Cerebrovascular Testing and Interpretation
Cerebrovascular Testing and Interpretation
This domain tests carotid and vertebral duplex acquisition, waveform interpretation, stenosis grading, and recognition of hemodynamic confounders.
Core rules
- Identify the ICA by its low-resistance waveform, larger caliber, and posterior/lateral course; the ECA is high resistance, has branches, and shows temporal-tap pulsatility.
- A validated carotid duplex scale uses ICA peak systolic velocity (PSV), end-diastolic velocity (EDV), and ICA/CCA PSV ratio. A common consensus scheme: ICA PSV <125 cm/s and ratio <2.0 is <50% stenosis; PSV 125-230 cm/s, EDV <100 cm/s, ratio 2.0-4.0 is 50-69%; PSV >230 cm/s, EDV >100 cm/s, ratio >4.0 is ≥70%. Near occlusion can produce a low-velocity string sign, not a normal study.
- NASCET stenosis = (1 - residual lumen/distal normal ICA lumen) × 100; do not apply it to a bulb measurement.
- Vertebral flow should be antegrade; reversal suggests subclavian steal. TCD vasospasm uses MCA velocity with Lindegaard ratio (MCA/extracranial ICA); >3 supports vasospasm.
Worked example
An ICA shows PSV 260 cm/s, EDV 110 cm/s, ICA/CCA ratio 5.0, and heterogeneous plaque. These meet ≥70% criteria. Check for near-occlusion, very low PSV with high resistance, and contralateral disease before finalizing. Elevated cardiac output or contralateral occlusion can raise velocities; a tandem lesion can lower them.
Traps
Do not sample the ECA and call it ICA, do not use an angle >60°, and do not grade stenosis from color alone. Aortic regurgitation and high-output states inflate EDV. A low PSV in a tight lesion is a trap for underestimation.