A note on Transcranial doppler (TCD) in subarachnoid haemorrhage (SAH)

 TCD has a significant application in SAH to differentiate between vasospasm and hyperaemia in patients. The Lindegaard ratio is calculated. The Lindegaard ratio is the flow velocity of the middle cerebral artery divided by the velocity measured in the extracranial internal carotid artery. A high flow velocity (>120 cm s−1) in association with a Lindegaard ratio of <3 implies hyperaemia. A Lindegaard ratio >3 is likely to imply vasospasm.