Arteriovenous Fistula Ultrasound Assessment

Abstract

Arteriovenous fistula (AVF) ultrasound (US) assessment is the first-line tool in addition to the physical examination to surveil, screen, and diagnose complications of the vascular access (VA) of chronic hemodialysis (HD) patients. The VA currently recommended by all guidelines as the “gold standard” for chronic HD patients is the AVF; therefore, creating long-lasting, complication-free fistulas is one of the targets considered by all nephrologists. US assessment is a noninvasive technique that offers detail regarding vascular morphology and hemodynamics, and may be performed at the patient’s bedside. The clinical settings in which US assessment is currently performed are the presurgical screening of veins and arteries (presurgical mapping) or the diagnosis of AVF complications, usually when there are relevant clinical signs and symptoms. Moreover, AVF point-of-care US examination (POCUS) is used to assist and aid nurses in the cannulation procedure, with the scope of minimizing accidents. The current chapter aims to provide a guidebook regarding the multiple situations in which AVF US may be performed and to highlight the importance of this tool for the purpose of maintaining the long-term functionality of the VA of HD patients.

1. Introduction

End-stage renal disease (ESRD) is a rising global healthcare burden leading to an increasing need for renal replacement therapies (RRT). Currently, ESRD prevalence has surged by 43% from 2003 to 2016 [1], while chronic kidney disease (CKD) is expected to be the world’s fifth leading cause of mortality by 2040 [2]. Even though renal transplantation is the most desirable therapeutic option for the patient with ESRD, kidney donation availability varies greatly among countries and cultures. The most used RRT modality worldwide nowadays is in-center HD. Survival rates on maintenance HD therapy are poor when compared to the age-matched general population, but they have increased steadily recently, reaching a 5-year survival of 42% [3]. However, the life span of HD patients may vary largely, depending on age, comorbidities, dialysis adequacy, vascular access, and treatment compliance.

Performing HD therapy requires a functional VA, accomplished either through central venous catheters (CVCs), AVFs, or arteriovenous grafts (AVGs). Initiating HD sessions on CVCs has been reported to carry a larger mortality risk, HR 1.55 (95%CI: 1.09, 2.21) [4, 5], with AVFs remaining the gold standard for vascular access in hemodialysis nowadays.

Referral to AVF formation should start when the estimated glomerular filtration rate (eGFR) drops below 15–20 ml/min/1.73 m², which, in general, is within at least 6 months of the estimated dialysis initiation [6]. Moreover, timely preservation of the vascular structures of the arm (no venipuncture or intravenous cannulation procedures) has proven of benefit in accomplishing a better primary AVF patency.

There are several types of autogenous AVFs, usually performed upon the upper limbs, such as radio-cephalic, brachio-cephalic, and brachio-basilic. The types of AVGs usually performed include the antecubital forearm loop graft, the upper-arm AVG, and only in rare cases lower extremity AVG or HeRO grafts. The choice of VA site should be based not only according to the current KDOQI vascular access guidelines [7] regarding the patient’s end stage kidney disease (ESKD) life plan but also on presurgical ultrasound mapping criteria [8]. The current consensus is that, if permitted, the AVF site should be performed on the nondominant arm, starting distally, at the wrist. The anastomosis preferred is end-to-side (vein to artery) due to a lower incidence of distal venous hypertension, as compared to side-to-side anastomosis [9].

After maturation, AVFs should be able to withstand repeated cannulations, roughly 24 cannulations/week and should ensure proper blood flow (Qb) toward the HD machine to ensure adequacy. Cannulation techniques are very important to preserve complication-free VA. Typically, needle sizes start from 17G, increasing up to 15G, depending on AVF/AVG characteristics. The preferred cannulation techniques are rope ladder (which implies the use of the entire VA, by alternating needling sites) or the buttonhole technique (only for AVFs), while the area technique leads to multiple complications (aneurysms, long bleeding time) and is not recommended anymore.

2. Presurgical clinical examination and ultrasound mapping

2.1 Presurgical clinical examination

Clinical examination of both the arterial and venous systems prior to surgery is mandatory, and several studies have shown that a correct preoperative evaluation may have an approximately 70% success rate in predicting fistula maturation [10]. The patient’s history of diabetes mellitus, presence of peripheral artery disease, and ipsilateral previous catheter placement may be useful in placing the patient in a high risk of VA failure category.

2.1.1 Arterial clinical examination

The preoperative arterial clinical examination should assess the patency of the arterial tree and potential detections of steno-occlusive disease. Therefore, the following should be performed:

• Assessment of the presence and quality of arterial pulses in the arterial, ulnar, and brachial arteries, with attention to potential asymmetries.

• Allen’s Test: To confirm the patency of the palmar arch, the Allen test involves the following steps:

  • Manual compression of the arteries at the wrist until distal pallor and lack of capillary refill occur.

  • Decompression of the radial artery and assessment of the speed in which revascularization occurs. A normal arm flush occurs in 5–15 seconds.

  • Repetition of the maneuver for the ulnar artery.

2.1.2 Venous clinical examination

The clinical examination of the veins implies following the cephalic and basilic veins at the forearm and antecubital fossa with and without a proximal tourniquet. The evaluation includes trajectory, tortuosity, and increase in diameter after tourniquet placement.

2.2 Ultrasound mapping

The latest clinical VA Society guidelines recommend that preoperative ultrasound examination should succeed in every clinical assessment. The patient should be placed lying supine, with their arms relaxed, in a warm room. The ultrasound scanning probe used is a high-frequency US probe, 7–12 MHz (5Mhz or higher for Doppler), and the modes involved in the examination are B-mode (for morphological information), spectral Doppler (for flow assessment), color Doppler (for flow detection and direction assessment). Grayscale (B-mode) is used to characterize anatomical features such as blood vessel diameters, depth from the skin surface, collateral veins, and blood vessel trajectories. The spectral Doppler technique allows the analysis of the blood flow velocity changes over time (regarded as cardiac cycles) in a small volume sample, therefore generating a spectrum with positive or negative curves. To achieve an accurate result, the technique requires consideration of the following aspects:

• Placing the transducer longitudinally, set the sample volume (“gate” size) for interrogation. The size should be 2/3 of the width of the vessel.

• Set the insonation angle lower than 60 degrees; usually, this means aligning it parallel to the long axis of the vessel.

• Adjust the gain setting to allow a clearly visible Doppler flow in the region of interest.

• Adjust the PRF (pulse repetition frequency, which changes the number of pulses transmitted by the transducer in 1 second) according to the velocity ranges. Higher PRF levels will allow the detection of higher velocities.

• Set the wall filter (if available) to eliminate any low-velocity frequency shifts.

The following parameters should be recorded after color and spectral Doppler analysis:

• Direction of flow.

• Type of curve (high resistance in the presurgical setting and low resistance in the functioning AVF).

• Peak systolic velocity (PSV), end-diastolic velocity (EDV), and resistive index (RI).

• Time average mean velocity (TA mean)—only for the AVF/AVG.

• Blood flow (Qb-ml/min) in the feeding artery.

The pressure placed on the ultrasound transducer should be minimal, and there should be plenty of ultrasound conduction gel to avoid the underestimation of the superficial venous calibers, which may be easily compressed. The insonation angle of the spectral Doppler scan should be lower than 60 degrees.

2.2.1 Presurgical mapping of the arterial system

The examination of the arterial system should start distally and should continue proximally. The following steps should be employed:

• B-mode examination in the transverse section at the wrist—note the internal diameter of the radial artery and the depth. Current guidelines (7) recommend a diameter of >2 mm for the radial artery (Figure 1)

• B-mode examination in longitudinal section—note calcifications, intimal irregularities, and wall thickening.

• Color and spectral Doppler examination in longitudinal scan—note peak systolic velocity (PSV) in the radial artery (which should be >50 cm/s) [11]. Perform hyperemic response (with PSV, end diastolic velocity (EDV), and resistive index (RI) quantifications) in the radial artery, which implies assessing the change in the spectral waveform after inducing distal ischemia by clenching the fist of the patient for 2 minutes. The waveform should be high resistance during ischemia and afterward should change into a moderate-resistance waveform during unclenching the fist (Figures 2 and 3). A study has found that a RI > 0,7 during reactive hyperemia predicted the failure of a future AVF [12]. The RI is an indirect indicator that the radial artery can provide an increased blood flow, as a response to a future anastomosis.

Figure 1.

B-mode transverse section of the radial artery (noted caliber and depth).

Figure 2.

Radial artery spectral Doppler scan with a clenched fist.

Figure 3.

Radial artery spectral Doppler scan after releasing the fist.

The RI may be calculated using the formula: RI = PSV-EDV/PSV.

• B-mode examination of the brachial artery in transverse and longitudinal sections assess wall morphology, diameter, and presence of calcifications.

• Color and spectral Doppler examination of the brachial artery include PSV and EDV measurements.

There is no current consensus on brachial artery measurement thresholds that predict the patency of a brachio-cephalic or brachio-basilic AVF.

2.2.2 Presurgical mapping of the venous system

The examination of the venous system should start distally and should continue proximally. All measurements should be performed before and after proximal tourniquet placement with sufficient gel use and without significant compression of the transducer. The increase in size of the vein after 2 min tourniquet placement reveals the venous capacity of distension, which should be at least 40%. The following steps should be employed:

• B-mode examination in transverse and longitudinal sections of the cephalic vein at the wrist—assess compressibility by pressing on the vein until it fully collapses. A minimum diameter of 2 mm is considered adequate for a future radio-cephalic AVF. Assess continuity, collaterals, depth, and caliber variations in its course toward the antecubital fossa.

• B-mode examination in transverse and longitudinal sections of the cephalic (Figure 4) and basilic veins of the arm—assess compressibility, depth, and diameters. A minimum venous diameter of 3 mm is considered sufficient for a brachio-cephalic or brachio-basilic vein. Follow the venous trajectories proximally. For the basilic vein, the depth is of relevance, because of its profound situation. At a depth of over 8 mm, a second surgical intervention is needed to raise the vein at a more superficial level.

• Color and spectral Doppler examination of the proximal veins shows respiratory phasicity. The lack of increase in flow during inspiration may be an indirect sign of stenosis or occlusion in the subclavian vein, brachio-cephalic trunk, or superior vena cava.

Figure 4.

B-mode transverse scan of a cephalic vein in the arm (noted caliber and depth).

3. Arteriovenous fistula ultrasound assessment

The surveillance of AVFs through ultrasound assessment in the postoperative setting is not routinely performed. Usually, the first ultrasound scan occurs at 4–6 weeks after creation, to assess the level of maturation it has reached. Cannulations of immature AVFs may lead to premature AVF failure or thrombosis. The ideal AVF should fulfill the rule of 6 s: minimum 600 ml/min flow, minimum 6 mm diameter, and maximum 6 mm depth. There is no recommendation regarding routine, scheduled US surveillance; however, the current VA guidelines suggest AVF ultrasound be performed as a first-line investigation if there are suspected complications. The clinical examination that may indicate the need for US evaluation includes the following signs:

• Distal edema, cyanosis and multiple collateral veins may suggest central vein stenosis, thrombosis, or occlusion.

• Hand pain, cold fingers, steal syndrome, or distal ischemia.

• Low blood flow on dialysis, inflow artery stenosis, anastomotic stenosis

• Indurated AVF—hematoma, collections, thrombosis

• Lack of thrill and bruit complete thrombosis

• AVF with high pulsatility, low perceptible thrill, AVF stenosis

• Aneurysms and dilatations may suggest high-flow AVFs.

• Blood clots removed upon cannulation, segmental, or total AVF thrombosis.

The examination should be performed with a relaxed and supported extended arm, using a 7–12 MHz linear transducer. A widely recognized, systematic protocol has not been agreed on by the current VA or Nephrology guidelines; however, the available literature proposes the following methodological approach for the VA Duplex US examination [13, 14].

3.1 Inflow arterial examination

• B-mode, transverse section follows the feeding artery in a proximal direction toward the clavicle. Any variations in caliber should be noted. The brachial artery bifurcation should be identified because blood flow should be measured proximal to the bifurcation (regardless of the type of AVF).

• B-mode longitudinal section applies color and spectral Doppler on brachial artery and proceed to measure blood flow. The typical spectral curve involves high flow and low resistance. It is important to maintain an insonation angle of 60 degrees and a sample volume of minimum of 2/3 of the vessel parallel to the vessel walls. If the sample volume is too small, the flows tend to be overestimated. Once a stable curve is obtained, freeze the image and measure the diameter of the artery (the brachial artery radius is the diameter divided by 2). The US machine software will calculate the time averaged mean velocity (TA mean) and the resistive index (RI), and in some cases, the AVF flow as well (Figure 5). The AVF flow may be calculated using the following formula, as well:

Figure 5.

Measuring the TA mean, RI in the brachial artery using color and Doppler scans.

AVF flow (ml/min) = 3,14 × brachial artery radius 2 × TA mean × 60.

Due to the variability of the AVF flow measurement, it is best to assess it three times, to ensure that the calculation is accurate and reproducible. The normal blood flow values for native AVFs are >500 ml/min and for AV grafts >600 ml/min.

The resistivity index is usually calculated by the US machine but has the following formula:

IR = PSV-EDV/PSV. If it is increased over 0,6, the AVF should be scanned thoroughly for the presence of stenosis.

3.2 Anastomosis examination

• B-mode transverse and longitudinal sections—measurement of the largest diameter of the anastomosis (Figure 6), to exclude an anastomotic stenosis.

Figure 6.

End-to-side anastomosis in a B-mode transverse scan.

3.3 Venous examination

• B-mode transverse section—follow the course of the vein from the anastomosis proximally, until its drainage in the subclavian system. The following should be noted: vein caliber and caliber variation, depth, and possible sites for cannulation.

• B-mode longitudinal section—identify any anomalies or complications inside and outside the vessels such as stenosis (vessel caliber appears reduced), aneurysms/pseudoaneurysms, thrombi, hematomas, and edema. Describe them in relation to the surrounding anatomical structures. If luminal caliber differences are noted, they should be recorded before the stenosis and inside the stenosis. Using color and Doppler scans, the PSVs should be measured in the same places as the calibers.

4. Arteriovenous fistula complications

US is the first-line imagistic method to use when suspecting a malfunction or complicated AVF in a patient, as per current VA Society guidelines [7, 8]. The most frequent complications include stenosis, thrombosis (partial or occlusive), calcifications, steal syndrome, aneurysms or pseudoaneurysms, and high-flow AVF.

4.1 Stenosis

Stenoses are defined as vascular intraluminal narrowing that leads to hemodynamic and functional consequences. The hemodynamic modifications induced by a stenosis in the vascular wall are turbulences at the site of the stenosis and decreased flow in the post-stenotic segment. The causes of AVF stenoses are atherosclerotic calcified plaques (usually occurring in longstanding AVFs), partial thrombosis, outer compression due to hematomas or edema, and lastly intimal hyperplasia. The latter situation occurs when the walls of the vein cannot withstand the shear wall stress due to the increased blood flow. That is why, in most cases, AVF stenoses occur in the juxta-anastomotic segment (first 5 cm), but may occur in the middle segment as well, or in the central veins. For AVGs, stenoses may occur at the distal (venous) anastomosis usually. For brachio-basilic AVFs, a typical site for stenosis is the proximal swing segment, due to kinking of the vessel [15]. According to a study [16], the most frequent sites for native AVF stenosis are brachio-basilic proximal swing segment stenoses, juxta-anastomotic stenoses, and puncture site stenoses.

The clinical examination that supports the suspicion of a stenotic lesion in the AVF includes abnormal thrill (weak) in the stenotic area, abnormal high-pitched bruit in the stenotic area, reduction/failure of the AVF collapse during arm elevation, lack of pulse augmentation during arm elevation, ipsilateral extremity edema, prolonged bleeding (central stenosis), not achieving dialysis efficiency (eKt/v), and cannulation difficulty and low Qb delivered at the HD machine.

Ultrasound has an over 50% sensitivity for stenosis diagnosis, with respect to angiography which has a sensitivity of 76–95%. The advantages of the US are that it allows the visualization of the surrounding structures as well as the calculation of blood flow. Current literature is quite heterogeneous in defining clear criteria for AVF stenosis, due to small-scale studies, retrospective studies, lack of control groups, etc. Even so, there are several aspects to assess when an AVF stenosis is suspected.

The following steps should be employed when assessing a suspected AVF stenosis:

• B-mode transverse scan—visualize whether the caliber of the AVF is decreasing abruptly. Rotate the transducer in a longitudinal section and measure the intraluminal diameters in the pre-stenotic and stenotic regions.

% stenosis = (original lumen- residual lumen)/original residual lumen × 100.

This measurement is useful in AVGs; however, in a native AVF lumen reduction alone must be interpreted with caution due to possible wall caliber changes.

• Color and spectral Doppler longitudinal section scan—determine the peak systolic velocities in the pre-stenotic segment and in the stenotic segment. A PSV ratio between 2 and 3 (pre-stenosis/stenosis) indicates a hemodynamically significant stenosis (50–75%), while a ratio > 3 reflects a stenosis of >75%.

The Prague group has defined the following criteria for stenosis diagnosis, including both hemodynamic and morphological changes on which the examinator may rely when judging if a stenosis needs immediate action or just watchful waiting. The Prague group criteria are presented in Table 1 [17].

Significant	Borderline
Main Criteria; Diameter reduction by >50%; Peak systolic velocity increase >2–3 ×
+ Additional criteria (≥ 1)
Residual diameter < 1.9–2 mm	No additional criterion
Flow volume decreases by >25% (if previously >1000 ml/min)
Flow volume < 600 ml/min for AVGs, < 500 ml/min for AVFs

Table 1.

Prague group criteria for stenosis.

In order to diagnose a significant stenosis, two main criteria and at least one additional criterion are needed; if less than two criteria are present, the patient needs to be reevaluated in 6–8 weeks. If only one main criterion is present, the stenosis is not considered significant.

Indirect arterial measurements in the brachial artery, highly suggestive of AVF stenosis, are high resistance Doppler wave, RI >0,6, and reduced flow.

4.2 Thrombosis

Ultrasonography is the first-line imaging technique employed when an AVF thrombosis is suspected. The clinical signs and symptoms that may lead to the suspicion of AVF thrombosis are AVF with no thrill or bruit but with high pulsatility, AVF pain, redness, cannulation difficulties, drawing clots from the AVF after cannulation, and high HD machine arterial or venous pressures. AVF thrombosis may occur as a postoperative complication (usually during the first week after surgery) or after prolonged AVF use. The current existing data is conflicting regarding certain risk factors (demographics and comorbidities) that would absolutely contraindicate performing AVF. The risk factors associated with AVF thrombosis are:

• For early AVF thrombosis: hypotension, hypovolemia, and vascular trauma due to prior needling.

• For late AVF thrombosis: persistent hypotension, AVF stenosis, AVF vicinity compressions (seromas, hematomas, lymphatic collections, and compressive wound dressings), and hypercoagulability states.

A study by Yii et al. [18] assessed 511 AVFs and analyzed the ROC curves for TA mean, PSV, and EDV as predictive for thrombotic flow-related AVF dysfunction. They concluded that TA mean cutoffs of 45 cm/s for forearm AVFs and 49 cm/s for arm AVFs yielded the best AUC values (0.95).

When performing AVF ultrasound, the following findings suggest thrombosis:

• B-mode examination: The vein is not fully compressible, and intraluminal echogenic material can be noticed. The thrombus increases in echogenicity as it becomes older.

• Color and spectral Doppler examination—based on how occlusive the thrombus is, the flow will be reduced accordingly. In complete thrombosis, no AVF flows will be determined at the site of thrombosis (Figure 7).

Figure 7.

Color Doppler transverse scan incomplete thrombosis of an aneurysmal brachio-cephalic AVF.

Indirect signs or complete AVF occlusion due to thrombosis are similar to those of critical stenosis, that is, the high resistance triphasic Doppler wave in the feeding artery and a high resistive index.

4.3 Aneurysms and pseudoaneurysms

Diagnosing AVF aneurysms is typically clinical. An AVF aneurysm is a fusiform or saccular dilatation of the AVF, with a diameter exceeding two times the diameter of the undilated vessel. It usually occurs in longstanding fistulas, due to the reactivity of the endothelium to repeated cannulations, and usually grows gradually over time. The aneurysmal dilatation comprises all three layers of the wall, which makes it less prone to ruptures (Figure 8). Even if most aneurysms are stable lesions, other complications may occur inside the aneurysms, such as calcifications or segmental thrombosis (due to nonlinear local blood flow circulation).

Figure 8.

Aneurysmal dilation of a brachio-cephalic AVF.

The pseudoaneurysm is a false vessel dilatation, which is not delimitated by the AVF walls, but by surrounding structures. It usually occurs abruptly, due to cannulation accidents, and implies blood flow from the AVF to a false saccular structure, adjacent to the AVF. It may occur next to the posterior or anterior walls and generally requires treatment as the risk of rupture is much higher than with true aneurysms. Anterior wall pseudoaneurysms are clinically present as a subcutaneous pulsating collection; the skin is lucent and thin, while posterior wall pseudoaneurysms may be asymptomatic.

From the US evaluation point of view, the aneurysm reveals only a dilated AVF. In the B-mode, the blood flow may describe a cigarette smoke-like pattern due to low blood flow velocities. In color Doppler, a red-blue vortex may appear (Korean flag appearance). Sometimes, local, punctiform calcifications may be visualized, appearing as hyperechogenic wall deposits.

In contrast, pseudoaneurysms require identifying an additional cavity or communication of the AVF with the adjacent structures. In B-mode, the communicating structure may be identified, while color and spectral Doppler show flows in a “to and fro” pattern [19]. When the sample volume of the spectral Doppler is placed at the collar region of the pseudoaneurysm, the flow curve shows both positive and negative waveforms, corresponding to the recirculation of the blood in the lesion. Angiography is the gold standard method for pseudoaneurysm diagnosis; however, it does entail more periprocedural complications. CT angiography also demonstrates a high sensitivity (95%) and specificity rate (99%) regarding diagnosis [20]. However, because the US is easy and quick to perform, it is noninvasive and has no contraindications as well as high (94%) sensitivity and high (97%) specificity rates; it is regarded as an essential tool for pseudoaneurysm diagnosis.

When identifying pseudoaneurysms, the treatment is either surgical repair, endovascular covered stenting, or percutaneous thrombin injections, which induce local thrombosis.

4.4 Hematomas

Hematomas typically occur in tissues surrounding the AVF due to improper cannulation or improper hemostasis. From a clinical point of view, the appearance is that of a localized subcutaneous blood suffusion. Its color varies according to the timing of the vessel injury, varying from red-bluish to green and ultimately yellowish. Typically, AVF hematomas do not require US confirmation; however, if additional, more severe complications (stenosis due to compression, AVF dysfunction) are suspected, it is reasonable to perform AVF US.

In the B-mode US evaluation, hematomas appear as local collections, not communicating with the AVF vessel walls. The margins of the hematomas are vaguely delimitated, many times accompanied by local edema in the surrounding tissue. If the color or spectral Doppler is implied, even when adjusting PRF for low velocities, no active blood flow should be detected in the hematoma. Most often, hematomas have a favorable evolution; however, some of them may become infected. US surveillance of AVF combined with US-guided cannulations reduced the occurrence of AVF hematomas from 2.63 to 1.71 episodes/patient/year in a prospective study [21]. The differential diagnosis between a local purulent collection and an uninfected hematoma is mainly clinical and does not have clear US criteria (Figures 9 and 10).

Figure 9.

Color Doppler transverse scan—Large hematoma surrounding the anastomotic site and the inflow artery (hematoma dimensions noted D1, D2). Note no color Doppler flow inside the hematoma.

Figure 10.

Color Doppler longitudinal scan—Small hematoma over the anterior AVF wall (hematoma dimension noted D1). Note no color Doppler flow inside the hematoma.

4.5 Maturation deficit

The maturation period (meaning the time from ABF/AVG creation to its first cannulation) differs significantly worldwide [22]. Higher rates of AVF maturation occur in patients with lower body mass index, no peripheral vascular disease, and lack of smoking or diabetes. Diseases associated with hypercoagulation are at higher risk of maturation failure, and so are patients with high inflammatory status or hypoalbuminemia. The ideal and mature AVF according to the NKF-KDOQI guidelines presents the outflow vein diameter ⩾of 6 mm and blood flow ⩾of 600 mL/min.

In a study by Caputo et al. [23], the addition of Doppler US evaluation at 4–6 weeks after surgical creation had a better sensitivity of detecting AVF failure than physical examination alone. The detection of poor AVF flow (<500 ml/min) had a 67% sensitivity of predicting AVF maturation deficit, in comparison with only 42% sensitivity of the physical examination (p < 0.001).

According to Wilson [24], the US evaluation at 4 weeks after surgical creation divides AVFs into the following categories (Table 2):

Type of AVF	AVF blood flow	Cannulation segment length	Other	Observations
Mature	>600 ml/min	At least 10 cm	>6 mm in diameter,; < 6 mm in depth	May be cannulated
Still maturing	400–600 ml/min	<10 cm	Depth < 6 mm	Needs follow-up in max. 4 weeks
Unusable	>600 ml/min	>10 cm	Too deep to cannulate	Require s surgical superficialization
Requiring intervention	>400 ml/min	—	Stenosis detected	Requires percutaneous transluminal angioplasty

Table 2.

Types of AVFs at the 4-week US evaluation.

4.6 Steal syndrome

Steal syndrome occurs when the structures situated distally from the AVF are poorly vascularized, and thus symptoms of ischemia occur. The risk factors associated with steal syndrome are: brachio-basilic or brachio-cephalic fistulas, diabetes mellitus, and coronary and peripheral artery disease. The hemodynamics governing AVF steal syndrome are complex and may include inflow arterial pathology, outflow vein low resistance, or the collateral arteries [25]. Therefore, the following situations should be considered:

• Proximal artery stenosis in the subclavian, axillary, or brachial arteries, leading to low inflow.

• Distal artery pathology: peripheral artery disease and vasculitis, leading to high distal arterial resistance in the ulnar and radial territory.

• Enlargement of the arteriovenous anastomosis or of the outflow vein—leading to low venous resistance.

The signs and symptoms associated with AVF steal syndrome include imperceptible radial and ulnar pulses, a digital brachial index lower than 0.6, and digital pressures under 50 mmHg. If steal syndrome occurs in the acute setting the hand is cold and painful, the patient experiences tingling or numbness. In a chronic setting, the symptoms include muscular atrophy, nail changes, cutaneous ulcers, and gangrene.

The ultrasound examination of steal syndrome may include the following findings:

• Color and spectral Doppler scan: Abnormal waveforms in the radial or ulnar arteries, showing decreased PSV.

• Inverse flow in the distal artery, usually with an antegrade flow during systole and a retrograde flow during diastole.

• High flow in the brachial artery feeding the AVF (usually over 2–3 l/min).

4.7 High-flow AVF

High-flow AVFs are rarely diagnosed vascular access complications because the symptoms are unspecific and are usually related to right heart decompensation. High-flow AVFs are defined by an AVF blood flow of over 1500 ml/min [26]. The symptoms of this VA complication are shortness of breath, palpitations, orthopnea, and dyspnea upon exertion. Patients undergoing chronic HD treatment have a high rate of cardiovascular comorbidities, and high-flow AVFs add to this cardiovascular burden. A study by Saleh et al. [27] showed that high-flow AVFs lead to an increase in ventricular mass by 12,7% and a reduction in the ejection fraction.

The ultrasound examination of high-flow AVFs reveals spectral Doppler brachial flows over 1500 ml/24 h. Most authors recommend evaluating this blood flow in conjunction with the cardiac output. Thus, if the AVF blood flow/cardiac output ratio exceeds 40%, the patient may present symptoms of heart failure [28]. The procedures that may be implied to reduce AVF flows are either surgical or endovascular, venous banding, Miller banding technique using a PTA balloon, anastoplasty (reducing the size of the anastomosis), graft interposition, and graft inclusion.

4.8 Calcifications

Usually, calcifications occur in the inflow arteries and may be observed when performing presurgical mapping US. Even though arterial calcifications may reflect a poor vascular bed, two studies have shown that the presence of arterial wall calcifications determined on preoperative mapping was not associated with poor AVF maturation [29, 30].

Studies involving AVF calcifications are scarcer, even though the prevalence is high. The diagnosis of calcifications is mainly based on ultrasound examinations as they are asymptomatic in clinical practice. Upon B-mode ultrasound examination, these complications occur as intensely hyperechogenic areas, either punctiform or gross, usually at the cannulation sites. There is no current consensus regarding calcification quantification in terms of dimensions or position and the prospective impact on AVF functionality.

Most of the calcifications present with posterior wall shadowing (Figures 11 and 12). One study [31] showed that AVFs presenting calcifications had a higher 1-year chance of complete thrombosis and are associated with other VA complications as well, such as stenosis.

Figure 11.

B-mode longitudinal scan- hyperechogenic areas with posterior wall shadowing on the posterior AVF wall.

Figure 12.

B-mode transverse scan- hyperechogenic area with posterior wall shadowing on the posterior AVF wall.

5. Point-of-care ultrasound use (POCUS) for AVF evaluation

The aim of POCUS for AVF evaluation is to provide an additional tool in extension to the physical examination. Taking into account that in almost all centers, the cannulations are performed by the HD nurses, having expertise in POCUS may be regarded as a supporting technique for difficult cannulations or new AVFs. Therefore, the scope of POCUS is to answer quick questions regarding the morphology of the AVF (where to cannulate, at what depth, whether there are side branches, and whether there are acute ongoing complications). It is not meant for in-depth analysis, such as stenosis quantification or detailed flow measurements.

AVF POCUS is implied in the following situations, and in current practice, there are portable devices or wireless transducers that make the technique easy to perform at the patient’s bedside:

5.1 First cannulation

The POCUS examination steps will include B-mode examination of the AVF morphology—depth, caliber, presence of site branches, and best site of cannulation (to avoid puncturing side branches) (Figure 13). Most centers use the “Rule of 6s” for the ideal AVF. It may also help draw a map of the AVF/AVG for the “rope ladder” cannulation technique.

Figure 13.

B-mode longitudinal scan of the AVF noted various depths and calibers.

5.2 Difficulty in cannulation

Most often, cannulation difficulties are encountered when the AVF is too deep and the needle does not reach it properly. In these situations, besides statically assessing AVF depth, an US-guided cannulation may be performed. US-guided cannulation requires the use of sterile gloves, sterile transducer gel, and a sterile sheath, to not risk contaminating the needling sites. Using a B-mode longitudinal scan, the long axis of the probe faces parallel to the vessel. The needle will be inserted below the ultrasound probe (example of cannulation position in Figure 14, using a mannequin).

Figure 14.

Transverse probe and needle positioning on a mannequin model.

The needle trajectory can be noticed on the screen as it progresses toward the middle of the vessel.

• Screening for hematomas or pseudoaneurysms—when clinically, there is swelling or blood infiltration around the AVF/AVG and may help avoid cannulation in a traumatized site.

6. Conclusions

To conclude, AVF US is an invaluable tool in daily clinical practice when managing chronic HD patients. It is an inexpensive and noninvasive technique that provides a quick diagnosis and assists in therapeutic management. Moreover, AVF US has the important advantage that it may be performed at the patient’s bedside, helping to reduce AVF complications due to miscannulations. More programs should include VA US training for nephrologists so that HD patients will benefit from the advantages of this technique.