Graftula: a composite access consisting of a graft used to repair a dysfunctional hemodialysis fistula.

Nonmaturing or dysfunctional hemodialysis fistulas are often repaired with interposition grafts placed either within the fistula (fistula-to-fistula configuration), or connected to another vein (fistula-to-vein configuration). The goal of this study was to compare the survival and usefulness of the composite accesses thus created, which we call "graftulas," with upperarm grafts. This was a retrospective study wherein we determined the survival and thrombosis rates of graftulas (n=24) and upper arm grafts (n=31) placed 1/1/07 through 12/31/09 and followed through 11/30/10. Graftulas resembled grafts as most (96%) were successfully cannulated in 65 ± 43 days. Survival of graftulas was also similar to grafts (58%, 47%, and 32% vs. 56%, 47%, and 39% at 1, 2, and 3 years respectively, p=0.60). However, graftulas had a lower thrombosis rate than grafts (0.5 vs. 1.2 per patient year, p=0.04), and in the fistula-to-fistula configuration, a 2-year thrombosis-free survival of 78%. Total survival of the access site (fistula+graftula) was 92%, 73%, and 42% at 1, 3, and 5 years, respectively. Graftulas possess certain beneficial properties of fistulas and grafts that allows for continued use of the original access site.

Thigh grafts contribute significantly to patients' time on dialysis.

BACKGROUND AND OBJECTIVES: Thigh grafts are placed in hemodialysis patients who have exhausted all arm access sites. The goal of this study was to compare the survival, complication rates, and overall contribution of thigh grafts with arm grafts and fistulas in patients with at least one functional thigh graft during their dialysis history. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: This longitudinal review of a prospectively acquired clinical database included 85 thigh graft recipients. The rates of survival, thrombosis, infection, and other complications were determined for a total of 268 fistulas, arm grafts, and thigh grafts placed in these patients. RESULTS: In this patient subset, thigh graft primary failure rate was lower than arm grafts and fistulas (3 versus 13 and 61%, respectively). Excluding primary failures, thigh grafts survived longer than both arm grafts and mature fistulas (53 versus 14 and 32%, at 3 years; 47 versus 3 and 11% at 5 years). Thigh grafts had a lower thrombosis rate than arm grafts (0.543 versus 1.457/patient-year) but similar rates of loss as a result of infection and surgical revision. In patients with previous arm accesses, thigh grafts contributed 51% of total dialysis time compared with 38 and 11% for arm grafts and fistulas. CONCLUSIONS: Thigh grafts provide long-term, thrombosis- and infection-free dialysis access for patients with exhausted arm access sites. The decision for thigh graft placement should, therefore, be made as soon as there is evidence for unavailability of arm access sites so that catheter use can be minimized.

Thigh grafts: a preferable alternative to catheters when upper extremity access sites are exhausted.

Hemodialysis patients with failed upper extremity (arm) access sites comprise 5-10% of the dialysis population. In these patients, arm vessels are either unsuitable for access placement due to trauma or peripheral vascular disease, or have been exhausted following dialysis usage. Synthetic grafts in the lower extremity (thigh) would benefit these patients, but surgeons are often reluctant to place them, due to concerns of infection and vascular complications. As a result, these patients receive tunneled central venous catheters as their permanent dialysis access. Recent studies have shown, however, that survival and complication rates of thigh grafts are similar to arm grafts and fistulas. Moreover, thigh grafts have lower infection and mortality rates than catheters and provide higher blood flows and dialysis adequacy. In this editorial we argue that thigh grafts are the better option in patients who have lost all arm access sites; they should be placed in preference to tunneled central venous catheters.

Risk of hemodialysis graft thrombosis: analysis of monthly flow surveillance.

BACKGROUND: During clinical application of flow surveillance of hemodialysis grafts, the risk of thrombosis is assessed month after month, rather than after one or several measurements, as has been done in published studies. Adequate assessment of risk should consider the many measurements obtained over time. STUDY DESIGN: Prospective cohort diagnostic test study. SETTING & PARTICIPANTS: 176 patients with hemodialysis grafts from 2 university-affiliated dialysis units during a 6-year period. INDEX TESTS: Monthly measurement of graft blood flow or change in flow. OUTCOME: Graft thrombosis. RESULTS: We used logistic regression analysis to compute the risk of thrombosis and used receiver operating characteristic (ROC) curves to assess the accuracy in predicting thrombosis within 1 month. Newer grafts were most likely to thrombose, whereas older grafts were unlikely to thrombose even at low flows or large decreases in flow. Areas under the ROC curves were 0.698 for flow and 0.713 for change in flow measured over 2 months. Flow predicted thrombosis with a sensitivity of 53% at a specificity of 79%, and change in flow had a sensitivity of 58% at a specificity of 75%. More than half the thromboses lacked a change in flow measurement, usually because thrombosis occurred before a change could be measured. Thus, the effective predictive accuracy of change in flow was much less than the ROC curves indicated because the curves do not consider missing measurements. LIMITATIONS: Performance characteristics of index tests may vary across patient populations. CONCLUSION: Flow and change in flow are inaccurate predictors of thrombosis. Many thromboses are not predicted, and intervention based on surveillance likely yields many unnecessary procedures. Thus, this study does not support routine application of surveillance to prevent thrombosis.

Inflow stenosis obscures recognition of outflow stenosis by dialysis venous pressure: analysis by a mathematical model.

BACKGROUND: Recent studies have shown that inflow stenosis of haemodialysis grafts is more common than previously realized. The influence of inflow stenosis on graft haemodynamics and venous pressure (VP) surveillance has not been previously systematically studied. METHODS: We used a well-established mathematical model to determine the relation between inflow stenosis and static VP (adjusted for mean arterial pressure, VP/MAP), outflow stenosis and artery and vein luminal diameters. We applied low, median and high ratios of artery/vein diameters from 94 patients with grafts. The median ratio was 0.77, indicating that the artery was generally narrower than the vein. RESULTS: The model shows that inflow stenosis reduces VP/MAP. More importantly, however, as outflow stenosis progresses, fixed inflow stenosis causes a delayed increase in VP/MAP followed by a rapid increase at critical outflow stenosis. When both stenoses progress together, their relative rates determine whether and how rapidly VP/MAP increases. The increase in VP/MAP is remarkably abrupt when the rate of inflow stenosis approaches that of outflow stenosis. No increase occurs when inflow stenosis progresses as fast or faster than outflow stenosis. CONCLUSION: Inflow stenosis exerts its most important haemodynamic effect through its interaction with outflow stenosis. As outflow stenosis progresses, inflow stenosis causes a delayed and then rapid increase in VP/MAP at critical outflow stenosis. This increase may not be detected before thrombosis unless measurements are very frequent. Inflow stenosis has an important impact on graft haemodynamics and surveillance because of its location in the relatively narrow inflow tract.

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.

Influence of luminal diameters on flow surveillance of hemodialysis grafts: insights from a mathematical model.

Randomized controlled trials have not shown that surveillance of graft blood flow (Q) prolongs graft life. Because luminal diameters affect flow resistance, this study examined whether the influence of diameters on Q can explain the limitations of surveillance. Inflow artery and outflow vein diameters were determined from duplex ultrasound studies of 94 patients. These diameters were applied to a mathematical model for determination of how they affect the relation between Q and stenosis. Also determined was the correlation between Q (by ultrasound dilution) and diameters, stenosis, and mean arterial pressure in 88 patients. Artery and vein diameters varied widely between patients, but arteries generally were narrower than veins. The model predicts that the relation between Q and stenosis is sigmoid: as stenosis progresses, Q initially remains unchanged but then rapidly decreases. A narrower artery increases flow resistance, causing a longer delay followed by a more rapid reduction in Q. In a multiple regression analysis of data from patients, Q correlated with artery and vein diameters, sum of largest stenoses from each circuit segment, and mean arterial pressure (R = 0.689, P < 0.001). This study helps to explain why Q surveillance predicts thrombosis in some patients but not others. Luminal diameters control the relation between Q and stenosis, and these diameters vary widely. During progressive stenosis, the delay and then rapid reduction in Q may impair recognition of low Q before thrombosis occurs. Surveillance outcomes might be improved by taking frequent measurements so that there is no delay in discovering that Q has decreased.

Thresholds for significant decrease in hemodialysis access blood flow.

During hemodialysis access surveillance, referral for evaluation and correction of stenosis is based upon determination that a significant decrease in blood flow (Q) has occurred. However, criteria for determining when a decrease is statistically significant have not yet been established. In this study we established such criteria by analyzing Q variation with the glucose pump test (GPT). We took nine Q measurements in each of 25 patients (18 grafts, 7 fistulas) during three dialysis sessions within a 2-week period (predialysis and during hours 1 and 3). We determined thresholds that define a significant percentage decrease in Q (deltaQ) for various p values. In order to confirm the general applicability of these thresholds, we computed the average within-patient Q variation during the three sessions (computed as a coefficient of variation and referred to as short-term variation). We then determined the relative influences of biological (true) variation and analytical error on short-term variation. We found that deltaQ must be > 33% to be significant at p < 0.05, whereas the threshold is > 17% for p < 0.20. Measuring Q at uniform versus different times during the sessions did not significantly reduce these thresholds. We also found that biological variation was nearly as large as short-term Q variation, whereas analytical error contributed minimally to short-term variation. In conclusion, this study defines thresholds for a significant deltaQ that have wide application in determining access referral for evaluation and correction of stenosis. Selection of a particular threshold should consider the relative importance of avoiding thrombosis versus avoiding unnecessary procedures. If avoiding unnecessary procedures is a priority, then we recommend a threshold of > 33%. These thresholds apply to other methods of measuring Q, provided analytical error is significantly less than biological variation.

Stenosis surveillance of hemodialysis grafts by duplex ultrasound reduces hospitalizations and cost of care.

Most recent randomized controlled trials (RCTs) have found that hemodialysis graft surveillance combined with preemptive correction of stenosis does not prolong graft survival. Nevertheless, such programs may be justified if they reduce other adverse outcomes or decrease the cost of care. This study tested this hypothesis by applying a secondary analysis to our original RCT. This study of 101 patients evaluated correction of stenosis based upon blood flow (Q) and stenosis surveillance. Patients were randomly assigned to control, flow, or stenosis groups, and were followed for up to 28 months. Q was measured monthly by ultrasound dilution; stenosis was measured quarterly by duplex ultrasound. Stenosis of 50% was corrected by percutaneous transluminal angioplasty (PTA) after referral for angiography. Referral criteria were: control group, clinical criteria; flow group, Q < 600 ml/min or clinical criteria; stenosis group, stenosis > 50% or clinical criteria. We compared access-related hospitalizations and cost of care, and use of central venous dialysis catheters (CVCs), among the three groups. Hospitalization rates were higher in the control and flow groups than in the stenosis group (0.50, 0.57, 0.18/patient-year, respectively [p < 0.01]), and hospitalization costs were lowest in the stenosis group (p = 0.026). The stenosis group had a trend toward lowest CVC rates (0.44, 0.32, 0.20/patient-year, respectively [p = 0.20]). The costs of care were higher in the control and flow groups than in the stenosis group (dollar 3727, dollar 4839, dollar 3306/patient-year, respectively [p = 0.015]). The costs of stenosis (dollar 142/patient-year) and Q (dollar 279/patient-year) measurements were minimal compared to the total cost of access-related care. In conclusion, stenosis surveillance by duplex ultrasound combined with preemptive correction yielded reduced hospitalization rates and costs, reduced total cost of access-related care, and a trend of reduced CVC rates. In contrast, flow surveillance did not yield a significant benefit. Stenosis surveillance provides important benefits that may justify application of such programs.

In vivo validation of glucose pump test for measurement of hemodialysis access flow.

BACKGROUND: The glucose pump test (GPT) is a recently introduced method of measuring hemodialysis access blood flow (Qa). A validation of GPT during dialysis has not yet been done, and performance characteristics of the method have not yet been fully analyzed. METHODS: The authors studied 33 patients (25 synthetic grafts, 8 autogenous arteriovenous fistulae). Qa measurements by ultrasound dilution (UD) and GPT were done in triplicate during dialysis. In GPT, a baseline blood sample (C(1)) was obtained, followed by infusion of a 10% glucose solution (C(i)) through the arterial needle into the access at 16 mL/min (Q(i)). After 11 seconds, a downstream blood sample (C(2)) was aspirated from the venous needle. C(1) and C(2) glucose were measured by glucometer. Qa was computed by the equation: Qa = Q(i)(C(i) - C(2))/(C(2) - C(1)). A model of the access vascular circuit was used to determine the influence of C(2) aspiration on the Qa measurement. RESULTS: Mean Qa was 1413 mL/min by UD versus 1,496 mL/min by GPT (P = 0.11). There was a strong linear correlation between the 2 methods (r = 0.905; P <0.001). The pooled coefficient of variation was 6.4% for UD and 9.6% for GPT. The circuit model showed that aspiration of C(2) causes an increase in Qa (DeltaQa) that depends on the aspiration rate (Q(ASP)) and fraction of resistance in the circuit that is downstream to the venous needle: DeltaQa = Q(ASP)(Downstream resistance)/(Total resistance). The model predicts the overestimate is approximately 62 mL/min for grafts and 120 mL/min for fistulae but may vary depending on the balance of resistances upstream and downstream to the venous needle. CONCLUSION: This study shows that GPT closely correlates with UD, and the method has adequate precision. GPT is an inexpensive method that may help make Qa measurements more widely available than previously possible.

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.

Association between blood pressure, ultrafiltration, and hemodialysis graft thrombosis: a multivariable logistic regression analysis.

Although a low blood flow (Q(a)) is the most important cause of graft thrombosis, several studies have shown that Q(a) measurements do not accurately predict thrombosis. This suggests that additional variables may influence thrombosis. Identification of such variables may be essential to designing surveillance protocols that accurately predict thrombosis. In this nested case-control study, we prospectively followed 105 patients for up to 2.5 years in order to test the association of a number of variables with thrombosis. These included Q(a) (monthly by ultrasound dilution), percentage stenosis (quarterly by duplex ultrasound), mean arterial pressure (MAP), percentage ultrafiltration (%UF) during dialysis (%UF = 100[liters]/[kilogram of weight]), and other variables that defined patient and graft characteristics. Patients were divided into patent (n = 53) and thrombosed groups (n = 52), and MAP and %UF from seven consecutive dialysis sessions were analyzed. In the thrombosed group, the last session was the final session before thrombosis. A multivariable logistic regression model showed that Q(a), MAP (the predialysis average of seven sessions), and %UF (from the last session) were independently associated with thrombosis, whereas all other variables were not. The model yielded the following odds ratios for thrombosis: for a single Q(a) value (reduction of 1,000 mL/min), 12.0 (P < 0.01); for %UF (increase of 4%), 5.3 (P < 0.01); for MAP (reduction of 30 mm Hg), 4.1 (P = 0.02); and for percentage decrease in Q(a) (> or =20% versus <20%), 2.4 (P = 0.12). We conclude that in addition to Q(a), both %UF at the last session before thrombosis and average predialysis MAP from seven sessions are independently associated with thrombosis. These results help explain why Q(a) alone does not accurately predict thrombosis. A prospective study is needed to determine whether %UF at each session and a moving average MAP from seven sessions improve the prediction of thrombosis. However, it should be recognized that a large %UF is a preterminal event that likely provides too short a warning for intervention before thrombosis.

Vascular access: anatomy, examination, management.

A systematic approach to managing vascular access problems is the key to reducing current high rates of access thrombosis and failure. This approach begins with a thorough knowledge of vascular access anatomy that, when combined with the physical examination, can help optimize access planning and maintenance. Because of the high complication rate of synthetic grafts, there has been increased emphasis on creating autogenous arteriovenous (AV) fistulae, which, once established, are more trouble-free. The benefit of increased fistula creation will not be realized, however, until the high rate of early fistula failure is reduced. It is widely recommended that graft surveillance programs be implemented and that stenosis be corrected when accompanied by graft dysfunction. Graft blood flow (Q(a)) is the preferred surveillance method, but has a poor accuracy in predicting thrombosis. Most studies that have evaluated the benefit of Q(a) surveillance have used historical control groups, or have been retrospective or nonrandomized. Consequently, we believe it is not currently possible to make definitive, evidence-based recommendations concerning Q(a) surveillance. The most important factor in access survival may be a team approach with an organized commitment to access planning followed by recognition and treatment of access problems.