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Breaking news Jan 11, 2011

Ultrasound monitoring of arteriovenous fistula for dialysis

Olivier Pichot, Grenoble, France

Dysfunction of vascular access (VA) is the first cause of morbidity in end-stage renal disease patients undergoing hemodialysis. Maintenance of adequate flow in VA to ensure delivery of the prescribed dialysis dose is a priority in dialysis units.1 Arteriovenous fistula (AVF) is the most reliable method of ensuring VA, but complications remain frequent and of these venous stenosis is the first cause of dysfunction and thrombosis.2
VA monitoring is based on physical examination by nephrologists and nurses, and often by the patient him- or herself. Surveillance uses different tests to measure arterial and venous intra-access pressure, recirculation rate and access flow by the dilution technique. Duplex ultrasound (DU) measures the VA flow even in patients whose VA is not yet used for dialysis delivery and provides an accurate anatomical and hemodynamic analysis of the VA.
DU examination can be performed as part of a routine surveillance program, or in the case of VA patency or suspected dysfunction of the VA. This technique can detect asymptomatic venous stenosis, and also determine the cause of:
• Nonmaturing AVF
• Difficulties using a patent AVF (puncture problems, increasing bleeding)
• Inadequate dialysis (low flow, high venous pressure, recirculation)
• Low access flow demonstrated by a dilution test or by decreased VA flow compared with previous measurements.
Used routinely after AVF creation, DU checks the quality of VA and specifies VA characteristics for further follow-up. It can also be used to check quality after surgical or endovascular revision.
Physical examination should in all cases be performed beforehand so the DU examination can be focused on pathological findings. The DU examination should routinely include anatomical and hemodynamic analysis of the feeding arteries, the anastomosis, and the draining veins, as well as the measurement of AVF flow and brachial artery resistance index (RI), to evaluate global function of the AVF.
Because the draining vein of the AVF presents commonly with diameter discrepancies, morphological venous stenoses are very frequent. Significant venous stenosis is usually described with DU as a lumen diameter below 3 mm or a maximum systolic peak velocity over 3 to 4 m/s or both.3 The key risk factor for VA thrombosis is a flow below 400 mL/minute, as well as RI above 0.7. 4
To maximize relevance and especially optimize timing for angioplasty, interpretation of the DU data must include local characteristics of the venous stenosis, data of prognostic value (access flow, RI), and analysis of clinical and dialysis problems. For moderate venous stenosis, angioplasty is proposed only if the stenosis is responsible for clinical problems or low access flow or both. If not, angioplasty can be delayed subject to closer surveillance and monitoring. Conversely, any venous stenosis associated with a critically low flow must be treated quickly even if asymptomatic.
DU diagnosis of venous stenosis has a high sensitivity and is a valuable alternative to angiography.5,6 Because DU is noninvasive and easily available, it appears to be ideal for the first examination. In current practice, DU circumvents radiological examinations when clinical suspicion of venous stenosis is unfounded. [Figures 1-4].


Figure 1 (morphology):
Stenosis of the cephalic vein in a longitudinal view. Retraction and thickening of the vein wall show that the stenosis involves fibrosis. The diameter of the residual lumen is less than 3 mm.


Figure 2 (color Doppler):
Color duplex ultrasound view. Aliasing is observed at the site of the stenosis despite the maximum setting of the PRF.


Figure 3 (peak systolic velocity – Doppler):
Duplex allows measurement of the velocities at the site of the stenosis. Here, peak systolic velocity is higher than 6 m/sec and end-diastolic velocity higher than 4 m/sec.


Figure 4 (assessment of vascular access flow rate):
Vascular access flow is estimated by measuring brachial artery flow. Flow below 400 mL/min indicates a high risk of thrombosis in the arterio-venous fistulae, suggesting the need for revision.

DU information on a venous stenosis is sufficient to decide whether or not to treat it and to choose between surgical and endovascular revision. [Figure 5]


Figure 5 (power Doppler):
Power Doppler provides a valuable “ultrasound angiography” (when settings are appropriate)

Even if the indication for angioplasty is clinically obvious, preliminary DU examination optimizes the practical modalities of the angioplasty.7 [Figure 6]


Figure 6 (assessment before percutaneous transluminal angioplasty):
A balloon type appropriate for angioplasty of the stenosis is chosen in light of measurements of the diameter of the cephalic vein below the stenosis and of the stenosis length.

Recently, ultrasound-guided angioplasty has become an option, thus avoiding contrast injection and X-ray exposure. 8 Furthermore, DU analysis of the AVF, including depth and characteristics of the course of the vein, is helpful for the dialysis team in the event of moderate or intermittent vascular access problems. It allows optimization of the AVF, especially in terms of puncture sites.

Diagnosis of AVF thrombosis is usually obvious, but DU is useful in defining the location and extension of the thrombus, and the quality of the wall of the obliterated vessels, in order to choose the most suitable treatment modality: thrombus removal alone if the vein is normal, or combined with angioplasty if a venous stenosis is responsible. If the vein appears to be fibrotic all along its course, creation of a new VA could be a better option. In such a case, DU can be used to check the remaining vessels of the upper limbs in order to define the more suitable type and location of the new AVF.

Distal ischemia in patients with AVF is poorly quantified by DU. The main goal of DU is to analyze causes of ischemia. DU examination must include access flow measurement and the search for proximal and/or distal upper limb artery occlusion or stenosis, and steal. Compression tests are used to determine which mechanisms are responsible for ischemia

Because it is inexpensive, widely available, and noninvasive, DU is very convenient for VA examination in patients undergoing hemodialysis.  It should be used in combination with clinical monitoring and other surveillance testing, but is informative enough to substitute for angiography in current practice.

References:
1. Besarab A, Asif A, Roy-Chaudhury P, Spergel LM, Ravani P. The native arteriovenous fistula in 2007. Surveillance and monitoring. J Nephrol. 2007;20:656-667.
2. National Kidney Foundation: KDOQI clinical practice guidelines for vascular access: update 2006. Am J Kidney Dis. 2006;48(Suppl 1):176-322.
3. Pichot O, Kuentz F, Sessa C, Bosson JL, Hachache T. Principes de l’exploration par écho-doppler des fistules artério-veineuses pour hémodialyse. Phlébologie. 2001;1:99-106.

4. Tonelli M, Hirsch DJ, Chan CT, et al. Best threshold for diagnosis of stenosis within 6 months of access flow measurement in arteriovenous fistulae. J Am Soc Nephrol.   2003;14:3264-3269.
5. Wittenberg G, Schindler T, Tschammler A, Kenn W, Hahn D. Value of color-coded duplex ultrasound in evaluating arm blood vessels arteries and hemodialysis shunts. Ultraschall Med. 1998;19:22-27.
6. Gadallah MF, Paulson WD, Vickers B, Works J. Accuracy of Doppler ultrasound in diagnosing anatomic stenosis of haemodialysis arteriovenous access as compared with fistulography. Am J Kidney Dis. 1998;32:273-277.
7. Doelman C, Duijm LE, Liem YS, et al. Stenosis detection in failing hemodialysis access fistulas and grafts: comparison of color Doppler ultrasonography, contrast-enhanced magnetic resonance angiography, and digital subtraction angiography. J Vasc Surg. 2005;42:739-746.
8. Ascher E, Hingorani A, Marks N. Duplex-guided balloon angioplasty of failing or nonmaturing arterio-venous fistulae for hemodialysis: a new office-based procedure. J Vasc Surg. 2009;50:594-599.