Vascular Access Monitoring and Surveillance

In the late 1980s and early 1990s, frequent hemodialysis (HD) access complications, particularly with arteriovenous grafts (AVG), lead to the development of vascular access monitoring protocols1. The universal goal of access monitoring is to identify access stenosis and enable intervention prior to thrombosis; thereby, maximizing access longevity and minimizing morbidity2. The advent and use of techniques including dynamic and static venous pressure monitoring, physical examination, access flow measurement, imagining and combined imaging and flow monitoring by duplex ultrasound demonstrate that it is possible to predict which accesses are at high risk for future thrombosis1. When vascular access monitoring is coupled with a program of elective stenosis correction, access thrombosis rates decline approximately 50-75%3. However, the long-term impact on access survival remains undetermined due to a lack of high powered randomized trials3.

The publication of the Dialysis Outcomes Quality Initiative (DOQI)4 and Kidney Disease Outcomes Quality Initiative (KDOQI)5 vascular access guidelines marked a major step in improving and standardizing a comprehensive approach to vascular access management6. The updated 2006 KDOQI Clinical Practice Guidelines for Vascular Access recommend that each dialysis facility have in place an organized program for the prospective diagnosis of venous stenosis by means of monitoring and surveillance5. The KDOQI guidelines are reinforced as part of the Fistula First Initiative (Change Concept #9), which recommends routine access monitoring and maintenance in conjunction with the adoption of standard criteria and an access surveillance plan for each patient7.

Preferred methods of vascular access surveillance vary according to the access type. For surveillance of arteriovenous fistulae (AVF), the 2006 KDOQI work group prefers5:

  • Direct access flow measurements
  • Physical examination to evaluate for findings of persistent swelling of the arm, presence of collateral veins, prolonged bleeding after needle withdrawal, or altered characteristics of pulse or thrill in the outflow vein
  • Duplex ultrasound

Acceptable AVF surveillance methods include recirculation using a non–urea-based dilutional method and static pressures, direct or derived.

For surveillance of grafts, the 2006 KDOQI guidelines prefer5:

  • Intra-access flow using sequential measurements with trend analysis
  • Directly measured or derived static venous dialysis pressure
  • Duplex ultrasound

Physical examination

Physical examination is the cornerstone of clinical monitoring. The elements of access physical examination include inspection (arm, shoulder, breast, neck, and face), palpation (from artery graft anastomosis to chest wall), and auscultation. Pulse augmentation and arm elevation tests, to evaluate the inflow and outflow tract, is also recommended8. Performing physical examination during an angioplasty procedure can assist in assessing the hemodynamics of the access and gauging the response to balloon dilatation8. KDOQI recommends physical examination, at least monthly by a qualified individual, to detect dysfunction in AVF and AVG5. Physical examination of the vascular access is simple to perform and readily available9; however, it does require training. A report involving only AVF, demonstrated the impact of training upon the accuracy of physical examination between an experienced interventionalist (IN) and a nephrology fellow (NF) with one month of training in physical examination of dialysis access10,11. Forty-five and 142 consecutive cases of AVF dysfunction were examined by the NF and IN, respectively10,11. Evaluation of the data revealed that the differences between NF and IN were not significant for outflow or inflow stenosis. However, the NF performed significantly better than the IN regarding central vein stenosis. NF [strong agreement (79%), Kappa value = 0.44]; IN [weak agreement (11%), Kappa value = 0.17]10.

Access flow measurement

The best validated and most widely recommended method of access surveillance for detecting hemodynamically significant stenoses3,12 is monthly access blood flow (Qa) measurement. A prospective study of 91 HD patients assessed the correlation between thrombosis and changes in Qa over a period of 18 months; Qa was assessed every six months via the ultrasound dilution technique13, 14. Only accesses with flows of more than 800ml/min were included. Among the accesses that thrombosed, a 22 and 41% decrease in Qa was found during the first and second observation periods, respectively13.

By comparison, a 4% decrease and a 15% increase in Qa were observed among accesses that did not thrombose. Accesses with a greater than 35% decrease in Qa had an approximate 14 fold increased risk of thrombosis compared to those without change14.

In a recent controlled cohort study, adding Qa surveillance to unsystematic clinical monitoring of mature AVF, increased the rate of stenosis detection and elective treatment, thereby reducing the need for temporary central venous catheters. This resulted in decreased thrombosis and access-related costs, and improved access patency rates in the first 3 years after fistula maturation12.

The measurement of Qa should be considered early in the HD treatment to eliminate error caused by decreases in cardiac output related to ultrafiltration. In one series of 32 patients, serial Qa measurements were performed within 30, 90 and 150 minutes after the start of the HD treatment13,15. The mean Qa decreased significantly over time: 1,344 ± 486 mL/min (range, 600 to 2,525 mL/min), decreased to 1,308 ± 532 mL/min (range, 560 to 2,905 mL/min) at 90 minutes, and decreased further to 1,250 ± 552 mL/min (range, 465 to 2,905 mL/min) at 150 minutes15. There was a statistically significant difference between first and final measurements (p = 0.03), corresponding to an overall decrease of 7% in Qa between initial and final measurements during HD15.

Several techniques are available for Qa measurement. These include ultrasound dilution (UD), conductivity dialysance, and duplex ultrasound. The most common indirect technique for Qa is UD. In this technique, an indicator (saline) is infused distally into the dialysis access after line reversal. Ultrasonic sensors measure changes in the protein concentration producing dilution curves used for the calculation of Qa3. Sands et al16 demonstrated a tight correlation (r=.83) between Qa measured by duplex ultrasonography and UD. Studies have shown that AVG with an access flow of <700 cm3/min have a 59% increase in the relative risk of developing thrombosis within the subsequent 3 months3. This technique has been available commercially for the past decade. However, the challenges of the UD technique are multiple. There needs to be at least 1 available UD machine, a trained technician, a quality assurance and maintenance program, provisions for travel as well as a backup plan if the trained technician becomes unavailable17.

In the early 1990s, Gotch et al18 developed a novel method for long-term serial measurement of Qa. This method used online (real-time) conductivity (OLC) dialysance. Conductivity dialysance measures the dialysance of sodium which can be considered equivalent to the dialysance of urea18. A recent study by Lacson et al17 has found a highly significant correlation between paired measurements of OLC dialysance and UD. In this 50 patient (27 AVG; 23 AVF) single center pilot study, mean UD access flow was 1086 ± 629 mL/min, whereas mean OLC access flow was 951± 575 mL/min, with significant correlation (0.93; p<0.0001) for all accesses combined17. OLC is an attractive and practical alternative to UD measurement. OLC is readily available within the HD machine which allows for measurement within the patient’s HD treatment. It can be repeated in successive HD treatments without the pre-planning necessary for specific technicians. Any trained clinical staff can perform the measurement. Using OLC may overcome the barriers posed by the cost and program requirements of instituting a UD vascular access surveillance program17.

Duplex ultrasound

Color-flow duplex ultrasound provides anatomic imaging in addition to physiologic data (Qa measurement) reflecting the function of the dialysis access16. In the late 1980s, Tordoir et al19 compared imaging of 64 accesses by color flow duplex ultrasound and angiography. Color flow Doppler ultrasound accurately identified significant stenosis in polytetrafluoroethylene (PTFE) grafts (accuracy 86%, sensitivity 92%, specificity 84%) and in AVF (accuracy 81%, sensitivity 79%, specificity 81%)16. Identification of venous outflow stenosis, the most common site of graft pathology, was even more impressive (accuracy 96%, sensitivity 95%, specificity 97%)16. Abbreviated Doppler ultrasound limited to measurement of Qa and imagining of the graft venous anastomosis is also an effective means of graft surveillance. In over 1300 cases, abbreviated studies identified over 85% of PTFE grafts with a significant abnormality on a complete Doppler ultrasound examination and had a 79.7% sensitivity and an 88.6% specificity for > 50% stenosis anywhere in the PTFE graft or venous runoff16. Color flow Doppler ultrasound has proven accuracy in the identification of patients at high risk for thrombosis, is mobile and noninvasive. Yet, direct QA measurements by means of Doppler ultrasound or magnetic resonance are expensive and can be impractical to perform routinely during HD treatment.

Venous pressure monitoring

Dynamic venous pressure (DVP) is measured by the dialysis machine pressure transducer at the beginning of hemodialysis using 15 gauge needles at a blood flow of 200. Measurements > 125-150 mmHg (different on each brand of hemodialysis machine) on 3 consecutive treatments are considered abnormal20. Although DVP is relatively simple and inexpensive to perform, static venous pressure is more predictive than DVP because it eliminates many of the confounding variables of needle size, machine type and blood flow associated with DVP measurement13. In the mid to late 1990’s, Besarab and colleagues21, 22 demonstrated that elevated static venous pressure was highly sensitive and specific for detecting venous stenosis in AVG. This was questioned by a more recent study by Dember et al23 which found that static venous pressure measurement had a poor sensitivity and specificity for predicting AVG thrombosis and was therefore not an optimal screening test for identifying AVG at risk for thrombosis.

The 2006 KDOQI guidelines recommend that patients should be referred for further evaluation if venous segment static pressure ratio is >0.5 in grafts or fistulas or if the arterial segment static pressure ratio is >0.75 in grafts5. With static pressures, analysis of trends is more informative than any one measurement5,13. Overall, recent trials have shown that dynamic and static venous pressures are less effective than access flow-based monitoring protocols3.

Signs and symptoms of access dysfunction

Other indicators that may be suspect in pending access failure include:

  • Edema: Indicative of potential infection or venous outflow impairment
  • Palpable graft pulsation: May be predictive of venous anastomotic stenosis and stenosis in fistulae
  • Decreases in delivered dialysis dose: May be indicative of either insufficient blood flow or high recirculation possibly due to stenosis
  • Excessive negative pressures (below -200 mm Hg with 15 gauge needles and a blood flow of 400ml/min): May indicate a failure of the fistula to provide the flow demanded by the blood pump due to a potential inflow stenosis in the fistula
  • Prolonged bleeding time after needle withdrawal (without excessive anticoagulation): Should be measured and documented as a potential indicator for graft or fistula stenosis. Prolonged bleeding of > 10 minutes or a change from current baseline with no change in anticoagulation may indicate stenosis particularly in PFTE grafts and requires evaluation

Prolongation of access survival

Although regular Qa surveillance is recommended to detect graft stenosis, there is little evidence that monitoring and correcting with angioplasty improves AVG survival24. Moist et al24 studied time to graft thrombosis and graft loss, comparing monthly Qa plus standard surveillance (DVP and physical examination) (treatment group) to standard surveillance alone (control group). In this blinded, randomized, controlled trial of 112 patients, there was no difference in time to graft loss (p = 0.890)24. However, monitoring Qa in AVF does show promise. Tessitore et al12 demonstrated that regular Qa surveillance is associated with significantly more access imaging, stenosis detection and elective repair, and significantly fewer temporary central venous catheter placements.

Conclusions

Despite the varying data regarding access monitoring techniques, early identification of failing accesses does provide the opportunity for elective repair of vascular access sites. Access monitoring allows planning, coordination of effort, and elective intervention for the correction of access dysfunction, rather than as urgent procedures with the potential need for hospitalization or catheter placement or access replacement25. Furthermore, some studies have reported that access flow-based surveillance programs are cost effective26. Additional prospective studies are necessary to further clarify these issues.

References:

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  2. Sirken GR, Shah C, Raja R. Slow-flow venous pressure for detection of arteriovenous graft malfunction. Kidney Int 63:1894-1898, 2003
  3. Sands JJ. Vascular access monitoring improves outcomes. Blood Purif 23:45-49, 2005
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  5. KDOQI Clinical Practice Guidelines for Vascular Access. Am J Kidney Dis 48(Suppl 1):S176-S273, 2006
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  7. Fistula First Change Concepts. Retrieved from https://esrdncc.org/en/fistula-first-catheter-last/ on 10/07/2020
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P/N 101052-01 Rev. 00 3/2009