US and Arteriovenous Fistulas for Hemodialysis

Fig. 56.1

Preoperative arterial mapping. DUS allows thorough assessment of the arterial circulation of the arm based on morphological and functional parameters. Left: B-mode evaluates vessel diameter, wall thickness, wall alterations, vessel course, and possible steno-obstructive lesions. Right: ECD allows blood flow direction assessment by codifying red flow moving toward the probe and blue flow moving away

AVF failures were found to be quite frequent when small-caliber (<1.5–1.6 mm) arteries were used to create the fistula. Malovrh et al. reported immediate and early failure rates of 55 and 64 %, respectively, when the arteries used had diameters of ≤1.5 mm, whereas much lower rates (8 and 17 %, respectively) were observed when the arterial diameters were >1.5 mm [17] Silva et al. proposed a minimal diameter of 2 mm, which was associated with an early failure rate of 8 % and a 1-year primary patency rate of 83 % [5]. However, AVF success rates of approximately 50 % have been reported even when the arterial diameter is <1.5 mm [18]. Definitively, the likelihood of AVF patency and survival increases with the diameter of the artery used to create the fistula [5, 10, 11, 17, 19].

Indeed, diameter is only one of the factors that affect the probability of successful AVF creation, and it has to be evaluated in conjunction with the functional status of artery and with the optimal site for AVF construction [9]. Arterial wall changes are common in patients with chronic renal insufficiency, diabetes, and atherosclerosis [10]. An increased thickness seems to be closely correlated with fistula failure [20]. Calcifications are easy to identify as areas of hyperechogenicity (with or without posterior shadowing) within the arterial wall and irregularities of the intimal lamina. They do not represent contraindications to the creation of a fistula although they can influence its outcome and/or render surgery more difficult [14].

DUS is also a very accurate method for identifying stenotic arterial lesions, obstructive arterial lesions [21], and vascular abnormalities such as brachial artery bifurcation in the most proximal portion of the arm.

Functional study involves the assessment of blood flow and the artery’s ability to dilate. Blood flow can be evaluated by measuring the vessel diameter and mean flow velocity (cm/s) on longitudinal scans. Malovrh et al. [10] found that successful radial-cephalic AVF construction was associated with radial artery flow exceeding 50 ml/min, and in the study by Sato et al. [22], a preoperative radial artery flow of 20 ml/min was associated with an increased risk of “primary AVF failure” within 8 months of surgery. After surgery, adequate fistula maturation is associated with dilation of the artery that feeds the AVF. As a result, blood flow within the vascular access increases, and the previously triphasic (high resistance) arterial spectrum becomes biphasic (low resistance).

The artery’s ability to increase its caliber (distensibility) can be estimated preoperatively on the basis of variations in the radial artery Doppler spectrum during the reactive hyperemia test [10]. Ischemia is induced by having the patient make a fist for 2 min, and the increase in arterial flow (reactive hyperemia) is observed immediately after the hand is reopened [10]. During the phase of ischemia, the Doppler spectrum of the artery is normally triphasic, reflecting high resistance. If the vessel is capable of dilatation, the arterial spectrum becomes biphasic during the phase of reactive hyperemia [10] (Fig. 56.2). Spectral variation can be quantified by calculating the resistance index (RI) [RI = (peak systolic velocity – end diastolic velocity)/peak systolic velocity]: the greater the intensity of the reactive hyperemia, the lower the RI will be [10]. Malovrh et al. [10] demonstrated that the absence of reactive hyperemia (reflected by an RI > 0.7 after the fist is opened) indicates insufficient increase in arterial flow during the test, which is predictive of immediate postoperative AVF failure.

Fig. 56.2

Reactive hyperemia test. Left: ischemic phase (closed fist) with high-resistance triphasic Doppler spectrum. Right: reactive hyperemia (opened hand) with low-resistance biphasic Doppler spectrum (arrows)

The reactive hyperemia test provides an excellent index of artery’s functional status and is particularly useful for selecting the surgical site (wrist, forearm, and elbow) for AVF construction.

56.2.3 Preoperative Venous Mapping

Preoperative venous DUS involves evaluation of the superficial and deep venous systems of the upper limb from the wrist up to the central veins. With ultrasound, direct visualization of the proximal portions of the subclavian vein and the innominate vein is not always possible [13]. A tourniquet is placed around the root of the arm, and the superficial venous circulation is examined with transverse scans, beginning with the cephalic vein, from the wrist to the point where it drains into the deep venous system [9]. The full course of the basilic vein should also be examined, but this is often done only if the cephalic vein is not suitable for AVF creation [9]. A map of the whole superficial venous circulation can be drawn (Fig. 56.3).

Fig. 56.3

Examples of preoperative vascular mapping. Left: arterial mapping with radial artery Doppler analysis showing a normal high-resistance flow. Right: superficial vein mapping is enabled by a tourniquet placed around the root of the arm; cephalic vein is characterized by continuous, low velocity flow

Several ultrasound parameters can be helpful in deciding whether a superficial vein can be used to create an AVF. They include the appearance of the vein wall, the course of the vessel, its patency, caliber and distensibility, and the presence of collateral circuits [9, 13].

A normal vein is characterized by thin, regular walls, and a completely anechoic lumen [23]. The course of the vein must be sufficiently linear (for a distance of at least 8–10 cm), and it should lie less than 6 mm below the skin surface to facilitate venipuncture [24]. Vein patency is assessed by exerting intermittent pressure with the transducer which causes complete collapse of the vessel walls [9]. Noncompressibility of the vein under the transducer’s pressure is a sign of obstruction and is often associated with the presence of echogenic material in the lumen [23]. Patency can be confirmed using the color Doppler module with a low pulse repetition frequency or verifying the presence of the Doppler trace in a longitudinal scan [23]. A normal venous Doppler spectrum is characterized by continuous, low velocity flow, which becomes increasingly phasic as the examination proceeds toward the central veins; the absence of such flow confirms the presence of an obstruction. The presence at the level of the subclavian and internal jugular veins of flow that varies in velocity with the respiratory and cardiac activity is an indirect index of the patency of the ipsilateral innominate vein and the superior vena cava, whereas a monophasic curve is indicative of steno-occlusion [10, 13, 14].

Fistulas created with small-caliber veins (<1.6 mm) are at high risk for early failure [11], but there is no consensus on the minimum cephalic vein diameter that will ensure good maturation of a radial-cephalic AVF. Silva et al. [5] suggest a minimum diameter of ≥2.5 mm when a tourniquet has been applied; in the absence of a tourniquet, Mendes et al. [25] propose a diameter of >2 mm. Well-documented indications on the minimum diameter for the veins of the arm are also lacking, but a value of at least 3 mm is recommended [14]. After the AVF has been created, the vein tends to dilate as a result of the increased blood flow. The vein’s ability to dilate (venous distensibility) can be evaluated during preoperative mapping. The diameter of the vessel is measured before and at least 2 min after placement of a tourniquet (or a sphygmomanometer cuff inflated to a pressure of 50–60 mmHg), and the percentage of increase is evaluated [14, 26]. Malovrh et al. [10] concluded that venous distensibility is a predictor of outcome since the mean percentage of vein dilatation observed in veins used for successfully constructed AVFs was 48 versus 11 % in those used for fistulas that ended in immediate failure. Lockhart et al. [27, 28] reported that cephalic veins with a pretourniquet diameter of ≥2.5 mm and smaller veins with a post-tourniquet diameter ≥2.5 mm were equally useful for creating dialysis fistulas. They concluded that distensibility testing should be used mainly to identify the actual maximum diameter of apparently small-caliber arm veins.

The presence of accessory veins less than 5 cm from the site chosen for the anastomosis can alter the functionality of the fistula [11], while higher frequencies of non-maturation are reported when the AVF is near large collateral veins [29].

56.3 AVF Maturation and Calculation of Blood Flow

Arteriovenous anastomoses with native vessels have been associated with a high incidence of early occlusion and failure to mature (FTM) during the postoperative period. The incidence of FTM for radiocephalic AVFs ranges from 30 to 60 % [15, 30]. When AVF does not appear clearly mature on inspection, ultrasound examination and assessment of hemodynamic parameters (AVF blood flow, RI) can help to determine whether the AVF is suitable for cannulation or whether it has instead failed to mature. In obese subjects, even well-developed veins can be difficult to visualize or palpate; in these cases, DUS can reveal whether the fistula is mature, and US mapping of the outflow veins can facilitate the first cannulation and simplify subsequent punctures [15]. “The Rule of Six,” incorporated by the K-DOQI guidelines [24], identifies ultrasound features that confirm fistula maturation: flow volume of >600 ml/min, outflow vein diameter of ≥6 mm, and outflow vein depth of ≤6 mm below the skin surface.

For slowly maturing AVFs, it is important to assess maturation with periodic calculation of flow volume at the brachial artery level. Serial measurement of AVF flow volumes during the first month after surgery can help distinguish fistulas that will mature correctly from those destined to fail. Maturation is likely if blood flow through the fistula is 250–500 ml/min on postoperative day 1 and 500–900 ml/min 1 month after construction of the anastomosis [33]. If lower flow rates are encountered, proper maturation is unlikely, and the fistula will probably become unsuitable for use in dialysis wing to problems of thrombosis or low flow.

56.3.1 Calculation of AVF Flow Volume

Calculation of the AVF flow volume by DUS is a simple procedure that can be completed in a few minutes with high reproducibility. The following formula is used to calculate flow volumes: area x mean velocity x 60, where area is the cross-sectional area of the vessel in square centimeters (since the vessel is cylindrical, its section is a circle whose area is calculated as the square of the radius × 3.14). Mean velocity (in cm/s) is that of the red blood cells measured from the Doppler trace recorded at the site used to measure area, and 60 is the number of seconds in a minute (since flow volumes are expressed in milliliters per minute) [13, 32]. Vessel diameter and mean flow velocity can be measured on a single longitudinal scan of the vessel. The vessel diameter is measured on the appropriately enlarged B-mode image. The pulsed Doppler module is then activated and the PRF adjusted to eliminate artifacts, and the mean flow velocity is calculated from the time/velocity curve (using the time-averaged velocity option available on most scanners) (Fig. 56.4).

Fig. 56.4

Calculation of the AVF flow volume. Left: theoretical basis of the formula used to calculate blood flow volume of a blood vessel. Right: example of AVF flow volume calculated at the level of the brachial artery, manually with the proposed formula (AVF flow volume) and with scanner software (Flow by diameter)

Measuring the flow volume at the level of the inflow artery improves accuracy and reproducibility. In clinical practice, brachial artery is the preferred site for the flow volume measurement of distal and proximal/proximalized AVFs [13, 24, 32, 33]. It is easy to sample and does not collapse under normal transducer pressures. Just above the elbow crease, there is an oblique segment of the brachial artery, where the sample volume can be easily positioned at an appropriate insonation angle. Finally, its laminar flow allows one to record suitable tracings for precise calculation of the mean velocity. In prosthetic grafts, the flow volume can be measured directly in the prosthetic conduit, which is more regular in caliber than a native outflow vein and more resistant to pressure exerted with the transducer.

Sample volume should be oriented parallel to the direction of blood flow and the angle of insonation maintained at <60°. The sample volume must always be positioned at the center of the vessel, but the amplitude should be adjusted to allow sampling of 50–70 % of the vessel lumen. Acquisition of velocity data must be as precise as possible; this can be achieved by careful regulation of the PRF to eliminate all types of artifacts.

56.4 AVF Monitoring/Surveillance (Follow-Up and Early Detection of Complications)

Measurement of blood flow is considered the best means of surveillance for a vascular access [24]: reduced flow volumes or values that decrease over time are predictive of thrombosis for both native and prosthetic AVFs [32, 34, 35]. DUS can document a low AVF flow volume and simultaneously explore possible causes [13, 15, 32]. Anyway, AVF examination through DUS should be used when monitoring/surveillance methods have revealed anomalies or when problems arise that prevent regular dialysis (difficult venipuncture, insufficient blood flow, high venous pressure, and prolonged bleeding after removal of fistula needles) (Fig. 56.5).