Basilar artery blood flow in subclavian steal.

Subclavian "steal", when blood siphons from one vertebral artery to the other, has been suggested as a cause of brain stem ischaemia and stroke. We investigated 33 patients using transcranial Doppler to determine the direction and velocity of basilar blood flow. All patients had severe subclavian stenosis with reversed vertebral blood flow in the ipsilateral artery previously demonstrated by extracranial Doppler. Basilar flow was normal in direction in all cases, but its velocity was significantly increased (p less than 0.0008) compared to age- and sex-matched controls. These findings, in conjunction with previous observations using extracranial Doppler techniques, suggest that subclavian steal is little more than a harmless haemodynamic phenomenon.