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

Bioimpedance as a measure of fluid status in critically ill patients: A systematic review

OBJECTIVE

Precise measurements of fluid status lack valid methods. Bioimpedance is an attractive diagnostic tool because it is noninvasive, quick, and relatively cheap. This systematic review aims to assess the existing evidence of bioimpedance as an accurate measure of fluid status in critically ill patients.

DATA SOURCES

PubMed and Embase up till March 2021 were systematically searched (PROSPERO: CRD42020157436).

STUDY SELECTION

Eligibility criteria were studies reporting original data from cohorts of adult patients in intensive care units and doing at least one whole-body bioimpedance and one reference test. In addition, studies assessing internal reproducibility were included.

DATA EXTRACTION

An extraction form was designed for the purpose.

DATA SYNTHESIS

Nine hundred five studies were screened for eligibility, and 28 studies, comprising 1482 individual patients, were included in the final analysis. Eight studies compared bioimpedance with a gold standard, and two of those reported the results adequate. We found a low mean difference, but the 95% limits of agreements had wide limits. The remaining studies applied different surrogates as reference tests. Correlations ranged from 0.05 to 0.99. The Grading of Recommendations Assessment, Development and Evaluation (GRADE) certainty of evidence for all outcomes was very low.

CONCLUSIONS

The accuracy of bioimpedance as a measure for fluids in critically ill patients in the intensive care unit cannot be determined. Due to the lack of a gold standard, numerous studies compared bioimpedance with surrogate outcomes with great variability in both designs and results. Assessing the internal reproducibility of bioimpedance had the same limitations, but the studies overall reported good internal reproducibility.

Solutes removal characteristics at various effluent rates during different continuous renal replacement therapy modalities

Background:

Some studies suggest the effluent as a surrogate solute removal indicator in continuous hemodialysis or hemofiltration, but the delivered clearance is frequently smaller than prescribed. This study aims at testing whether the effluent, represented by mL/kg/h, could measure solute clearance and whether increasing effluent increases clearance proportionately in continuous hemodialysis or hemofiltration.

Methods:

Patients treated with continuous renal replacement therapy for various diagnoses were included. The range of dialysate flow rate or substitution fluid flow rate was 1–5 L/h; solutes in the effluent and in the plasma entering the filter were measured, and the ratio of solutes in the effluent and in the plasma entering the filter and the clearance of blood urea nitrogen, creatinine, phosphate, and β2-microglobulin were calculated.

Results:

The ratio of solutes in the effluent and in the plasma entering the filter showed a decreasing trend with increased dialysate flow rate or substitution fluid flow rate ( p  < 0.05), but solute clearance showed an increasing trend. The increase in solute clearance was less than expected from the increased effluent ( p < 0.01), and actual delivered clearance was always below the corresponding prescribed clearance ( p < 0.001).

Conclusion:

With increasing prescribed clearance of continuous renal replacement therapy, effluent rate overestimated the delivered clearance.

Renal replacement therapy for AKI: When? How much? When to stop?

Severe acute kidney injury (AKI) requiring renal replacement therapy (RRT) is a serious clinical disorder in the intensive care unit (ICU), occurring in a significant proportion of critically ill patients. However, many questions remain about the optimal administration of RRT with regard to several important considerations, including treatment dose, timing of treatment initiation and cessation, therapy mode, type of anticoagulation, and management of fluid overload. While Level 1 evidence exists for RRT dosing in AKI, all the studies contributing to this evidence base employed fixed-dose regimens throughout a patient's continuous RRT (CRRT) course, without regard for the possibility of individualizing treatment dose according to the clinical status of a given patient at a specific time. As opposed to CRRT dose, no consensus about the timing of RRT in critically ill AKI patients exists currently. While numerous clinical trials over the past 40 years have attempted to assess "early" versus "late" initiation of RRT, they have been plagued by a myriad of methodological problems, including their largely observational nature and the widely varying definitions of early and late initiation. Although questions about the appropriate timing of CRRT discontinuation arise very frequently in clinical practice, even less information is available in the literature to guide this important decision. The aim of this review is to provide a comprehensive update on RRT delivery to critically ill AKI patients, with specific attention paid to treatment dose and timing and emphasis on addressing the practical questions that arise in daily clinical practice.

Quantification and Dosing of Renal Replacement Therapy in Acute Kidney Injury: A Reappraisal

BACKGROUND/AIMS

Delivered dialysis therapy is routinely measured in the management of patients with end-stage renal disease; yet, the quantification of renal replacement prescription and delivery in acute kidney injury (AKI) is less established. While continuous renal replacement therapy (CRRT) is widely understood to have greater solute clearance capabilities relative to intermittent therapies, neither urea nor any other solute is specifically employed for CRRT dose assessments in clinical practice at present. Instead, the normalized effluent rate is the gold standard for CRRT dosing, although this parameter does not provide an accurate estimation of actual solute clearance for different modalities.

METHODS

Because this situation has created confusion among clinicians, we reappraise dose prescription and delivery for CRRT.

RESULTS

A critical review of RRT quantification in AKI is provided.

CONCLUSION

We propose an adaptation of a maintenance dialysis parameter (standard Kt/V) as a benchmark to supplement effluent-based dosing of CRRT. Video Journal Club "Cappuccino with Claudio Ronco" at http://www.karger.com/?doi=475457.

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.

Advances in peritoneal dialysis in acute kidney injury

Peritoneal dialysis (PD) is a simple, safe, cheap, and efficient renal replacement therapy method. It can correct metabolic disorders and fluid overload in acute kidney injury (AKI) patients both in and out of the intensive care unit. Use of PD in AKI is enhanced by placement of a Tenckhoff catheter, which can be safely accomplished at the bedside. Some PD modalities, such as high-volume PD and continuous-flow PD, can provide dialysis doses and efficiency comparable to extracorporeal blood purification methods. PD is particularly suitable for neonates, children, and patients with refractory heart failure or who are otherwise hemodynamically unstable. PD should be considered in situations where systemic anticoagulation and/or vascular access are problematic. PD is limited by a lower efficiency that may produce inadequate renal replacement in larger and/or severely hypercatabolic patients. Fluid removal can be unpredictable, there is a risk of infection, and possible issues with mechanical ventilation. In this article, we discuss the use of PD in AKI, with emphasis on recent advances.

Preventing harm during treatment of acute kidney injury: what do we really know?

Acute kidney injury (AKI) affects approximately 5% of all hospitalized patients, and its incidence continues to increase. The treatment of AKI involves tremendous financial costs, estimated to exceed $10 billion in the United States annually. Although our understanding of the pathophysiology of AKI has progressed at a tremendous pace, mortality remains high at 50% to 80%, with no improvement during the past several decades. More questions than answers currently exist regarding the optimal dialysis dose, optimal modality, and optimal timing of the initiation of renal replacement therapy in the setting of AKI, making it particularly difficult for the practicing clinician to both optimize treatment and practice cost-effective medicine. This article will review current evidence and concerns regarding these issues and identify areas of future research.

Dosing of renal replacement therapy in acute kidney injury

The impact of the intensity of renal replacement therapy on outcomes in patients with acute kidney injury has been studied intensively during the past decade. In this review, we consider the concept of dose of renal replacement therapy in acute kidney injury and summarize the recent clinical trials addressing this topic. Although several single-center trials suggest that more intensive therapy is associated with improved outcomes, 2 large multicenter randomized trials do not find a benefit with higher doses of therapy. Based on these studies, we provide recommendations for the delivered intensity of renal replacement therapy in acute kidney injury.

Creatinine generation is reduced in patients requiring continuous venovenous hemodialysis and independently predicts mortality

BACKGROUND

Existing systems for grading severity of acute kidney injury (AKI) rely on a change of serum creatinine concentration over a defined time interval. The rate of change in serum creatinine increases by degree of reduction in glomerular filtration rate, but is mitigated by low creatinine generation rate (CGR). Failure to appreciate variation in CGR may lead to erroneous conclusions regarding severity of AKI and distorted predictions regarding patient outcomes based on AKI severity.

METHODS

Cohort study of 103 patients who received continuous venovenous hemodialysis (CVVHD) over a 2-year period in a tertiary care hospital setting. Study participants entered the cohort when they were anuric, receiving a stable and uninterrupted dose of CVVHD with serum creatinine in steady state. They were followed until hospital discharge. CGR was measured based on dialyzate effluent volume and effluent creatinine concentration (prospective cohort) and via effluent volume and serum creatinine concentration (retrospective cohort).

RESULTS

CGR (mean 10.5, range 1.7-22.4 mg/kg/day) was substantially lower in this patient population than what would be predicted from existing equations. Correlates of CGR in multivariable analysis included the length of hospitalization prior to measurement and presence of an oncologic diagnosis. Lower CGR was independently associated with in-hospital mortality in unadjusted analysis and after multivariable adjustment for measures of severity of illness.

CONCLUSIONS

Grading systems for severity of AKI fail to account for variation in CGR, limiting their ability to predict relevant outcomes. Calculation of CGR is superior to other risk metrics in predicting hospital mortality in this population.

Clinical review: timing of renal replacement therapy

Acute kidney injury is common in intensive care patients and continuous renal replacement therapy is the preferred treatment for this in most centres. Although these techniques have been adopted internationally, there remains significant variation with regard to their clinical application. This is particularly pertinent when one considers that the fundamental questions regarding any treatment, such as initiation, dose and length of treatment, remain a source of debate and have not as yet all been fully answered. In this narrative review we consider the timing of renal replacement therapy, highlighting the relative paucity of high quality data regarding this fundamental question. We examine the role of the usual biochemical criteria as well as conventional clinical indications for commencing renal replacement therapy together with the application of recent classification systems, namely RIFLE and AKIN. We discuss the potential role of biomarkers for acute kidney injury as predictors for the need for renal support and discuss commencing therapy for indications other than acute kidney injury.

Catheter dysfunction and dialysis performance according to vascular access among 736 critically ill adults requiring renal replacement therapy: a randomized controlled study

OBJECTIVE

To compare dialysis catheter function according to catheter site.

DESIGN

Multicenter, open, randomized controlled trial.

SETTING

Nine university-affiliated hospitals and three general hospitals in France.

PATIENTS

Seven hundred thirty-six patients in intensive care units who required a first venous catheterization to perform either intermittent hemodialysis (470 patients with 1275 sessions) or continuous renal replacement therapy (266 patients with 1003 days).

INTERVENTION

Patients randomly received either femoral (n = 370) or jugular (n = 366) catheterization. For the jugular site, right-side position (n = 252) was recommended.

MEASUREMENTS AND MAIN RESULTS

Time to catheter ablation for dysfunction, urea reduction ratio (intermittent hemodialysis), and downtime (continuous renal replacement therapy) were assessed for all participants and evaluated by randomly assigned catheterization site (femoral or jugular). Baseline demography and dialysis prescriptions were similar between the site arms. In modified intent-to-treat, catheter dysfunction occurred in 36 of 348 (10.3%) and 38 of 342 (11.1%) patients in the femoral and jugular groups, respectively. The risk of catheter dysfunction did not significantly differ between randomized groups (hazard ratio, 1.06; 95% confidence interval, 0.67-1.68; p = .80). Compared to the femoral site, the observed risk of dysfunction decreased in the right jugular position (15 of 226; 6.6%; adjusted hazard ratio, 0.58; 95% confidence interval, 0.31-1.07; p = .09) and significantly increased in the left jugular position (23 of 118; 19.5%; adjusted hazard ratio, 1.89; 95% confidence interval, 1.12-3.21; p < .02). The postintermittent hemodialysis mean urea reduction ratio per session was 50.8% (standard deviation, 16.1) for femoral vs. 52.8% (standard deviation, 15.8) for jugular (p = .30) sites, and the median continuous renal replacement therapy downtime per patient-day was 1.17 hrs (interquartile range, 0.75-1.50) for both sites (p = .98).

CONCLUSIONS

In terms of catheter dysfunction and dialysis performance among critically ill adults requiring acute renal replacement therapy, jugular site did not significantly outperform femoral site placement.

Comparing body composition assessment tests in long-term hemodialysis patients

BACKGROUND

Protein-energy wasting is common in chronic kidney disease and is associated with decreases in body muscle and fat stores and poor outcomes. The accuracy and reliability of field methods to measure body composition is unknown in this population.

STUDY DESIGN

Cross-sectional observational study.

SETTING & PARTICIPANTS

118 maintenance hemodialysis patients were seen at the General Clinical Research Center at Harbor-UCLA Medical Center, Torrance, CA.

INDEX TESTS

Triceps skinfold, near-infrared interactance, and bioelectrical impedance analysis using the Segal, Kushner, and Lukaski equations.

REFERENCE TEST

Dual-energy x-ray absorptiometry (DEXA).

RESULTS

Participants (42% women, 52% with diabetes, 40% African Americans, and 38% Hispanics) were aged 49.4 +/- 11.5 (mean +/- SD) years, and had undergone dialysis therapy for 41.1 +/- 32.9 months. Body mass index was 27.0 +/- 6.0 kg/m(2). Using DEXA as the reference test, the bioelectrical impedance analysis-Kushner equation, triceps skinfold, and near-infrared interactance were most accurate of the index tests in estimating total-body fat percentage, whereas bioelectrical impedance analysis-Segal equation and bioelectrical impedance analysis-Lukaski equation overestimated total body fat percentage. Bland-Altman analyses and difference plots showed that bioelectrical impedance analysis-Kushner and near-infrared interactance were most similar to the reference test. Bioelectrical impedance analysis-Kushner, triceps skinfold, and near-infrared interactance had the smallest mean differences from DEXA, especially in women (1.6%, 0.7%, and 1.2%, respectively). Similar results were observed in African American participants (n = 47).

LIMITATIONS

Measurements were performed 1 day after a hemodialysis treatment, leading to more fluid retention, which may have affected the reference and index tests differently.

CONCLUSIONS

Using DEXA as the reference test, both near-infrared interactance and bioelectrical impedance analysis-Kushner method yield more consistent estimates of total body fat percentage in maintenance hemodialysis patients compared with the other index tests. Near-infrared interactance is not affected by skin color. Field methods with portable devices may provide adequate precision.

Dosing of renal replacement therapy in acute kidney injury: lessons learned from clinical trials

Prescribing dialysis to manage acute kidney injury (AKI) is common and recently has become a controversial area for physicians. The concept of dialysis "dose" initially was developed for end-stage renal disease and has been extended to AKI in the last decade. Urea kinetic modeling has been the mainstay of dose quantification in end-stage renal disease. Extrapolation of these techniques to critically ill patients with AKI is difficult because of a non-steady state leading to a variable increase in urea generation rate, alterations in total-body water and its compartmental distribution, and changing renal excretory capacity. Additional challenges are imposed when dose is considered for different modalities of dialysis that vary in operational characteristics (diffusion, convection, and adsorption), duration (intermittent and continuous), and frequency. The purpose of this article is to review the concept of dialysis dose, perform a critical assessment of the most important clinical trials of dialysis dose in AKI, summarize clinical evidence from these trials, and define key research issues that should be addressed in the future.

Comparison of methods for estimating glomerular filtration rate in critically ill patients with acute kidney injury

BACKGROUND

In critically ill patients with acute kidney injury, estimates of kidney function are used to modify drug dosing, adjust nutritional therapy and provide dialytic support. However, estimating glomerular filtration rate is challenging due to fluctuations in kidney function, creatinine production and fluid balance. We hypothesized that commonly used glomerular filtration rate prediction equations overestimate kidney function in patients with acute kidney injury and that improved estimates could be obtained by methods incorporating changes in creatinine generation and fluid balance.

METHODS

We analysed data from a multicentre observational study of acute kidney injury in critically ill patients. We identified 12 non-dialysed, non-oliguric patients with consecutive increases in creatinine for at least 3 and up to 7 days who had measurements of urinary creatinine clearance. Glomerular filtration rate was estimated by Cockcroft-Gault, Modification of Diet in Renal Disease, Jelliffe equation and Jelliffe equation with creatinine adjusted for fluid balance (Modified Jelliffe) and compared to measured urinary creatinine clearance.

RESULTS

Glomerular filtration rate estimated by Jelliffe and Modification of Diet in Renal Disease equation correlated best with urinary creatinine clearances. Estimated glomerular filtration rate by Cockcroft-Gault, Modification of Diet in Renal Disease and Jelliffe overestimated urinary creatinine clearance was 80%, 33%, 10%, respectively, and Modified Jelliffe underestimated GFR by 2%.

CONCLUSION

In patients with acute kidney injury, glomerular filtration rate estimating equations can be improved by incorporating data on creatinine generation and fluid balance. A better assessment of glomerular filtration rate in acute kidney injury could improve evaluation and management and guide interventions.

Rapid measurement of total body water to facilitate clinical decision making in hospitalized elderly patients

BACKGROUND

Bioelectrical impedance analysis (BIA) is a noninvasive rapid and simple bedside technique that can be used to predict total body water (TBW), extracellular water (ECW), and intracellular water (ICW) and identify altered fluid distribution following critical illness.

METHODS

An equivalence study of BIA in 32 hospitalized elderly patients was compared with reference standard dilutional measurements of deuterated water (TBW) and sodium bromide (ECW). The results were compared with anthropometric equations commonly used to predict TBW.

RESULTS

There was variability in TBW content among the participating hospitalized elderly patients. This variability was within (+/-5 L) and the percent difference between the standard and BIA was as follows: mean (range) -4.1% (-18.5 to 11.2). BIA reliably predicted TBW and ECW in individual participants, whereas standard prediction equations uniformly over- or underestimated TBW in individuals and whole group population.

CONCLUSION

TBW in hospitalized elderly patients can be estimated noninvasively by bedside BIA. Standardized anthropometric equations have to be used with caution in this population.

Diagnosis, epidemiology and outcomes of acute kidney injury

Acute kidney injury is an increasingly common and potentially catastrophic complication in hospitalized patients. Early observational studies from the 1980s and 1990s established the general epidemiologic features of acute kidney injury: the incidence, prognostic significance, and predisposing medical and surgical conditions. Recent multicenter observational cohorts and administrative databases have enhanced our understanding of the overall disease burden of acute kidney injury and trends in its epidemiology. An increasing number of clinical studies focusing on specific types of acute kidney injury (e.g., in the setting of intravenous contrast, sepsis, and major surgery) have provided further details into this heterogeneous syndrome. Despite our sophisticated understanding of the epidemiology and pathobiology of acute kidney injury, current prevention strategies are inadequate and current treatment options outside of renal replacement therapy are nonexistent. This failure to innovate may be due in part to a diagnostic approach that has stagnated for decades and continues to rely on markers of glomerular filtration (blood urea nitrogen and creatinine) that are neither sensitive nor specific. There has been increasing interest in the identification and validation of novel biomarkers of acute kidney injury that may permit earlier and more accurate diagnosis. This review summarizes the major epidemiologic studies of acute kidney injury and efforts to modernize the approach to its diagnosis.

Disease severity adversely affects delivery of dialysis in acute renal failure

BACKGROUND/AIMS

Methods of intermittent hemodialysis (IHD) dose quantification in acute renal failure (ARF) are not well defined. This observational study was designed to evaluate the impact of disease activity on delivered single pool Kt/V(urea) in ARF patients.

METHODS

100 patients with severe ARF (acute intrinsic renal disease in 18 patients, nephrotoxic acute tubular necrosis in 38 patients, and septic ARF in 44 patients) were analyzed during four consecutive sessions of IHD, performed for 3.5-5 h every other day or daily. Target IHD dose was a single pool Kt/V(urea) of 1.2 or more per dialysis session for all patients. Prescribed Kt/V(urea) was calculated from desired dialyzer clearance (K), desired treatment time (t) and anthropometric estimates for urea distribution volume (V). The desired clearance (K) was estimated from prescribed blood flow rate and manufacturer's charts of in vivo data obtained in maintenance dialysis patients. Delivered single pool Kt/V(urea) was calculated using the Daugirdas equation.

RESULTS

None of the patients had prescription failure of the target dose. The delivered IHD doses were substantially lower than the prescribed Kt/V values, particularly in ARF patients with sepsis/septic shock. Stratification according to disease severity revealed that all patients with isolated ARF, but none with 3 or more organ failures and none who needed vasopressive support received the target dose.

CONCLUSION

Prescription of target IHD dose by single pool Kt/V(urea) resulted in suboptimal dialysis dose delivery in critically ill patients. Numerous patient-related and treatment-immanent factors acting in concert reduced the delivered dose.

[Extracorporeal blood purification in the intensive care units]

Mortality remains high in intensive care patients with renal failure requiring extra corporeal blood purification. This article reviews the recent data that have led to the improvement of the care for such patients. We will discuss the criteria to determine the choice of the technique (intermittent or continuous), of the membrane, of the prescribing dose, and the type of anticoagulation and when to initiate such a treatment.

Ionic dialysance: a new valid parameter for quantification of dialysis efficiency in acute renal failure?

OBJECTIVE

Several studies have reported a close relationship between an increased dose of dialysis and survival in patients treated for acute renal failure. Unfortunately, the quantification of dialysis in critically ill patients based on the urea nitrogen formula Kt/V is not applicable. Ionic dialysance is a new parameter calculated in real time from the dialysate conductivity and correlated with the effective urea clearance in chronic hemodialysis patients. The aim of our study was to evaluate ionic dialysance in the quantification of dialysis in critically ill patients with acute renal failure.

DESIGN

Prospective open-label study.

SETTING

An 18-bed medical intensive care unit.

PATIENTS

Thirty-one patients with multiple organ dysfunction syndrome and acute renal failure requiring intermittent hemodialysis were included.

MEASUREMENTS

Using the first dialysis session of each patient, we compared the delivered dose of dialysis based on ionic dialysance measurement (Kt(ID)) with the well-accepted gold standard method based on fractional dialysate sampling (Kt(dialysate)). The data were analyzed using linear regression and Bland-Altman analysis.

RESULTS

Thirty-one intermittent hemodialysis sessions were performed in 31 critically ill patients (mean age 58+/-12 years, SAPS II score 56+/-10). We found a close correlation between Kt(dialysate) and Kt(ID) (Kt(dialysate) = 36.3+/-11.4 l; Kt(ID)=38.4+/-11.8; r=0.96) with excellent limits of agreement (-2.2 l; 6.4 l).

CONCLUSION

The feasibility of dialysis quantification based on ionic dialysance in the critically ill patient is good. This method is a simple and accurate tool for the determination of dialysis dose in critically ill patients.

Current status of dosing and quantification of acute renal replacement therapy. Part 1: mechanisms and consequences of therapy under-delivery

The dosing and quantification of acute renal replacement therapy has emerged as one of the most pressing issues in the management of critically-ill patients with acute kidney injury. Although there is ongoing debate as to the best marker of uraemic injury in this setting, several landmark studies have identified clearance-related expressions of acute renal replacement therapy dose as important determinants of survival. Part 1 of this review examines the factors affecting delivery of prescribed acute renal replacement therapy dose. The review continues in Part 2 and examines the implications of recent advances in this area for clinical practice.

Evaluating a new method to judge dialysis treatment using online measurements of ionic clearance

New technology now supports direct online measurements of total dialysis dose per treatment, Kt. An outcome-based, nonlinear method for estimating target Kt in terms of ionic clearance measurements and body surface area (BSA) has been described recently. This is a validation study of the new method that evaluates the relationship between the (actual Kt-target Kt) difference and death risk. Patients with Kt measurements during March 2004 were identified (N=59,644). Target Kt was determined for each patient using the new method. Patients were then grouped by (actual Kt-target Kt) decile. They were also grouped by (actual URR-target URR) decile. Cox analysis-based risk profiles were constructed using those groupings. The (actual Kt-target Kt) difference profiles suggested improving death risk as Kt increased from below target to equal target. Risk ratios then flattened and remained so until (actual Kt-target Kt) reached the highest decile at which it appeared to improve, suggesting a possible biphasic profile. The (URR-target URR) risk profile was U-shaped. Death risk was related to the difference between the actual Kt and a target Kt value selected using the new nonlinear method. The method is therefore valid for prescribing and monitoring hemodialysis treatment.

The online measurement of hemodialysis dose (Kt): Clinical outcome as a function of body surface area

BACKGROUND

Recent advances enable the direct measurement of small molecule clearance, Kecn, during each dialysis. Average Kecn and treatment length, t, are multiplied giving total clearance, Kt. The body surface area (BSA) is a fixed transformation of height and weight and is a well recognized measure of body size. This project was conceived to search for clinical outcome-based functions for measured Kt in terms of BSA to enable simple Kt prescription guidelines for clinicians who are able to measure Kecn, and to provide foundations for future clinical research.

METHODS

The data came from Fresenius Medical Care (NA) files and included more than 32,000 patients with height, weight, and paired Kecn and t measurements during December 2002. Measurements were averaged for the month and used as predictor measures in Cox models of survival time during 2003. Candidate Kt values from 30 L/treatment through 70 were examined to determine the best statistical fit for quintile and decile delimited BSA groups evaluating the best fit Kt treatment target for each group. Functional forms representing the relationship between target Kt values and mean BSA of the groups were then evaluated to determine the best fit.

RESULTS

Kt targets increased with BSA in a curvilinear way such that the rate of increase is greater at low BSA than high. The best statistical fit was a double reciprocal form, Kt = 1/(a + b/BSA); "a" and "b" are statistically derived coefficients. The form has an appealing mathematical property; Kt approaches 0 as BSA approaches 0. Other forms fit the data nearly as well, however, and can be used to estimate Kt targets for patients with different BSA.

CONCLUSION

Empirical, outcome-based functions of measured Kt in terms of BSA exist and can be used as aids for prescribing and judging hemodialysis treatment.

Clinical review: Renal tubular acidosis--a physicochemical approach

The Canadian physiologist PA Stewart advanced the theory that the proton concentration, and hence pH, in any compartment is dependent on the charges of fully ionized and partly ionized species, and on the prevailing CO₂ tension, all of which he dubbed independent variables. Because the kidneys regulate the concentrations of the most important fully ionized species ([K⁺], [Na⁺], and [Cl^-]) but neither CO₂ nor weak acids, the implication is that it should be possible to ascertain the renal contribution to acid–base homeostasis based on the excretion of these ions. One further corollary of Stewart's theory is that, because pH is solely dependent on the named independent variables, transport of protons to and from a compartment by itself will not influence pH. This is apparently in great contrast to models of proton pumps and bicarbonate transporters currently being examined in great molecular detail. Failure of these pumps and cotransporters is at the root of disorders called renal tubular acidoses. The unquestionable relation between malfunction of proton transporters and renal tubular acidosis represents a problem for Stewart theory. This review shows that the dilemma for Stewart theory is only apparent because transport of acid–base equivalents is accompanied by electrolytes. We suggest that Stewart theory may lead to new questions that must be investigated experimentally. Also, recent evidence from physiology that pH may not regulate acid–base transport is in accordance with the concepts presented by Stewart.

Renal replacement therapy and acute renal failure

PURPOSE OF REVIEW

Acute renal failure (ARF) is a syndrome that occurs when there is a sudden decline in the glomerular filtration rate. The purpose of this review is to examine new developments and clinical applications of renal replacement therapies including hemodialysis, continuous renal replacement therapy, the bioartificial kidney, and peritoneal dialysis in the management of this complicated syndrome.

RECENT FINDINGS

New developments in hemodialysis include in-line hematocrit monitoring and improved biocompatible dialyzer membranes. While recent studies indicate that increased delivery of dialysis improves the outcome of patients with ARF, the optimal regimen of intermittent dialysis or continuous renal replacement therapy remains to be determined. The bioartificial kidney, combining hemofiltration with a device containing human tubular cells, is currently in clinical trials and represents another alternative in the management of ARF. In peritoneal dialysis, new solutions using icodextrin may improve fluid removal and blood pressure.

SUMMARY

The optimal choice of renal replacement therapy depends on many factors. Use of new options in renal replacement therapy and early initiation of dialysis may help to improve survival and outcome of patients with ARF.

Factors associated with urea reduction ratio in acute renal failure

Prescription and delivery of hemodialysis (HD) in acute renal failure (ARF) may be affected by patient-related factors such as hemodynamic instability, catabolism, variable extracellular fluid volume, and coagulation disturbances. This study was undertaken in a cohort of patients with ARF requiring HD, to quantify patient- and dialysis-related variables that influence dialysis delivery. The urea reduction ratio (URR) was calculated for each HD session. Patient-related variables included age, gender, weight, mean arterial pressure, and Acute Physiological and Chronic Health Evaluation (APACHE) II and Multiple Organ Failure (MOF) scores. Dialysis-related variables were dialyzer characteristics (membrane type, surface area, KoA, and K(UF)), blood flow rate (Qb), session length, anticoagulant use, vascular access, and ultrafiltration volume. The analysis of factors associated with URR was performed using mixed linear statistical models. The cohort consisted of 81 adult patients with ARF who underwent 419 consecutive dialysis sessions. Mean (+/- SD) age was 60 +/- 18 years; 57% were male. At dialysis initiation, APACHE II score was 23 +/- 8 and MOF score 2 +/- 1. The number of HD treatments averaged 5.5 +/- 6.1/patient and 0.8 +/- 0.2/patient/day, mean URR was 54 +/- 15%, and session length 3.2 +/- 1.1 h; 58% used a femoral venous catheter, and 92% polysulfone dialyzers. Among patient-related variables, the only independent predictor of delivered dose of dialysis, as measured by URR was the predialysis weight (P < 0.01). Among the dialysis-related variables, treatment time (P < 0.01), dialyzer surface area (P < 0.01), dialyzer K(UF) (P = 0.04), blood flow rate (P < 0.01), and the use of a femoral venous catheter (P < 0.01) were also independently associated with URR. An interaction between vascular access site and blood flow rate was also found to be significant (P < 0.01). This study underscores the importance of the dialysis prescription parameters and vascular access site in influencing the dialysis dose in critically ill patients, and argues against the importance of patient-related characteristics such as disease severity.

Adequacy of dialysis in acute renal failure

Renal replacement therapy (RRT) is currently the mainstay of management for patients with acute renal failure (ARF). Adequacy of dialysis in the setting of renal failure is defined poorly and encompasses multiple domains of clinical and biochemical outcomes. Multiple operational factors influence the delivery of adequate dialysis. No current standards exist for RRT for ARF; current RRT practices for ARF generally have been extrapolated from end-stage renal disease (ESRD) literature. The heterogeneity of patient population, variation in RRT practices, and differences in outcomes studied have made it difficult to define or study adequate dialysis in ARF or its impact on clinical outcomes.

POOR NUTRITIONAL STATUS AND INFLAMMATION: Anthropometric and Body Composition Assessment in Dialysis Patients

Anthropometric and body composition assessments provide important information about the nutritional status of dialysis patients. Anthropometric measurements describe body size, fatness, and leanness in dialysis patients and have been collected in the Modification of Diet in Renal Disease (MDRD) and HEMO studies. Dialysis patients present special problems for anthropometry, including decreased functional status and increased comorbidity, that challenge nutrition assessment methodology. Recumbent anthropometric techniques are recommended and stature is estimated from knee height. Measures of weight, stature, calf circumference, arm circumference, and triceps and subscapular skinfolds have recently been reported for dialysis patients, who tend to be shorter, lighter, and have less adipose tissue than healthy persons of the same age. The HEMO study anthropometric data provide a clinical reference for assessing the nutritional status of dialysis patients. The most common body composition methods used with dialysis patients are dual energy X-ray absorptiometry (DEXA), bioelectrical impedance, total body water (TBW), and prediction equations, but they are not recommended for assessment of predialysis patients, as estimates are best obtained postdialysis. The TBW volume used in calculating the dose of dialysis has commonly been predicted from the limited, out-of-date equations of Watson, based on nonrepresentative samples. New prediction equations are available for white, black, and Mexican American children and adults. Watson's data are not representative of the TBW of U.S. men and women. The greater TBW in non-Hispanic black men and women and Mexican American women reflects the greater levels of obesity in the U.S. population.

Measurement of dialyzer clearance, dialysis time, and body size: Death risk relationships among patients

BACKGROUND

Urea pharmacokinetic equation systems have contributed to better understanding of treatment dose among hemodialysis patients. The methods are indirect, however, and require the measurement of blood urea nitrogen (BUN) concentration before and after a dialysis session to estimate the total treatment dose that clinicians prescribe [urea clearance x dialysis time (Kt)] indexed to an estimate of body size [the volume of urea distribution in the body (V)] yielding the ratio, Kt/V. New technology permits direct on-line measurement of average small molecule clearance (Kecn) during each dialysis treatment that can be multiplied by time (t) to give a direct measurement of total treatment dose (Kt). This study evaluated the relationship of measured Kt with death risk. It also evaluated the relationship of simple body size measures to risk and also the combination of one such measure [body surface area (BSA)] with Kt to death risk.

METHODS

The data were taken from the Fresenius Medical Care (NA) (FMCNA) clinical database that included patients who had outcome data, height and weight measurements, and at least one average Kecn and t measurement during April 2002. Kecn, t, and the body size measures [body weight, body mass index (BMI), and BSA)] were averaged during the month. Those values were used as predictors of survival during the next 1 year in unadjusted and case mix adjusted proportional hazards (Cox) models.

RESULTS

Increasing values of Kecn, t, Kt and all of the body size measures were associated with lower death risk. The body size measure most closely associated with risk was the BSA that was used in subsequent models. Kt and BSA were independent risk predictors. There was a significant interaction between Kt and BSA in the case mix but not the unadjusted model indicating that the risk burden of lower total dialysis dose, Kt, may be greater among small than large patients.

CONCLUSION

The direct measurement of dialysis dose during each treatment is practical and the values reported by it are clinically relevant. Higher dose was associated with better survival in both small and large patients treated three times weekly. Furthermore, smaller patients may require proportionately greater total dose than larger patients to achieve comparable survival.