A left vertebral artery showing a to/fro pattern indicates what condition?

The presence of a to/fro flow pattern in the left vertebral artery is indicative of the phenomenon known as subclavian steal syndrome. This occurs when there is significant stenosis or obstruction in the proximal subclavian artery, which can lead to a diversion of blood flow. In this situation, the vertebral artery may reverse its flow direction during periods of arm activity or because of compromised blood supply.

This condition arises when the demand for blood flow exceeds the supply due to the blocked subclavian artery, causing the vertebral artery to draw blood from the vertebrobasilar system to compensate. Consequently, you may observe alternating patterns of flow direction—hence the to/fro pattern in the Doppler ultrasound readings of the vertebral artery, reflecting this compensatory mechanism.

In contrast, stenosis, normal flow, and distal obstruction would produce different flow patterns. For instance, stenosis would most likely show disturbed flow with possible increased velocities but not the distinct reversal of flow that characterizes subclavian steal. Normal flow would present with consistent antegrade flow patterns, while distal obstruction would typically lead to reduced or absent flow at the site of obstruction rather than the alternating flow seen in subclavian steal.