In subclavian steal, obstruction in which vessel triggers vertebral flow reversal?

• A. Aorta
• B. Carotid
• C. Subclavian
• D. Renal

Answer: (C)

When the proximal subclavian artery is obstructed, the pressure distal to that blockage falls, creating a siphon-like gradient that favors blood moving from the vertebrobasilar system down the ipsilateral vertebral artery into the subclavian to perfuse the arm. In other words, the vertebral artery reverses its flow to supply the ischemic limb, which is the hallmark of subclavian steal. The obstruction must be in the subclavian artery, near where the vertebral artery originates, to produce this retrograde flow. Occlusion or narrowing of the aorta, carotid, or renal arteries wouldn’t create the same hemodynamic setup to drive vertebral artery flow backward into the subclavian.