Transcranial Doppler (TCD) Ultrasound
OVERVIEW
TCD is a non-invasive technique for monitoring blood flow velocity (FV) in the basal cerebral arteries using Doppler ultrasound
USES
- assessment of blood flow velocity through cerebral arteries is useful for:
- emboli
- stenosis
- vasospasm post SAH
- assessment of cerebral blood flow
DESCRIPTION
- pulsed Doppler probe -> graphs velocity over time
METHOD OF USE
Technique
- 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
OTHER INFORMATION
Interpreting flow
- 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
Advantages
- non-invasive
- quick and easily repeatable
- performed at the bedside
- can be used to monitor spasm post-treatment
Disadvantages
- 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.
Critical Care
Compendium
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.
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