Wednesday, September 1, 2010

Duplex Evaluation of the Carotid Arteries

Degree of stenosis ICA PSV
Plaque ICA/CCA PSV ratio ICA EDV
Normal <125 None < 2.0 <40
< 50% <125 <50% < 2.0 <40
50%-69% 125-230 ≥50% 2.0-4.0 40-100
≥70<near occlusion >230 ≥50% > 4.0 > 100
Near-occlusion Anything goes Visible Anything goes Anything goes
Total occlusion Undetectable No detectable lumen NA NA

A tight stenosis in an internal carotid artery (ICA) can cause falsely elevated peak systolic and end diastolic velocities in the contralateral ICA evaluated by duplex ultrasound. This artifactually increased velocity can lead to a false positive diagnosis of a stenosis. One explanation put forward is that the increased flow is caused by cross filling via the circle of Willis to the cerebral hemisphere on the same side as the tight stenosis. Unfortunately, this has not been confirmed by angiography. In any case, if you see a velocity increase out of proportion to real-time vessel characteristics (i.e., large plaque), you should do a careful evaluation for high-grade stenosis of the contralateral ICA.

Aortic regurgitation can present with different findings on carotid duplex. Aortic regurgitation results in reflux of blood from the aorta back into the left ventricle, and causes a widened pulse pressure and increased stroke volume. Ejection fraction may be normal or high early on, but as left ventricular decompensation occurs with longstanding aortic regurgitation, the ejection fraction normalizes and then drops.

A bisferious pulse, also known as pulsus bisferiens, is the presence of two systolic peaks that can be seen in pressure tracings of the the ascending aorta, the aortic arch, and the carotid artery in patients with aortic regurgitation with or without concurrent aortic stenosis. It can also be seen in patients with severe obstructive hypertrophic cardiomyopathy.

Reversal of diastolic flow direction can also be seen in the carotid arteries of patients with aortic regurgitation

Aortic stenosis can cause a characteristic tardus-parvus waveform: prolonged systolic acceleration time (tardus) with low peak systolic velocity (parvus). Bilateral tardus-parvus waveforms in the carotids can be seen with aortic stenosis. In general a tardus-parvus waveform is indicative of a severe stenosis proximal to the point of measurement. For example when sampling distal renal artery branches in a patient with renal artery stenosis.

Right subclavian steal can be reflected in the carotids as a tardus-parvus waveform of the right common and internal carotid arteries.

Cardiac dynamic factors can also affect the carotid waveform. Hypertension can result in high flow (> 135 cm/s) in both common carotid arteries, whereas poor cardiac output can lead to low flow (< 45 cm/s) in both common carotid arteries.

Bradycardia, by allowing longer diastolic filling of the left ventricle, produces increased stroke volume and increased systolic velocities in the carotids. The prolonged diastolic time also leads to a longer diastolic runoff and spuriously decreased end diastolic values.


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