 2.5% for urea, creatinine, vitamin B12, inulin, and beta(2)-microglobulin, respectively. Calculated stdKt/V values predicted more substantial increases in dose for all solutes. Increasing the time of dialysis from 4 to 8 hours three times weekly resulted in substantially greater stdKt/V and EKR values compared with both conventional and short daily HD. Solute clearances during short daily HD could be enhanced to approach those during long HD if treatment time was increased or very high surface area dialyzers were used with very high blood flow rates.

CONCLUSION

Dose measures for all molecules larger than urea increase with either increased frequency or, even more so, increased duration of dialysis. Prediction results of these models require confirmation in clinical studies. Furthermore, the relationship between increased dialysis dose and long-term clinical outcome during more intensive HD regimens requires examination in clinical trials.