Where should velocity be measured to assess a stenosis?

• A. Proximal to stenosis
• B. Poststenotic segment
• C. External carotid artery
• D. Through the narrowed lumen

Answer: (D)

Velocity should be measured through the narrowed lumen at the site of greatest stenosis. When the artery constricts, cross-sectional area drops and flow must speed up to maintain forward flow, so the peak systolic velocity is highest precisely at the stenotic point. This makes the reading there the most meaningful indicator of how severe the narrowing is. If you measure upstream of the stenosis, you sample pre-stenotic flow, which can appear normal and underestimate the blockage. Post-stenotic segments can show disturbed or decelerated flow that doesn’t reliably reflect the degree of narrowing. The external carotid artery isn’t the vessel used to evaluate the stenosis in the internal circulation. So, taking velocity readings through the narrowed lumen gives the best correlation with stenosis severity.