Arterial Shunt on Ultrasound: Restenosis Criteria and Control Points

Clinical Task

Duplex scanning of an arterial shunt is the primary method for non-invasive postoperative monitoring of peripheral bypass graft patency. The practical question is not whether 'flow is visible,' but whether there is hemodynamically significant narrowing, low-flow state, or local anastomotic issue that increases the risk of thrombosis. In the AIUM Practice Parameter for Peripheral Arterial Ultrasound, 2025, postoperative shunt assessment is considered part of peripheral arterial ultrasound examination: it is necessary to document grayscale, color, and spectral signs throughout the reconstruction.

What is Considered an Arterial Shunt in This Context

This refers to a peripheral arterial bypass shunt: autogenous vein, prosthetic, or composite graft connecting arterial inflow to the distal arterial bed. Criteria apply to duplex assessment of the graft itself and anastomoses. They should not be automatically applied to arterial stents, dialysis arteriovenous fistulas, or carotid reconstructions, as separate protocols and validated scales are used for them.

Mandatory Control Points

The protocol should be anatomically consistent. Recording a single velocity in the middle of the graft is insufficient: significant stenoses often localize in anastomotic zones, valve remnants of autogenous veins, kinked areas, transitions to prosthetic material, and outflow arteries.

• Inflow artery proximal to the shunt: spectral shape, PSV, signs of proximal stenosis.
• Proximal anastomosis: grayscale assessment, color aliasing, PSV in the most accelerated segment.
• Proximal, middle, and distal thirds of the shunt: PSV and spectral shape in each segment.
• Suspicious zones: local narrowing, kinking, turbulence, post-stenotic dilation, mural material.
• Distal anastomosis: frequent zone of restenosis, mandatory recording of PSV and spectrum.
• Outflow arteries: spectral shape, preservation of distal bed, additional stenoses.

PSV Measurement Technique

AIUM emphasizes the need for correct spectral Doppler. Velocities should be measured with a Doppler correction angle ≤60°, the cursor should be aligned with the flow direction, and the sample volume should be placed in the central flow or the most accelerated jet at stenosis. If the angle is set incorrectly, absolute PSV becomes unreliable, and the velocity ratio may be distorted.

For a suspicious zone, it is important to record at least two velocities: PSV at maximum acceleration and PSV in the nearest pre-stenotic segment. The PSV ratio is calculated as PSV stenosis / PSV prestenotic. This local ratio helps distinguish true stenosis from physiologically high velocities in a small diameter autogenous vein segment.

Restenosis Criteria and Shunt Threats

Focal PSV >300 cm/s and PSV ratio >3.5 are best interpreted together. Isolated elevation of absolute velocity is possible with small caliber graft, kink, or technical error of angle. Conversely, the local velocity ratio shows that flow truly accelerates relative to the nearest normal segment.

What Restenosis Looks Like on Ultrasound

Grayscale stenosis may manifest as local lumen narrowing, intimal thickening, echogenic mural material, valve remnant in a venous graft, or anastomosis deformation. In color mode, typical findings include aliasing, mosaic flow, local jet, and post-stenotic turbulence. In spectral mode, high PSV in the narrowing zone, spectral broadening, and waveform changes distal to stenosis are expected.

With significant proximal inflow lesions, the shunt may not exhibit focal high velocity: spectra become dampened, PSV decreases throughout the graft. Therefore, low PSV <45 cm/s is an independent alarming criterion. It is especially important with complaints of worsening perfusion or changes in the clinical status of the limb.

Typical Interpretation Pitfalls

• Measurement only in the middle third of the graft. Such a protocol misses anastomotic restenoses.
• Comparison with previous study without considering the angle. Different correction angles can mimic PSV increase.
• Assessment of only absolute PSV. For a suspicious area, the velocity ratio must be calculated.
• Ignoring inflow and outflow. Stenosis of iliac, femoral, or distal artery can alter spectra in the shunt.
• Incomplete visualization in deep or edematous segment. Technical limitation should be noted, and a categorical conclusion about the entire graft's patency should not be made.

What to Include in the Conclusion

The conclusion should be clinically applicable rather than descriptive. Ideally, specify the type and course of the shunt, its patency, measurement zones, maximum PSV, PSV ratio in the suspicious segment, presence or absence of low-flow state, condition of anastomoses, and outflow arteries. If criteria PSV >300 cm/s, ratio >3.5, or diffuse PSV <45 cm/s are identified, they should be highlighted in the final line as signs of significant stenosis or shunt threat.

Example formulation: “Shunt patent. Local flow acceleration at distal anastomosis: PSV 340 cm/s, PSV ratio 4.0; color aliasing and post-stenotic turbulence. Ultrasound signs of hemodynamically significant restenosis of distal anastomosis.” For low flow: “No focal acceleration detected, however, PSV throughout the shunt is diffusely <45 cm/s; signs of low-flow shunt, risk of failure.”

When to Consider the Study Incomplete

The study is incomplete if one of the anastomoses, a significant graft segment, or outflow arteries are not visualized, and if velocities are measured without correct angle ≤60°. In such cases, the protocol should explicitly state which segment is inaccessible and why: wound, dressing, gas, significant edema, calcinosis, depth, or pain factor. This reduces the risk of a false negative conclusion.