Carotid Doppler Interpretation
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Direct answer
Peak systolic velocity in the internal carotid artery carries most of the diagnostic load: below 125 cm/s suggests under 50% stenosis, 125-230 cm/s corresponds to 50-69%, and above 230 cm/s indicates 70% or greater stenosis — the grade that earns revascularisation alongside symptoms. End-diastolic velocity above 100 cm/s and an ICA-to-common carotid PSV ratio above 4 corroborate the severe grade, guarding against the false elevations that contralateral occlusion or hyperdynamic circulation produce. Beyond numbers, the waveform tells physiology — a tardus-parvus pattern downstream signals a significant inflow lesion — and plaque character (echolucent, ulcerated) refines stroke risk. This velocity ladder, aligned with the SRU consensus, is among the most frequently quoted numeric stems in NEET-PG Radiology.
What you must remember
- PSV ladder: under 125 cm/s means less than 50% stenosis; 125-230 cm/s means 50-69%; above 230 cm/s means 70% or more — the core table to reproduce.
- Corroborating numbers: end-diastolic velocity above 100 cm/s and ICA/CCA ratio above 4 support severe (70%+) stenosis; a ratio near 2 flags the 50-69% band.
- Stenosis measurement reference: NASCET uses the narrowest residual lumen versus distal internal carotid (beyond the bulb); ECST used the estimated original bulb — a definitional one-liner.
- Waveform reading: tardus-parvus (slow rise, low peak) distal to a lesion implies significant proximal narrowing; dampened common carotid waveforms suggest aortic or innominate disease.
- Plaque morphology: echolucent and ulcerated plaques are more stroke-prone than uniformly echogenic calcified ones; heavy calcification produces shadowing and widens inter-observer variability.
- Vertebral arteries: document direction of flow — retrograde vertebral flow suggests subclavian steal, a favourite physiological stem.
- Confounding states: contralateral carotid occlusion or high-output states inflate velocities and can overgrade stenosis; arrhythmia degrades the spectral trace.
- Doppler grades, angiography confirms: ultrasound is screening; CT or MR angiography (and catheter angiography when discordant) confirms before endarterectomy or stenting.
Walking through a symptomatic case
A 66-year-old smoker with a recent transient right hemiparesis and a left carotid bruit. Scan the common carotid first: normal waveform, PSV 90 cm/s. At the bulb, spectral broadening appears; in the internal carotid just beyond, PSV climbs to 260 cm/s with end-diastolic velocity of 105 cm/s — the severe band. Calculate the ICA/CCA ratio: 260 divided by 90 is about 2.9, which is below 4, so the corroboration is partial, and the plaque is heavily calcified with distal shadowing. This is exactly where blind number-quoting fails: the sonographer adjusts the angle correction, samples further distally where velocities fall to 180 cm/s, and the study is reported as 50-69% with technical limitation, triggering CT angiography. The lesson for the exam — and for the viva — is that velocity criteria are consensus thresholds, not measurements of anatomy; they must be internally consistent (PSV, EDV and ratio) and paired with imaging before a surgeon touches the neck. A symptomatic patient with confirmed 70% or greater stenosis benefits from endarterectomy; the same lesion found incidentally in an asymptomatic patient tips the balance toward best medical therapy, the decision the numbers ultimately serve.
When the numbers mislead
Treating PSV as infallible is the recurring fail: contralateral occlusion diverts flow and inflates velocities on the studied side, and a hyperdynamic or hypertensive patient overgrades without pathology. The second error is measuring stenosis with the wrong denominator — NASCET percentages run lower than ECST for the same lesion because the bulb denominator is larger, so a stem mixing the two definitions is testing precisely that awareness. Finally, do not report a percentage where no stenosis exists: a "percent stenosis" at the bulb without a plaque or velocity abnormality is meaningless, and occlusion (no flow, no waveform — pre-occlusive "string sign" with low, tardus flow) is a category of its own that changes management acutely.
Frequently asked questions
What PSV indicates 70% or greater carotid stenosis?
Peak systolic velocity above 230 cm/s in the internal carotid artery, supported by end-diastolic velocity above 100 cm/s and an ICA/CCA ratio above 4.
What do PSV values of 125 to 230 cm/s represent?
The 50-69% stenosis band — moderate disease where velocity alone should be corroborated by ratio and plaque findings before grading.
How does NASCET differ from ECST in measuring stenosis?
NASCET compares the residual lumen with the normal distal internal carotid, while ECST compared it with the estimated original bulb diameter, so ECST percentages read higher for the same lesion.
What is a tardus-parvus waveform?
A slowly rising, low-amplitude, delayed-peak waveform seen downstream of a haemodynamically significant stenosis, indicating dampened flow beyond the lesion.
What does retrograde vertebral artery flow indicate?
Flow reversal in the vertebral artery, classically towards a stenotic or occluded subclavian artery proximal to the vertebral origin — subclavian steal, producing vertebrobasilar symptoms on arm exercise.