In most healthy adults, peak systolic velocity in the internal carotid artery falls somewhere between about 54 and 88 cm/s when measured in the straight segment just past the bulb, though readings up to 120 cm/s can still be normal. That range sounds tidy, but in practice “normal” shifts depending on where in the artery the measurement is taken, how the ultrasound probe is angled, the patient’s age and sex, and what is happening on the opposite side of the neck. Understanding the number on your report means understanding all the things that nudge it up or down.
The Typical Range in a Healthy Adult
The most widely referenced normal values come from Doppler ultrasound studies of the postbulbar internal carotid artery, which is the straight segment just above the widened portion at the vessel’s origin. In that region, peak systolic velocity (PSV) generally ranges from 54 to 88 cm/s, with readings as high as 120 cm/s still falling within normal limits in some individuals.1European Society of Radiology. Normal findings at spectral and color Doppler ultrasound (US) imaging: What the beginner needs to know The bulb itself, where the common carotid artery flares open before splitting, typically registers lower velocities and can even show brief flow reversal, which is a normal quirk of the anatomy rather than a sign of disease.
End-diastolic velocity (EDV), the speed at the lowest point of the cardiac cycle, matters too. In a vessel with less than 50% narrowing, EDV generally stays below 40 cm/s.2NeurologÃa (English Edition). Ultrasound measurement of carotid stenosis: Recommendations from the Spanish Society of Neurosonology The reason clinicians care about both numbers is that PSV tends to climb first as a narrowing develops, while EDV rises later and signals more advanced disease. Together, they help distinguish mild plaque buildup from something that deserves closer attention.
Why Velocities Differ Between People
A 25-year-old woman and a 70-year-old man can both have perfectly healthy carotid arteries and still show meaningfully different velocity readings. Sex is one factor: across all degrees of stenosis, women average about 9% higher PSV and 6% higher EDV than men.3PubMed. Gender differences in blood velocities across carotid stenoses The difference is thought to relate partly to smaller vessel diameters in women, which speeds blood through the same cross-section the way pinching a garden hose makes water shoot farther.
Age plays a role as well. In children, normal PSV and EDV values are higher than in adults, and they decline as the child grows.4Journal for Vascular Ultrasound. Age, Race, and Gender Influence Internal Carotid Artery Doppler Velocities in Children That same study found that Caucasian children had higher velocities than African-American children, suggesting that racial and ethnic variation in vessel geometry or cardiac output may also contribute. Among adults, the ratio of ICA velocity to common carotid artery velocity tends to increase with age in men but remains relatively stable in women, and at younger ages the ratio is higher in women than in men.5PubMed. Normal reference values of ratios of blood flow velocities in internal carotid artery to those in common carotid artery using Doppler sonography
Arterial stiffness adds another layer. As arteries stiffen with age or hypertension, the elastic “cushioning” effect of the aorta and large vessels weakens. Researchers have found that a waveform feature called the Decay Index, which reflects how quickly the velocity drops after the systolic peak, correlates with measures of aortic stiffness.6PubMed. Decay Index: a new carotid Doppler waveform measure associated with the Windkessel function of elastic arteries In practical terms, stiffer arteries often produce higher systolic peaks and lower diastolic flow, making the waveform look “spikier” even without any narrowing of the vessel.
Where in the Artery the Measurement Is Taken
The internal carotid artery is not a uniform pipe. At its very beginning sits the carotid bulb, a natural widening where blood slows and swirls. Doppler studies of healthy people show that flow separation, a zone where blood briefly reverses direction, occurs in the bulb in more than 90% of cases.7PubMed. Diagnostic significance of flow separation in the carotid bulb This reversed flow typically occurs in the posterolateral portion of the sinus and is most pronounced just after the systolic peak.8Atherosclerosis. Flow separation and velocity profiles in the carotid bifurcation Because the swirling slows the overall forward velocity, readings taken in the bulb look quite different from those taken a centimeter or two farther up in the straight postbulbar segment.
Recent high-resolution imaging has confirmed that even young, healthy people show vortex patterns during the downstroke of each heartbeat, with younger subjects tending to display more pronounced vortices than older ones.9PubMed. Quantification of Blood Flow in the Carotid Bifurcation of Healthy Subjects These complex flow patterns are a natural consequence of the bifurcation geometry, not a warning sign. But they do mean that a velocity reading taken a few millimeters in one direction versus another can look dramatically different, which is one reason standardized measurement locations matter so much.
Anatomical variants can also shift velocities. Some people have a kinked or coiled carotid artery, where the vessel bends sharply. In those individuals, peak systolic velocity at the bend is higher than in people whose arteries merely curve gently.10PubMed. Carotid artery tortuosity, kinking, coiling: stroke risk factor, marker, or curiosity? This can create the appearance of a stenosis on ultrasound where none actually exists, a frustrating diagnostic trap that sonographers have to watch for.
How the Ultrasound Angle Changes Your Number
Doppler ultrasound does not measure blood speed directly. It measures the frequency shift of sound waves bouncing off moving blood cells, and the angle between the ultrasound beam and the direction of blood flow changes the math. Even a small change in that angle can meaningfully change the velocity on the screen. In one study, PSV and EDV obtained at a 60-degree angle of insonation were about 24% higher than the same measurements taken at 45 degrees.11PubMed. Effect of Doppler angle in diagnosis of internal carotid artery stenosis A pilot study found that within a single vessel, the maximum difference between a 45-degree and a 60-degree reading was 29 cm/s, with an average variation across four angles of about 14 cm/s.12PubMed. Changes in Internal Carotid Artery Doppler Velocity Measurements With Different Angles of Insonation: A Pilot Study
This is not a small shift. Imagine someone whose true PSV sits at 115 cm/s. At one angle, the reading might be 105; at another, 134. The first number looks comfortably normal. The second crosses the 125 cm/s threshold many labs use to flag possible stenosis. Same artery, same moment, different angle, different clinical conclusion. Research has shown that using a constant Doppler angle for both the common and internal carotid arteries reduces this problem: when the same angle is held for both measurements, the ICA-to-CCA velocity ratio stays stable, but when different angles are used, the ratio drifts.13Journal for Vascular Ultrasound. Is a Constant Doppler Angle of Insonation Worthwhile during Carotid Duplex Ultrasound?
Equipment and operators add more variability. A study comparing two different duplex systems found that raw velocity measurements were significantly different between machines, though velocity ratios were similar.14PubMed. Standardization of carotid ultrasound: a hemodynamic method to normalize for interindividual and interequipment variability Among trained technologists working in the same accredited lab, the good news is that operator-to-operator variability accounted for less than 1% of total measurement variance; the overwhelming majority of variance came from genuine patient-to-patient differences.15PubMed. Interobserver variability of carotid Doppler peak velocity measurements among technologists in an ICAVL-accredited vascular laboratory In well-trained hands, the technique is quite reproducible. The problem arises when comparing readings across different labs, different machines, or different angle protocols.
When a Normal-Looking Artery Runs Fast
One of the trickiest situations in carotid ultrasound is the contralateral compensation effect. If one internal carotid artery is severely narrowed or blocked, the opposite artery picks up the slack and carries more blood to the brain. That extra volume means higher velocities, sometimes much higher. A study of patients with known stenosis on one side found that a severe contralateral narrowing (70% to 99%) artificially elevated PSV in the open artery by an average of 84 cm/s.16PubMed. Effect of contralateral carotid artery stenosis on carotid ultrasound velocity measurements That is enough to push a truly normal artery into a range that looks like moderate stenosis. Less severe contralateral disease produced smaller bumps of 11 to 21 cm/s.
This matters because a patient being monitored after treatment on one side might appear to have new disease developing on the other side when in reality the artery is simply doing more work. Clinicians who interpret carotid ultrasound need to know what is happening on both sides of the neck before drawing conclusions from a single velocity number.
The Diagnostic Thresholds for Stenosis
The most widely cited cutoffs come from a 2003 consensus conference of the Society of Radiologists in Ultrasound. That consensus classified an ICA as normal when PSV is below 125 cm/s and no plaque is visible, as less than 50% stenosed when PSV is below 125 cm/s but plaque is present, as 50% to 69% stenosed when PSV is between 125 and 230 cm/s, and as 70% or more stenosed when PSV exceeds 230 cm/s.17PubMed. Carotid artery stenosis: gray-scale and Doppler US diagnosis–Society of Radiologists in Ultrasound Consensus Conference The consensus also recommended using supplementary measures, including the ICA-to-CCA PSV ratio and end-diastolic velocity, when the PSV alone might not be representative.
Those cutoffs are treated as a standard, but in practice, American vascular labs have adopted a surprisingly wide range of thresholds. A survey of accredited centers found 60 distinct PSV thresholds in use for grading stenosis. For moderate disease (at least 50% narrowing), PSV thresholds ranged from 110 to 245 cm/s, with a median of 125. For severe disease (at least 70%), the range was 175 to 340 cm/s, with a median of 230.18PubMed. Variation in Ultrasound Diagnostic Thresholds for Carotid Stenosis in the United States The variation means that the same patient scanned at two different hospitals could be classified differently, a problem the field has acknowledged but not fully resolved.
Part of the reason for the spread is that some labs have internally validated their thresholds against CT angiography or catheter angiography and found that slightly different numbers work better for their particular equipment and patient population. One center found that a PSV threshold of about 155 cm/s combined with an ICA/CCA ratio of at least 2 gave a positive predictive value of 97% for at least 50% stenosis when compared against high-resolution CT angiography.19PubMed Central. Reappraisal of velocity criteria for carotid bulb/internal carotid artery stenosis utilizing high-resolution B-mode ultrasound validated with computed tomography angiography Another group calculated slightly different optimal thresholds depending on whether CT angiography or catheter angiography was used as the gold standard.20Journal for Vascular Ultrasound. Should Carotid Duplex Velocity Thresholds be Modified for Patients Undergoing Computed Tomographic Angiography? The takeaway for patients is that a velocity number alone, pulled from a report without context, can be misleading. The threshold it is measured against matters just as much as the number itself.
Exercise, Blood Loss, and Other Acute Shifts
Carotid velocity is not a fixed property of your arteries. It fluctuates with your cardiac output, which itself changes with activity, hydration, and stress. During moderate-intensity exercise, ICA blood flow increases by roughly 23% compared to rest.21PubMed Central. The distribution of blood flow in the carotid and vertebral arteries during dynamic exercise in humans At higher intensities, however, something interesting happens: ICA flow actually plateaus or drops back toward resting levels as the external carotid artery diverts more blood toward the skin for cooling. The body, in effect, starts prioritizing thermoregulation over brain blood flow at near-maximal effort.
Blood volume loss shifts the picture even more dramatically. Carotid Doppler measurements track closely with stroke volume during simulated hemorrhage, making the carotid artery a potential bedside window into how much blood the heart is pumping.22PubMed Central. The correlation between carotid artery Doppler and stroke volume during central blood volume loss and resuscitation For everyday clinical scans, this means that a patient who is dehydrated, in pain, anxious, or has just walked briskly to the ultrasound suite could produce velocity readings that differ from what they would show in a calm, rested state. Most labs try to account for this by having patients lie quietly for a few minutes before scanning, but it is worth knowing that some day-to-day variation is inevitable.
MRI Versus Ultrasound
Patients sometimes receive velocity measurements from 4D flow MRI instead of Doppler ultrasound, and the two technologies do not produce interchangeable numbers. A direct comparison found that MRI underestimated systolic ICA velocity by about 19% and common carotid velocity by about 26% compared with ultrasound, though diastolic velocities were similar between the two methods.23American Journal of Neuroradiology. Comparison of Blood Flow Velocity Quantification by 4D Flow MR Imaging with Ultrasound at the Carotid Bifurcation The mismatch comes partly from how each technique samples flow: ultrasound captures the fastest-moving cells in a small sample volume, while MRI averages velocity across the entire cross-section of the vessel. Neither is wrong, but the “normal” benchmarks built from decades of ultrasound data cannot be directly applied to MRI readings.
Velocities After Stenting or Endarterectomy
Patients who have had a carotid stent placed or plaque surgically removed often wonder what their follow-up velocities should look like. The short answer is that the usual rules change. After carotid stenting in particular, velocities tend to run higher than the pre-procedure anatomy would predict, even when the stent is wide open and functioning well.24PubMed. Carotid artery velocity characteristics after carotid artery angioplasty and stenting The stent alters the compliance of the arterial wall, making it stiffer and changing the way the pulse wave travels through the treated segment. Applying the same velocity thresholds used for native, untreated arteries would overestimate the degree of any new narrowing developing inside the stent.
Vascular labs that monitor post-stent patients often develop separate internal criteria, sometimes raising the PSV threshold by 40 to 80 cm/s above what they would use for an untreated artery. For patients, the practical implication is simple: if your post-procedure scan shows velocities that look “high” compared to the textbook numbers you found online, it does not automatically mean the stent is failing. Ask your vascular specialist whether they are using adjusted thresholds for stented arteries.
How These Thresholds Developed
The velocity criteria used today trace back to pioneering work at the University of Washington in the late 1970s and early 1980s. Before duplex ultrasound existed, the only noninvasive way to assess the carotid arteries was through indirect tests like periorbital Doppler exams, which inferred disease by measuring flow patterns around the eye rather than looking at the artery itself. The duplex scanner combined real-time imaging with pulsed Doppler in one instrument, allowing clinicians for the first time to place a sample volume at a precise spot inside the vessel and record the velocity waveform there.25PubMed. Carotid duplex criteria: What have we learned in 40 years? Through a series of validation studies comparing these waveforms against catheter angiography, threshold values for PSV and EDV were established and refined.
Four decades of accumulated data have strengthened confidence in the overall framework, but the field is still grappling with the fact that thresholds validated on one population with one generation of equipment may not transfer perfectly to another. Newer ultrasound machines have better resolution and different signal-processing algorithms, which can shift measured velocities slightly. Some researchers have argued that every accredited lab should periodically revalidate its thresholds against a modern anatomic gold standard like CT angiography, rather than relying on criteria inherited from earlier eras. Whether that level of local calibration becomes standard practice remains an open question.
The Resistive Index and Small Vessel Disease
Beyond raw velocity, some clinicians look at the resistive index (RI), a ratio derived from the difference between systolic and diastolic velocities divided by systolic velocity. It gives a rough sense of how much resistance the downstream brain vessels are putting up against incoming flow. In healthy people, the ICA resistive index typically sits in the range of about 0.55 to 0.70. Research on patients with markers of cerebral small vessel disease, including tiny deep brain strokes and microbleeds, has found that their ICA resistive indices tend to run higher than those of the general population.26Journal of Hypertension. 62. RESISTIVE INDEX OF INTERNAL CAROTID ARTERY AND COMMON CAROTID ARTERY IN PATIENTS WITH CEREBRAL SMALL VASCULAR DISEASE An elevated RI does not diagnose small vessel disease on its own, but in the context of other clinical findings it can be another piece of the puzzle. For patients undergoing carotid screening who show no significant stenosis but have a strikingly high RI, it may prompt further investigation into what is going on inside the brain’s smaller blood vessels.