TCD is a non-invasive technique for monitoring blood flow velocity (FV) in the basal cerebral arteries using Doppler ultrasound
- assessment of blood flow velocity through cerebral arteries is useful for:
- vasospasm post SAH
- assessment of cerebral blood flow
- pulsed Doppler probe -> graphs velocity over time
METHOD OF USE
- uses a 2 MHz probe (low frequency)
- probe on temporal bone -> measure flow in MCA
- phase shift is proportional to the speed of blood
- during vasospasm there is an increase in flow velocity through the narrowed segment that is proportional to the reduction in vessel diameter.
Flow in MCA
- normal flow: mean = 55cm/sec
- mild: > 120cm/sec
- moderate: > 160cm/sec
- severe: > 200cm/sec
Lindegaard Ratio = mean velocity in the MCA / mean velocity in ipsilateral extracranial internal carotid artery
- high velocities in the MCA (>120cm/s) may be due to hyperaemia or vasospasm.
- the Lindegaard Ratio helps distinguish these conditions.
- <3 = hyperaemia
- >3 = vasospasm
-> 3-6 mild
-> >6 severe
- increases in flow velocity may be vasospasm or hyperaemia (to differentiate compare flow through MCA with flow through ICA)
- reverberant flow (flow forward during systole and backward during diastole) = no sustainable cerebral perfusion pressure
- quick and easily repeatable
- performed at the bedside
- can be used to monitor spasm post-treatment
- sensitivity 80% compared to angiography
- operator dependent
- poor for distal vessels (i.e. other than MCA and ICA)
- potential confounders include hypo/hypercapnia, haematocrit, BP
- edema and vasospasm may be difficult to distinguish post-op
- difficult views in some patients (e.g. thick skulls!)
References and Links
- Bathala L, Mehndiratta MM, Sharma VK. Transcranial doppler: Technique and common findings (Part 1). Ann Indian Acad Neurol [serial online] 2013 [cited 2013 May 27];16:174-9. Available from: http://www.annalsofian.org/text.asp?2013/16/2/174/112460
- Moppett IK, Mahajan RP. Transcranial Doppler ultrasonography in anaesthesia and intensive care. Br J Anaesth. 2004 Nov;93(5):710-24. PMID: 15220174.
- White H, Venkatesh B. Applications of transcranial Doppler in the ICU: a review. Intensive Care Med. 2006 Jul;32(7):981-94. PMID: 16791661.