Prediction of hemodialysis synthetic graft thrombosis: can we identify factors that impair validity of the dysfunction hypothesis?

Regular ultrasonographic screening significantly prolongs patency of PTFE grafts

BACKGROUND

Polytetrafluoroethylene (PTFE) dialysis grafts have considerably shorter patency than native arteriovenous fistulas, despite the use of a complex of screening monitoring methods (venous pressure, access flow). PTFE grafts are used often in subjects with depleted subcutaneous veins after previous abandoned accesses, so keeping the access patent is crucial. We hypothesized that regular duplex Doppler ultrasound screening for access stenoses, together with their sooner treatment, would prolong PTFE graft patency.

METHODS

We performed a randomized, prospective study of PTFE grafts' cumulative patency in 192 subjects. In group 1, regular ultrasound examinations performed every 3 months was added to traditional screening (i.e., regular access examination at hemodialysis unit, monitoring of venous pressure and access flow). Group 2 was screened only traditionally (without ultrasound). Interventions of suspected stenoses were indicated by nephrologists, vascular surgeon, and, in group 1, also by ultrasonography. Classic ultrasound criteria for significant stenosis were used, even if the access flow had not been decreased. The mean follow-up lasted 392 +/- 430 days.

RESULTS

Groups were similar with respect to age, gender, diabetes status, and number of previous abandoned accesses. Group 1 had significantly longer access patency (P < 0.001). Number of interventions per graft was 2.1 +/- 1.8 and 1.3 +/- 1.0 in group 1 and group 2.

CONCLUSIONS

Regular screening duplex Doppler ultrasonography results in significantly longer PTFE graft patency due to early detection of access stenosis and, thus, more frequent elective interventions of access stenoses.

Thrombophilia and the Risk for Hemodialysis Vascular Access Thrombosis

Vascular access thrombosis is the most common and costly complication in hemodialysis patients. The role of thrombophilia in access thrombosis is not established. A case-control study was conducted of 419 hemodialysis patients to determine whether thrombophilia was associated with arteriovenous fistula or graft thrombosis. Participants were enrolled from three in-center and five satellite dialysis units associated with a Canadian academic health science center that provides dialysis services in a catchment area of one million. Patients were tested for factor V Leiden, prothrombin gene mutation, factor XIII genotype, methylenetetrahydrofolate reductase genotype, lupus anticoagulant, anticardiolipin antibody, factor VIII, homocysteine, and lipoprotein (a) concentrations. Overall, 59 (55%) patients with access thrombosis had at least one thrombophilia compared with 122 (39%) patients without access thrombosis (unadjusted odds ratio [OR], 1.91; 95% confidence interval [CI], 1.23 to 2.98). After controlling for important risk factors, the association between any thrombophilia and access thrombosis remained (adjusted OR, 2.42; 95% CI, 1.47 to 3.99). For each additional thrombophilic disorder, the odds of access thrombosis increased significantly (adjusted OR, 1.87; 95% CI, 1.34 to 2.61). This study suggests that thrombophilia is associated with access thrombosis in dialysis patients. Large, multicenter, prospective cohort studies are needed to confirm the observations from this case-control study.

Are hemodialysis access flow measurements by ultrasound dilution the standard of care for access surveillance?

Maintenance of vascular access function is vital to the delivery of adequate hemodialysis therapy. Failure of function is associated with significant morbidity and cost. Thus, access surveillance programs are suggested. The most common cause for access dysfunction is stenosis formation within the graft fistula. This may lead to reduced blood flow. The measurement of access blood flow has thus been recommended as the preferred method for surveillance. This article reviews blood flow among other methods for the screening of access dysfunction, the techniques used to measure it, the predictability of access flow measurements in determining the presence of access stenosis and allowing successful; intervention and finally the cost-effectiveness of such surveillance. Review of available evidence would suggest that access flow measurements are the best tests currently available to screen for access dysfunction, and as preventative interventions, such as angioplasty and surgery, are successful, they should be regarded as the present standard of care. This would appear to be a cost-effective strategy. Furthermore, the method of choice for access flow measurement is by ultrasound dilution technology.

Blood flow surveillance of hemodialysis grafts and the dysfunction hypothesis

It is widely recommended that hemodialysis graft surveillance programs should be implemented and that significant stenosis should be corrected when it is accompanied by graft dysfunction. The rationale for surveillance depends on the dysfunction hypothesis, which states that stenosis causes graft dysfunction [such as a decrease in graft blood flow (Qa)], and this dysfunction reliably precedes and accurately predicts thrombosis. The usefulness of Qa surveillance depends on accurate prediction of thrombosis so that stenosis can be corrected prior to thrombosis. An analysis of the dysfunction hypothesis indicates that some or all of its underlying assumptions are invalid. Most importantly, the presence of wide hemodynamic variation during Qa measurements makes Qa a relatively inaccurate predictor of thrombosis. A number of studies have evaluated the value of surveillance with intervention in reducing thrombosis rates and prolonging graft life. Review of these studies show that few have been prospective and randomized, and many have included historical control groups. It is debatable whether these studies have established that Qa surveillance with intervention should be applied to all grafts. Data from several studies suggest that severity of stenosis may be at least as accurate as Qa in predicting thrombosis. Consequently, inclusion of stenosis measurements (e.g., by duplex ultrasound) may improve the results of surveillance. These unresolved issues indicate it is premature to recommend routine Qa surveillance with intervention of all hemodialysis patients with grafts.

Hemodynamic reproducibility during blood flow measurements of hemodialysis synthetic grafts

We have previously shown that graft blood flow (Qa) has a poor accuracy in predicting graft thrombosis. In this study, we determined whether hemodynamic variation helps explain this poor predictive accuracy. We also determined whether standardized timing of Qa measurements, which is widely recommended, will promote measurement reproducibility. We analyzed variations in mean arterial pressure (MAP) in seven consecutive dialysis sessions for 51 patients and determined the influence of MAP on Qa (by ultrasound dilution). We used a pooled coefficient of variation (CV) to summarize MAP variation within individual patients (computed as +/-2 CVs). MAPs from the seven sessions varied widely, and most variation was present with the first MAPs at the beginning of the sessions. These first MAPs varied by +/-23%, whereas variation for the entire session was +/-28%. The influence of MAP on Qa was determined by measuring the two together during consecutive thirds of a single session. The percentage of change in MAP (DeltaMAP) and Qa (DeltaQa) from the first to middle or last thirds of the session varied over wide ranges: -37% to 86% and -43% to 78%, respectively. The DeltaQa versus DeltaMAP correlation was relatively strong for changes between the first and middle thirds (r = 0.666) and first and last thirds (r = 0.646) of the session (both P: < 0.01). We conclude that MAP varies far more widely during dialysis than previously recognized. This variation is associated with large changes in Qa that may impair accuracy in predicting thrombosis. This wide MAP variation also indicates hemodynamic reproducibility is not feasible when measuring Qa. Thus, we do not recommend standardized timing of Qa measurements during dialysis. A practical method of addressing poor Qa reproducibility may be to take frequent measurements so that trends can be recognized before thrombosis occurs.