Transcranial Doppler monitoring guided by cranial two-dimensional ultrasonography
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KeywordsDistal Internal Carotid Artery Montreal Heart Institute Cerebral Falx Temporal Bone Window Foramen Lacerum
To the Editor,
Transcranial Doppler (TCD) imaging was introduced in 1982 as a noninvasive tool to evaluate the velocity of blood flow in the basal cerebral arteries.1 Since then, TCD has been used in a wide variety of situations, including detection of vasospasm after subarachnoid hemorrhage, finding microemboli during cardiac surgery, and intracranial pressure monitoring. Transcranial Doppler uses low frequency (2 MHz) focused pulsed waves to insonate major cerebral vessels. The red blood cell velocity can be calculated by integrating the emitted and reflected frequencies along with the angle subtended by the respective ultrasonic beam and blood flow directions.
Transcranial Doppler monitoring can be easily performed through the temporal window for the arteries of the circle of Willis, submandibular window for the distal internal carotid artery, transorbital window for the ophthalmic artery, and suboccipital window for the vertebral and basilar arteries. The skull is composed of two layers of compact bone separated by a porous diploë layer that allows propagation of ultrasound waves by creating an acoustic interface.2 Nevertheless, in up to 38% of patients, Doppler signals cannot be acquired because of an inadequate temporal bone acoustic window.3 Blind placement of a TCD probe can be time consuming, and an inadequate signal might result from an insufficient temporal bone window. The use of two-dimensional (2D) cranial ultrasonography has the potential to facilitate localization of the temporal acoustic window prior to TCD probe placement.
Since 2015, the above TCD technique has been used at our institution as part of a multimodal neurologic monitoring strategy. We recently reviewed our last 100 consecutive patients in the transesophageal echocardiography/hemodynamic database of the Montreal Heart Institute (previously approved by the local ethics and research committee [#11-1277]).
Ninety-five of these patients had at least an adequate unilateral temporal bone window available for TCD monitoring. An adequate bilateral window was found in 70 patients, and in five patients, neither a right nor a left temporal bone window was present and TCD could not be used.
This novel report describes the use of 2D cranial ultrasound to facilitate TCD. Use of this technique to identify the temporal acoustic window can facilitate TCD monitoring in most patients. As the reported failure rates in TCD monitoring are quite high,3 adding cranial 2D ultrasonography to optimize placement of the TCD probe might be beneficial without being time consuming. Further studies are required to compare the yield of positioning conventional vs echo-guided TCD probes.
Conflicts of interest
This submission was handled by Dr. Hilary P. Grocott, Editor-in-Chief, Canadian Journal of Anesthesia.
VIDEO Transcranial two-dimensional and colour video of Figure D showing the right middle cerebral artery. (WMV 2410 kb)