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

Hemodialysis adequacy and the hospitalized end-stage renal disease patient--raising awareness

Assessment of hemodialysis adequacy may require different approaches for the stable, outpatient with end-stage renal disease (ESRD) and for the sick, inpatient with acute kidney injury (AKI). Variability of urea distribution volume, urea generation, and treatment schedule, for instance, complicates dialysis dosing in the latter group although progress has been made in our understanding of their needs. There is a third population, however, for whom hemodialysis dosing requirements remain unclear--the hospitalized ESRD patient. This commentary discusses the key urea kinetic differences between stable ESRD and AKI to give the context to where, on the intervening spectrum, the hospitalized ESRD patient might lie. The limited literature examining hemodialysis dosing in this population is discussed along with those outstanding questions that might form the basis of a future research agenda.

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.

Assessing and delivering dialysis dose in acute kidney injury

Assessing and delivering dialysis dose in acute kidney injury (AKI) has emerged as an important issue in the management of critically ill patients. There is ongoing debate on how dose of dialysis should be expressed and measured. Most studies have focused on clearance of small molecules (blood urea nitrogen) as a marker of delivered dose and for establishing dose-outcome relationships. Recent evidence has shown that other markers may also be important to consider, as acid-base balance and fluid overload have emerged as important factors contributing to outcomes. In this review, we provide an evaluation of current approaches to prescribing and delivering dialysis dose in AKI, identify gaps in practice and propose an integrated approach to optimize dose delivery in dialysis with a goal to improve outcomes.

Urea kinetics and intermittent dialysis prescription in small animals

Hemodialysis improves survival for animals with acute kidney injury beyond what would be expected with conventional management of the same animals. Clinical evidence and experience in human patients suggest a role for earlier intervention with renal replacement to avoid the morbidity of uremia and to promote better metabolic stability and recovery. For a large population of animal patients, it is the advanced standard for the management of acute and chronic uremia, life-threatening poisoning, and fluid overload for which there is no alternative therapy.

Dosing intermittent haemodialysis in the intensive care unit patient with acute renal failure--estimation of urea removal and evidence for the regional blood flow model

BACKGROUND

Blood-side dosing methods may overestimate urea removal in comparison to dialysate-side measurements during intermittent HD (IHD) for acute renal failure (ARF). The present study sought to quantify this mass balance error (MBE) and explore potential explanatory factors.

METHODS

Prospective, formal, blood-side urea kinetic modelling was performed in serial sessions (n = 42) in 18 intensive care unit ARF patients. Three blood-side estimates of urea removal were calculated and these were compared to urea removal derived from fractional dialysate sampling and use of an on-line urea monitor. We also examined urea rebound in these patients, as expressed by the intercompartmental urea clearance (Kc), and in a subset of patients examined the relation of Kc to cardiac output and systemic vascular resistance (SVR).

RESULTS

The mean % MBE (MBE = blood - dialysate-estimated urea removal) was about 9% using conventional two-pool modelling based on a 60-min post-dialysis blood urea nitrogen (BUN) with or without the use of one or more intra-dialytic BUN values. The extent of MBE could not be explained by the clinical or dialytic variables that were measured. Part of the MBE error was due to overestimation of the intradialytic BUN profile, because model-independent profiling of intra-dialytic BUN values to compute urea removal reduced the MBE to approximately 6%. The log Kc was correlated with cardiac output and showed trends towards an inverse correlation with SVR.

CONCLUSIONS

Classical, two-pool, blood-side UKM produces a modest overestimate of urea removal in IHD for critically ill ARF patients. The source of this small, residual MBE is unknown. The amount of urea rebound, as reflected by Kc, varied among patients and associated with cardiac output and SVR, as predicted by the regional blood flow model.

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.

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.

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.

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.