| 2008 JASN IMPACT FACTOR 7.505 | HOME AUTHOR INFO EDITORIAL BOARD SUBSCRIBE FEEDBACK ALERTS HELP | |||
| CURRENT ISSUE | ARCHIVES | JASN Express | ONLINE SUBMISSION | |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
CLINICAL RESEARCH |

* Departments of Medicine and
Emergency Medicine, University of Alabama at Birmingham, Birmingham, Alabama
Correspondence: Dr. Ashita Tolwani, ZRB 604, 1530 3rd Avenue S., Birmingham, AL 35294-0007. Phone: 205-975-2021; Fax: 205-996-2156; atolwani{at}uab.edu
Received for publication November 7, 2007. Accepted for publication December 29, 2007.
| Introduction |
|---|
|
|
|---|
The mortality associated with acute renal failure (ARF) in the intensive care unit (ICU) has remained greater than 50% during the past three decades, despite improvements in renal replacement technology.1–5 Continuous renal replacement therapy (CRRT) has emerged as the predominant form of renal replacement therapy (RRT) in the ICU as a result of slow continuous fluid removal, steady acid-base and electrolyte correction, and beneficial effects on hemodynamic stability.6–8 There are no standardized protocols for prescribing or quantifying the adequacy of solute removal with CRRT.5
The available CRRT modalities differ according to whether solute clearance is accomplished primarily by diffusion, convection, or a combination of these techniques. Diffusion clears small molecular weight solutes efficiently across a concentration gradient but is relatively ineffective in clearing larger molecular weight solutes (>5000 Da). Convection removes water by mass transport across a pressure gradient, thereby removing both small and larger molecular weight solutes dissolved in the transported water. There is no consensus as to which one of these clearance techniques is best.
The optimal dosage of CRRT in ICU patients with ARF has not been established. Although three randomized trials using differing CRRT modalities have evaluated the impact of dialysis dosage (defined by effluent rate) on patient survival in this population, they all were single-center studies with differing designs and compared different CRRT strategies.9–12 Continuous venovenous hemodiafiltration (CVVHDF), a CRRT technique that uses both convection and diffusion, is the only CRRT technique used at the University of Alabama at Birmingham (UAB). To determine the impact of CRRT dosage on patient outcomes, we conducted a prospective, randomized study comparing CVVHDF with prefilter replacement fluid using an effluent rate of 35 ml/kg per h versus an effluent rate of 20 ml/kg per h on patient survival in ICU patients with ARF.
| RESULTS |
|---|
|
|
|---|
|
|
|
|
|
| DISCUSSION |
|---|
|
|
|---|
Systematic efforts to quantify CRRT dosage in patients with ARF have only recently occurred. Three randomized studies evaluated the impact of CRRT dosage, defined by effluent rate, on patient survival (Table 4). Using an end point of 15-d survival after continuous venovenous hemofiltration (CVVH) discontinuation, a purely convective CRRT modality, Ronco et al.11 found that a dosage of 35 ml/kg per h had a survival benefit compared with 20 ml/kg per h. Postdilution CVVH was initiated relatively early in the course of ARF. The majority of patients had postsurgical ARF, and relatively few had sepsis or preexisting CKD. At least 85% of prescribed dosage was achieved by all patients. For compensation for treatment time interruptions, dosage was increased in subsequent hours to match the targeted dosage.
|
We did not detect a significant difference in survival between the high-dosage and standard-dosage arms in this study. This study was designed to detect an absolute 20% difference in survival rates with 80% power. It is possible either that a true difference exists but was not detected because of sample size or that the true survival difference was less than 20%. Second, our patient population also had higher rates of CKD and sepsis, which may have had a negative impact on outcomes. Third, although the targeted high dosage in this study was 35 ml/kg per h, the actual delivered dosage was 29 ml/kg per h. The high-dosage arms in the previous studies prescribed higher dosages, suggesting that a critical dosage threshold may not have been reached in our study. Furthermore, only 77% of the 200 patients achieved greater than 80% of the prescribed dosage. Last, convective clearance is more effective for removal of middle molecules, although the clinical importance of this remains unclear.13 Compared with previous studies, lower convective rates in this study might have resulted in higher mortality in patients with sepsis, although this is speculative.
Renal recovery is an important secondary end point in CRRT outcomes. There were no differences in rates of renal recovery at ICU or hospital discharge for survivors (Table 3), and there was a NS trend toward worsening renal recovery in the high-dosage group. None of the previous three studies had greater rates of renal recovery in the high-dosage arms. Renal recovery is influenced by multiple factors, including preexisting CKD and nephrotoxins.
Taken together, these studies (including this one) underscore the difficulty of CRRT research in the ICU. Factors such as sepsis, preexisting CKD, delivered dosage, solute clearance techniques (diffusion versus convection), and time of initiation all are important characteristics that may influence outcome and limit generalizing conclusions. All four CRRT studies discussed thus far are also single-center studies. There are two large, multicenter, randomized trials in progress that should help to resolve some of these issues.
In conclusion, we did not observe a significant difference in either survival to ICU discharge or 30 d between a CVVHDF dosage of 35 versus 20 ml/kg per h. Large, multicenter trials will help to address more definitively the impact of CRRT dosage on the survival of ICU patients with ARF.
| CONCISE METHODS |
|---|
|
|
|---|
80 mg/dl), or (5) hyperkalemia (K+
6.5 mmol/L) and/or an increase in serum creatinine >2.5 mg/dl from normal values or a sustained rise in serum creatinine of
1 mg/dl over baseline. Patients were excluded when they had ESRD, when they had had previous IHD, or when they had >24 h of CRRT at time of enrollment. Patients were also excluded when they weighed >125 or <50 kg because of limitations of the Prisma machine to deliver study doses for those weights. The study protocol was approved by the local institutional review board. Written informed consent was obtained by B.S.S. from all study participants or from their next of kin or legal guardian. Patients were followed prospectively from time of enrollment through hospital discharge.
Treatment Assignments
CVVHDF was initiated at the discretion of the consulting nephrologists, without consideration of the patient's eligibility for this study. CVVHDF was performed with the COBE Prisma (Lakewood, CO) M100 set and AN69 dialyzer (effective surface area 0.9 m2) through a double-lumen 12F catheter inserted into the internal jugular, subclavian, or femoral vein. Hemodiafiltration was accomplished using blood flow rates of 100 to 150 ml/min and predilution replacement fluid. Regional citrate or no anticoagulation was used at the consulting nephrologists' judgment.
Patients were randomly assigned to the treatment dosage by a computer-generated block randomization scheme, using a 1:1 ratio between treatment dosages. Treatment assignments were kept in numbered, sealed envelopes that were opened at the time of enrollment. The treatment assignments were stratified by sepsis and oliguria to ensure balanced randomization, because both parameters are independent predictors of patient survival.5,14 The four stratification categories were (1) sepsis + oliguria, (2) sepsis + nonoliguria, (3) nonsepsis + oliguria, and (4) nonsepsis + nonoliguria. Each time a patient was enrolled, the next available envelope was opened by the study coordinator and the allocated treatment communicated to the consulting nephrologists. Blinding was impossible for logistic reasons.
In all CRRT modalities, the "effluent" represents the end product of filtration and comprises the ultrafiltrate in convective therapies, the spent dialysate in diffusive therapies, and the sum of both in combined therapies. CRRT solute clearance is determined by the ratio between the concentration of the solute in the effluent and in the plasma multiplied by the effluent rate. Because urea is a small molecular weight solute, it reaches complete equilibrium in the effluent; thus, the ratio of the concentration of urea in the effluent to plasma is 1. Urea clearance becomes equal to the effluent rate, provided that the replacement fluid is given after dilution. For this study, the prescribed amount of effluent was used as proxy for treatment dosage. The two treatment dosages were an effluent rate of 20 ml/kg per h (standard) or 35 ml/kg per h (high).
On the Prisma, effluent rate (ml/h) is the sum of the replacement fluid rate, dialysate rate, and fluid removal rate. For example, a 70-kg patient assigned to the high dosage would require an effluent rate of 2450 ml/h (70 kg x 35 ml/kg per h). The replacement fluid rate, dialysate rate, and fluid removal rate for that patient would be adjusted to achieve an effluent rate of 2450 ml/h per d for the study duration. Dosage was calculated only once per patient and based on the patient's actual body weight on the day of CVVHDF initiation. This dosage remained constant throughout the treatment period and was not adjusted for body weight changes. Convective clearance is the sum of the replacement fluid and fluid removal rates. Diffusive clearance equals the dialysate rate. Every attempt was made to divide the effluent rate equally between convective and diffusive clearances. The actual delivered dosage of CVVHDF was measured directly by obtaining effluent BUN and creatinine levels daily. Total time of actual CRRT treatment (minutes/24 h period) was recorded daily, along with time off CRRT secondary to clotting, procedures, or other events. No compensation was made for therapy interruptions; however, interruptions were factored into the average percentage of prescribed therapy achieved.
Patients were transitioned to IHD at the judgment of the treating nephrologists. This usually occurred when the patient was dialysis dependent and transferred from the ICU to the ward or when the patient was being mobilized in the ICU. Dosage and timing of IHD were decided by the treating nephrologists.
Outcome Measurements
The primary outcome measure was survival to the earlier of either ICU discharge or 30 d. Secondary end points included renal recovery at ICU discharge, renal recovery at hospital discharge, ICU survival, hospital survival, ICU length of stay, and hospital length of stay. Renal recovery was defined as freedom from any RRT after CRRT discontinuation. Subanalyses were performed for each of the strata.
Statistical Analyses
Sample size calculation was based on a power analysis that assumed an expected improvement in patient survival of 20% in the high-dosage arm, compared with the standard-dosage arm. On the basis of preliminary data demonstrating 65% mortality in this patient population at UAB, we calculated that 200 patients would be needed to detect an absolute survival difference of 20%, assuming a power of 80%, a significance level of 5%, and a two-sided
2 test.
Analysis was done on an intention-to-treat basis. The primary analysis of the study compared the proportion of patients who survived to the earlier of either ICU discharge or 30 d in each study arm. The two proportions were compared using the Pearson
2 test or Fisher exact test when the
2 test was not valid. The secondary analyses compared the proportion of patients who recovered renal function at ICU discharge and at hospital discharge, ICU survival, hospital survival, and hospital length of stay, using similar methods as the primary analysis. Baseline characteristics and outcome measures were compared using the two-group t test or the Wilcoxon rank-sum test for continuous variables and the Pearson
2 test or Fisher exact test for categorical variables.
Logistic regression analysis was used to determine which baseline factors were associated with (significant predictors of) survival to ICU discharge or 30 d, hospital survival, and ICU survival. The Kaplan-Meier method was used to obtain hospital survival estimates by prescribed CVVHDF dosage, and the log-rank test was used to compare survival curves of the two dosage arms. All statistical tests were two-sided and performed using a significance level of 5%. Statistical analyses were performed using SAS 9.1 (SAS Institute, Cary, NC).
| DISCLOSURES |
|---|
|
|
|---|
| Acknowledgments |
|---|
This work was presented in abstract form at the annual CRRT meeting; March 7 to 10, 2007; San Diego, CA.
We thank Drs. Michael Allon and Anupam Agarwal for their critical appraisal and thoughtful suggestions and Dr. Earl Francis Cook, Jr., for his mentorship.
| Footnotes |
|---|
See related editorial, "Dialysis Dosage in Acute Kidney Injury: Still a Conundrum?" on pages 1046–1048.
| REFERENCES |
|---|
|
|
|---|
Related Article
This article has been cited by other articles:
![]() |
The RENAL Replacement Therapy Study Investigators Intensity of Continuous Renal-Replacement Therapy in Critically Ill Patients N. Engl. J. Med., October 22, 2009; 361(17): 1627 - 1638. [Abstract] [Full Text] [PDF] |
||||
![]() |
P. M. Palevsky Renal Support in Acute Kidney Injury -- How Much Is Enough? N. Engl. J. Med., October 22, 2009; 361(17): 1699 - 1701. [Full Text] [PDF] |
||||
![]() |
J. Bouchard, E. Macedo, S. Soroko, G. M. Chertow, J. Himmelfarb, T. A. Ikizler, E. P. Paganini, R. L. Mehta, and DM, FACP, FASN. Program to Improve Care in Acute R Comparison of methods for estimating glomerular filtration rate in critically ill patients with acute kidney injury Nephrol. Dial. Transplant., August 13, 2009; (2009) gfp392v1. [Abstract] [Full Text] [PDF] |
||||
![]() |
M. Elahi, S. Asopa, A. Pflueger, N. Hakim, and B. Matata Acute kidney injury following cardiac surgery: impact of early versus late haemofiltration on morbidity and mortality Eur. J. Cardiothorac. Surg., May 1, 2009; 35(5): 854 - 863. [Abstract] [Full Text] [PDF] |
||||
![]() |
Z. Ricci and C. Ronco Kidney diseases beyond nephrology: intensive care Nephrol. Dial. Transplant., February 1, 2009; 24(2): 391 - 395. [Full Text] [PDF] |
||||
![]() |
R. L. Mehta and J. Bouchard Dialysis Dosage in Acute Kidney Injury: Still a Conundrum? J. Am. Soc. Nephrol., June 1, 2008; 19(6): 1046 - 1048. [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
HOME
CURRENT ISSUE
ARCHIVES
JASN Express
ONLINE SUBMISSION
AUTHOR INFO
EDITORIAL BOARD SUBSCRIBE FEEDBACK ALERTS HELP |