Doppler diagnosis of partial vertebral/subclavian steals convertible to full steals with physiologic maneuvers.

Subclavian steal syndrome has been classically described as a unidirectional phenomenon in which retrograde flow occurs in the ipsilateral vertebral artery in cases of high-grade proximal subclavian stenosis. Pulsed Doppler examination in cases of subclavian stenosis shows that partial steals with to-and-fro flow do occur. A model based on fluctuating pressure gradients to explain the partial steal phenomenon is discussed. The usefulness of physiologic maneuvers such as ipsilateral arm exercise and postocclusive hyperemia in converting partial steals to full steals is demonstrated. In cases in which surgical bypass to correct partial steals is carried out, a relative steal can be demonstrated by Doppler examination before and after physiologic maneuvers.