Sunday, September 30, 2012

Crouching Bunny, Hidden Stenosis

Carrying on with the innominate artery theme, Zweibel notes in his textbook that a biphasic (or "crouching bunny") right common carotid waveform can result from innominate stenosis.

First, a normal right common carotid waveform for comparison:

Normal right common carotid artery Doppler waveform:   Moderately broad systolic peaks and a moderate amount of flow throughout diastole. Uniphasic.

If there is enough stenosis in the innominate artery, then a steal phenomenon can result, with resulting biphasic waveforms, indicating back-and-forth blood flow.

If there is biphasic blood flow bilaterally, the best consideration is pulsus bisferiens (next post) from aortic insufficiency... but if it's only on the right side, it indicates a steal phenomenon in the right common carotid artery; during systole, blood flows forward into the carotid system and in diastole it flows backward into the right subclavian and arm.

In severe cases of innominate artery stenosis, there is complete flow reversal, but in less severe cases, the flow remains cephalad although an atypical biphasic waveform results... the "crouching bunny."

An example of the biphasic "crouching bunny" waveform showing midsystolic deceleration with antegrade late-systolic velocities .  Although this is is an image of the right vertebral artery, the principle and the Doppler waveform are the same in the carotid.

If the innominate artery were completely occluded, then obviously there would be complete flow reversal in the common carotid artery.

Two other points: 1) in a normal situation, carotid steal can only occur on the right side. 2) if you see abnormal right common carotid (or vertebral) waveforms, check the subclavian arteries and measure the BP in both arms.

1. "Introduction to Vascular Ultrasonography" 5th ed.(2005) Zweibel, Pellerito, et al.
2. Tahmasebpour HR, Buckley AR, Cooperberg PL, Fix CH. "Sonographic Examination of the Carotid Arteries." RadioGraphics, 25, 1561-1575.