SSS results when the short low resistance path (along the subclavian artery) becomes a high resistance path (due to narrowing) and blood flows around the narrowing via the arteries that supply the brain (left and right vertebral artery, left and right internal carotid artery). The blood flow from the brain to the upper limb in SSS is considered to be stolen as it is blood flow the brain must do without.
As in vertebral-subclavian steal, coronary-subclavian steal may occur in patients who have received a coronary artery bypass graft using the internal thoracic artery (ITA). As a result of this procedure, the distal end of the ITA is diverted to one of the coronary arteries (typically the LAD), facilitating blood supply to the heart. In the setting of increased resistance in the proximal subclavian artery, blood may flow backward away from the heart along the ITA causing myocardial ischemia. Vertebral-subclavian and coronary-subclavian steal can occur concurrently in patients with an ITA CABG.
In SSS a reduced quantity of blood flows through the proximal subclavian artery. As a result, blood travels up one of the other blood vessels to the brain (the other vertebral or the carotids), reaches the basilar artery or goes around the cerebral arterial circle and descends via the (ipsilateral) vertebral artery to the subclavian (with the proximal blockage) and feeds blood to the distal subclavian artery (which supplies the upper limb and shoulder).