What TCD finding indicates vasospasm following a subarachnoid hemorrhage?

The indication of vasospasm following a subarachnoid hemorrhage is characterized by greatly increased mean velocities in the middle cerebral artery (MCA). Vasospasm occurs when there is a tightening or constriction of blood vessels, which can reduce blood flow to the brain. The physiological response to vasospasm is reflected in Transcranial Doppler (TCD) findings.

When there is vasospasm, the narrowed arteries lead to increased blood flow velocity as the same volume of blood is forced through a smaller lumen. This results in significantly elevated peak systolic velocities in affected vessels, such as the MCA, which is why a finding of greatly increased mean velocities is indicative of vasospasm.

In contrast, decreased mean velocities would suggest a reduction in blood flow, while no change in velocities would indicate that the vessel is not experiencing significant changes that are typically associated with vasospasm. Finally, consistent velocities across cerebral arteries would imply that there is no focal disturbance in blood flow or vascular tone, which would also not correlate with the presence of vasospasm.