Subclan aery steal is caused by stenosis of the subclan aery at:

Subclan aery steal is caused by stenosis of the subclan aery proximal to the takeoff of the veebral aery. This causes a retrograde flow in the ipsilateral internal mammary aery and the veebral aeries. Symptoms of subclan aery steal may include veigo and syncope with left arm exeion, angina, or ulcerated or gangrenous fingers. Blood pressures measured in the upper extremities are nearly always unequal. The average difference is a 45-mm Hg decrease in systolic pressure in the arm supplied by the stenotic vessel. Ref: Stone C., Strecker-McGraw M., Juergens A. (2011). Chapter 18. Syncope. In R.L. Humphries, C. Stone (Eds), CURRENT Diagnosis & Treatment Emergency Medicine, 7e.