Original studies that affirm (28) or challenge (6) the use of ultrasound dilution for vascular access surveillance.
New method introduced to measure access flow during dialysis. Measurements tracked a calibrated in-line flow sensor within 3.25 +/- 0.34%; repeatable within 3%. Data showed that access flow can be accurately measured by ultrasound dilution.
Blood flow is a fundamental property of the hemodialysis access device. A comparison with access flow measured by color Doppler technique gave a mean error of 9.2 +/- 7.2% in paired studies. These data show that blood flow in peripheral arteriovenous grafts and fistulas can be measured accurately during hemodialysis using ultrasound velocity dilution.
“Measurements correlated well with flow rates determined by magnetic resonance angiography and by a technique based on intra-access flow-pressure curves. …Access flow can be measured easily, noninvasively, and reliably by the ultrasound dilution device… The method requires little investment in time making it superior to other methods.”
“Mean access flow was 1,086 +/- 505 ml/min by ultrasound dilution and 1,026 +/- 513 ml/min with duplex ultrasonography (NS). … Measurement of hemodialysis access flow by
ultrasound dilution was essentially equivalent to that obtained by duplex ultrasound.
Three-center study of 170 patients over six months. “The blood flow by Dilution (for grafts) was the best predictor of thrombosis within the subsequent three months. Multi-variate analysis showed a significantly increasing risk of thrombosis with decreasing access blood flow.”
There is a 13.6-fold increase in the relative risk of thrombosis for accesses with more than 35% decrease in vascular access blood flow. Study prospectively determined that measurement of blood flow plays an important role in evaluation and detection of PTFE grafts at higher risk of thrombosis.
Repeated measurements of access flow have the potential to predict future access failure in PTFE grafts.
“We believe that monthly access flow measurement will ensure the lowest incidence of thrombosis and decrease the cost of access maintenance.”
(83 grafts, 80 patients over 12 months)
“Study found a Qa less than 600 ml/min or net ΔQa of 20% or greater had a sensitivity of 77% with an FPR of 23%...Thrombosis was predicated to occur one month after a measurement period.” This study is largely referred to as a negative study, however, the study findings were positive for ultrasound dilution’s sensitivity to detect flow-limiting stenosis. The study’s conclusions were later debated.
“Sequential measurement of AV access flow is an acceptable means of both monitoring for the development of access stenoses and assessing response to therapy."
“Vascular access blood flow monitoring along with preventative interventions should be the standard of care in chronic hemodialysis patients. … The comprehensive cost is markedly reduced due to the decreased number of hospitalizations, catheters placed, missed treatments, and surgical interventions.”
Data demonstrate that standardized monitoring of venous pressure or access flow or the combination and subsequent corrective intervention can reduce thrombosis rate in grafts to below the 0.5 patient per year (NKF-DOQI), although it was noted that VP does not detect inflow stenosis.
The current study is the first to evaluate the accuracy of ultrasound dilution predicting hemodialysis access stenosis in pediatric hemodialysis population. Study supports the use of monthly measurement to prevent access thrombosis in children receiving hemodialysis.
“Over a period of one month, simultaneous flow and venous pressure monitoring [was performed]… in 71 dialysis patients with (PTFE) grafts. These patients were prospectively followed for one year… During the period of follow-up, there were 71 graft failures [in 38 patients]… Failed grafts had lower blood flow rate… when compared with those without failure… Conclusions: Although dialysis graft blood flow rates are statistically different in patients who have graft failure… versus those who do not, the performance characteristics [of a single month of patient measurements] preclude clinical decision-making [for the ensuing year].” Transonic notes that the methods studied in this article are in no way recommended for access surveillance by KDOQI or any other groups as conclusions are not meant to be drawn from a single instance.
(49 PTAs on 32 grafts) This study, on the effectiveness of intervention rather than the accuracy of surveillance, indicates that preemptive angioplasty of graft stenoses results in an initial doubling of ABF, but the effect is temporary, with the average ABF decreasing to baseline values by three months. Study underscores the need for more investigation on intervention techniques and controls.
Study established separate baseline flows for radiocephalic and brachiocephalic fistulae. “Newly created native AVFs have an initial Q(ac) that does not vary significantly during the first six months and may already be maximal at six weeks or at the time of first needle puncture in our hands.”
Prompt referral for angioplasty of VA with corrected vascular access flow rates <650 mL/min/1.73 m2 leads to decreased thrombosis rates in children.
This study shows that prophylactic PTA of stenosis in a functioning forearm AVF improves access survival and decreases access-related morbidity, supporting the usefulness of preventive correction of stenosis before the development of access dysfunction. It also strongly supports surveillance program for early detection of stenosis.
“In the treatment group, the positive predictive value of Qa to detect stenosis larger than 50% was 87%…The observation that there is no significant difference in thrombosis rate or graft patency between groups in spite of an improvement in detection of graft stenosis calls into question not the monitoring technique but the success of the PTA intervention.” This study is considered a challenge to UD but it explains its own shortfall of unsuccessful PTA and lack of controlling for PTA success in the body of the article itself. The data are positive and indicate that ultrasound dilution is effective at indicating flow-limiting stenosis.
“This study does not support the concept that Qa or stenosis surveillance are superior to aggressive clinical monitoring… Clearly, further studies are needed to evaluate the effectiveness of surveillance in reducing thrombosis and improving graft survival.”
The measurement sensitivity of access flow surveillance in AVFs was 73.3% and specificity was 91%. In AVGs, the sensitivity was 68.8% and the specificity was 87.5%
A three-year study, 300-400 patients. Low flow rates detected using Transonic monitoring were associated with increased thrombosis, while stenosis detected using Duplex ultrasonography was not a strong predictor of incipient thrombosis.
Ultrasound Dilution measurements were used as the gold standard to compare other surveillance methodologies. “The very high reproducibility seen in UD, both for measurements at the same extracorporeal blood flow (QB) and for measurements at two different QB justifies its current status of a reference method in vascular access flow.”
Ultrasound dilution surveillance increased the rate of angioplasty procedures shortening primary unassisted patency but did not decrease the thrombosis rate or improve cumulative fistula patency when compared to historical controls.
Rebuttal: Study limitations cited by the authors include: Historic, rather than concurrent, controls were used. The study was not randomized thereby risking an uncontrolled bias. For instance, changes in access management practices such as the trend toward increased upper arm fistulas or surgical expertise or better preoperative vascular mapping could explain why primary and cumulative fistula patency prior to initiation of UDT monitoring was better in Group 2 than Group 1.
“A quality improvement programme based on periodical access flow measurement reduced the number of acute vascular access failures due to thrombotic events and also significantly reduced health care costs in patients with AVG, but not in patients with AVF.”
Adding Qa surveillance to monitoring in mature AVFs is associated with a better detection and elective treatment of stenosis, and lower thrombosis rates and access-related costs.
(24 center, 2300 patient Quality Improvement Plan) Findings suggest that a vascular access care Quality Improvement Plan is worthwhile to improve dialysis patients’ care and access morbidity.
Stringent flow surveillance policy coupled with prompt intervention has proven effective in maintaining AVG long-term patency.
Huge statistical study linked Medicare beneficiaries in Fresenius clinics to study 41,132 patients across 1,342 facilities in 48 states and determined “The benefits of PTA interventions are most seen in newer accesses or accesses with insufficient flow.” This underscores the importance of flow measurement to guide PTA.
“Fistula stenosis can be detected and located during dialysis with a moderate-to-excellent accuracy using Physical Exam and vascular access flow measurement to screen for stenosis.”
“The variation in access flow during HD is relatively small. They found that 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.”
"Thrombosis rate dropped from 13.5 per 100 patient-months on HD during the baseline period to 3.5 per 100 patient-months on HD during the surveillance period. Ultrasound Dilution surveillance is very sensitive in detecting hemodynamically significant stenosis and can decrease AV access thrombosis rates.”
A three-year multicenter, prospective, open-label, controlled RCT found that the measurement of access flow combining Duplex Ultrasound and Ultrasound Dilution shows a reduction in thrombosis rate and an increased assisted primary patency rate in AVF after one-year follow-up.
QA-based surveillance combining Doppler ultrasound and ultrasound dilution reduces the frequency of thrombosis, is cost effective, and improves thrombosis-free and secondary patency in autologous AV.