Blood flow velocities in basilar artery during rotation of the head.

The authors measured blood flow velocities with transcranial Doppler ultrasonography in the basilar artery after extreme rotations of the head. Studied subjects were divided in two groups according to the degree of asymmetry of the vertebral artery lumens. Rotation of the head caused significant reduction of the mean flow velocity in the basilar artery in the subjects with asymmetry of the vertebral arteries of more than 75%. The decline occurred only after the rotation ipsilaterally to the hypoplastic vertebral artery. This study confirms the importance of the collateral flow in the compensation of dynamic compression of the vertebral artery.