Arterial inflow may be inadequate due to atherosclerotic occlusive disease, dissection, or other arterial conditions that narrow the upper extremity arteries. Doppler spectral waveforms from the inflow artery to a widely patent autogenous fistula or prosthetic graft are characterized by increased peak systolic velocity (PSV) with low pulsatility (high diastolic flow)—features typical of a “low-resistance” flow pattern (Fig. 23.1a). However, with a poorly functioning or occluded access site, the inflow arteries will display a typical “high-resistance” multiphase flow pattern similar to that of a normal peripheral artery (Fig. 23.1b). Significant arterial stenosis at any level in the vasculature supplying the access site may reduce the pulsatility and PSV within the autogenous fistula or prosthetic graft and lead to access failure.

The subclavian, axillary, brachial, radial, or ulnar arteries proximal to the access site should be evaluated for stenosis. Flow patterns in the native arteries distal or peripheral to the anastomosis should also be recorded to document distal perfusion to the hand. Distal to the arterial anastomosis, the flow waveform may return to a high-resistance pattern or it may show alternating or retrograde flow direction suggestive of an arterial steal, as discussed later in this chapter. Spectral waveforms for all PSV measurements should be obtained using a Doppler angle of 60° between the ultrasound beam and the vessel wall. If a 60° angle is not possible, a smaller angle is acceptable; however, larger angles approaching 90° should be avoided.

The site of the anastomosis is identified and the adjacent artery and vein segments are scanned. The diameter of the arteriovenous anastomosis is measured and is typically in the range of 4–5 mm (Fig. 23.2), since larger diameters have a higher risk of causing an arterial steal. The PSV at the anastomosis is measured from Doppler spectral waveforms, recognizing that relatively high velocities are common at anastomotic sites followed by turbulence due to caliber change and angulation.

Because high velocities are common at an arteriovenous anastomosis, one method of identifying anastomotic stenosis by duplex ultrasound is based on the PSV ratio (Vr) which is defined as the maximum PSV within the anastomosis divided by the PSV in the inflow artery approximately 2 cm proximal to the anastomosis. A Vr of 3.0 or greater and a PSV of 400 cm/s or greater are suggestive of a stenosis of at least 50 % diameter reduction at the anastomosis [8, 9]. However, B-mode confirmation of an intraluminal defect at the anastomosis should also be obtained, since the geometry of the vessels may cause a velocity increase without a true stenosis (Fig. 23.3).

The main superficial venous outflow in an autogenous fistula is usually through either the cephalic or the basilic vein. The entire length of the outflow vein is evaluated with B-mode, color-flow Doppler, and Doppler spectral waveforms. B-mode imaging in long-axis and transverse views will help identify intraluminal defects such as thrombus, chronic webbing, a fibrotic valve, or caliber change (Fig. 23.4). Color-flow images may show aliasing along the length of the outflow vein which helps to quickly identify focal velocity increases signifying a possible stenosis. Doppler spectral waveforms are also obtained along the length of the outflow vein. Within the outflow vein of the fistula, PSV is typically in the range of 150–300 cm/s, although velocities are highly variable. Some laboratories use a twofold focal velocity increase (Vr of 2.0) with associated poststenotic turbulence as a threshold to indicate a significant stenosis within the autogenous vein fistula [9]. Occlusion is identified by the presence of intraluminal echoes with no obtainable color Doppler flow or Doppler spectral waveforms.

The duplex evaluation of a prosthetic graft continues after the inflow arteries are assessed, measuring the diameter of the arterial anastomosis and the maximum PSV at that site. As with autogenous vein conduits, high velocities are common at the arterial anastomosis of a prosthetic graft, and flow disturbances related to angulation are frequently present; however, the diameters of graft anastomoses are less variable than those of autogenous vein anastomoses. Although velocities at the anastomotic sites of prosthetic grafts are extremely variable, the general threshold criteria for stenosis listed previously for autogenous vein fistulas can be applied to grafts. A PSV of 400 cm/s or greater and a focal velocity increase with a Vr of 3.0 or greater are consistent with a significant (≥50 % diameter reduction) anastomotic stenosis [9].

An initial B-mode image evaluation of the prosthetic graft in transverse and long-axis views will identify any intraluminal abnormalities that may be masked by the color-flow display. Abnormalities in the soft tissues around the graft, such as fluid collections, may also be identified by B-mode imaging. Color-flow Doppler is helpful in detecting anatomic defects such as pseudoaneurysms (Fig. 23.5). The prosthetic graft should be examined throughout its length using the pulsed Doppler and spectral waveforms for focal increases in PSV which may be due to intraluminal thrombus, neointimal hyperplasia, or stenosis at a revision site. Velocities in prosthetic grafts are quite variable; however, some criteria for significant (≥50 % diameter reduction) stenosis that have been applied include a PSV of 300–400 cm/s or greater, end-diastolic velocity (EDV) of 240 cm/s or greater, and Vr of 2.0 or greater [8, 9].

Graft occlusion is suggested by the absence of a palpable thrill or audible bruit over the graft on physical examination. Duplex confirmation of graft occlusion is based on the presence of intraluminal echoes on B-mode imaging and absence of flow on Doppler spectral waveforms and color-flow Doppler (Fig. 23.6). Some synthetic graft materials, including PTFE, contain small amounts of air that impede the transmission of ultrasound for a period of time after implantation. This causes acoustic shadowing across the lumen of the graft and prevents direct interrogation by Doppler. In this situation, Doppler spectral waveforms from the adjacent inflow artery and outflow vein can provide indirect evidence of graft patency. With a patent prosthetic graft, the inflow artery will show a low-resistance flow pattern, and the flow pattern may return to a high-resistance waveform in the artery distal to the anastomosis.

The venous outflow anastomosis should be identified and its diameter and maximum PSV measured. A common site for a stenosis is at or just beyond the venous anastomosis of a prosthetic graft, as indicated by a focal velocity increase with poststenotic turbulence.

Duplex evaluation of the upper extremity outflow veins can provide information on the presence or absence of central venous obstruction as well as the status of the autogenous vein fistula or prosthetic graft. Obstruction in the central veins is a common finding in long-term dialysis access patients, particularly those with a history of multiple central venous catheters. The flow patterns in the innominate and subclavian veins provide indirect information about graft or fistula outflow. With a functioning access site, spectral waveforms from the innominate and subclavian veins will show “arterialized” pulsatility (Fig. 23.7); if the access site is occluded, the central vein flow pattern will be non-pulsatile and more phasic with respiration.

A central outflow vein stenosis produces a focal velocity increase in the “arterialized” vein segment. The peak vein velocity (PVV) ratio has been defined as the poststenotic velocity divided by the pre-stenotic velocity, and a value >2.5 has been shown to correlate with a central venous stenosis severe enough to result in a pressure gradient [10]. Some central venous occlusions may be difficult to visualize directly with duplex scanning due to their location under the clavicle or in the mediastinum. The presence of visible, well-developed venous collaterals around the shoulder is suggestive of a central vein stenosis or occlusion.