(Circulation. 2000;102:826.)
© 2000 American Heart Association, Inc.
Editorial |
Monitoring Embolism in Real Time
From the Department of Clinical Neuroscience, St George’s Hospital Medical School, London, UK.
Correspondence to Hugh Markus, Clinical Neuroscience, St George’s Hospital Medical School, Cranmer Terrace, London, UK. SW17 ORE. E-mail h.markus@sghms.ac.uk
Key Words: embolism • carotid artery diseases • cardiovascular diseases • ultrasonics
We have known since the 17th century that emboli can cause stroke but, despite their occasional visualisation in the retinal circulation, a diagnosis of embolic stroke is usually one of "guilt by association," which is made by the detection of an appropriate embolic source in a patient with stroke. Frequently, >1 potential embolic source exists; determining which is clinically relevant may be impossible. The indirect diagnosis of embolic stroke also results in management difficulties in stroke prevention. For example, in atrial fibrillation, treatment failure can only be determined by the onset of stroke or systemic embolization. Recently, studies have evaluated a technique that allows the direct visualisation of circulating emboli. This offers exciting potential applications in both the diagnosis and management of patients at risk of cerebral and systemic embolism.
Since the 1960s, we have known that gaseous emboli can be detected in
blood using Doppler ultrasound. The large acoustic impedance
difference between air and blood results in a scattering of ultrasound
at the blood-air interface and a marked increase in received ultrasound
intensity as the bubble passes. This results in a brief, high-intensity
signal. This technique was applied to develop safe decompression limits
in divers and to investigate air embolism during
cardiopulmonary bypass. In 1990, while recording for
air emboli during carotid endarterectomy, similar
signals, but of lower intensity, were noted during manipulation of the
carotid bifurcation.1 This was before arterial
opening and, therefore, these signals could not represent air
emboli; it was suggested that they represented thrombus and
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