Dynamic venous access pressure ratio test for hemodialysis access monitoring

Association of risk stratification score with dialysis vascular access stenosis

BACKGROUNDS

Clinical monitoring is the recommended standard for identifying dialysis access dysfunction; however, clinical monitoring requires skill and training, which is challenging for understaffed clinics and overburdened healthcare personnel. A vascular access risk stratification score was recently proposed to assist in detecting dialysis access dysfunction.

PURPOSE

Our objective was to evaluate the utility of using vascular access risk scores to assess venous stenosis in hemodialysis vascular accesses.

METHODS

We prospectively enrolled adult patients who were receiving hemodialysis through an arteriovenous access and who had a risk score ⩽3 (low-risk) or ⩾8 (high-risk). We compared the occurrence of access stenosis (>50% on ultrasonography or angiography) between low-risk and high-risk groups and assessed clinical monitoring results for each group.

RESULTS

Of the 38 patients analyzed (18 low-risk; 20 high-risk), 16 (42%) had significant stenosis. Clinical monitoring results were positive in 39% of the low-risk and 60% of the high-risk group (p = 0.19). The high-risk group had significantly higher occurrence of stenosis than the low-risk group (65% vs 17%; p = 0.003). Sensitivity and specificity of a high score for identifying stenosis were 81% and 68%, respectively. The positive predictive value of a high-risk score was 65%, and the negative predictive value was 80%. Only 11 (58%) of 19 subjects with positive clinical monitoring had significant stenosis. In a multivariable model, the high-risk group had seven-fold higher odds of stenosis than the low-risk group (aOR = 7.38; 95% CI, 1.44-37.82; p = 0.02). Positive clinical monitoring results and previous stenotic history were not associated with stenosis. Every unit increase in the score was associated with 34% higher odds of stenosis (aOR = 1.34; 95% CI, 1.05-1.70; p = 0.02).

CONCLUSIONS

A calculated risk score may help predict the development of hemodialysis vascular access stenosis and may provide a simple and reliable objective measure for risk stratification.

ACR Appropriateness Criteria® Dialysis Fistula Malfunction

The creation and maintenance of a dialysis access is vital for the reduction of morbidity, mortality, and cost of treatment for end stage renal disease patients. One's longevity on dialysis is directly dependent upon the quality of dialysis. This quality hinges on the integrity and reliability of the access to the patient's vascular system. All methods of dialysis access will eventually result in dialysis dysfunction and failure. Arteriovenous access dysfunction includes 3 distinct classes of events, namely thrombotic flow-related complications or dysfunction, nonthrombotic flow-related complications or dysfunction, and infectious complications. The restoration of any form of arteriovenous access dysfunction may be supported by diagnostic imaging, clinical consultation, percutaneous interventional procedures, surgical management, or a combination of these methods. This document provides a rigorous evaluation of how variants of each form of dysfunction may be appraised and approached systematically. The American College of Radiology Appropriateness Criteria are evidence-based guidelines for specific clinical conditions that are reviewed annually by a multidisciplinary expert panel. The guideline development and revision process support the systematic analysis of the medical literature from peer reviewed journals. Established methodology principles such as Grading of Recommendations Assessment, Development, and Evaluation or GRADE are adapted to evaluate the evidence. The RAND/UCLA Appropriateness Method User Manual provides the methodology to determine the appropriateness of imaging and treatment procedures for specific clinical scenarios. In those instances where peer reviewed literature is lacking or equivocal, experts may be the primary evidentiary source available to formulate a recommendation.

Intradialytic techniques for automatic and everyday access monitoring

Vascular access dysfunction is associated with reduced delivery of dialysis, unplanned admissions, patient symptoms, and loss of access, making assessment of vascular access a fundamental part of routine care in dialysis. Clinical trials to predict the risk of access thrombosis based on accepted reference methods of access performance have been disappointing. Reference methods are time-consuming, affect the delivery of dialysis, and therefore cannot repeatedly be used with every dialysis session. There is now a new focus on data continuously and regularly collected with every dialysis treatment, directly or indirectly associated with access function, and without interrupting or affecting the delivered dose of dialysis. This narrative review will focus on techniques that can be used continuously or intermittently during dialysis, taking advantage of methods integrated into the dialysis machine and which do not affect the delivery of dialysis. Examples include extracorporeal blood flow, dynamic line pressures, effective clearance, dose of delivered dialysis, and recirculation which are all routinely measured on most modern dialysis machines. Integrated information collected throughout every dialysis session and analyzed by expert systems and machine learning has the potential to improve the identification of accesses at risk of thrombosis.

Detection of Hemodialysis Venous Needle Dislodgment Using Venous Access Pressure Measurements: A Simulation Study

Key Points

Hemodialysis machine pressure alarms may not detect venous needle dislodgment when patients have changes in venous pressure. A cross-sectional analysis of hemodialysis treatment data identified the occurrences of venous pressure changes that would make it hard to trigger a machine alarm. A proof-of-concept use of a data analytic–derived algorithm for the detection of venous needle dislodgments was demonstrated.

Background:

In rare instances, hemodialysis venous needles may become dislodged, and when left undetected, this can lead to severe injury or death. Although dialysis machines have alarms to detect venous needle dislodgment (VND), their range of detection is limited. An understanding of the clinical conditions that may lead to missed needle dislodgments is needed for the development of more robust detection systems.

Methods:

We created a sham dialysis circuit with a Fresenius 2008K dialysis machine for in vitro simulation testing of machine alarm behavior under variable conditions. The circuit used a blood substitute and mimicked a patient's venous access site. We varied blood flow rate, venous pressure (VP), and upward drift in VP and analyzed the time to alarm for the machine and an improved alarm algorithm. We also performed a cross-sectional retrospective study to identify the clinical occurrence of VP upward drift between September 1, 2016, and November 1, 2016, in patients on hemodialysis with an arteriovenous fistula.

Results:

Of 43,390 VP readings for 147 patients on hemodialysis, 16,594 (38%) showed an upward drift in VP (range 20–79 mmHg), with a mean±SD increase of 11±18 mm Hg within 20±14 minutes. A total of 19 VND simulations under different VP and blood flow parameters resulted in 19 (100%) algorithm alarm activations. Only eight simulations (42%) activated a machine alarm, and machine alarm activation time was longer than the algorithm activation time for all eight machine alarms (range 1–13 seconds).

Conclusions:

Patients can experience changes in VP during hemodialysis which may not trigger a machine alarm in the case of a VND. Our simulations showed that current dialysis machine alarm systems may not compensate for upward drift in VP, and improved algorithms for detecting needle dislodgment during hemodialysis are needed.

Development and validation of a risk score to prioritize patients for evaluation of access stenosis

Background

Access flow dysfunction, often associated with stenosis, is a common problem in hemodialysis access and may result in progression to thrombosis. Timely identification of accesses in need of evaluation is critical to preserving a functioning access. We hypothesized that a risk score using measurements obtained from the Vasc‐Alert surveillance device could be used to predict subsequent interventions.

Methods

Measurement of five factors over the preceding 28 days from 1.46 million hemodialysis treatments (6163 patients) were used to develop a score associated with interventions over the subsequent 60 days. The score was validated in a separate dataset of 298,620 treatments (2641 patients).

Results

Interventions in arteriovenous fistulae (AVF; n = 4125) were much more common in those with the highest score (36.2%) than in those with the lowest score (11.0). The score also was strongly associated with interventions in patients with an arteriovenous graft (AVG; n = 2,038; 43.2% vs. 21.1%). There was excellent agreement in the Validation datasets for AVF (OR = 2.67 comparing the highest to lowest score) and good agreement for AVG (OR = 1.92).

Conclusions

This simple risk score based on surveillance data may be useful for prioritizing patients for physical examination and potentially early referral for intervention.

Comparison of dynamic arterial and venous pressure between metal needles and plastic cannulas in incident hemodialysis patients with arteriovenous graft

Background and objects:

We hypothesized that plastic cannulae with 17-gauge inner needle compared to 16-gauge metal needles can have stable dynamic venous and arterial pressure maintaining prescribed blood flow safely during a hemodialysis treatment.

Design, setting, participants, and measurements:

A single-center, prospective, randomized, open-label clinical trial was conducted involving 16 incident hemodialysis patients who had arteriovenous graft placed as their first arteriovenous access. Subjects were randomized to metal needle group (n = 8) versus plastic cannula group (n = 8). We measured serial dynamic pre-pump arterial and venous pressure under five different blood pump flow rates (150, 200, 250, 300, and 350 mL/min).

Results:

The mean age of patients was 67.6 ± 8.5 (range: 51–81) years, and six patients (37.5%) were male. Patients with plastic cannula showed less negative arterial pre-pump pressures and lower venous pressures than those with metal needles at all prescribed blood flow rates, and the difference was statistically significant (P < 0.001).

Conclusion:

The plastic cannula had more stable arterial and venous pressures at prescribed blood pump flow rates than those pressures of the metal needle in incident patients with arteriovenous graft during hemodialysis.

KDOQI Clinical Practice Guideline for Vascular Access: 2019 Update

The National Kidney Foundation's Kidney Disease Outcomes Quality Initiative (KDOQI) has provided evidence-based guidelines for hemodialysis vascular access since 1996. Since the last update in 2006, there has been a great accumulation of new evidence and sophistication in the guidelines process. The 2019 update to the KDOQI Clinical Practice Guideline for Vascular Access is a comprehensive document intended to assist multidisciplinary practitioners care for chronic kidney disease patients and their vascular access. New topics include the end-stage kidney disease "Life-Plan" and related concepts, guidance on vascular access choice, new targets for arteriovenous access (fistulas and grafts) and central venous catheters, management of specific complications, and renewed approaches to some older topics. Appraisal of the quality of the evidence was independently conducted by using a Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach, and interpretation and application followed the GRADE Evidence to Decision frameworks. As applicable, each guideline statement is accompanied by rationale/background information, a detailed justification, monitoring and evaluation guidance, implementation considerations, special discussions, and recommendations for future research.

Screening for hemodialysis graft stenosis and short-term thrombosis risk: A comparison of the available tools

INTRODUCTION

Guidelines recommend hemodialysis graft screening to identify and repair significant (>50%) stenosis at high risk of thrombosis, but there is insufficient evidence to prefer one or other screening tool due to the lack of studies comparing all available options.

METHODS

Seeking an optimal screening approach, we compared the performance of all currently used tools (duplex ultrasound to detect significant stenosis (StD) and measure access blood flow (QaD), ultrasound dilution access blood flow (QaU), static venous intra-access pressure ratio (VAPR), dynamic arterial and venous pressures measurement, and monitoring) for diagnosing significant angiography-proven stenosis (StA) and predicting incipient thrombosis (occurring within 4 months) in 62 grafts. All thrombotic and symptomatic acute hypotension episodes were recorded during follow-up.

RESULTS

VAPR > 0.70 and QaU < 1600 mL/min were the best indicators to angiography for those aiming to identify the majority of StA (91% sensitivity) and QaU < 1000 mL/min or StD for those aiming to avoid unnecessary angiograms (95%-93% positive predictive value). At Cox's analysis, the only significant thrombosis predictors were acute hypotension episodes (relative risk = 4.4 (95% confidence interval = 2.2-8.8), p < 0.0001) and QaU or QaD (14% (95% confidence interval = 8-21) or 16% (95% confidence interval = 6-25) increased risk per 100 mL/min drop in Qa, p < 0.003). Thrombosis risk (adjusted for acute hypotension) became significantly higher at QaU = 1000-700 mL/min (relative risk = 3.6 (95% confidence interval = 1.6-8.2), p < 0.001) and QaD = 1300-1000 mL/min (relative risk = 3.1 (95% confidence interval = 1.1-12.8), p = 0.031). The proportion of thromboses attributable to acute hypotension was 40% (95% confidence interval = 24-57).

CONCLUSIONS

Our comparative study showed that an effective screening for graft stenosis and short-term thrombosis risk can rely on Qa surveillance alone, and suggested that avoiding acute hypotension and correcting stenosis at QaU < 1000 mL/min or QaD < 1300 mL/min can contain thrombosis risk.

Sentinel vascular access monitoring after endovascular intervention predicts access outcome

BACKGROUND AND OBJECTIVES

The vascular access pressure ratio test identifies dialysis vascular access dysfunction when three consecutive vascular access pressure ratios are >0.55. We tested whether the magnitude of the decline in vascular access pressure ratio 1-week post-intervention could alert of subsequent access failure.

DESIGN, SETTING, PARTICIPANTS, AND MEASUREMENTS

The retrospective study included all vascular access procedures at one institution from March 2014 to June 2016. Data included demographics, comorbidities, vascular access features, %ΔVAPR = ((Pre-Post)/Pre] × 100% assessed within the first 2 weeks post-percutaneous transluminal balloon angioplasty, time-to-next procedure, and patency. The log-rank test compared the area under the curve, receiver operating curve, Kaplan-Meier arteriovenous graft and arteriovenous fistula survival curves. A multivariable Cox proportional hazard (CP) model was used to determine the association of %ΔVAPR with access patency.

RESULTS

Analysis of 138 subjects (females 51%; Black 87%) included 64 arteriovenous fistulas with 104 angioplasties and 74 arteriovenous grafts with 134 angioplasties. The area under the receiver operating characteristic curve for fistula failure at 3 months was 0.59, with optimal screening characteristics of 33.3%, sensitivity of 56.1%, and specificity of 63.2%. Arteriovenous fistula with <33.3% decline compared to >33.3% required earlier subsequent procedure (136 vs 231 days), lower survival on Kaplan-Meier analysis (P = 0.01), and twofold greater risk of failure (P = .006). Area under the receiver operating characteristic for arteriovenous graft failure at 3 months had a sensitivity of 52.3% and specificity of 67.4%. Arteriovenous graft with a post-intervention vascular access pressure ratio decline of <28.8% also required earlier subsequent procedure (144 vs 189 days), lower survival on Kaplan-Meier (P = 0.04), and a 59% higher risk for failure. The area under the receiver operating characteristic curve for combined access failure (arteriovenous fistula + arteriovenous graft) at 3 months had an optimal cut-point value of 31.2%, a sensitivity of 54.6%, and a specificity of 63.1%. Access with a <31.2% drop had a 62% increase in the risk of failure (hazard ratio 1.62; confidence interval 1.16, 2.27; P = 0.005).

CONCLUSION

The magnitude of post-intervention reduction in vascular access pressure ratio provides a novel predictive measure of access outcomes.

Substitution-rate based screening model to assess stenosis progression in experimental stenotic arteriovenous grafts

An arteriovenous graft (AVG) has a higher patency rate in stenosis progression at the venous anastomosis site, which causes coexisting inflow and outflow stenoses. This leads to increases in blood pressure, flow velocity, and flow resistance, resulting in hemodialysis (HD) vascular access dysfunction from early clots and thrombosis to the progression of coexisting stenoses. To prevent vascular access complications such as inflow or outflow stenoses, this study proposes a novel examination method in an experimental AVG system using a substitution-rate based screening model. In our practical measurements, we found that inflow and outflow channeled through a narrowed access indicated both pressure and resistance differences as the degree of stenosis (DOS) gradually increased. A substitution-rate matrix was conducted to replace bilateral pressure variations, while a transition probability matrix was calculated. Differences in transition probabilities were then used to distinguish between normal conditions and flow instabilities using the distance estimation method. The joint probability decayed from < 0.81 to 0.00 could be specified to identify the progression in stenosis levels from a DOS% = 50.0-95.0%. Average joint probabilities were found to be inversely related with the DOS using a non-linear regression (R>2 0.90). Hence, the joint probability could be specified as a critical threshold, < 0.81, to identify the severity stenosis level, DOS% ⩾ 70%, in the assessment of coexisting inflow and outflow stenoses. Experimental results suggest that the proposed model is superior to hemodynamic analysis and traditional intelligent method, and can be used for dysfunction screening during HD treatment.

Comparison of access blood flow and venous pressure measurements as predictors of arteriovenous graft thrombosis

Purpose

The purpose of the present study was to prospectively compare the predictive accuracy of static venous pressure (SVP); dynamic venous pressure (DVP) and access blood flow (ABF) in determining subsequent graft thrombosis and/or failure.

Methods

This study included 43 patients with functional arteriovenous grafts (AVG's) who underwent monthly serial measurements of SVP, DVP and ABF for 3 consecutive months. Patients were then followed for an additional 6 months. The primary end point was graft thrombosis.

Results

Six patients were excluded from the final analysis. Of the 37 patients completing the study, 7 episodes of graft thrombosis occurred within 6 months of follow up. Neither SVP nor DVP exhibited satisfactory sensitivity or specificity for graft thrombosis. Ten patients either began with or developed an ABF < 600 during the 3 months of measurements, but only 5 clotted. Δ ABF of >20% provided the best combination of sensitivity (86%) and specificity (90%) for graft thrombosis. In AVG's that have an ABF<600, it is those grafts with falling ABF that appear most likely to clot in the short term.

Conclusion

The study supports the concept that it is a falling level of access flow rather than the absolute level that is the most potent predictor of graft thrombosis.

Functional diagnostic parameters for arteriovenous fistula

The inability to detect the arteriovenous fistula (AVF) dysfunction in a timely manner under the current surveillance programs, which are based on either diameter (d), flow rate (Q), or pressure (p) measurements, is one of the major challenges of dialysis treatment. Thus, our aim is to introduce new functional diagnostic parameters that can better predict AVF functionality status. Six AVFs were created between the femoral arteries and veins of three pigs, each pig having two AVFs on either limb. Flow fields and pressure drop (Δp) in AVFs were obtained via numerical analysis utilizing the CT scan and Doppler ultrasound data at 2D (D: days), 7D, and 28D postsurgery. The dataset included 16 (two pigs [four AVFs] for three time points, and one pig [two AVFs] for two time points) repeated measurements over time, and the statistical analysis was done using a mixed model. To evaluate the nature of pressure drop-flow relationships in AVFs, the Δp was correlated with the average velocity at proximal artery (v) and also the corresponding scaled velocity (v*) by the curvature ratio of anastomotic segment. Based on these relationships, two new functional diagnostic parameters, including the nonlinear pressure drop coefficient (Cp ; pressure drop divided by dynamic pressure at proximal artery) and the linear resistance index (R; pressure drop divided by velocity at proximal artery), were introduced. The diagnostic parameters that were calculated based on scaled velocity are represented as R* and Cp *. A marginal (P = 0.1) increase in d from 2D (5.4 ± 0.7 mm) to 7D (6.8 ± 0.7 mm), along with a significant increase in Q (2D: 967 ± 273 mL/min; 7D: 1943 ± 273 mL/min), was accompanied by an almost unchanged Δp over this time period (2D: 16.42 ± 4.6 mm Hg; 7D: 16.40 ± 4.6 mm Hg). However, the insignificant increase in d and Q from 7D to 28D (d = 7.8 ± 0.8 mm; Q = 2181 ± 378 mL/min) was accompanied by the elevation in Δp (24.6 ± 6.5 mm Hg). The functional diagnostic parameters, R and Cp , decreased from 2D (R = 22.4 ± 2.8 mm Hg/m/s; Cp  = 12.0 ± 2.6) to 7D (R = 20.8 ± 2.8 mm Hg/m/s; Cp  = 8.1 ± 2.6), and then increased from 7D to 28D (R = 35.5 ± 5.7 mm Hg/m/s; Cp  = 17.5 ± 3.6) with a marginal significance. However, when the scaled velocity was used to calculate R* and Cp *, the increase in diagnostic parameters from 7D to 28D achieved statistical significance (P < 0.05). In summary, although the differences in the hemodynamic parameters (d, Q, and Δp) from 7D to 28D were insignificant, changes in their combined effects in the form of diagnostic parameters were significant. Therefore, the functional diagnostic parameters are capable of better distinguishing changes in the hemodynamic variations, and thus, could be promising endpoints to diagnose the functionality of AVFs over time.

Vascular access for hemodialysis: postoperative evaluation and function monitoring

Vascular access (VA) survival is a crucial issue associated with morbidity and mortality of patients undergoing maintenance hemodialysis. The development of stenosis is the major factor that leads to VA failure. Strategies for early detection of lesions within a VA system before serious complications arise are therefore crucial. The implementation of a VA surveillance program could lead to timely detection of VA dysfunction and referral for correction, reduction in central venous catheter use and decrease in hospitalization and VA-related cost. Suggested methods for arteriovenous fistulae and grafts surveillance include blood flow measurement, static pressure evaluation and duplex ultrasonography. Physical examination is an accepted method in contrast to nonstandardized dynamic pressure measurement for grafts. Access recirculation (not urea based) and dynamic pressure measurements are accepted methods for fistulae. Decreasing URR or Kt/V (otherwise unexplained) and increased (negative) arterial pressure in the dialysis machine are methods of limited sensitivity and specificity for both fistulae and grafts. Measurement of access blood flow has been proposed as the gold standard for the screening of all types of VA. Access flow can be measured by various techniques which are direct or indirect. Several studies about VA surveillance programs have demonstrated conflicting results. Larger, randomized controlled trials need to be carried out in order to clarify whether surveillance programs are necessary and which is the best surveillance strategy for each type of VA.

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.

Surveillance and monitoring of dialysis access

Vascular access is the lifeline of a hemodialysis patient. Currently arteriovenous fistula and graft are considered the permanent options for vascular access. Monitoring and surveillance of vascular access are an integral part of the care of hemodialysis patient. Although different techniques and methods are available for identifying access dysfunction, the scientific evidence for the optimal methodology is lacking. A small number of randomized controlled trials have been performed evaluating different surveillance techniques. We performed a study of the recent literature published in the PUBMED, to review the scientific evidence on different methodologies currently being used for surveillance and monitoring and their impact on the care of the dialysis access. The limited randomized studies especially involving fistulae and small sample size of the published studies with conflicting results highlight the need for a larger multicentered randomized study with hard clinical end points to evaluate the optimal surveillance strategy for both fistula and graft.

Surveillance of hemodialysis vascular access

A mature, functional arteriovenous (AV) access is the lifeline for a hemodialysis (HD) patient as it provides sufficient enough blood flow for adequate dialysis. As the chronic kidney disease (CKD) and end-stage renal disease (ESRD) population is expanding, and because of the well-recognized hazardous complications of dialysis catheters, the projected placement and use of AV accesses for HD is on the rise. Although a superior access than catheters, AV accesses are not without complications. The primary complication that causes AV accesses to fail is stenosis with subsequent thrombosis. Surveying for stenosis can be performed in a variety of ways. Clinical monitoring, measuring flow, determining pressure, and measuring recirculation are all methods that show promise. In addition, stenosis can be directly visualized, through noninvasive techniques such as color duplex imaging, or through minimally invasive venography. Each method of screening has its advantages and disadvantages, and several studies exist which attempt to answer the question of which test is the most useful. Ultimately, to maintain the functionality of the access for the HD patient, a team approach becomes imperative. The collaboration and cooperation of the patient, nephrologist, dialysis nurse and technician, vascular access coordinator, interventionalist, and vascular surgeon is necessary to preserve this lifeline.

Bedside screening for fistula stenosis should be tailored to the site of the arteriovenous anastomosis

BACKGROUND AND OBJECTIVES

Given different sites of stenosis and access blood flow rates (Qa), the criteria for diagnosing fistula stenosis might vary according to anastomotic site. To test this, we analyzed the database of a prospective blinded study seeking an optimal bedside screening program for fistula stenosis.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Several methods used during dialysis (physical examination [PE], dynamic and derived static venous pressure [VAPR], dialysis blood pump flow/arterial pressure ratio, and Qa measurement) to diagnose angiographically-proven >50% stenosis were assessed in an unselected population of hemodialysis patients with mature fistulae (43 at the wrist [distal fistulae], 76 at mid-forearm or the elbow [proximal fistulae]).

RESULTS

Prevalence of inflow stenosis was uninfluenced by anastomotic site, whereas outflow stenoses were more prevalent in proximal fistulae. The best test for inflow stenosis was Qa <650 ml/min in distal fistulae and a combination of a positive PE and Qa <900 ml/m in proximal fistulae. In proximal fistulae, PE and VAPR >0.5 were both equally highly diagnostic of outflow stenosis. Tailoring choice of test to site of the anastomosis may also contain the screening-associated workload, by reducing the need to perform PE and measure VAPR, compared with a screening approach regardless of the access location.

CONCLUSIONS

Our study shows that an effective bedside screening program with ≥85% accuracy for fistula stenosis can be tailored to the site of the anastomosis, Qa being the tool of choice for the wrist, and PE alone or combined with Qa and VAPR measurements for more proximally-located accesses.

In search of an optimal bedside screening program for arteriovenous fistula stenosis

BACKGROUND AND OBJECTIVES

Guidelines recommend systematically screening for stenosis using various methods, but no studies so far have compared all of the options. A prospective blinded study was performed to compare the performance of several bedside tests performed during dialysis in diagnosing angiographically proven >50% fistula stenosis.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In an unselected population of 119 hemodialysis patients with mature fistulas, physical examination (PE) was conducted; dynamic and derived static venous pressure (VAPR), blood pump flow/arterial pressure (Qb/AP) ratio, recirculation (R), and access blood flow (Qa) were measured; and angiography was performed.

RESULTS

Angiography identified 59 stenotic fistulas: 43 stenoses were located upstream from the venous needle (inflow stenosis), 12 were located downstream (outflow stenosis), and 4 were located at both sites. The optimal tests for identifying an inflow stenosis were Qa < 650 ml/min and the combination of a positive PE "or" Qa < 650 ml/min (accuracy 80% and 81%, respectively), the latter being preferable because it was more sensitive (85% versus 65%, respectively) for a comparable specificity (79% versus 89%, respectively). The best tests for identifying outflow stenosis were PE and VAPR, with no difference between the two (accuracy 91% and 85%, sensitivity 75% and 81%, specificity 93% and 86%, respectively), the former being preferable because it was more reproducible, easier to perform, and applicable to all fistulas.

CONCLUSIONS

This study showed that fistula stenosis can be detected and located during dialysis with a moderate-to-excellent accuracy using PE and Qa measurement as screening procedures.

Access Monitoring Does Not Really Improve Outcomes

It is widely accepted that hemodialysis access monitoring combined with preemptive percutaneous transluminal angioplasty (PTA) improves outcomes. The many studies that have evaluated monitoring during the last decade provide an opportunity to examine whether this hypothesis is valid. Because synthetic grafts are more likely than autogenous arteriovenous fistulas to benefit from monitoring, this review is restricted to grafts. Recent studies show that monitoring does not accurately predict graft thrombosis or failure, nor does it prolong graft life. However, monitoring can reduce thrombosis, and thereby reduce access-related hospitalizations and use of central venous dialysis catheters. Because preemptive PTA is expensive, however, monitoring does not reduce the cost of access-related care. The limited benefit that monitoring provides emphasizes the urgent need to develop better approaches to solving the problem of graft thrombosis and failure.

Automated intravascular access pressure surveillance reduces thrombosis rates

Although monitoring of vascular accesses by physical examination is nearly as sensitive as surveillance measurements by vascular access pressure when performed by examiners, the frequency of examinations is limited by time. We developed intravascular access pressure surveillance as a surrogate to physical examination. Using real-time data from hemodialysis machines, we derived intravascular access pressure ratios for each dialytic procedure. An automated, noninvasive surveillance algorithm that generated a "warning" list of patients at risk for thrombosis was formulated. We hypothesized that this algorithm would reduce access thrombosis frequency. We designed a study comparing thrombosis rates during a baseline 6-month interval to three subsequent 6-month periods of active surveillance. Referrals for interventions during this 18-month period were based on persistently abnormal elevated vascular access pressure ratio tests (VAPRT) >0.55. Thrombosis rates declined progressively for arteriovenous grafts (AVG) during the intervention period compared with the baseline period. Arteriovenous fistula (AVF) thrombosis rates decreased during postintervention months 13-18 during employment of the VAPRT. We conclude that use of VAPRT can reduce thrombosis rates in vascular accesses, and the magnitude of the effect is larger and more consistent in arteriovenous grafts (AVGs) than autologous AVFs.

The natural history of vascular access for hemodialysis: a single center study of 2,422 patients

BACKGROUND

Our objective is to provide provision of primary and secondary patency rates data and incidence of complications. Despite the publication of some review articles and small prospective trials about vascular accesses, controversy still exists regarding the choice of the outflow conduit and especially the choice of the fistula to be formed in secondary and tertiary access procedures.

METHODS

This is a retrospective study of 2,422 consecutive patients who underwent 3,685 vascular access procedures in a tertiary care hospital, including radial-cephalic (RCAVF), brachial-cephalic (BCAVF), brachial-basilic (BBAVF), and prosthetic graft (PTFE) fistulas. Maximum follow-up period was 20 years. Actuarial patency rates were obtained by Kaplan-Meier analysis.

RESULTS

The median primary patency (days) of the most common 1st choices for vascular access were 712 (95% CI: 606, 818), 1,009 (95% CI: 823, 1,195), and 384 (95% CI: 273, 945) days for RCAVF, BCAVF, and PTFE, respectively. The median secondary patency was 1809 days (95% CI: 1,692, 1,926) for the RCAVF. The median primary patency of BBAVF (2nd or 3rd choice for vascular access) was 1,582 days (95% CI: 415, 2,749). The cumulative incidence of clinically important complications for the patients who received a RCAVF, BCAVF, BBAVF, and u-PTFE was 0.25, 0.57, 0.33, and 0.61 per patient-year, respectively.

CONCLUSION

We advocate maximal use of autogenous conduits, except probably the case of the older diabetic patient, in whom access at the antecubital fossa should be the first choice. BBAVF is an excellent fistula and should probably be constructed before prosthetic graft placement.

A method for monitoring vascular access function during hemodialysis

We tested a new bedside method to determine the function of native arteriovenous fistula in 16 patients performed during hemodialysis without stopping the treatment. We initially measured vascular access flow (Q(a)) in each patient using the Transonic HD01(plus) device. We then measured the pressure in arterial and venous drip chambers at different blood pump flow rates (Q(bset)=0, 50, 100, 250, 300, 350 ml/min). The intravascular blood pressure gradient (P(f)) between arterial and venous puncture sites was estimated by a mathematical model. P(f) was positive for low Q(bset), but became negative when Q(bset) overcame the threshold value (Q(Inv)). Such critical flow showed a high correlation with Q(a), even if it was systemically lower. Computer analysis of fluid dynamics showed that when the blood pump flow overcame the Q(Inv) threshold, a critical transition from laminar flow to vortex circulation took place downstream of the venous needle, causing a dangerous shearstress on the vessel wall. Our results show that Q(Inv) provides an indication of the maximal blood pump flow rate needed to be reached to maximize blood flow supply in order to limit hemodynamic stress on the vascular access.

Analysis of novel geometry-independent method for dialysis access pressure-flow monitoring

Background

End-stage renal disease (ESRD) confers a large health-care burden for the United States, and the morbidity associated with vascular access failure has stimulated research into detection of vascular access stenosis and low flow prior to thrombosis. We present data investigating the possibility of using differential pressure (ΔP) monitoring to estimate access flow (Q) for dialysis access monitoring, with the goal of utilizing micro-electro-mechanical systems (MEMS) pressure sensors integrated within the shaft of dialysis needles.

Methods

A model of the arteriovenous graft fluid circuit was used to study the relationship between Q and the ΔP between two dialysis needles placed 2.5–20.0 cm apart. Tubing was varied to simulate grafts with inner diameters of 4.76–7.95 mm. Data were compared with values from two steady-flow models. These results, and those from computational fluid dynamics (CFD) modeling of ΔP as a function of needle position, were used to devise and test a method of estimating Q using ΔP and variable dialysis pump speeds (variable flow) that diminishes dependence on geometric factors and fluid characteristics.

Results

In the fluid circuit model, ΔP increased with increasing volume flow rate and with increasing needle-separation distance. A nonlinear model closely predicts this ΔP-Q relationship (R² > 0.98) for all graft diameters and needle-separation distances tested. CFD modeling suggested turbulent needle effects are greatest within 1 cm of the needle tip. Utilizing linear, quadratic and combined variable flow algorithms, dialysis access flow was estimated using geometry-independent models and an experimental dialysis system with the pressure sensors separated from the dialysis needle tip by distances ranging from 1 to 5 cm. Real-time ΔP waveform data were also observed during the mock dialysis treatment, which may be useful in detecting low or reversed flow within the access.

Conclusion

With further experimentation and needle design, this geometry-independent approach may prove to be a useful access flow monitoring method.

Effect of online haemodialysis vascular access flow evaluation and pre-emptive intervention on the frequency of access thrombosis

Introduction. Guidelines advocate surveillance of vascular access to reduce incidences of thrombosis. However, the value of online vascular access flow monitoring is still under debate.

Methods. Through a systematic literature search, the effect of online access flow surveillance combined with pre-emptive intervention on thrombosis frequency is reviewed.

Results. Due to methodological differences, adequate comparison of the individual study results is not possible. Moreover, the methodological quality of most of the included studies is not suitable for an adequate statistical analysis of the results.

Conclusion. Until now, there is no conclusive evidence that online access flow evaluation has a significant effect on the rate of thrombosis. Future large-scale studies with adequate study design, adequate surveillance and intervention protocols and, possibly, better pre-emptive intervention alternative(s) are necessary.

Automation, decision support, and expert systems in nephrology

Increasing data suggest that errors in medicine occur frequently and result in substantial harm to the patient. The Institute of Medicine report described the magnitude of the problem, and public interest in this issue, which was already large, has grown. The traditional approach in medicine has been to identify the persons making the errors and recommend corrective strategies. However, it has become increasingly clear that it is more productive to focus on the systems and processes through which care is provided. If these systems are set up in ways that would both make errors less likely and identify those that do occur and, at the same time, improve efficiency, then safety and productivity would be substantially improved. Clinical decision support systems (CDSSs) are active knowledge systems that use 2 or more items of patient data to generate case specific recommendations. CDSSs are typically designed to integrate a medical knowledge base, patient data, and an inference engine to generate case specific advice. This article describes how automation, templating, and CDSS improve efficiency, patient care, and safety by reducing the frequency and consequences of medical errors in nephrology. We discuss practical applications of these in 3 settings: a computerized anemia-management program (CAMP, Henry Ford Health System, Detroit, MI), vascular access surveillance systems, and monthly capitation notes in the hemodialysis unit.

Noninvasive Methods to Identify Early Vascular Access Dysfunction

A functional vascular access is of critical importance to the hemodialysis patient, the patient's healthcare providers, and the hemodialysis treatment center. A poorly functioning or thrombosed vascular access can lead to increased morbidity, hospitalization, length of stay, and cost. This article reviews the increasing evidence supporting surveillance of arteriovenous (AV) hemodialysis access and the various strategies and techniques available for detection of a failing access.

Mathematical model demonstrates influence of luminal diameters on venous pressure surveillance

BACKGROUND

The reliability of dialysis venous pressure (VP) in detecting stenosis is controversial. A mathematical model may help to resolve the controversy by providing insight into the factors that influence static VP.

DESIGN, SETTING, PARTICIPANTS, AND MEASUREMENTS

This study used inflow artery and outflow vein luminal diameters from duplex ultrasound studies of 94 patients. These diameters were applied to a mathematical model, and how they affect the relation among VP, mean arterial pressure (MAP), blood flow, and stenosis was determined. Whether VP/MAP is a valid adjustment for the influence of MAP on VP, and whether the standard VP/MAP referral threshold of 0.50 is valid, were also determined.

RESULTS

It was found that there is an approximate one-to-one relation between MAP and VP, so VP/MAP is a valid adjustment. Also, the 0.50 threshold successfully identifies most grafts with stenosis of 65% or more. However, the ratio of artery/vein diameters varied widely between patients, and the ratio independently influences VP/MAP. When the inflow artery is relatively narrow, the VP/MAP increase is delayed followed by a more rapid increase as critical stenosis is reached.

CONCLUSIONS

VP/MAP is a valid adjustment for the influence of MAP on VP, and the standard VP/MAP threshold of 0.50 warns of the transition to critical stenosis. However, relatively narrow arteries cause a delay followed by a rapid increase in VP/MAP that may not be detected before thrombosis unless measurements are very frequent. Clinical trials that emphasize trend analysis with frequent measurements are needed to evaluate the efficacy of VP surveillance.

Routine Surveillance in Vascular Access for Hemodialysis

There is increasing evidence that surveillance of AV access for haemodialysis prevents access thrombosis and improves the quality of care. This article reviews the evidence for surveillance and the various strategies and techniques available for detection of the failing access.

Mathematical Model for Pressure Losses in the Hemodialysis Graft Vascular Circuit

Stenosis-induced thrombosis and abandonment of the hemodialysis synthetic graft is an important cause of morbidity and mortality. The graft vascular circuit is a unique low-resistance shunt that has not yet been systematically evaluated. In this study, we developed a mathematical model of this circuit. Pressure losses ΔPs were measured in an in vitro experimental apparatus and compared with losses predicted by equations from the engineering literature. We considered the inflow artery, arterial and venous anastomoses, graft, stenosis, and outflow vein. We found significant differences between equations and experimental results, and attributed these differences to the transitional nature of the flow. Adjustment of the equations led to good agreement with experimental data. The resulting mathematical model predicts relations between stenosis, blood flow, intragraft pressure, and important clinical variables such as mean arterial blood pressure and hematocrit. Application of the model should improve understanding of the hemodynamics of the stenotic graft vascular circuit.

Static intra-access pressure ratio does not correlate with access blood flow

BACKGROUND

Access flow (Qa) measurement is recommended by Kidney Disease Outcomes Quality Initiative (K/DOQI) as the preferred method for access surveillance. Static intra-access pressure ratio (SIAPR) measurement is the second surveillance method of choice. The purpose of this prospective multicenter study was to investigate the relationship between SIAPR and Qa and to examine the premise upon which SIAPR surveillance is based-namely, that high SIAPR is a surrogate for low Qa associated with hemodynamically significant stenosis.

METHODS

SIAPR and Qa (HD01; Transonic Systems, Inc., Ithaca, NY, USA) were simultaneously measured monthly in 242 patients [146 prosthetic arteriovenous bridge grafts (AVG), 96 autogenous arteriovenous fistulas (AVF)] from three centers. SIAPR was measured according to the K/DOQI protocol.

RESULTS

There was no correlation between Qa and venous or arterial SIAPR in AVGs (R(2)= 0.0037 and R(2)= 0.006, respectively, N= 730), or in AVFs (R(2)= 0.0247 and R(2)= 0.0329, respectively, N= 431). Of the high SIAPR measurements in AVGs, 81% and 50% were associated with Qa > or =600 and Qa > or =1000 mL/min, respectively. Of the AVGs studied, 41% (60/146) had consistently high Qa > or =1000 mL/min. Seventy percent (42/60) of these high-Qa AVGs had at least two consecutive sessions with high SIAPR measurements, thereby meeting the K/DOQI SIAPR criteria for referral. In addition, 78% (14/18) of new AVGs with Qa > or =1000 mL/min, and 86% (6/7) of AVGs with the highest Qa (> or =2000 mL/min), had high SIAPR. As a result, these high-Qa AVGs, which represented the best functioning AVGs by K/DOQI Qa standards, were erroneously targeted for referral based on SIAPR measurements.

CONCLUSION

SIAPR does not correlate with Qa or discriminate between high and low Qa. Therefore, because the utility of SIAPR surveillance for detection of clinically significant stenosis depends on a correlation with Qa, the current use of absolute K/DOQI SIAPR thresholds for intervention based on the presumption that such thresholds are indicative of low Qa is not justified, and should be discontinued. Studies need to be done to examine the utility of SIAPR for trend analysis.

A randomized controlled trial of blood flow and stenosis surveillance of hemodialysis grafts

BACKGROUND

It is widely accepted that hemodialysis graft surveillance combined with correction of stenosis reduces thrombosis and prolongs graft survival. Nevertheless, few randomized controlled trials have evaluated this approach.

METHODS

In this randomized controlled trial, 101 patients were assigned to control, flow (Qa), or stenosis groups, and were followed for up to 28 months. All patients had monthly Qa measured by ultrasound dilution and quarterly percent stenosis measured by duplex ultrasound. Referral for angiography was based on the following criteria: (1) control group (N = 34), clinical criteria; (2) flow group (N = 32), Qa <600 mL/min or clinical criteria; and (3) stenosis group (N = 35), stenosis>50% or clinical criteria. Stenosis >or=50% during angiography was corrected by preemptive percutaneous transluminal angioplasty (PTA).

RESULTS

The preemptive PTA rate in the control group (0.22/patient year) was two thirds the rate in the flow group (0.34/patient year), and was highest in the stenosis group (0.65/patient year, P < 0.01). The percentage of grafts that thrombosed was similar in the control (47%) and flow groups (53%), but reduced in the stenosis group (29%, P = 0.10). Two-year graft survival was similar in the control (62%), flow (60%), and stenosis groups (64%) (P = 0.89).

CONCLUSION

Qa and stenosis surveillance were not associated with improved graft survival, although thrombosis was reduced in the stenosis group. The most important factors in this result may be that monthly Qa and quarterly stenosis measurements were not accurate or timely indicators of risk of thrombosis or progressive stenosis. This study does not support the concept that Qa or stenosis surveillance are superior to aggressive clinical monitoring.