The determination of hemodialysis blood recirculation using blood urea nitrogen measurements

Spirulina platensis mediated the biochemical indices and antioxidative function of Nile tilapia (Oreochromis niloticus) intoxicated with aflatoxin B1

Aflatoxicosis is one of the threats that cause severe mortalities in fish farms. The dietary functional additives are a friendly approach attributed to beneficial effects on aquatic animals. The study aimed at evaluating the impact of Spirulina platensis (SP) on the biochemical indices and antioxidative function of Nile tilapia (Oreochromis niloticus) intoxicated with aflatoxin B₁ (AFB₁). A control diet and 3 test diets were enriched with 0% SP/0 mg AFB₁/kg (control), 1% SP (SP), 2.5 mg AFB₁/kg diet (AFB₁), and 1% SP+2.5 mg AFB₁/kg diet (SP/AFB₁). The diets were supplied to three aquaria for each group twice daily at the rate of 2.5% for 30 days. The blood alanine transaminase (ALT), alkaline phosphatase (ALP), and aspartate transaminase (AST) were significantly increased by AFB₁ toxicity with regards to fish fed the control and SP diets (P < 0.05). The inclusion of SP in the diet of tilapia intoxicated with AFB₁ lowered the levels of ALT, AST, and ALP in comparison to fish contaminated with AFB₁ without SP (P < 0.05). The total blood protein and albumin were decreased in fish contaminated with AFB₁ (P < 0.05); however, the dietary SP resulted in improving the blood protein and albumin with similar levels with the control and SP diets. The urea and creatinine were increased in tilapia fed AFB₁ diet without SP (P < 0.05); however, the inclusion of SP reduced the levels of urea and creatinine with similar levels with the control and SP diets. The antioxidative capacity of Nile tilapia fed SP and contaminated with AFB₁ is expressed by superoxide dismutase (SOD), glutathione (GSH), and malondialdehyde (MDA) concentration. The activities of SOD and GSH were decreased by AFB₁ (P < 0.05); however, dietary SP increased the SOD and GSH in fish fed AFB₁. On the other hand, the concentration of MDA was increased in tilapia fed AFB₁ (P < 0.05); however, SP decreased the level of MDA in fish fed AFB₁. In conclusion, the application of SP in the aquafeed seems to be an innovative approach to relieve the toxic influences of AFB₁ on aquatic animals.

Potassium-Based Dilutional Method to Measure Hemodialysis access Recirculation

Background

Assessment of access recirculation (AR) is crucial to dialysis efficiency and there is thus a need for a method yielding a highly accurate, fast, easy and economical measurement that can be applied in any busy dialysis clinic. Non-urea based dilutional methods are more accurate than urea based methods and avoid problems with cardiopulmonary recirculation, but they require expensive specialized devices, which limit their applicability.

Methods

We developed a simple dilutional method of AR which does not require any specific device, based on the determination of serum potassium [K+] in two samples. Briefly, a basal sample is drawn at the time of needle insertion (basal [K+]); needles are connected to blood lines and blood flow rate is quickly increased to 300 ml/mm; a second sample (arterial [K+]) is drawn from the arterial line port within 5 to 10 seconds, to avoid errors due to cardiopulmonary recirculation of the normal saline entering the blood stream. At this time, if recirculation is present, part of the normal saline will enter the arterial line and dilute the serum [K+]. The AR formula is: AR (%) = 100 x [1 - arterial K+ / basal K+] We compared our method with the two-needle urea and ultrasound velocity dilution methods. Results: AR values by the ultrasound method > 10% were hypothesized as gold standard for AR, against which values obtained with the potassium method were compared. The potassium based method showed: sensitivity (100%,); specificity (95%); predictive value, positive (91%); predictive value, negative (100%). In addition, the potassium based method appears to be more reliable than the two-needle urea based method.

Conclusion

Our method, similar to other dilutional methods, is not influenced by cardiopulmonary recirculation or veno-venous disequilibrium and is fast and accurate. Moreover it is very simple, economical, and can easily be performed in any dialysis unit.

Continuous On-Line Determination of Recirculation by Thermodilution in Hemodialysis Patients

The Consistency Of The Determination Of A-V Fistula Recirculation (R) Using The Thermodilution Method (T) With A New Probe (Blood Temperature Monitor, Btm Fresenius A.G.) Was Studied In 32 Patients (Avf: Proximal 34%, Distal 63%, Graft 3%). We Compared R Calculated By T With Both The Traditional Three-Sample Method (C) And The Low-Flow Three-Sample Method (L); Both Bun And Creatinine (Cr) Were Measured In All Samples At The Beginning And At The End Of The Session. T Was Also Determined At The 2Nd And 3Rd Hour. There Was A Significant Correlation Between T And Either C Or L At The Start Of The Session (Bun And Cr) As Well As At The End (Only Cr). R Was Higher (11.9±10) In Proximal Avf Than In The Distal (5+3.1%; P0.01) When Measured By T At The Same Blood Flow (Qb: 313±45 Vs 343+52 Mls/Min, P=Ns). T Increased But Not Significantly By Increasing Qb From 150 To 300 Mls/Min In Ten Patients. No Correlation Was Found During The Session Between Blood Pressure And T Variations. In Conclusion, T And L Give Very Similar Results While C Overestimates Recirculation. R Is Easy To Perform Repeteadly By T With Results Available Online.

Urea-based recirculation validation of the symmetrical palindrome catheter

BACKGROUND

High blood flow and low recirculation rates are central for adequate haemodialysis. A new symmetrical tip has been invented promising efficient haemodialysis even if the ports are reversed.

OBJECTIVE

To evaluate access recirculation of the 'palindrome' catheter and to report initial experiences in a clinical setting.

MATERIAL AND METHODS

After implantation of the new catheter in 20 patients (male: 14; female: 6; mean age 72 ± 12.2), access recirculation was evaluated using the urea-based recirculation test. After 30 minutes of haemodialysis, ultrafiltration was stopped and arterial and venous samples were taken. Afterwards, the blood flow rate was reduced to 120 ml/min. Another systemic arterial blood sample was taken 10 seconds after the blood pump was switched off.

RESULTS

All 20 interventions were performed successfully without complications. The average recirculation rate was 8.1% with a median of 2.5% ranging from 0 to 85.8%. Recirculation rates under 5% were measured in 13 patients and more than 10% recirculation were found in two patients. The median of days between catheter implantation and recirculation assessment was the day following implantation.

CONCLUSION

The new symmetrical catheter presented low recirculation rates in a clinical setting. Since there is just a single tip, fluoroscopic placement in the right atrium is facilitated.

Selenate inhibits adipogenesis through induction of transforming growth factor-β1 (TGF-β1) signaling

Selenium is essential for many aspects of human health. While selenium is known to protect against cancer and cardiovascular diseases, the role of selenium in adipose development is unknown. Here we show that selenate at non-toxic concentration exhibits an anti-adipogenic function in vitro and ex vivo. In addition, selenate induced a morphological change of these cells from fibroblast-like to spindle cell shape. However, other forms of selenium, including selenite and methylseleninic acid, showed either toxic or no effect on adipogenesis and morphology change of preadipocytes. The effects of selenate on adipogenesis and cell morphology change were blunted by the treatment with SB431542, a specific inhibitor of transforming growth factor-β1 (TGF-β1) receptor, neutralization TGF-β1 by its antibody, and knockdown of TGF-β1 in preadipocytes, suggesting a requirement of TGF-β signaling for the anti-adipogenic function of selenate. Among tested forms of selenium, selenate appears to be an effective activator of TGF-β1 expression in preadipocytes. These results indicate that selenate is a novel dietary micromineral that activates TGF-β1 signaling in preadipocytes and modulates adipogenesis.

Diagnosis of arteriovenous fistula dysfunction

Arteriovenous fistula (AVF) dysfunction remains a major contributor to the morbidity and mortality of hemodialysis patients. The failure of a newly created AVF to mature and development of stenosis in an established AVF are two common clinical predicaments. The goal is to identify a dysfunctional AVF early enough to intervene in a timely manner to either assist with the maturation process or to prevent thrombosis. The currently available tools in our armamentarium include clinical evaluation, physical examination of the AVF, and surveillance tests. Physical examination has been recognized as a simple and cost-effective tool, but is often not implemented either because of lack of training or time constraints. Surveillance tests include measurement of access flow or pressure as a surrogate marker of AVF dysfunction. Surveillance tests often require expensive equipment, additional personnel, and are controversial. Currently, there are guidelines and recommendations to include all of these measures while evaluating an AVF. Implementing judicious use of these tools in clinical practice can facilitate early diagnosis for timely intervention in the appropriate population. Furthermore, this strategy may avoid unnecessary interventions and assist with healthcare cost containment.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

HCO3 increment in arterial line can reveal significant vascular access recirculation in high-flux hemodialysis: A preliminary report

We report a new and simple way that can reveal the presence of vascular access recirculation (VAR) in patients undergoing hemodialysis (HD). Acid-base and blood gas parameters (pH, pO(2), pCO(2), and HCO(3)) were measured in blood samples drawn from an arterial fistula needle before the initiation of HD and from arterial and venous lines simultaneously 5 min later, in 31 patients (group A). Vascular access recirculation was measured using the glucose infusion test (GIT) immediately after the withdrawal of the 5-min samples. The same study was repeated in 30 patients in whom HD lines were reversed (group B). A comparison with baseline (predialysis) values of an analysis of the arterial line in group A at 5 min revealed that pCO(2) increased by 1.14+/-2.5 mmHg and HCO(3) by 0.6+/-0.6 mM/L (p<0.02 and p<0.00001, respectively). The corresponding pO(2) and pH values did not show significant differences. Glucose infusion test at 5 min (GITa) was -0.058+/-0.03%. A comparison with baseline (predialysis) values of an analysis of the arterial line in group B at 5 min revealed that pCO(2) increased by 7.7+/-3.5 mmHg and HCO(3) by 2.9+/-1.0 mM/L (p<0.000001 in each case). The pH level was significantly lower in comparison with baseline values (p<0.00001), while pO(2) did not show a significant difference. Glucose infusion test at 5 min (GITb) was 12.0+/-6.1% (p<0.000001 in comparison with GITa values). Clinically significant VAR was defined as HCO(3) increment >1.8 mM/L, based on the receiver-operating characteristics curve, which showed a threshold value of HCO(3) increment >1.8 mmol/L as a predictor of GIT recirculation. Five minutes after the initiation of high-flux HD with a 0 ultrafiltration rate, there is a small increment in arterial HCO(3) values relative to predialysis values. Clinically significant VAR is present when this increment is higher than 1.8 mM/L.

Comparison of Recirculation Percentage of the Palindrome Catheter and Standard Hemodialysis Catheters in a Swine Model

PURPOSE

The purpose of this study was to evaluate in vivo recirculation of a new bi-directional dialysis catheter with a symmetric tip design and compare it to currently marketed staggered and split tip catheters.

MATERIALS AND METHODS

Twelve catheters were placed in three 70-kg male swine. The catheters were connected to a roller type peristaltic pump. Blood flow and recirculation rates were measured with ultrasound (US) dilution. Recirculation was measured in blood flow rates of 300 mL/min, 400 mL/min, and 500 mL/min. The dialysis lines were then reversed and recirculation was measured in flow rate of 400 mL/min. Measurements were conducted with the catheter tip in the right atrium (RA) and repeated with the catheter tip in the superior vena cava (SVC).

RESULTS

Recirculation rates measured with the Palindrome catheter tip (Tyco Healthcare Group LP, Mansfield, MA) in the SVC and in the RA were 0%. When the dialysis lines were reversed, the Palindrome catheter average recirculation was 0% with the tip in the SVC and 1% (0%-3%) with the tip in the RA. The average recirculation rates measured with the split tip catheter were 1% (0%-10%) in the SVC and 0% when the catheter tip was in the RA. When the dialysis lines were reversed split tip catheter recirculation was 11% (9%-16%) with the tip in the SVC and 13% (7%-18%) with the tip in the RA. With the staggered tip catheter, 1% (0%-5%) recirculation was measured. When the dialysis lines were reversed, the average recirculation in the staggered tip catheter measured 23% (18%-30%). Recirculation rate with reversal of dialysis lines was significantly lower with the Palindrome catheter when compared to the split tip and staggered tip designs (P < .05).

CONCLUSIONS

Preliminary results in vivo indicate that the Palindrome catheter demonstrates minimal recirculation with reversal of the dialysis lines compared to currently marketed split tip and staggered tip catheters.

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 (dKt/V), which is denoted nKt/V, in contrast to the equilibrated Kt/V (eKt/V). This work clarifies the significance of dKt/V as a complement to eKt/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 (spKt/V) and the eKt/V are also analyzed from gspUKM in this work, with the aim of achieving a better interpretation of the results. Indices nKt/V, eKt/V and spKt/V have been compared with the dKt/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 nKt/V and 2pKt/V were -0.077+/-0.72% (percentage of the dKt/V mean), while between eKt/V and 2pKt/V were -13.75+/-17.39% and between spKt/V and 2pKt/V were -1.61+/-6.54%. These scores prove that the nspUKM model is able to provide a very accurate estimate of 2pKt/V and thus dKt/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.

Renal telehealthcare system based on a patient physiological image: a novel hybrid approach in telemedicine

This paper presents a novel renal telemedicine system, Virtual Center for Renal Support (VCRS), focused on the end-stage renal disease (ESRD) population. The VCRS design modifies the telemedicine paradigm, currently centered on communication technologies and monitoring devices, by emphasizing the way that biosignals are used to extract on-line knowledge to be used by physicians to solve current needs of this population. We begin with an ESRD review, from which a summary of major limitations of current renal health assistance programs is obtained. This is used to form the basis for the VCRS. This work is focused on a theoretical description of the technological architecture of VCRS, followed by a simulation experiment showing some preliminary results from a prototype of a patient physiologic image (PPI) computer component, the major knowledge creator of VCRS. Preliminary results show that PPI technology provides the ability to supervise internal variables representing the patient's dynamic behavior. The demonstrated relation between adequate control of extracellular volume and blood pressure suggests that VCRS is able to generate hypovolemia warnings before their occurrence during a hemodyalysis session delivered remotely. However, PPI is not restricted to kinetic models, which were initially chosen because of their successful results in the provision of dialysis. Preliminary results suggest the ability of this telemedicine system to enhance remote patient supervision and care.

Virtual center for renal support: technological approach to patient physiological image

The patient physiological image (PPI) is a novel concept which manages the knowledge of the virtual center for renal support (VCRS), currently being developed by the Biomedical Engineering Group of the University of Seville. PPI is a virtual "replica" of the patient, built by means of a mathematical model, which represents several physiological subsystems of a renal patient. From a technical point of view, PPI is a component-oriented software module based on cutting-edge modeling and simulation technology. This paper provides a methodological and technological approach to the PPI. Computational architecture of PPI-based VCRS is also described. This is a multi-tier and multi-protocol system. Data are managed by several ORDBMS instances. Communications design is based on the virtual private network (VPN) concept. Renal patients have a minimum reliable access to the VCRS through a public switch telephone network--X.25 gateway. Design complies with the universal access requirement, allowing an efficient and inexpensive connection even in rural environments and reducing computational requirements in the patient's remote access unit. VCRS provides support for renal patients' healthcare, increasing the quality and quantity of monitored biomedical signals, predicting events as hypotension or low dialysis dose, assisting further to avoid them by an online therapy modification and easing diagnostic tasks. An online therapy adjustment experiment simulation is presented. Finally, the presented system serves as a computational aid for research in renal physiology. This is achieved by an open and reusable modeling and simulation architecture which allows the interaction among models and data from different scales and computer platforms, and a faster transference of investigation models toward clinical applications.

Effect of reversal of catheter ports on recirculation: comparison of the PermCath with Tesio Twin Catheter

To study the effect of the reversal of the blood ports on blood flow rate (QB), percentage recirculation, and blood urea nitrogen (BUN) clearance, we compared 12 well functioning chronic hemodialysis catheters (7 PermCaths and 5 Tesio Twin Catheters) in both standard and reversed blood flow setups. The reversal of PermCath ports caused no change in the QB (307+/-20 ml/min vs. 314+/-9 ml/min, standard vs. reversed hook-up, respectively), but a significant increase in percentage recirculation (2.5+/-1.8% vs. 12+/-4.6%, standard vs. reversed hook-up, respectively, p = 0.02). Reversal of the Tesio Twin Catheter ports caused a significant decline in QB (296+/-13 ml/min vs. 250+/-16 ml/min, standard vs. reversed hook-up, respectively, p = 0.02), but no significant change in percentage recirculation (2.8+/-1.4% vs. 3.8+/-2.5%, standard vs. reversed hook-up, respectively, p = not significant). Reversal of the ports caused no significant change in BUN clearance with the PermCath (264+/-18 ml/min vs. 257+/-17, standard vs. reversed hook-up, respectively, p = 0.8), but a significant decline in BUN clearance with the Tesio Twin Catheter (247+/-11 ml/min vs. 216+/-13.5 ml/min, standard vs. reversed hook-up, respectively, p = 0.015). In conclusion, our results suggest that reversed hook-up of a well functioning Tesio Twin Catheter is associated with a significant decline in QB and BUN clearance, but no change in percentage recirculation; however, inadvertent reversed hook-up of a well functioning PermCath can lead to a considerable increase in percentage recirculation but no change in QB or BUN clearance.

Recirculation in double lumen catheter veno-venous extracorporeal membrane oxygenation measured by an ultrasound dilution technique

Recirculation is a limiting factor for oxygen delivery in double lumen catheter veno-venous extracorporeal membrane oxygenation (DLVV-ECMO). This study compares three different methods for the determination of the recirculation fraction during double lumen catheter veno-venous ECMO at ECMO flow rates of 150, 125, 100, 75, and 50 ml/kg.min in nine lambs: (1) an ultrasound dilution method, in which the change in ultrasound velocity in blood after injection of a saline bolus as a marker is used for determination of recirculation; (2) an SvO2 method using real mixed venous blood oxygen saturation, the gold standard, for determination of recirculation fraction; and (3) the CVL method, in which oxygen saturation of a blood sample of the inferior vena cava is considered to represent mixed venous oxygen saturation. In all methods, the recirculation fraction increased with increasing ECMO flow rate. The correlation coefficient between the ultrasound dilution method and the SvO2 method was 0.68 (p < 0.01); mean difference was -2.4% (p = 0.6). Correlation coefficient between the ultrasound dilution method and the CVL method was 0.48 (p < 0.01); mean difference was -18.1% (p < 0.01). The correlation coefficient between the SvO2 method and the CVL method was 0.51 (p < 0.01); mean difference was -15.7% (p < 0.01). The ultrasound dilution method is a useful method for measurement of the recirculation fraction in DLVV-ECMO and is easier to use than the other methods.

Glucose infusion test: a new screening test for vascular access recirculation

BACKGROUND

Vascular access recirculation is an important cause of diminished dialysis efficiency. We propose a new screening test based on glucose infusion as a tracer for recirculation.

METHODS

The glucose infusion test (GIT) protocol comprises a basal blood sample (A) from the arterial port, a 5 mL bolus of 20% glucose into the venous chamber (time 0), followed by a second sample (B) in four seconds (from 13 to 17 s with QB 300 mL/min) from the same port. The blood glucose level is determined at the bedside on A and B with a reflectance photometer (CV 1.8%). Interpretation of the test is straightforward: If B = A, there is no recirculation, whereas if B > A, recirculation can be calculated from the regression equation: 0.046 x (B - A) + 0.07, obtained from in vitro tests reproducing artificial recirculation at 0, 5, and 10%. To validate this new method in vivo, we compared GIT and the urea test on 39 hemodialysis patients, obtaining a good correlation (r = 0.93). The two tests were considered positive (recirculation present) when the lower 95% confidence intervals were more than zero.

RESULTS

Our patients were divided into two groups: those with (22 out of 39, mean recirculation 11.8%) or without recirculation (17 out of 39, mean 0.06%). The urea test did not recognize 7 out of 22 patients because they had a small recirculation below the urea test limit of detection.

CONCLUSIONS

GIT was more sensitive (detection limit 0.3%), simpler, and immediate in showing the results than the urea test. It is an accurate and low-cost technique for screening and follow-up of vascular access in a dialysis unit.

All currently used measurements of recirculation in blood access by chemical methods are flawed due to intradialytic disequilibrium or recirculation at low flow

Blood flows and recirculations with standard and reversed direction of lines were measured by chemical (urea and creatinine) and ultrasound dilution (saline) methods in 47 chronic hemodialysis patients. Thirty-seven patients had 47 dual-lumen, central vein (CV) catheters: 32 were PermCath (Quinton Instruments Company, Seattle, WA), 6 were Access Cath (MEDCOMP, Harleysville, PA), 3 were Soft Cell PC (Vas Cath, Mississauga, Ontario, Canada) and 6 were SNIJ (experimental catheters). Three of these last catheters had the tip staggered 7 mm, and three had flush tips; PermCath, Access Cath, and Soft Cell PC catheters have the tips staggered 23 to 25 mm. Forty-six catheters were implanted into the superior vena cava/right atrium, and one catheter was implanted through the left saphenous vein into the left iliac vein. The catheters were studied 1 to 31 months after implantation (median, 3.0 months). Ten patients with arteriovenous (AV) graft access were also studied. The stop-flow method was used in catheter dialysis, and the slow-flow method was used to calculate recirculations in AV access dialysis with samples for systemic blood concentrations taken from arterial line both before and after samples from the arterial and venous lines. At 500 mL/min pump speed, actual blood flow was 436+/-18 mL/min (mean+/-SD; range, 407 to 464 mL/min) with standard direction of catheter lines. At 500 mL/min pump speed, the arterial chamber pressure was -330+/-48 mm Hg (mean+/-SD; range, -380 to -225 mm Hg, and the venous chamber pressure was 259+/-48 mm Hg (mean+/-SD; range, 140 to 310 mm Hg). Arterial chamber pressure was less negative, and venous chamber pressure was less positive with SNIJ catheters, which had larger internal diameter (2.1 mm) compared with the other catheters (2.0 mm). Recirculation varied with the catheter design and the location of the catheter tip. In the catheters with tip staggered more than 20 mm and with standard line connection at pump speeds of 50 mL/min and 500 mL/min, recirculations were approximately 1 % and 5%, respectively, when measured by the chemical method. In the same catheters with reversed lines, the recirculations were approximately 5% and 27%, respectively. Inflow failure catheters with reversed lines had similar recirculation values to those of well-functioning catheters with reversed lines. In catheters with tips staggered 7 mm, and with standard connection of lines, recirculations were approximately 3% and 8%, respectively, at pump speeds of 50 and 500 mL/min. With reversed lines, at the same pump speeds, the values were 7% and 12%, respectively. In flush-tip catheters, the recirculation was higher at a 50 mL/min pump speed (approximately 17%) than at a pump speed of 500 mL/min (approximately 13%). The ultrasound dilution method usually gave lower values than the chemical methods, most likely because of overestimation of recirculation by chemical methods. At least triplicate measurements are needed because single measurements by the ultrasound dilution method are associated with substantial variation. We conclude that both currently used methods (stop flow and slow flow) of taking systemic samples for measurements of recirculation by chemical methods are flawed because of disequilibrium and recirculation at low flow.

Central venous dialysis catheter dysfunction

Central venous catheter dysfunction is a limiting factor in regard to renal replacement therapy efficiency and can thus influence patient morbidity. Early catheter dysfunction is frequently due to mechanical problems such as inadequate positioning, kinking, or constriction, but early fibrin deposition can develop soon after insertion. Delayed dysfunction usually results from thrombus formation, either within the lumen, around the catheter ("fibrin sleeve"), or in the host vein. Catheter dysfunction is suspected clinically or documented by simple imaging studies. It is usually evident and manifested by failure to aspirate blood from the lumen(s), inadequate blood flow and/or high resistance pressures during hemodialysis. However, a more subtle dysfunction may lead to a high recirculation of dialyzed blood and be overlooked if dialysis adequacy is not monitored regularly. Local instillation of a fibrinolytic agent is usually successful in restoring catheter patency. Central venous dialysis catheters present intrinsic limitations consequent to their composition and design, whereas extrinsic limitations result from site of insertion, blood properties and anatomic particularities of a given individual. These characteristics largely determine overall catheter performances. Performance parameters to consider include maximal consistently achievable blood flow rate, resistance to blood flow indicated by arterial and venous pressures during hemodialysis, and blood recirculation rate. Catheter longevity is an important consideration for cuffed catheters implanted for long-term use. The tolerated blood recirculation within central venous dialysis catheters should be below 10% to 15%, and is ideally between 3% to 7% in most clinical settings. Several recent studies confirm that short femoral catheters recirculate significantly more than is desirable. Well functioning and nonreversed internal jugular and subclavian venous catheters have, in general, recirculation rates less than 5%. With regard to various performance criteria, the TwinCath (Medcomp, Harleysville, PA) appears particularly advantageous. In any case, a good catheter maintenance program is of critical importance for the prevention and the early detection of catheter dysfunction.

Validation of a revised slow-stop flow recirculation method

Slow flow/stop flow methods have replaced the three needle technique as methods of choice for measuring recirculation. However, the time delay after reducing blood flow may affect the BUN in the systemic (slow flow/stop flow arterial line) sample and therefore limit the accuracy of this methodology. It has been observed that recirculation does not occur in a properly cannulated access unless the access blood flow rate is less than the dialyzer blood flow rate (BFR). This suggests that the systemic sample could be obtained at a higher than usual blood pump rate. We studied 50 patients and compared a revised slow-stop flow (S/SF) recirculation technique in which the systemic sample was drawn after the blood pump rate was reduced to 120 ml/min for 10 seconds and then stopped, to a non-urea based method that utilized indicator velocity dilution (IVDM). Seven patients were found to have recirculation by IVDM; all had recirculation by S/SF of more than 10% (minimum 16.7%) and an access BFR that was less than the dialyzer BFR. In the 43 patients without recirculation by IVDM, the mean recirculation by S/SF was 1.9 +/- 3.2% (mean +/- SD). Five patients without recirculation by IVDM had more than 5% recirculation by S/SF (range, 5.9 to 8.3%). Although there was a small systematic tendency to overestimate recirculation, this modified urea based method was still able to detect recirculation with good reliability. Single values above 10% are highly likely to indicate the presence of true recirculation. Repeated values over 5%, are also likely to be significant, indicating the presence of true recirculation and its clinical correlate, marginal access blood flow.

Recirculation, urea disequilibrium, and dialysis efficiency: peripheral arteriovenous versus central venovenous vascular access

When accurate, non-urea-based methods of measuring recirculation are used, recirculation is usually absent in arteriovenous (AV) accesses. When urea-based methods are used to measure recirculation in AV accesses, falsely elevated recirculation rates are common. These errors are due to AV and venovenous disequilibrium (peripheral vein method), delayed systemic sampling (two-needle methods), and errors in urea measurement (all methods). The literature suggests that recirculation in central venovenous (CV) catheters is approximately 5%. The methods used for these determinations have all been urea based. However, there are few theoretical problems in using urea-based measurements for measuring recirculation in this setting, making it more likely that these values are accurate. When hemodialysis via CV and AV accesses are compared, equilibrated Kt/V values differ significantly for the same single-pool Kt/V when 15-second postdialysis blood urea nitrogen values are used for modeling, but differ minimally when 2-minute postdialysis samples are used. The impact of transient retrograde blood flow in the superior vena cava on recirculation and whether dialysis efficiency is influenced by the exact site of CV catheter placement (superior vena cava v right atrium) is uncertain.

Recirculation measures with urea and mannitol during hemodialysis

A simple, accurate, and reproducible method of measuring recirculation in grafts during hemodialysis is essential for improving the efficiency of dialysis. In our studies, plasma samples for plasma urea nitrogen (PUN) were taken from the arterial line of the dialyzer at blood flows (A) of 200, 300, and 400 ml/min, preceded by a 5-min period of equilibration, and at 15 s and 2 min after turning the flow down to 100 ml/min (S), the latter serving as systemic samples. Recirculation was calculated as (S - A)/(S - V). Total blood flow (Qb) through the grafts was measured by color Doppler ultrasound. We found a significant, inverse relationship between recirculation and total flow through the graft at dialyzer Qb of 400 but not 300 or 200 ml/min. The magnitude and prevalence of recirculation was always greater when the 2 min sample was used as S compared to the 15 s sample and as dialyzer Qb increased. As a qualitative, urea-independent measure of recirculation, we assayed the appearance of mannitol in the arterial line in blood drawn 15 s after initiating a mannitol push into the venous line. Blood obtained just prior to the mannitol push was used as the zero blank. Thirteen of 18 patients had a measurable, but low, level of mannitol, 5 did not, and 2 had inconsistent results in studies done on separate days. We conclude that the majority of patients receiving chronic hemodialysis have a low degree of recirculation and that methods relying on urea must be suspected of exaggerating the true degree of recirculation.

Techniques for prospective detection of venous stenosis

The vascular access device continues to be a bottleneck in the quest for improved dialysis efficiency and cost reduction. Stenotic lesions occur frequently in synthetic arteriovenous fistulas (AVFs), usually at the venous end, and less often in native AVFs. The reduction in blood flow and other thrombogenic effects of the stenosis, such as local turbulence, eventually lead to loss of the access. Before thrombosis occurs, reduced blood flow through the AVF limits inflow to the dialyzer and predisposes to local recirculation. Recirculation decreases the effective solute clearance of the dialyzer, jeopardizing the adequacy of treatment. Regular evaluation of the access using methods such as routine physical examination, measurement of recirculation, measurement of venous dialysis pressure, and radiographic or ultrasonic imaging when combined with percutaneous or surgical interventions have been shown to prolong access life and eliminate recirculation. Physical examination includes inspection and palpation to detect edema, palpation and auscultation to detect local increases in the intensity of a thrill or bruit, and optional occlusion of the fistula during dialysis to detect recirculation. Recirculation can be measured directly using classical solute dilution techniques or indicator dilution methods provided by a variety of devices now appearing on the market. Recirculation may also be detected indirectly from the results of urea modeling. The difference between modeled and expected urea clearance is a measure of recirculation provided no other error (eg, in blood flow) contributes to the difference. Pressure monitoring has proven useful in many centers. A strict protocol is required to normalize other influences such as blood flow and needle size that may alter pressure independently of access stenosis. Duplex Doppler ultrasonography has been evaluated and found useful in several studies but suffers from relatively high cost and operator dependency. Controlled studies of these screening techniques are needed, especially for those that incur high costs.

Recirculation reassessed: the impact of blood flow rate and the low-flow method reevaluated

The recognition that during hemodialysis the blood urea nitrogen level in a peripheral vein differs substantially from that in an artery has made invalid the conclusions of prior studies based on the standard (peripheral vein) means of calculating recirculation. The increase in recirculation associated with an increase in blood flow may be artifactual, as may the reported inaccuracy of the two-needle low blood flow technique for measuring recirculation. Using a stop-flow technique to measure recirculation, a study was performed to reassess these issues. Sixteen patients were each studied once within the first 30 minutes of a standard hemodialysis session (mean, 3.78 hour) at routine blood flow (mean, 419 mL/min) and at 200 mL/min. Dialyzer afferent and efferent blood line samples were obtained, as were afferent samples after sequential, rapid reductions in blood flow to 200, 50, and 0 mL/min. The final two samples were obtained within 30 seconds of reducing blood flow to less than 200 mL/min. Blood flow had a significant (P = 0.03) although clinically minor effect on recirculation (mean, 13.9% at 419 mL/min v 10.2% at 200 mL/min). The low blood flow technique yielded the same results as the stop flow technique (stop flow technique 13.9% v low blood flow technique 13.2% at 419 mL/min, P = 0.76). We conclude (1) that the increase in recirculation associated with increased blood flow is substantially less than has been reported previously and is minimal in a well-functioning access, and (2) that a modified low blood flow (50 mL/min) technique for measuring recirculation yields results equivalent to those obtained using the stop flow technique.