Low Velocity, High Risk: Interpreting a Near-Occlusion on Carotid Duplex
A 68-year-old man presents with two episodes of right amaurosis fugax in the past week and a right hemispheric transient ischemic attack. Carotid duplex is performed with a linear array at a 60° Doppler angle, a 2-mm sample volume, and a PRF of 4 kHz. The right internal carotid artery (ICA) shows a narrow color lumen with a peak systolic velocity (PSV) of 92 cm/s and end-diastolic velocity (EDV) of 24 cm/s; the right common carotid artery (CCA) PSV is 58 cm/s. The distal ICA waveform is damped and high resistance. On the left, the ICA PSV is 285 cm/s with an EDV of 105 cm/s. The patient has hypertension, a 45-pack-year smoking history, and a prior left carotid endarterectomy. The technologist notes that the right ICA velocity is lower than the left, and the ordering provider asks whether the right side is 'only mild' because the velocity is below 125 cm/s.
1. Calculate the right ICA/CCA ratio and explain why standard velocity criteria may misclassify this lesion.
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The right ICA/CCA ratio is 92 ÷ 58 = 1.6. By conventional velocity thresholds, a PSV below 125 cm/s and a ratio below 2.0 would suggest no significant stenosis, but those thresholds assume a focal stenosis with preserved proximal pressure and flow. In a near-occlusion, the residual lumen is so tight that volume flow falls; the PSV can drop even though the stenosis is critical. The clues here are the narrow color lumen, the low EDV of 24 cm/s, and the damped, high-resistance distal ICA waveform. The contralateral ICA at 285/105 cm/s confirms severe disease and makes a 'mild' right lesion biologically implausible in a symptomatic patient. Do not apply the usual velocity cut-points in isolation. The correct interpretation is suspected right ICA near-occlusion, with the caveat that a moderate stenosis can occasionally produce low velocity if there is tandem disease or poor cardiac output. The report should state that velocity criteria underestimate the lesion and that urgent confirmation is needed.
2. What additional duplex findings and maneuvers would you use to distinguish near-occlusion from true occlusion and from moderate stenosis?
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First, optimize the technique: lower the PRF and scale to detect slow flow, reduce the sample volume to 1–1.5 mm, use power Doppler, and scan in transverse and longitudinal planes. Near-occlusion shows a string-like residual color lumen, detectable low-velocity flow, and a high-resistance distal waveform; true occlusion shows no intraluminal color or spectral flow and no distal ICA flow. Look for external carotid collaterals and reversed ophthalmic artery flow, which support occlusion or near-occlusion. Moderate stenosis usually shows a focal velocity increase with a normal distal waveform and no string sign. Contrast-enhanced ultrasound or CTA/MRA can confirm residual flow when native Doppler is equivocal. Also evaluate for tandem lesions in the CCA or distal ICA, because they can alter the pressure gradient and velocity. The goal is not to force the case into a velocity category but to determine whether any residual lumen exists and whether the patient needs urgent revascularization evaluation.
3. How should this be documented and communicated to the ordering provider?
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Document the measurements, the narrow color lumen, the high-resistance distal waveform, and the contralateral severe stenosis. Use unambiguous language: 'Suspected right ICA near-occlusion; standard velocity criteria may underestimate stenosis; cannot exclude critical stenosis.' Avoid the word 'mild.' Because the patient is symptomatic, make a direct telephone communication to the ordering provider or vascular surgeon and document the time, the person contacted, and the recommendation for urgent CTA or catheter angiography. State that the low PSV is not reassuring in this context. Include representative images and a sentence that technical factors were optimized. This is a critical finding, so delay in communication is a patient-safety issue. The report should also note that the left ICA has a critical stenosis and that bilateral disease affects management.