Subclavian steal

Subclavian artery occlusion or a hemodynamically significant stenosis proximal to the origin of the vertebral artery results in lower pressure in the distal subclavian artery. As a result, blood is "stolen" from the cerebral circulation to perfuse the arm. Blood flows up the contralateral vertebral artery to the basilar artery, and retrograde down the ipsilateral vertebral artery away from the brainstem.