What are the normal Internal Carotid Artery (ICA) Doppler velocities?

Normal Internal Carotid Artery Doppler Velocities

Normal ICA peak systolic velocity (PSV) is less than 125 cm/s, with an ICA/CCA ratio less than 2.0 and end-diastolic velocity (EDV) less than 40 cm/s. 1

Velocity Parameters for Normal ICA

The absence of hemodynamically significant stenosis is defined by specific velocity thresholds that distinguish normal flow from pathologic acceleration:

• Peak Systolic Velocity (PSV): <125 cm/s indicates minimal to no stenosis (0-49% stenosis) 1
• End-Diastolic Velocity (EDV): <40 cm/s in the normal ICA 1
• ICA/CCA PSV Ratio: <2.0 indicates less than 50% stenosis 1

Velocity Thresholds Indicating Abnormal Flow

Understanding when velocities transition from normal to abnormal is critical for clinical decision-making:

• PSV 125-230 cm/s indicates 50-69% stenosis (mild-moderate disease) 1
• PSV ≥230 cm/s indicates ≥70% stenosis (severe disease requiring intervention consideration) 1
• EDV ≥100 cm/s suggests ≥70% stenosis 1
• ICA/CCA ratio ≥4.0 indicates ≥70% stenosis 1

More recent research suggests slightly different optimal thresholds: PSV ≥130 cm/s for ≥50% stenosis and PSV ≥200 cm/s for ≥70% stenosis, with high diagnostic accuracy (95%) 2. However, the consensus guideline criteria remain the standard for clinical practice 1.

Critical Waveform Characteristics

Beyond absolute velocity values, waveform morphology provides essential diagnostic information:

• Normal waveforms are multiphasic (triphasic) in the common carotid and external carotid arteries 3
• Monophasic waveforms universally present indicate severe proximal disease (aortic arch or innominate/subclavian stenosis) or cardiac dysfunction, invalidating standard velocity criteria 4, 3
• The internal carotid demonstrates more pulsatile flow compared to the external carotid's continuous diastolic flow 4

Important Clinical Pitfalls

Several factors can falsely elevate or reduce velocities, leading to misclassification:

• Women have higher velocities than men for the same degree of stenosis 1
• Contralateral carotid occlusion elevates velocities in the patent artery through compensatory flow, potentially overestimating stenosis severity 1, 3
• Cardiac dysfunction reduces all velocities, potentially underestimating stenosis 4, 3
• In-situ carotid stents decrease vessel wall compliance and accelerate flow velocity 1
• Severe arterial tortuosity, high carotid bifurcation, obesity, and extensive calcification reduce ultrasound accuracy 1

Technical Considerations for Accurate Measurement

Proper technique is essential for obtaining reliable velocity measurements:

• Use a high-frequency linear array transducer (5-12 MHz) with pulsed-wave Doppler capability 4
• Position the sample volume in the center of the vessel lumen, 1-2 cm proximal to the carotid bifurcation 4
• Ensure the sample volume encompasses the full vessel diameter to capture peak velocities 4
• Apply minimal probe pressure to avoid vessel compression 4

When Additional Imaging Is Required

Duplex ultrasound has limitations that necessitate confirmatory imaging in specific scenarios:

• Subtotal versus complete occlusion cannot be reliably distinguished by ultrasound alone 1
• Discrepancies between velocity criteria and clinical presentation warrant CTA or MRA correlation 3
• Universal monophasic waveforms require arch vessel imaging (CTA or MRA) to identify proximal stenoses 3
• Quality assurance programs should compare ultrasound findings with other imaging modalities to maintain accuracy 3