Access Flow Measurement Research Articles

Management of vascular access inflow-outflow imbalance: A bimodal approach.

A more accurate descriptive and clinically useful diagnosis based upon pathophysiology for what is commonly referred to as venous outflow stenosis is inflow-outflow imbalance. In these cases, the total outflow capacity of the AV access is inadequate to handle the inflow volume (Qa) without an increase in pressure. The relative inadequacy of the access outflow capacity in comparison to Qa results in increased outflow resistance and a proportional increase in intraluminal pressure. The clinical indicators associated with venous stenosis are the resulting manifestations of this imbalance. The point at which this occurs is dependent upon variations in these two parameters-Qa and outflow resistance. The variations in these two parameters are considerable and reciprocal. Excessive Qa results in or can lead to an entire list of serious problems that adversely affect patient morbidity and mortality. Most studies dealing with AV access Qa reduction have been for the treatment of an existing condition rather than its prevention; however, prevention of disease rather than waiting for its development is an important tenet of medical practice. The resulting clinical picture of inflow-outflow imbalance is taken as an indication for corrective treatment. In the past, in most cases this has meant angioplasty to open the outflow if it is reduced; however, this clinical picture may be associated with an excessive Qa and angioplasty in these cases creates the risk for a further increase in Qa. It is the authors' opinion that access flow measurements should be a part of the evaluation of these cases prior to planning treatment. Using this information, a bimodal approach to primary treatment should be adopted involving either angioplasty for cases with a low or normal Qa or flow reduction in cases with an elevated Qa.

Hemodialysis access flow measurement: Comparison of ultrasound dilution and ultrafiltration method on NIKKISO DBB-EXA™ dialysis machine.

The methods of estimating vascular access (VA) flow rates are usually based on the indicator dilution theory by measuring recirculation during dialysis sessions. This is an observational study comparing the VA flow rates measured by NIKKISO DBB-EXA™ and Transonic®. Sixty-five patients (38 M/27 F, mean age 72 ± 10 years) participated in the study. We measured the VA flow rates during dialysis twice with each method and repeated the procedure 7 days later. In 130 double measurements for each method on the same day, mean flow with Transonic® was 1413±715 ml/min and with DBB-EXA™ 1297 ± 664 ml/min. In Bland-Altman analysis, the mean difference between the two methods was 159 ± 211 ml/min (limits of agreement: -274 and 572 ml/min). Eighty-one out of the 130 DBB-EXA™ measurements were within 25% of the Transonic® measurements (62% accuracy). Regarding reproducibility of each method on different days, mean difference in the Bland-Altman analysis was 29 ± 620 ml/min (limits of agreement: -1186 and 1244 ml/min) for the Transonic® measurements and 132 ± 625 ml/min (limits of agreement: -1092 and 1356 ml/min) for the DBB-EXA™ measurements. The measurements on two different days were within 25% of each other for 52 of the 65 patients (80%) with the Transonic® method, and for 35 of the 65 patients (54%) with the DBB-EXA™ method. In conclusion, the DBB-EXA™ method underestimates VA flow rates compared to the Transonic® technique, resulting in a limited accuracy of 62%. There was poor reproducibility for both methods in different day measurements with better performance of the Transonic® technique. The DBB-EXA™ method could be used as a simple tool for a rough estimate of VA flow rates but cannot replace the Transonic® reference method.

MO771: Haemodialysis Access Flow Measurement: Comparison of Ultrasound Dilution and Ultrafiltration Method on Nikisso DBB-EXA™ Dialysis Machine

Abstract BACKGROUND AND AIMS The regular measurement of the haemodialysis vascular access (VA) flow demonstrates a protective effect over thrombotic complications [1]. Transonic® Ultrasound dilution technique was first introduced in 1995 and progressively became a method of reference for recirculation and blood flow measurement on haemodialysis VA [2]. It is based on the technique of haemodilution with a certain volume of 0.9% NaCl and the measure of recirculation with inversed lines using the ultrasound technique. Consecutively the VA flow is estimated with a certain formula. Nikisso DBB-EXA™ dialysis machine has the capacity to measure the VA recirculation rate by photometry performing a controlled rise on ultrafiltration rate. The aim of the present study is to compare the VA flow rate between the Transonic® Ultrasound dilution technique and the Nikisso DBB-EXA™ Ultrafiltration method. METHOD The primary outcome of the study is the association of VA flow measured by Transonic® and the Nikisso DBB-EXA™. During the first half of the same dialysis session, we measured the VA recirculation and flow rate twice with the two methods under a constant dialysis blood pump flow of 250–300 mL/min with the ultrafiltration and haemodiafiltration interrupted during the procedure after having reversed the dialysis lines. RESULTS The values are represented as mean ± SD. A total of 18 chronic haemodialysis patients (13 male: 5 female, mean age: 70 ± 14 years) participated in the study. They were haemodynamically stable without acute clinical conditions and were dialysed with a well-functioning arteriovenous fistula for at least 6 months. The mean value of recirculation with inverted dialysis lines for the Transonic® method was 20.79 ± 7.58% and for the Nikisso DBB-EXA™ was 24.43 ± 7.77. The mean values of VA blood flow with the Transonic® method were 1229 ± 588 mL/min and for the Nikisso DBB-EXA™ 1096 ± 490 mL/min. The measured values of the two methods presented a significant positive correlation (by Pearson) (R = 0.920, P < 0.001) (Figure). CONCLUSION There is a significantly positive correlation concerning the VA flow measurement between the reference method of Transonic® and the Ultrafiltration method offered by the Nikisso DBB-EXA™. Larger studies are needed to confirm this correlation with an additional comparative analysis of the reproducibility of measurements.

Mathematical Models for Blood Flow Quantification in Dialysis Access Using Angiography: A Comparative Study.

Blood flow rate in dialysis (vascular) access is the key parameter to examine patency and to evaluate the outcomes of various endovascular interve7ntions. While angiography is extensively used for dialysis access–salvage procedures, to date, there is no image-based blood flow measurement application commercially available in the angiography suite. We aim to calculate the blood flow rate in the dialysis access based on cine-angiographic and fluoroscopic image sequences. In this study, we discuss image-based methods to quantify access blood flow in a flow phantom model. Digital subtraction angiography (DSA) and fluoroscopy were used to acquire images at various sampling rates (DSA—3 and 6 frames/s, fluoroscopy—4 and 10 pulses/s). Flow rates were computed based on two bolus tracking algorithms, peak-to-peak and cross-correlation, and modeled with three curve-fitting functions, gamma variate, lagged normal, and polynomial, to correct errors with transit time measurement. Dye propagation distance and the cross-sectional area were calculated by analyzing the contrast enhancement in the vessel. The calculated flow rates were correlated versus an in-line flow sensor measurement. The cross-correlation algorithm with gamma-variate curve fitting had the best accuracy and least variability in both imaging modes. The absolute percent error (mean ± SEM) of flow quantification in the DSA mode at 6 frames/s was 21.4 ± 1.9%, and in the fluoroscopic mode at 10 pulses/s was 37.4 ± 3.6%. The radiation dose varied linearly with the sampling rate in both imaging modes and was substantially low to invoke any tissue reactions or stochastic effects. The cross-correlation algorithm and gamma-variate curve fitting for DSA acquisition at 6 frames/s had the best correlation with the flow sensor measurements. These findings will be helpful to develop a software-based vascular access flow measurement tool for the angiography suite and to optimize the imaging protocol amenable for computational flow applications.

ASDIN white paper: Management of cephalic arch stenosis endorsed by the American Society of Diagnostic and Interventional Nephrology.

Brachiocephalic arteriovenous fistulas (AVF) makeup approximately one third of prevalent dialysis vascular accesses. The most common cause of malfunction with this access is cephalic arch stenosis (CAS). The accepted requirement for treatment of a venous stenosis lesion is ⩾50% stenosis associated with hemodynamically abnormalities. However, the correlation between percentage stenosis and a clinically significant decrease in access blood flow (Qa) is low. The critical parameter is the absolute minimal luminal diameter (MLD) of the lesion. This is the parameter that exerts the key restrictive effect on Qa and results in hemodynamic and functional implications for the access. CAS is the result of low wall shear stress (WSS) resulting from the effects of increased blood flow and the unique anatomical configuration of the CAS. Decrease in WSS has a linear relationship to increased blood flow velocity and neointimal hyperplasia exhibits an inverse relationship with WSS. The result is a stenotic lesion. The presence of downstream venous stenosis causes an inflow-outflow mismatch resulting in increased pressure within the access. Qa in this situation may be decreased, increased, or within a normal range. Over time, the increased intraluminal pressure can result in marked aneurysmal changes within the AVF, difficulties with cannulation and the dialysis treatment, and ultimately, increasing risk of access thrombosis. Complete characterization of the lesion both hemodynamically and anatomically should be the first step in developing a strategy for management. This requires both access flow measurement and angiographic imaging. Patients with CAS present a relatively broad spectrum as relates to both of these parameters. These data should be used to determine whether primary treatment of CAS should be directed toward the anatomical lesion (small MLD and low Qa) or the pathophysiology (large MLD and high Qa).

P1358THE MONITORING OF FIRST LEVEL IN DIAGNOSIS OF COMPLICATION OF VASCULAR ACCESS: MULTICENTRIC STUDY ON 900 HEMODIALIZED PATIENTS WITH FOLLOW UP TO 5 YEARS

Abstract Background and Aims Vascular Access (VA) is the lifeline of hemodialysis patient. The universal goal of access monitoring is to identify access stenosis and enable intervention prior to thrombosis; thereby, maximizing access longevity and minimizing morbidity. The advent and use of techniques including dynamic and static venous pressure monitoring, physical examination, access flow measurement, imagining and combined imaging and flow monitoring by duplex ultrasound demonstrate that it is possible to predict which accesses are at high risk for future thrombosis. Currently arteriovenous fistula (AVF) and arteriovenous graft (AVF) have been recognized as the permanent access. This study takes advantage of the opportunity to utilize data from 19 associated Dialysis Clinics to examine trends in VA use, trends in patient characteristics and practice associated with VA. Determine the status quo of the overall information related to the VA to start a five years follow up study with the aims to reduce VA complications 8stenosis and thrombosis) and related hospitalization days to improve patient’s quality of life. Method VA data were collected for each patient at study entry. Practice pattern data from the facility medical director, nurse manager and VA surgeon were also analyzed. We have developed a mask on the management database to implement the first level monitoring of access and collected data at each treatment (Fig.1) Results Average age of 801 patients enrolled was 73,5 years. Native AVF was used by 79%, AVG by 2% and CVC by 19%. As the age increase as well as he use of CVC move from 6% (15-39(years) to 50,5% in patients with more than 85 years. Most frequent complications were: Thrombosis 16,5 %, Infectios 5,5%. Related VA hospitalization days are 28,7 % of total days. Average dialysis goals achieved were: QB 290 ml/Min; blood processed 69,7 L; KT/V:1,35. Conclusion By interpreting collecting data for specific performance measures using accurate reports allow health care professionals to highlight the VA performance/inefficiences and provide correct information to the clinical staff to support them in their daily clinical practice and decision making.

SAT-049 HOW GOLDEN IS THE STANDARD? MEASUREMENT ERRORS IN THE DETECTION OF HIGH FLOW ARTERIOVENOUS ACCESS

The development of a high flow state in a surgically created arteriovenous (AV) access can lead to patient harm as a consequence of reduced clearance due to cardiopulmonary recirculation, cardiac failure and vascular steal. Access flow (Qa) measurement using saline indicator dilution has been proposed as the ‘gold standard’ for measurement of Qa however has not been validated for accuracy at high Qa rates in vivo. A Qa value of >2L per minute has been proposed as a marker of a high flow state at risk of cardiac dysfunction.

Efecto de la dirección de la aguja arterial en la medición del flujo del acceso vascular con métodos dilucionales

Introducción: La medida del flujo del acceso vascular a través de métodos dilucionales exige la punción retrograda de la aguja arterial. Las últimas recomendaciones de canalización del acceso defienden la punción anterógrada. Objetivo: Cuantificar el error de medición producido por la punción anterógrada y analizar su repercusión clínica. Material y Método: Estudio transversal multicéntrico. Se analizaron medidas de flujo con Termodilución y Ultrasonografia dilucional en punciones anterógradas y retrogradas. Se utilizó el método Bland-Altman, el coeficiente de correlación intraclase y el índice Kappa para contraste con Eco-Doppler. Resultados: Se analizaron 37 fístulas. La punción anterógrada respecto a la retrograda generó medidas que sobrestimaban su diagnóstico en un 65% con Termodilución y 59% con Ultrasonografia Dilucional. El coeficiente de correlación intraclase fue 0,831 para Termodilución y 0,724 para Ultrasonografía Dilucional. El grado de acuerdo clínico entre ambos modos de punción para catalogar a las fístulas con flujo menor de 500ml/min fue del 95% (Índice Kappa=0,841) para Termodilución y del 91% (Índice Kappa=0,760) para Ultrasonografía Dilucional. El grado de acuerdo con Eco-Doppler fue del 86% (Índice Kappa=0.586) y 92% (Índice Kappa=0.720) con Termodilución y Ultrasonografía Dilucional del 78% (Índice Kappa=0.349) y 81% (Índice Kappa=0.400) utilizando la punción anterógrada y retrograda respectivamente. Conclusión: La punción anterógrada presenta un sesgo significativo incrementándose en flujos altos. Clínicamente, el grado de concordancia con ambos métodos dilucionales en los distintos tipos de punción y Eco-Dopler es bueno para descarte de intervención.

Flow reduction in high-flow arteriovenous fistulas improve cardiovascular parameters and decreases need for hospitalization.

High output heart failure (HF) and pulmonary hypertension have been demonstrated in patients with prevalent arteriovenous (AV) fistulas. Fistulas with flow >2000 mL/minutes are more likely to induce changes in cardiac geometry and pulmonary artery pressure. The effects of reducing flow in AV access and its implications on HF decompensation and hospitalizations have not been studied. Retrospective analysis of 12 patients who needed hospitalization for acute Congestive Heart Failure (CHF) decompensation with AV access flow of 2 L/minutes (as defined by Kidney Disease Outcomes Quality Initiative (KDOQI)) or more were included in the study. All the patients underwent banding of their inflow at the anastomosis with perioperative access flow measurement. Follow-up period was 6 months. 2D echo was done at 6 months postbanding in addition to access flow and clinical evaluation. Complete data was available for all the 12 patients. Study data was collected on all the 12 patients. Mean age was 64.7 years. The mean access flow pre and postbanding were 3784 mL/minutes and 1178 mL/minutes, respectively (P < 0.001). Eighty percent of the patients had diabetes and 41% had coronary artery disease. There was a statistically significant decrease in cardiac output (pre = 7.06 L/minutes, post = 6.47 L/minutes P = 0.03), pulmonary systolic pressure (pre = 54 mmHg, post = 44 mmHg P = 0.02), left ventricular mass index (LVMI) (pre = 130 g/m(2) , post = 125 g/m(2) P = 0.006) and need for rehospitalization for CHF decompensation. The New York Heart Association (NYHA) staging improved by 1 stage postbanding (P = 0.002). The hospitalization rate was 3.75 ± 1.2 in the 6 months before banding and was decreased to 1.08 ± 1.2 (P = 0.002) postbanding. The hemoglobin level, predialysis systolic blood pressure, calcium phosphorous product and the use of Renin Angiotensin Aldosterone System (RAAS) blockade agents and calcium channel blockers were comparable before and after inflow banding. Flow reduction in high flow fistulas are associated with decreased LVMI and pulmonary artery pressures. There is also a significant reduction in the risk for hospitalization due to acute HF and an improvement in NYHA heart failure stage.

The use of tissue plasminogen activator as continuous infusion into an arteriovenous hemodialysis access in the hemodialysis unit: a case series.

BackgroundThrombolytics such as tissue plasminogen activator are often used by interventional radiologists in the radiology suite to salvage thrombosed hemodialysis arteriovenous fistulae. Thrombolytics are also commonly used in dialysis facilities as a dialysis catheter lock solution or as an infusion into a dialysis catheter when dysfunctional. However, the use of tissue plasminogen activator as a continuous infusion into an arteriovenous fistula in the dialysis facility to treat clot burden is not commonly done.ObjectiveThe aim of our case series is to demonstrate the successful use of tissue plasminogen activator to decrease clot burden in an arteriovenous fistula in the dialysis unit.DesignObservational case series.SettingAn outpatient dialysis facility in a tertiary care hospital.PatientsThree non- consecutive patients were diagnosed with an acute thrombosed arteriovenous fistula either on physical exam or by imaging.MeasurementsA thrombosed fistula as well as successful resolution of clot was illustrated by either physical exam, fistulogram or ultrasound. A functioning fistula demonstrated successful continuation of dialysis and adequate access flow measurements in follow up.MethodThis is a case series that describes the technique of using an infusion of tissue plasminogen activator into an arteriovenous fistula on the dialysis unit by vascular access nurses to treat or diminish clot burden.ResultsWe have described three separate cases where treatments with tissue plasminogen activator infusions on the dialysis unit resulted in objective evidence of decrease in clot burden on ultrasound or fistulogram.LimitationsThe small numbers of our case series requires that the results need to be verified in a larger study. Inclusion and exclusion criteria need to be defined before widespread application of this technique.ConclusionsTo our knowledge this small case series is the first to describe the procedure whereby low dose tissue plasminogen activator is directly infused into the fistula by the vascular access nurse in the dialysis unit during dialysis and not in the interventional suite. This provides additional information to the existing literature that there is an alternative option for dialysis units to diminish clot burden until a more permanent solution is established through angioplasty.

Why vascular access trials on flow surveillance failed.

Since the introduction of access flow surveillance technology for routine patient screening in 1995, more than 30 clinical trials have been presented in peer reviewed journals. Despite overall positive outcomes, some trials, including randomized control trials (RCTs), failed to produce positive outcomes for access surveillance. The purpose of this study is to analyze published data related to the main component of access surveillance--adequate increase of access flow after percutaneous transluminal angioplasty (PTA). A total of nine studies for arteriovenous grafts (AVGs) that include 350 accesses and nine studies for arteriovenous fistula (AVF) that included 503 accesses were considered for analysis from 14 publications. Practically, all reference data find high sensitivity (>90%) of access flow measurement to predict 50% stenosis. Mean access flow increase after PTA in AVGs was 319 ml/min (from 238 to 524 ml/min). Mean access flow increase in AVFs was 331 ml/min (from 195 to 402 ml/min). Relative flow increase in AVFs was 1.6 times greater than in AVGs. The authors of failed RCT for AVGs either did not select patients for PTA based on KDOQI guidelines and did not provide/analyze PTA flow results data, or reveal data that obviously show failure of PTA to adequately improve access flow. Access flow surveillance successfully identifies patients with hemodynamically significant stenosis. PTA performed on AVFs produce better hemodynamic results than in AVGs. Inadequate flow increases during PTA and not following KDOQI guidelines are major contributing factors for failed AVG randomized tails. Radiologists should use objective means for flow evaluation during PTA.

Transonic, thermodilution, or ionic dialysance to manage vascular access: Which method is best?

Regularly monitoring blood flow through a vascular access (Qa) can predict a dysfunction and dramatically reduce the number of thromboses. The aim of our study was to compare two integrated access flow devices, thermodilution (Qa-BTM: BTM(®), Fresenius Medical Care, Bad Homburg, Germany) and ionic dialysance (Qa-ID: OCM(®), Fresenius Medical Care, Bad Homburg, Germany), with the "gold standard" saline dilution (Qa-T: Transonic(®), Systems Inc., Ithaca, NY, USA). Measurements were performed sequentially and were repeated in the first 90 minutes of a single dialysis session in 24 long-term hemodialysis patients with a vascular access. Bland-Altman, linear regression (r(2)), and intraclass correlation coefficients (ICC) assessed reproducibility, correlations, and concordance between the techniques. Average access flow for Qa-T was 1549 (± 844) mL/minute, Qa-BTM was 1530 (± 856) mL/minute (P = NS), and Qa-ID was 1619 (± 1085) mL/minute (P = NS). Respectively, ICC, (r(2)), and bias were 0.99, (0.98), and -19 mL/minute for Qa-BTM, and 0.75, (0.65), and +69 mL/minute for Qa-ID. The limits of agreement were -287 to +250 mL/minute for Qa-BTM and -1647 to +1785 mL/minute for Qa-ID. Reproducibility of thermodilution and ionic dialysance, expressed as relative differences, was not significantly different from saline dilution. Recirculation, measured by saline dilution, was 0% (0-4%), the same as the 0% measured by thermodilution, with correct placement of bloodlines and corrected for cardiopulmonary recirculation. The integrated access flow measurement devices, thermodilution and ionic dialysance, are reasonable alternatives to using saline dilution to measure Qa: Thermodilution showed better precision and correlation. They are reliable, make monitoring of vascular access easier, incur no extra costs, and use no additional consumables.

Effect of intradialytic change in blood pressure and ultrafiltration volume on the variation in access flow measured by ultrasound dilution

BackgroundProspective access flow measurement is the preferred method for vascular access surveillance in hemodialysis (HD) patients. We studied the effect of intradialytic change in blood pressure and ultrafiltration volume on the variation in access flow measured by ultrasound dilution. MethodsAccess flow was measured 30minutes, 120minutes, and 240minutes after the start of HD by ultrasound dilution in 30 patients during 89 HD sessions and evaluated for variation. ResultsThe mean age of the 30 patients was 62±11 years: 19 were male. The accesses comprised 16 fistulae and 14 grafts. The mean access flow over all sessions decreased by 6.1% over time (1265±568mL/min after 30minutes, 1260±599mL/min after 120minutes, and 1197±576mL/min after 240minutes, P<0.01 by repeated measures ANOVA). In addition, a≥5% decrease in mean arterial pressure during HD significantly reduced access flow (P=0.014). However, no other variable (ultrafiltration volume, sex, age, presence of diabetes, type or location of access, body surface area, hemoglobin, serum albumin level) interacted significantly with the effect of time on access flow. Furthermore, mean arterial pressure did not correlate with ultrafiltration volume. ConclusionWe conclude that the variation in access flow during HD is relatively small. Decreased blood pressure is a risk factor for variation in access flow measured by ultrasound dilution. In most patients whose blood pressures are stable during HD, the access flow can be measured at any time during the HD treatment.

Impact of initial blood flow on outcomes of vascular access in hemodialysis patients

BackgroundDirect access flow measurements are considered the most useful surveillance method for significant stenosis, and ultrasound dilution has become the most popular and validated technique. The goal of this study was to evaluate access flow (Qa) at the time of first cannulation and its relationship to the survival of vascular access in Korean hemodialysis patients. MethodsWe conducted a prospective observational study from May 2004 to June 2011. We enrolled 60 patients (36 men) who underwent the first access operation between January 2004 and December 2005 and were followed-up for surveillance. ResultsMaturation failure occurred in nine patients (15%). Mean time to first use was 1.8±1.2 months after surgery. The patients were followed-up for a mean of 50.5±25.9 months. There were 25 deaths and six kidney transplants in patients with a functioning access. The total percutaneous transluminal angioplasty incidence was 50 in 27 patients (0.14/access-year). The initial Qa was 757.5±476.4mL/minute. First cannulation time was not significantly correlated with initial Qa (r=0.234, P=0.075). A total of 22 of the 60 patients (36.7%) had an initial Qa<500mL/minute. Maturation failure, initial Qa<500mL/minute, and the use of antiplatelet agents were risk factors for poor primary patency. Diabetic status and use of a graft were risk factors for low cumulative patency. ConclusionAn initial Qa<500mL/minute is a risk factor for poor primary patency, while an initial Qa<500mL/minute is not a risk factor for low cumulative patency or mortality.

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.

Measurement of Vascular Access Flow by Online Clearance Monitor in Online Hemodiafiltration

Background: Measurement of ionic dialysance has been shown to accurately determine urea dialyzer clearance and vascular access flow rate (Qa) in diffusive hemodialysis. Recently, online clearance monitoring (OCM) in an online hemodiafiltration (OL-HDF) machine could accurately measure urea dialyzer clearance. In the present study, OCM was utilized to assess the Qa in OL-HDF. Methods: In 32 stable end-stage renal disease patients undergoing predilution OL-HDF, the values of Qa measured by the OCM technique (Qa_OCMT) in a Fresenius 4008H HDF machine were compared with those determined by the standard ultrasound dilution technique (Qa_UDT). Results: The values of Qa_UDT and Qa_OCMT measured at the beginning of the HDF session were 800.5 ± 910.3 and 794.6 ± 895.4 ml/min (nonsignificant). There was a significant correlation between the two techniques (r = 0.95, p < 0.01). A Bland-Altman plot demonstrated a good agreement. Conclusion: The vascular access flow determined by OCM, which is integrated mostly in the current HDF machine, is highly accurate, easy to perform, and economical and can be used for vascular access surveillance in OL-HDF.

Validation of the Measurement of Haemodialysis Access Flow Using a Haemoglobin Dilution Test

Monitoring of blood flows in arteriovenous fistulae and arteriovenous grafts is recommended to predict access thrombosis. The ultrasound dilution technique (UDT) is the gold standard. We compare a recently described haemoglobin dilution technique (HDT) with the UDT in measurement of vascular access flow. Access blood flow was measured in 67 stable dialysis patients using HDT by bedside Hemocue (Hemocue AB, Ängelholm, Sweden) and laboratory measurement. Access blood flow was then measured by UDT in the same dialysis session. Median flow rate by UDT was 950 ml/min (IQR 490–1,440 ml/min), by Hemocue HDT 935 ml/min (IQR 475–1,395 ml/min, p = 0.534), and by laboratory haemoglobin HDT 920 ml/min (IQR 463–1,378 ml/min). Bland-Altman plots demonstrated poor agreement between UDT and HDT (limits of agreement for Hemocue HDT –22.7 to 20.1%, for laboratory HDT –21.2 to 20.4%). HDT can be used to measure vascular access flow but requires validation against clinical outcomes before being recommended as an alternative to UDT.

Comparison of Hemodialysis Access Flow Measurements Using Flow Dilution and In-Line Dialysance

Comparison of Hemodialysis Access Flow Measurements Using Flow Dilution and In-Line Dialysance

Comparison of Hemodialysis Access Flow Measurements Using Flow Dilution and In-Line Dialysance

Comparison of Hemodialysis Access Flow Measurements Using Flow Dilution and In-Line Dialysance

Hemodialysis Access Flow Measurements Using Flow Dilution and In-Line Dialysance Are Not Interchangeable for Hemodialysis Access Flow Surveillance

In Response: Our recent article1 compared hemodialysis access flow (QA) measurements using two methods: flow dilution (FD) and in-line dialysance (DD). The difference in the measured QA between the two methods was calculated, and the mean difference was reported as 29 ± 19 ml/min. Because the mean difference (rather than the difference itself) was provided, the standard error of the mean (SEM ∼19 ml/min) rather than standard deviation (SD ∼175 ml/min) was reported. Reporting the SEM follows the original Bland-Altman article2 because the intent here is to report the precision of estimating the mean. In addition, the SEM and SD are related by a simple formula, and hence, one can easily derive the SD (∼175 ml/min) from the SEM (∼19 ml/min). Some statisticians may prefer reporting the SD, and we understand that some readers may assume that the number reported might be the SD. However, by careful examination of our data (e.g., Figure 2), it would be apparent (given the data range shown in the figure) that the 19 ml/min cannot be the SD. We thank Dr. Krivitski (from Transonic Systems Inc., the manufacturer of the DD equipment) for pointing out that 19 ml/min cannot be the SD. The SD of 180 ml/min that he calculated is very close to the actual SD = 175 ml/min of our data. Because, it is common to show the mean ± 2 SD in Bland-Altman plots, we have added two dashed lines to our original Figure 2 showing these limits.Figure 2.: Bland-Altman analysis of flow dilution (FD) and in-line dialysance (DD). In the average, FD only slightly over estimates Q A compared with DD. The dashed lines show the mean and mean ± 2 SD of the difference (FD − DD).Comparison of our study1 results with an earlier similar study3 needs to be done with care. More specifically, the latter study reported a mean QA difference between the same two methods (using older models of the equipment) as 135 ± 229 ml/min. It is apparent from Figure 4 of Lacson et al.3 that 229 ml/min is the SD not SEM. Comparing QA differences between these two studies1,3 shows that the mean difference was 29 and 135 ml/min, respectively, and the SD was 175 and 229 ml/min, respectively. Although the SD values are comparable, the relatively smaller mean difference in our study1 suggests a possible improved agreement between DD and FD measurements. The mean difference represents a bias error that can be, relatively easily, reduced by calibration as earlier suggested.1 Comparison of these results suggests that the two studies1,3 are consistent and confirmatory. An important consequence of QA agreement/disagreement is the impact on imaging referral decisions. It can be seen that even with the larger QA bias reported,3 the QA differences resulted in nonsignificant differences in imaging referral decisions.3 Part of the referral differences is due to the QA bias in one technique vs. the other. The reduced QA bias found in our study1 suggests even less significant referral differences. Hence, it is our opinion that “FD and DD may be used interchangeably” as stated by Whittier et al.1 However, more studies are suggested to further address the referral issue.1,3 Regarding Dr. Krivitski's comment that we mistakenly “referenced a 1998 Gotch paper4 instead of Lacson et al. 2008,” we respectfully disagree because our study did not mention or refer to Gotch et al.4 In conclusion, there are no calculation or reference errors as Dr. Krivitski's letter suggested. Figure 2 was updated to reduce potential reader confusion about the value of SD (Figure 1). Although clear from context, we agree that it is better to clearly state that the numbers reported represent SEM not SD. Hansen A. Mansy William L. Whittier Richard H. Sandler Rush University Medical Center Chicago, IL