Single-pool Urea Kinetic Model Research Articles

Does subjective assessment of dialyzer appearance reflect dialyzer performance in online hemodiafiltration?

ABSTRACT Introduction: In post‐dilution online hemodiafiltration, a very thin balance subsists in preventing coagulation of the extracorporeal circuit (ECC) during treatment and bleeding in the patient, concerning dialyzer status and anticoagulation dose.The aim of this study was to assess whether there are clinical outcome differences between the visual aspect of the dialyzer's status in terms of clotted fibers at end of dialysis treatments, single‐pool urea kinetic modeling (spKt/V) and substitution volume (SubsVol). Methods: It is a multicenter, descriptive‐correlational study, involving 2829 patients during April 2016. Previous training was given to the Nursing staff to evaluate and classify both the dialyzer's and the venous chamber's appearance of the ECC venous line. Registration was performed at bedside immediately after the patient disconnection. Findings and discussion: Mean age was 68.96 years (SD = 13.75), 60.8% were men. The average hematocrit was 33.91% (SD = 3.45%). The average dry weight was 68.53 kg (SD = 13.27 kg). Mean unfractioned heparin (UFH) dose was 58.13 IU/kg. Only 32.4% of the patients had a clean dialyzer at the end of treatment. 19.4% of patients finished the treatment with more than 10% of clotted fibers. Patients with no residual blood (clean, 32.4%) presented a higher UFH dose (66.32 IU/kg) compared to overall average dose. UFH dose had a significant effect on dialyzer status. There were significant differences in average of spKt/V and SubsVol between the category clean and the other categories of dialyzer's status.Evaluating the dialyzer status represents an excellent opportunity to help the physicians to establish an ideal heparin dose. Only the category clean is significant to achieve the target. The nursing staff, by classifying the ECC appearance at patient's bedside and recording it in a centralized database, can be a major contributor to achieve an individualized and optimal UFH dose and subsequently better patient outcomes.

A Multidisciplinary Approach for the Management of Severe Hyponatremia in Patients Requiring Continuous Renal Replacement Therapy

IntroductionHyponatremia is a common electrolyte disorder in critically ill patients. Rapid correction of chronic hyponatremia may lead to osmotic demyelination syndrome. Management of severe hyponatremia in patients with acute or chronic kidney disease who require continuous renal replacement therapy (CRRT) is limited by the lack of commercially available hypotonic dialysate or replacement fluid solutions.MethodsThis was a single-center quality improvement project that consisted of the development and implementation of a multidisciplinary protocol for gradual correction of severe hyponatremia in patients who were admitted to the intensive care unit (ICU) and required CRRT. The protocol utilized a simplified method based on single-pool urea kinetic modeling and a hybrid technique of volume exchange, and addition of sterile water for sodium dilution of commercially available dialysate and replacement fluid solutions.ResultsWe report data of the first 3 ICU patients who required CRRT for acute kidney injury management, had severe hyponatremia (serum sodium <120 mEq/l), and were treated under the protocol. Targeted and gradual hyponatremia correction was achieved in all 3 patients. The observed versus the predicted serum sodium correction in each patient was concordant. No complications related to the protocol were reported. We detailed facilitators of and hindrances to the development and successful implementation of our multidisciplinary protocol.ConclusionWe demonstrated gradual and individualized rates of severe hyponatremia correction utilizing customized (sodium dilution) dialysate/replacement fluid solutions in ICU patients who required CRRT. It is not known whether the use of customized solutions to prevent hyponatremia overcorrection has a significant impact on patient outcomes. Further research in this susceptible population is needed.

Errors in Computing the Normalized Protein Catabolic Rate due to Use of Single-pool Urea Kinetic Modeling or to Omission of the Residual Kidney Urea Clearance

The protein catabolic rate normalized to body size (PCRn) often is computed in dialysis units to obtain information about protein ingestion. However, errors can manifest when inappropriate modeling methods are used. We used a variable volume 2-pool urea kinetic model to examine the percent errors in PCRn due to use of a 1-pool urea kinetic model or after omission of residual urea clearance (Kru). When a single-pool model was used, 2 sources of errors were identified. The first, dependent on the ratio of dialyzer urea clearance to urea distribution volume (K/V), resulted in a 7% inflation of the PCRn when K/V was in the range of 6mL/min per L. A second, larger error appeared when Kt/V values were below 1.0 and was related to underestimation of urea distribution volume (due to overestimation of effective clearance) by the single-pool model. A previously reported prediction equation for PCRn was valid, but data suggest that it should be modified using 2-pool eKt/V and V coefficients instead of single-pool values. A third source of error, this one unrelated to use of a single-pool model, namely omission of Kru, was shown to result in an underestimation of PCRn, such that each ml/minute Kru per 35L of V caused a 5.6% underestimate in PCRn. Marked overestimation of PCRn can result due to inappropriate use of a single-pool urea kinetic model, particularly when Kt/V <1.0 (as in short daily dialysis), or after omission of residual native kidney clearance.

Application of Cystatin C Reduction Ratio to High-Flux Hemodialysis as an Alternative Indicator of the Clearance of Middle Molecules

Background/AimsAlthough high-flux (HF) dialyzers with enhanced membrane permeability are widely used in current hemodialysis (HD) practice, urea kinetic modeling is still being applied to indicate the adequacy of both low-flux (LF) and HF HD. In comparison with urea (molecular weight, 60 Da) and β2-microglobulin (β2MG, 12 kDa), cystatin C (CyC, 13 kDa) is a larger molecule that has attractive features as a marker for assessing solute clearance. We postulated that CyC might be an alternative for indicating the clearance of middle molecules (MMs), especially with HF HD.MethodsEighty-nine patients were divided into LF and HF groups. Using single pool urea kinetic modeling, the urea reduction ratio (URR) and equilibrated Kt/Vurea (eKt/Vurea) were calculated. The serum CyC concentrations were measured using particle-enhanced immunonephelometry. As indices of the middle molecular clearance, the reduction ratios of β2MG and CyC were calculated.ResultsThe β2MG reduction ratio (β2MGRR) and CyC reduction ratio (CyCRR) were higher in the HF group compared to the LF group. However, the URR and eKt/Vurea did not differ between the two groups. The CyCRR was significantly correlated with the eKt/Vurea and β2MGRR (r = 0.47 and 0.69, respectively, both p < 0.0001).ConclusionsCompared to the LF dialyzer, the HF dialyzer removed CyC and β2MG more efficiently. Unlike the β2MGRR, the CyCRR was correlated with the eKt/Vurea and β2MGRR. This study suggests a role for the CyCRR as an alternative indicator of the removal of MMs.

Dialysis dose and nutrition assessment by optical on-line dialysis adequacy monitor

In light of the variability of dialysis sessions, on-line monitoring could improve hemodialysis (HD) adequacy. A new optical Dialysis Adequacy Monitor (DIAMON) prototype enables to estimate dialysis dose and protein nitrogen appearance (PNA) at every dialysis session. The aim of this study was to compare the adequacy of dialysis treatment and the patient's nutritional status by pre-and post-dialysis blood samples, the DIAMON prototype and Total Dialysate Collection (TDC). Ten patients were monitored during three consecutive hemodialysis sessions during one week. Blood samples were drawn before the start of dialysis and at the end of dialysis. The DIAMON prototype was connected to the fluid outlet of the dialysis machine with all spent dialysate passing through during the on-line experiments, and TDC was performed for all dialysis treatments. Equilibrated Kt/V (eKt/V) values were estimated from blood-urea (eKt/Vb) and from DIAMON (eKt/Va), and normalized PNA (nPNA) values from TDC and DIAMON, respectively. The variable volume single pool urea kinetic modeling (VVSP UKM) was also utilized for single-pool Kt/V (spKt/V) and nPNA estimation. The mean +/- SD given by eKt/Vb was 1.08 +/- 0.22 (n = 30), and eKt/Va 1.05 +/- 0.21 (n = 28). The mean +/- SD of nPNA was 0.73 +/- 0.15 g/kg/day (n = 29) from TDC, and 0.73 +/- 0.14 g/kg/day (n = 28) using DIAMON prototype. The mean values of eKt/V from blood samples and nPNA from TDC were not statistically different from the corresponding values estimated by DIAMON (p < 0.05). Generally the delivered dialysis dose and dietary protein intake of the patients observed during the study using the DIAMON prototype was very similar to that obtained by TDC and VVSP UKM. The optical dialysis adequacy sensor, DIAMON, provides continuous, on-line measurements of dialysis adequacy and permits longitudinal analysis of the delivered dialysis dose and patient's nutritional status, and can immediately identify, and alert to, any deviations in dialysis treatment.

Reduction in urea distribution volume over time in clinically stable dialysis patients

We have previously shown that, assuming urea distribution volume (V) remains constant for 1 month, ionic dialysance (ID) allows the dialysis dose to be calculated without the need for blood sampling. The aim of this multicenter study was to verify whether the assumption of a constant V can be extended to 1 year. In clinically stable patients receiving thrice-weekly hemodialysis at 13 dialysis centers, V and Kt/V were assessed during three dialysis sessions at baseline and 1 year later using ID as dialyzer urea clearance and the single-pool urea kinetic model. Baseline albumin, hemoglobin, and C reactive protein were prespecified covariates for predicting the change in V over time. Of the 52 enrolled patients, 40 (25 males; age 63.0+/-13.5 years) completed the study. Baseline end-dialysis body weight (62.4+/-13.7 kg) showed a non-significant 1% reduction during follow-up (-0.6+/-2.8 kg; P=0.175), whereas V significantly decreased from 29.0+/-6.8 to 27.4+/-6.0 l (-1.6+/-3.0 l or 4.5%; P=0.002). The reduction in V was greater when baseline albumin was lower (P=0.001) and baseline V was higher (P=0.005). The single-pool K(t)/V calculated using baseline V underestimated the actual value by 0.07+/-0.16 (P=0.008). The slight underestimate of Kt/V during follow-up suggests that annual V evaluations may be sufficient for dialysis dose quantification as the only risk is underestimating the actually delivered dialysis dose. However, the relationship between baseline albumin and the reduction in V over time may have nutritional value, and suggests more frequent V evaluations.

Do different haemodialysis regimen interfere with genomic damage in patients with end-stage-renal disease?

Continuous renal replacement therapies have practical and theoretical advantages compared with conventional intermittent hemodialysis in hemodynamically unstable or severely catabolic patients with acute renal failure (ARF). Sustained low-efficiency dialysis (SLED) is a hybrid modality introduced July 1998 at the University of Arkansas for Medical Sciences that involves the application of a conventional hemodialysis machine with reduced dialysate and blood flow rates for 12-hour nocturnal treatments. Nine critically ill patients with ARF were studied during a single SLED treatment to determine delivered dialysis dose and the most appropriate model for the description of urea kinetics during treatment. Five patients were men, mean Acute Physiology and Chronic Health Evaluation (APACHE) II score was 28.9 and mean weight was 92.5 kg. Kt/V was determined by the reference method of direct dialysate quantification (DDQ) combined with an equilibrated postdialysis plasma water urea nitrogen (PUN) concentration and four other methods that were either blood or dialysate based, single or double pool, or model independent (whole-body kinetic method). Solute removal indices (SRIs) were determined from net urea removal and urea distribution volume supplied from DDQ (reference method) and by mass balance using variables supplied from blood-based formal variable-volume single-pool (VVSP) urea kinetic modeling. Equivalent renal urea clearances (EKRs) were calculated from urea generation rates and time-averaged concentrations for PUN based on weekly mass balance with kinetic variables supplied by either DDQ (reference method) or formal blood-based VVSP modeling. Mean Kt/V determined by the reference method was 1.40 and not significantly different when determined by formal VVSP modeling, DDQ using an immediate postdialysis PUN, or the whole-body kinetic method. Correction of single-pool Kt/V by a Daugirdas rate equation did not yield plausible results. Mean SRI and EKR by the reference methods were 0.61 and 24.8 mL/min, respectively, and not significantly different when determined by blood-based methods. A single-pool urea kinetic model adequately described intradialytic PUN profiles, indicating that SLED was associated with minimal urea disequilibrium. This was supported by the parity between hemodialyzer and whole-body urea clearances, and the mean postdialytic urea rebound of 4.1% (P = 0.13 versus zero). Additional prospective studies are needed in this setting to define the optimal method for dialysis quantification, targets for azotemic control, and optimal modality of renal replacement therapy. In conclusion, SLED delivers a high dose of dialysis with minimal associated urea disequilibrium and can be quantified by Kt/V, SRI, and EKR from blood-based methods using single-pool urea kinetic models. © 2002 by the National Kidney Foundation, Inc.

A novel mathematical method based on urea kinetic modeling for computing the dialysis dose

A novel normalized single pool urea kinetic model (nspUKM) for the quantification of the urea removal, dialyzer urea clearance and urea generation rate during a dialysis session, is presented. Its major goal is the computation of an accurate estimate of the fractional dialyzer urea clearance (d Kt/ V), which is denoted n Kt/ V, in contrast to the equilibrated Kt/ V (e Kt/ V). This work clarifies the significance of d Kt/ V as a complement to e Kt/ V in hemodialysis (HD) prescription and quantification. This new model emerges from a generalization of the standard single pool urea kinetic model (spUKM) of the US National Cooperative Dialysis Study (NCDS), identified as gspUKM. Due to their significance, the standard single pool Kt/ V (sp Kt/ V) and the e Kt/ V are also analyzed from gspUKM in this work, with the aim of achieving a better interpretation of the results. Indices n Kt/ V, e Kt/ V and sp Kt/ V have been compared with the d Kt/ V computed from a published and validated two-pool urea kinetic model (2pUKM). We present the results obtained from a clinical study carried out on a group of 30 end stage renal disease (ESRD) patients. The limits of agreement (mean±2S.D. (standard deviation) of the difference) between n Kt/ V and 2p Kt/ V were −0.077±0.72% (percentage of the d Kt/ V mean), while between e Kt/ V and 2p Kt/ V were −13.75±17.39% and between sp Kt/ V and 2p Kt/ V were −1.61±6.54%. These scores prove that the nspUKM model is able to provide a very accurate estimate of 2p Kt/ V and thus d Kt/ V, even with high flux (HF) HD. The presented method joins the simplicity of single-pool models to the accuracy of double-pool models, when the target is the identification of the dialyzer urea clearance, urea removal and urea generation rate, although it does not provide a good prediction of the urea dynamics. Finally, we think that our analytical and experimental findings throw light on the behavior and applicability of the different Kt/ V indices analyzed.

Urea kinetics during sustained low-efficiency dialysis in critically ill patients requiring renal replacement therapy

Continuous renal replacement therapies have practical and theoretical advantages compared with conventional intermittent hemodialysis in hemodynamically unstable or severely catabolic patients with acute renal failure (ARF). Sustained low-efficiency dialysis (SLED) is a hybrid modality introduced July 1998 at the University of Arkansas for Medical Sciences that involves the application of a conventional hemodialysis machine with reduced dialysate and blood flow rates for 12-hour nocturnal treatments. Nine critically ill patients with ARF were studied during a single SLED treatment to determine delivered dialysis dose and the most appropriate model for the description of urea kinetics during treatment. Five patients were men, mean Acute Physiology and Chronic Health Evaluation (APACHE) II score was 28.9 and mean weight was 92.5 kg. Kt/V was determined by the reference method of direct dialysate quantification (DDQ) combined with an equilibrated postdialysis plasma water urea nitrogen (PUN) concentration and four other methods that were either blood or dialysate based, single or double pool, or model independent (whole-body kinetic method). Solute removal indices (SRIs) were determined from net urea removal and urea distribution volume supplied from DDQ (reference method) and by mass balance using variables supplied from blood-based formal variable-volume single-pool (VVSP) urea kinetic modeling. Equivalent renal urea clearances (EKRs) were calculated from urea generation rates and time-averaged concentrations for PUN based on weekly mass balance with kinetic variables supplied by either DDQ (reference method) or formal blood-based VVSP modeling. Mean Kt/V determined by the reference method was 1.40 and not significantly different when determined by formal VVSP modeling, DDQ using an immediate postdialysis PUN, or the whole-body kinetic method. Correction of single-pool Kt/V by a Daugirdas rate equation did not yield plausible results. Mean SRI and EKR by the reference methods were 0.61 and 24.8 mL/min, respectively, and not significantly different when determined by blood-based methods. A single-pool urea kinetic model adequately described intradialytic PUN profiles, indicating that SLED was associated with minimal urea disequilibrium. This was supported by the parity between hemodialyzer and whole-body urea clearances, and the mean postdialytic urea rebound of 4.1% (P = 0.13 versus zero). Additional prospective studies are needed in this setting to define the optimal method for dialysis quantification, targets for azotemic control, and optimal modality of renal replacement therapy. In conclusion, SLED delivers a high dose of dialysis with minimal associated urea disequilibrium and can be quantified by Kt/V, SRI, and EKR from blood-based methods using single-pool urea kinetic models.

Effects of Postdialysis Urea Rebound on the Quantification of Pediatric Hemodialysis

Urea rebound (UR) causes single pool urea kinetic modeling (UKM), which is based on end-dialysis urea instead of its equilibrated value (Ceq), to erroneously quantify hemodialysis (HD) treatment. We estimated the impact of postdialysis UR on the results of formal variable volume single pool (VVSP) UKM [Kt/V, urea distribution volume (V), urea generation rate (G), normalized protein catabolic rate (nPCR), and urea reduction ratio (URR)] in children on chronic HD. Thirty-eight standard pediatric HD sessions in 15 stable patients (9 female, 6 male) aged 14.5 ± (SD) 3.28 years were investigated. The HD sessions lasted 3.75 ± 0.43 h. The single pool urea clearance was 4.84 ± 1.25 ml/min/kg. All HD sessions were evaluated by VVSP and URR (%) with postdialysis urea taken at the end of HD and with Ceq taken 60 min after the end of HD, incorporating double pool effects and representing true double pool values. The anthropometric V was calculated by Cheek and Mellits formulae for children. VVSP significantly overestimated Kt/V by 0.26 ± 0.18 U (1.68 ± 0.36 vs. 1.42 ± 0.30, p < 0.0001), i.e., 19.05 ± 13.07%, G/V (0.20 ± 0.04 vs. 0.18 ± 0.04, p < 0.0001), nPCR (1.26 ± 0.23 vs. 1.18 ± 0.22 g/kg/day, p < 0.0001), and URR (73.92 ± 6.49 vs. 69.22 ± 7.06, p < 0.0001). VVSP significantly underestimated kinetic V in comparison to anthropometric V (18.74 ± 4.04 vs. 20.76 ± 4.43 liters or expressed as V/body weight: 58 ± 8 vs. 65 ± 9%, p < 0.05), while double pool kinetic V was more accurate (21.45 ± 4.34 liters, V/body weight: 64 ± 6%, p > 0.05). We conclude that UR has a significant effect on all results of UKM even after standard pediatric HD, and the degree of this efffect is documented. We suggest an increase of the minimum required prescribed single pool Kt/V in children and reduction of any delivered single pool Kt/V by approxiamtely 0.26 Kt/V U. Overestimation of nPCR by approximately 0.08 g/kg/day and underestimation of V by 8.5% should be kept in mind.

Blood Urea Levels 30 Minutes Before the End of Dialysis Are Equivalent to Equilibrated Blood Urea

The steady decline in blood urea during high efficiency hemodialysis is followed by a rebound phase after dialysis in which the level of urea rises to an equilibrium value (Ct + 30) that may be up to 20% higher than the immediate post dialysis (Ct) concentration. The artificially low urea concentration immediately after dialysis leads to an overestimate of the efficiency of the dialysis calculated by Kt/V if the true equilibrium blood concentration of urea is not used in the calculation by the single-pool urea kinetic model. The measurement of equilibrium urea concentration requires a blood sample approximately 30 min after hemodialysis, which is an encumbrance on dialysis patients. This study was undertaken to determine whether an intradialytic sample taken 30 min before the end of dialysis (Ct - 30) may be representative of the equilibrium sample, and to compare the Kt/V using the Ct - 30 and Ct + 30 samples. Thirty-six patients were studied and blood urea concentrations were measured half an hour before the end of dialysis (Ct - 30), at the end of dialysis (Ct), and half an hour after the end of dialysis (Ct + 30). Kt/V (Daugirdas method) was calculated using urea concentration 30 min before the end of dialysis (Kt/Vt - 30) and was compared with Kt/V calculated using equilibrium urea concentration (Kt/Vt + 30). There were no significant differences between the Kt/Vt - 30 and the KtVt + 30 (1.25 versus 1.22, p = 0.65). The correlation between Kt/Vt - 30 and Kt/Vt + 30 was excellent with r2 = 0.93, regression y = 1.05 x -0.033. Kt/Vt - 30 also compared favorably with the Kt/V double pool method (Kt/Vdp) described by Daugirdas (1.25 versus 1.19, p = 0.23). Using the Ct - 30 to calculate Kt/V by the percent urea reduction methods of jindal (Kt/Vpru) decreases the Kt/V value by 0.14 on average, but it remains significantly higher than the Daugirdas method. The authors conclude that calculations using urea concentration 30 min before the end of dialysis improves the accuracy of dose estimation in high efficiency dialysis, without inconveniencing the patient.

Effects of Different Sampling Methods for Measurement of Post Dialysis Blood Urea Nitrogen on Urea Kinetic Modeling Derived Parameters in Patients Undergoing Long-Term Hemodialysis

A simple and reliable sampling method for measuring post-dialysis blood urea nitrogen (C2) has not been well established. Therefore, the effects of different methods of sampling C2 on calculating urea kinetic modeling (UKM) derived parameters were studied in patients on hemodialysis. At first, C2 values were sampled at the end of hemodialysis at a blood flow rate of 300 ml per min (blood flow rate (Qb) 300 ml per min). They were then divided into two groups. In group 1 (n = 11), C2 samples were taken after slowing down Qb to 100 ml per min for 3 min (C2-100); the blood pump was then stopped for 1 min, and C2 was again sampled (C2-100-stop). Finally, C2 was sampled with Qb restored to 100 ml per min and the venous line clamped for another 12 sec (C2-100-clamp). In group 2 (n = 11), C2 samples were collected after Qb was slowed down to 50 ml per min for 3 min (C2-50), and C2-50-stop was then measured. Finally, C2-50-clamp was measured. Using C2-clamp as a reference standard, UKM parameters were calculated using a variable volume, single pool UKM. The authors found that Kt/V calculated by C2-300, C2-100, and C2-50 overestimated the reference Kt/V by 8.4-9.1%, 5.1%, and 3%, respectively. In contrast, protein catabolic rate and time-averaged BUN were affected only minimally. Therefore, a low flow technique with a Qb of 50 ml per min for 3 min can be used for the routine estimation of C2 and UKM-derived parameters with reasonable accuracy.

Estimation of treatment dose in high-efficiency haemodialysis.

Blood urea concentration is artificially low immediately following high-efficiency dialysis of duration T and will rise ('rebound') due to the continued diffusion of urea from the intracellular to the extracellular space. This leads to an overestimate of the efficiency of the dialysis given by KT/V (where V is the total distribution volume of urea and K is the urea clearance of the dialyser) if the true equilibrium blood concentration of urea is not used in the calculation of KT/V by the single-pool urea kinetic model (UKM). The measurement of the equilibrium urea concentration entails an additional blood sample 60 min after dialysis, but an estimate may be calculated using a blood sample taken 80 min following the start of dialysis together with an approximate formula derived from the 2-pool model. In 14 patients, an average error in the calculation of KT/V of 35% (range 19-75%) by the single-pool UKM was reduced to 13% (range 1-55%, but 8 measurements to < 7%) using the approximate technique. It is concluded that the approximate technique significantly improves the accuracy of dose estimation in high-efficiency dialysis without inconveniencing the patient.

An analysis of the single-pool urea kinetic model and estimation of errors

The single pool urea kinetic model assumes that urea is distributed within the body in a volume Vf at a uniform concentration. It may be used to describe the clearance of urea during and following haemodialysis, and to determine the value of the urea concentration at the end of dialysis Cf or the urea generation rate G, and the urea distribution volume Vf. The protein catabolic rate (PCR) is obtained using the ratio G/Vf. The sensitivity of the predictions of the model to small errors in the experimentally estimated model parameters is evaluated and suggests that the model prediction of Cf is relatively insensitive to errors in the estimate of G, but more sensitive to errors in Vf and K. The determination of G from urea concentration measurements made during dialysis alone is very sensitive to errors.

Mathematical modelling of haemodialysis in children.

The single-pool urea kinetic model (UKM), utilising "Kt/V" (the normalised whole body urea clearance), is widely used to help assess the adequacy of haemodialysis in adults. In the presence of an adequate dietary protein intake, a value of unity is acceptable for thrice weekly dialysis. Children could benefit from this approach but, with their relatively higher protein intakes and dialysis needs, this model may not be applicable. Urea kinetics, studies in six children with chronic renal failure by serial timed blood urea measurements during and after haemodialysis, were compared with the kinetics of a one-pool and a two-pool UKM. The two-pool UKM with intra- and extracellular pools best fitted the observed data, re-equilibration between pools accounting for the marked rebound increase in blood urea seen in the 1st h after dialysis (mu 17%, SD 5). Kt/V calculated using the end-dialysis blood urea was higher (mu 21%, SD 5) than when the more correct equilibrated value was used. The post-dialysis rebound indicates significant disequilibrium between the two pools at the end of dialysis. Dialysis efficiency may be substantially overestimated unless this is allowed for by using the rebounded post-dialysis blood urea when calculating Kt/V.

A Modified Algorithm of the Single Pool Urea Kinetic Model

Urea Kt/V, calculated according to the variable volume single pool urea kinetic model (UKM), has been accepted as the yardstick reflecting the adequacy of haemodialysis therapy. However, the classical algorithm of UKM requires great care in dialyser urea clearance (K) measurement in order to avoid major inaccuracies in estimating the urea distribution volume (V). Thus, we suggest a modified algorithm of UKM which avoids the measurement of K. It assumes an arbitrary V value and then calculates kinetically K as a function of the assumed V value. The rationale of the modified algorithm can be derived from the knowledge that the classical algorithm imposes a proportionality ration between K and V: given a particular set of data, a change in the attributed value of K leads to a proportional change in the calculated V value, so that the ratio K/V remains nearly constant. Aims of the study were (1) to validate the modified algorithm by comparing the resulting Kt/V and normalised protein catabolic rate (NPCR) values with the homologous ones obtained using the classical algorithm in a group of 33 patients on thrice-weekly haemodialysis; plasma water urea concentrations were used with the classical algorithm (CApw) and the modified algorithm (MApw); and (2) to verify the possibility of using plasma urea concentrations with the modified algorithm (MAp) instead of the more rigorous plasma water concentrations. NPCR (g/kg per day) was 1.33 +/- 0.05 in CApw, 1.29 +/- 0.05 in MApw and 1.28 +/- 0.04 in MAp. Kt/V was 1.27 +/- 0.03 in CApw, 1.25 +/- 0.03 in MApw and 1.26 +/- 0.03 in MAp.(ABSTRACT TRUNCATED AT 250 WORDS)

Transcellular Disequilibrium and Intradialytic Catabolism Reduce the Reliability of Urea Kinetic Formulas

To elucidate the limits of single-pool models as regard aberrations induced by urea transcompartmental disequilibrium during urea kinetics, mass balances and fractional clearances were studied, and original formulas developed to calculate urea clearance and distribution volume. In the early dialytic phases, aberrations were more evident, with low kinetic volume (VK) values and kinetic clearance (KK) values double those obtained by dialysate collection (KDC). Over the whole session, both VDC and VK were underestimated (9.9 and 8.2%), compared with anthropometric data (VA). In 3 patients, the comparison of VA and VDC, and the behavior of effective body water clearance (KE), agreed with the hypothesis of dialysis-induced catabolism. Both disequilibrium and hypercatabolism can affect the reliability of the single-pool urea kinetic model; because of overlap, their effects are difficult to separate, and became particularly important in high-efficiency dialysis. A modified model, using V and CtET as input is suggested in order to establish the most appropriate dialysis prescription for uremia therapy.

Transcellular disequilibrium and intradialytic catabolism reduce the reliability of urea kinetic formulas.

To elucidate the limits of single-pool models as regard aberrations induced by urea transcompartmental disequilibrium during urea kinetics, mass balances and fractional clearances were studied, and original formulas developed to calculate urea clearance and distribution volume. In the early dialytic phases, aberrations were more evident, with low kinetic volume (VK) values and kinetic clearance (KK) values double those obtained by dialysate collection (KDC). Over the whole session, both VDC and VK were underestimated (9.9 and 8.2%), compared with anthropometric data (VA). In 3 patients, the comparison of VA and VDC, and the behavior of effective body water clearance (KE), agreed with the hypothesis of dialysis-induced catabolism. Both disequilibrium and hypercatabolism can affect the reliability of the single-pool urea kinetic model; because of overlap, their effects are difficult to separate, and became particularly important in high-efficiency dialysis. A modified model, using V and CtET as input is suggested in order to establish the most appropriate dialysis prescription for uremia therapy.