In subclan steal syndrome there is reversal of blood flow in:

In subclan steal syndrome there is retrograde flow in the veebral aery due to an ipsilateral subclan aery block proximal to veebral aery origin. The block results in lower pressure in the distal subclan aery and upper limb ischemia. Consequently, blood flows from the contralateral veebral aery to the basilar aery and then in a retrograde direction down the ipsilateral veebral aery away from the brainstem.