Cerebrovascular Testing and Interpretation: Carotid and Vertebral Arteries
Core Concepts
Carotid duplex interpretation hinges on velocity criteria correlated with angiographic stenosis. Understand the normal spectral waveform (low-resistance, broad systolic peak, continuous forward diastolic flow) and how it changes with stenosis: focal velocity increase, spectral broadening, post-stenotic turbulence, and eventually a dampened distal waveform. Know the standard internal carotid artery (ICA) thresholds used in most labs: PSV, EDV, and the ICA/common carotid artery (CCA) ratio, plus the meaning of near-occlusion (string sign) and complete occlusion (no flow, thump).
Vertebral and Other Findings
Recognize vertebral artery waveform changes with subclavian steal (retrograde flow, systolic deceleration), and understand that the ophthalmic artery direction can indicate collateral flow. Know the significance of plaque characterization (echolucent vs calcified) and the difference between hemodynamically significant stenosis and ulceration.
Traps
Frequent traps: applying ICA criteria to the CCA or vertebral artery; confusing elevated velocity from contralateral occlusion or compensatory flow with true stenosis; missing tandem lesions; and interpreting a dampened distal waveform as occlusion when it reflects severe proximal disease. Another trap is calling occlusion when a low-velocity, high-resistance signal is present-near-occlusion can still show a faint string sign.
Worked Example
A patient has ICA PSV 280 cm/s, EDV 110 cm/s, and ICA/CCA ratio 4.5 with post-stenotic turbulence. Using standard criteria, this indicates a hemodynamically significant stenosis (commonly ≥70%). The trap: if the contralateral ICA were occluded, velocities could be elevated without severe stenosis-always check the contralateral side and the distal waveform before finalizing.
Rule of Thumb
Interpret the ICA in context: velocity plus ratio plus spectral broadening plus distal waveform. A single high velocity without corroborating features deserves a second look.