Colour Doppler ultrasound in dialysis access

Patrick Wiese and Barbara Nonnast-Daniel

Department of Medicine IV, Nephrology, University Erlangen-Nuernberg, Germany

Keywords: access stenosis; access thrombosis; colour Doppler ultrasound; hemodialysis; steal syndrome; vascular access



   Introduction
 Top
 Introduction
 Preoperative vascular evaluation
 Access volume flow
 Vascular access stenosis
 Thrombosis and aneurysm...
 Steal syndrome
 References
 
The native arteriovenous fistula (AVF) at the wrist [1] is generally accepted as the vascular access of choice in haemodialysis patients due to its low complication and high patency rates [2].

However, with an increasing number of elderly patients and patients with co-morbid conditions such as vascular disease and diabetes mellitus in the haemodialysis population, the creation and maintenance of a patent and well-functioning AVF has become a real challenge to nephrologists and vascular surgeons [3]. The DOQI guidelines state that creation of a primary AVF is possible in only 50% of the patients [4]. Therefore, in the US, synthetic polytetrafluoroethylene (PTFE) grafts are still the predominant form of permanent vascular access [5,6], despite the well known poorer outcome compared to AVFs [4,6,7].

To further increase the use of native AVFs, especially in the co-morbid patient group, a thorough preoperative evaluation with colour Doppler ultrasound (CDU) and mapping of the arterial and venous vascular system allows the placement of an AVF in a higher proportion of patients [8,9] and to achieve a better cumulative patency rate of fistulas [10]. After creation of access, periodic monitoring is recommended, since early detection of access dysfunction and subsequent intervention may help to reduce the rate of access failure [11,12]. Although angiography has been considered as the gold standard for imaging of vascular access abnormalities, duplex ultrasound may be superior in some aspects since it provides information both on the morphology and on the function of vascular access. In addition, CDU offers the advantage of a non-invasive bedside procedure with lower costs and with no need for radiocontrast.

The aim of this article is to provide an update on CDU and to give some new information on preoperative evaluation and routine monitoring.



   Preoperative vascular evaluation
 Top
 Introduction
 Preoperative vascular evaluation
 Access volume flow
 Vascular access stenosis
 Thrombosis and aneurysm...
 Steal syndrome
 References
 
Clinical criteria for the selection of veins and arteries for the successful placement of an AVF include a visible cephalic vein after tourniquet placement, a superficial course of the vein, absence of tortuous veins, an easily palpable radial pulse, a patent palmar arch (Allen's test) and the absence of significant pressure differences (≥20 mmHg) between both arms [13].

A more detailed identification of suitable upper extremity vessels to guide the surgeon is provided by preoperative vascular mapping using a linear CDU scanner with a 7.0 MHz imaging/5 MHz Doppler probe or higher. The patient should be studied in a supine position without angling of the elbow joint to avoid compression of the vessels under investigation. Both the superficial and the deep venous system of the left and right arm can be examined from the wrist to the central veins (that is axillary or distal subclavian vein) if this is technically possible. Silva et al. [10] identified certain sonographic criteria for the examination of veins before placement of a vascular access. Using a tourniquet, a venous luminal diameter of ≥2.5 mm is required for placement of an AVF and a diameter of ≥4.0 mm for grafts. Further criteria are the absence of segmental stenoses or occluded segments and a continuity with the deep venous system in the ipsilateral upper arm. In ultrasound measurements without the use of a tourniquet a minimal diameter of the cephalic vein of >2.0 mm results in a significantly higher proportion of well-matured fistulas [13].

The non-visualized part of the central veins can only be indirectly assessed sonographically by Doppler waveform analysis. Disturbances in respiratory venous filling, such as a missing increase of venous flow during deep inspiration, may point at a stenosis of the central venous system. Although CDU is a valuable and accurate screening tool for detection of such lesions [14], phlebography or magnetic resonance venography might be considered if central vein stenosis is suspected.

In elderly patients and patients with co-morbid conditions, e.g. diabetes or vascular disease, arterial problems with reduced arterial diameter and poor arterial flow become more important. Arterial wall calcifications can be detected well by ultrasound, which shows hyperechogenicity and wall irregularities. While these changes of wall structure influence outcome, they are difficult to quantify. Many authors postulate an arterial diameter ≥2.0 mm and a peak systolic velocity of at least 50 cm/s for AVF placement [10,15]. For a precise calculation of the vessel diameter, we recommend measuring the arterial lumen from inner edge to inner edge by a B-mode or M-mode technique. Using this method, Malovrh [16] demonstrated a significantly increased risk of AVF failure when the internal diameter of the radial artery was ≤1.6 mm.

The feeding arteries dilate during access maturation. Consequently, it is obvious that not only the initial diameter but also the arterial compliance affect access outcome. The distensibility of the arterial wall can be assessed preoperatively by evaluating the Doppler waveform in the radial artery during reactive hyperaemia, induced by reopening a fist that was clenched for 2 min. The high-resistance triphasic Doppler ultrasound signal with clenched fist (regular signal of peripheral arteries) changes to a low-resistance biphasic waveform after releasing the fist (Figure 1) and the resistance index (RI) at reactive hyperaemia can be calculated using the formula

A preoperative RI of ≥0.7 in the feeding artery after release of the fist indicates that arterial blood flow will not increase sufficiently so that the chance of successful creation of an AVF is reduced [16]. We recommend preoperative screening to exclude an inappropriate response to reactive hyperaemia. This manoeuver is especially helpful in planning the location of the initial operation, i.e. selecting the wrist/forearm or elbow region.



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Fig. 1. Preoperative colour Doppler ultrasound of the radial artery, showing a change from triphasic high-resistance to biphasic low-resistance Doppler waveform at reactive hyperaemia.

 
Table 1 summarizes the preoperative CDU criteria for the selection of suitable arteries and veins that predict AVF outcome.


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Table 1. Preoperative colour Doppler ultrasound criteria for AVF outcome

 


   Access volume flow
 Top
 Introduction
 Preoperative vascular evaluation
 Access volume flow
 Vascular access stenosis
 Thrombosis and aneurysm...
 Steal syndrome
 References
 
CDU assessment of access flow cannot be performed accurately during haemodialysis sessions and must be performed in the dialysis-free period. Measurement of access flow rates at the time of haemodialysis is feasible using the ultrasound dilution technique [17], a method which correlates well with CDU (correlation coefficient 0.83) [18]. However, colour Doppler imaging offers the advantage of additional morphologic and functional information and access flow can already be assessed prior to first cannulation to follow-up access maturation.

Ultrasound imaging of the feeding brachial artery in the middle of the upper arm with flow measurement [19] and Doppler waveform analysis gives a rapid indication of the quality of the access. The calculation of volume flow holds promise to become the most significant predictor of access dysfunction both in fistulas and grafts [20–23]. While blood flow in PTFE grafts can be investigated by Doppler ultrasound along the entire access, we recommend measurement of volume flow of native fistulas in the feeding brachial artery, which correlates very well with access flow rates [24]. Measurement in the AVF feeding radial artery only can underestimate flow volume, since a variable portion of fistula flow may be supplied by the distal collateral arteries via the palmar arch. Flow calculations in the venous outflow tract are often difficult because of curves, bifurcations, variations in vessel diameter, turbulent flow and vibrations due to the superficial localization and arterialization of the veins.

For the CDU evaluation of access flow the diameter of the feeding artery is determined by B-mode ultrasonography in a transverse plane from inner edge to inner edge. The cross-sectional area is calculated by equipment software. At the same site Doppler spectra for calculation of time averaged velocity (TAV) are obtained in a longitudinal plane with an insonating angle maintained at ≤60°. The sample volume size must be sufficiently large to include the entire luminal cross section (Figure 2). Access flow is determined by equipment software using the formula

In PTFE grafts, sonographically determined threshold access flow rates less than 500–800 ml/min are associated with a significantly increased risk of failure (Table 2) [22,23,25–27]. Clotting problems in grafts may be prevalent even at relatively high flow rates, but the lower the volume flow the more reliable it is as an indicator of thrombosis within a short time [23].



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Fig. 2. Colour Doppler ultrasound measurement of flow volume in the access feeding brachial artery.

 

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Table 3. Colour Doppler ultrasound characteristics of haemodynamically relevant stenoses

 

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Table 2. Colour Doppler ultrasound estimation of critical access flow rates

 
There is controversy whether in native fistulas, an access flow less than 300–500 ml/min as determined by CDU is predictive of AVF survival [21] or not [22]. It is obvious that fistulas generally remain patent at considerably lower flow than grafts (Table 2). But even in fistulas an access flow higher than 300 ml/min is usually required to deliver an adequate dose of haemodialysis without recirculation problems. Therefore, we recommend careful sonographical screening to exclude treatable causes of access dysfunction when flow rates are <500 ml/min.



   Vascular access stenosis
 Top
 Introduction
 Preoperative vascular evaluation
 Access volume flow
 Vascular access stenosis
 Thrombosis and aneurysm...
 Steal syndrome
 References
 
Graft stenoses usually develop in the venous outflow tract at the site of anastomosis between the graft and the vein. This is the result of intimal and fibromuscular hyperplasia, caused by shear stress. The frictional force generated by blood flow at sites of changing luminal diameter, e.g. at the site of graft anastomosis, can be estimated by the aliasing phenomenon seen in CDU (Figure 3). Aliasing is an artifact that lowers the frequency components when the pulse repetition frequency is less than twice the highest frequency of a Doppler signal. Stenoses in AVFs tend to be located more centrally in the outflow tract and are caused by bifurcations of the vein, by scarring of puncture sites or by venous valves [28,29].



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Fig. 3. Aliasing in the colour Duplex mode as a sign of turbulent flow at the critical vein graft anastomosis.

 
Ultrasound has proven to be a valuable and reliable screening tool in managing clinically suspected haemodialysis access stenosis. In a pre-selected patient population with an intermediate level of clinical suspicion of stenosis [including criteria such as low delivered dose of dialysis (Kt/V), cannulation problems, prolonged bleeding after cannulation, pain or elevated venous inflow pressure], Dumars et al. [14] showed that CDU imaging can substantially reduce the number of subsequent invasive angiographic procedures. Sonographically detected high grade stenoses predict thrombotic events and access failure both in PTFE grafts and AVFs [23,25,26].

For sonographic assessment of stenosis the vascular access is examined in the longitudinal and transverse plane from the feeding brachial artery across the anastomosis and the arterialized draining veins as far into the central venous system as possible. The perivascular space is also investigated because functional stenoses may be the result of extraluminal compression of the access by abscess, haematoma or seroma. CDU characteristics of haemodynamically relevant stenoses are listed in Table 3. For calculation of stenosis, the minimal intraluminal cross-sectional area is compared with the diameter of a nearby normal segment using the formula


While it is usually easy to measure the original lumen in PTFE grafts, AVFs often exhibit enormous pre- and post-stenotic dilatation of the vessel wall. When the original lumen cannot be determined adequately, a segment of the AVF located proximal to the stenotic area is substituted for the original lumen. The assessment of stenoses near the arteriovenous anastomosis may sometimes also be difficult due to the angle of the wrist and the highly turbulent flow in this area. In this case, assessment of indirect parameters (Table 3) in the feeding brachial artery can provide indirect evidence of high-grade access stenoses. Indirect criteria can also be used in grafts made of Thoralon (Thoratec Laboratories Corporation, Berkeley, CA), a special synthetic material that generally does not allow direct CDU monitoring of the access lumen [30].

The sensitivity of CDU for detecting high-grade stenoses (>50% diameter reduction) ranges generally from 76 to 87% against the gold standard of angiography [31,32]. Tordoir et al. [33] even report a sensitivity of 95% if the following threshold values of peak systolic Doppler frequency were selected: 10 kHz for grafts, 12 kHz for AVFs and 8 kHz for the venous outflow tract [33]. Nevertheless, unlike in the case of renal artery stenosis the peak systolic frequency or velocity is not routinely measured to quantify stenoses of the vascular access. The degree of stenosis is usually determined by searching for a reduction in the luminal diameter.

Although CDU is highly accurate when detecting access stenosis, there is currently no consensus concerning the optimal method to screen for haemodynamically significant stenoses in order to allow timely intervention. Sands and Miranda et al. [12] reported that elective procedures significantly improve long-term access outcome. In contrast, Lumsden et al. [31] found no improved access patency after prophylactic endovascular correction of stenoses detected by ultrasound. Therefore, the decision to intervene should not be based upon detection of high-grade stenosis, but on reduction in flow volume.



   Thrombosis and aneurysm formation in vascular access
 Top
 Introduction
 Preoperative vascular evaluation
 Access volume flow
 Vascular access stenosis
 Thrombosis and aneurysm...
 Steal syndrome
 References
 
The most common cause of vascular access failure is thrombosis [23,34]. In PTFE grafts, thrombosis is primarily the result of progressive venous outflow stenosis [21,25] with access blood flow decreasing progressively after creation of access. In contrast to grafts, thrombotic occlusions of AVFs often occur early because of inadequate flow resulting from small lumen of vessels or failure to dilate. Post-operative flow measurements by CDU help to predict access outcome and to consider early intervention if fistulas fail to mature. Once AVFs are fully matured, thrombosis is rare. As mentioned above, patency may be maintained even at substantially lower flow rates compared to grafts. In AVFs with severe stenoses, sonographic assessment often shows the development of collateral veins that drain the fistula and prevent complete thrombotic occlusion. CDU is the most accurate non-invasive method for direct detection of thrombus formation in AVFs and grafts. For early diagnosis of access thrombosis, mainly postoperatively, indirect CDU parameters such as a triphasic Doppler waveform and low flow values at the access feeding artery give a rapid answer. Additionally, direct visualization of the thrombosis should be aimed for, because indirect ultrasound parameters at the feeding artery are similar in cases with severe access stenosis and thrombosis. The localization and extension of an acute thrombosis is evaluated more easily by colour Doppler mode than by B-mode imaging, as fresh thrombotic material may still have the same echogenicity as blood. Older thrombotic material shows an increased echogenicity and therefore facilitates B-mode imaging of the thrombus (Figure 4). An inability of manual compression of the vein with the linear transducer is not sufficient to diagnose access thrombosis using B-mode imaging. Unlike native veins, the access draining veins react in this test like incompressible arteries.



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Fig. 4. B-mode image of a thrombosis in the venous outflow tract, showing both fresh (low echogenicity) and older (high echogenicity) thrombotic material.

 
Aneurysms and pseudoaneurysms usually develop at sites of vessel destruction after repeated cannulation. Colour Doppler flow imaging can especially distinguish pseudoaneurysms from haematoma, showing the so called ‘to-and-fro’ sign, a typical waveform characterized by the backflow of blood from the aneurysmatic sac into the original vessel lumen during diastole. With increasing diameter, wall stress rises and aneurysms tend to progress spontaneously according to Laplace's law. CDU is particularly suitable to determine the extension of the aneurysm and to demonstrate thrombotic material within the aneurysmatic sac. These are important criteria to decide whether surgical correction is required.



   Steal syndrome
 Top
 Introduction
 Preoperative vascular evaluation
 Access volume flow
 Vascular access stenosis
 Thrombosis and aneurysm...
 Steal syndrome
 References
 
Elderly patients and patients with co-morbid conditions (diabetes, vascular disease) in end-stage renal disease are at increased risk of developing a ‘steal’ syndrome, i.e. access-induced ischaemia. So to speak the newly created fistula steals blood, i.e. perfusion, from the distal extremity. Steal phenomenon is particularly frequent in patients with forearm and upper arm AVFs and in patients with prosthetic straight or loop grafts [35]. Because of low resistance in the venous outflow, the fistula sucks not only the antegrade flow into the feeding artery but also ‘steals’ retrograde flow from the hand via the palmar arch and jeopardizes adequate perfusion of the hand. Some ‘steal’ occurs in 75–90% of patients after creation of the vascular access [35–37]. Usually the steal phenomenon is clinically silent and the patient remains asymptomatic.

The steal phenomenon is converted into a steal syndrome when compensatory mechanisms to maintain peripheral arterial perfusion fail. Risk factors for a steal syndrome are female gender, age >60 years and diabetes mellitus [38]. The steal syndrome is characterized by pain at rest, pain during haemodialysis sessions, ulcerations, mostly acral necrosis and even tissue loss. The challenge is to identify patients at high risk for an access-induced ischaemic steal syndrome by CDU prior to creation of the fistula.

Preoperative investigation comprises assessment of flow patterns and the diameter of the brachial, radial and ulnar arteries to detect stenoses or occlusions. The Doppler spectrum, especially at reactive hyperaemia (Figure 1), is useful to predict the risk of low flow steal. Absent or low diastolic flow correlates with impaired capacity of the palmar arch arteries to vasodilate.

Postoperative investigation comprises assessment of the access-feeding artery by investigating the parts proximal and distal to the anastomosis. The direction of flow is very easy to demonstrate by pw- or colour Doppler. If colour or spectral Doppler ultrasound show a change in flow direction, a steal situation is documented (Figure 5).



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Fig. 5. ‘Steal phenomenon’ in the radial artery at the anastomotic region. Fistula flow is not exclusively supplied by the proximal radial artery (red, antegrade flow) but also by the distal radial artery (blue, retrograde flow) via distal collateral arteries and palmar arch.

 


   Acknowledgments
 
We would like to thank Professor E. Ritz for having reviewed this editorial and given his suggestions and approval.

Conflict of interest statement. None declared.



   References
 Top
 Introduction
 Preoperative vascular evaluation
 Access volume flow
 Vascular access stenosis
 Thrombosis and aneurysm...
 Steal syndrome
 References
 

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