Carotid Stents Raise Ultrasound Threshold for Detecting Stenosis

By Mitchell L. Zoler  

PHILADELPHIA -- The current standards for diagnosing carotid artery stenosis by ultrasound velocity are too low once a stent is placed in the vessel, according to the results of a study of 80 patients presented at the Vascular Annual Meeting.

Currently, the standard ultrasound threshold for diagnosing carotid stenosis of 50% or more is a peak systolic velocity (PSV) of at least 125 cm/s. But after a carotid artery is stented, its biomechanical properties change and the PSV rises.

In stented patients, the flag for carotid stenosis of 50% or more should be a PSV of at least 217 cm/s, Dr. Sam A. Zakhary said at the meeting, which was sponsored by the Society for Vascular Surgery.

A second marker of stenosis in a stented carotid artery is the ratio of the PSV in the internal carotid artery to that in the common carotid artery (ICA/CCA). The standard threshold for flagging stenosis of 50% or more is a ratio of at least 2.0, but in the 80 patients in the study, the best threshold was found to be a ratio of at least 2.98, said Dr. Zakhary, a vascular surgeon at Baylor University Medical Center in Dallas.

In the series of 80 patients, carotid angiography was done within 30 days after carotid artery stenting, and ultrasound examinations were done within 30 days of angiography. A subset of 28 patients with less than 50% carotid stenosis after stenting was followed for 6 years, during which time 5 of the 28 developed stenosis of 50% or more.

Among the 80 patients, 40 were misdiagnosed with a residual carotid stenosis of 50% or more after stenting because their PSV was at least 125 cm/s. None of these 40 patients had this high a level of carotid stenosis when they were assessed with carotid angiography.

In contrast, a PSV of at least 217 cm/s showed high sensitivity and specificity for diagnosing 50% or more stenosis in the 80 patients (see box). And an ICA/CCA ratio of at least 2.98 diagnosed 50% or more stenosis with good sensitivity and specificity, Dr. Zakhary reported.

When asked to comment on this article, Dr. R. Eugene Zierler,of the University of Washington Division of vascular Surgery, Seattle, stated: "This is another in a growing number of studies showing that the standard Doppler velocity criteria for native internal carotid arteries cannot be applied once a stent has been placed. As noted above, relatively high velocities can be found in widely patent internal carotid arteries after stenting, resulting in overestimating the severity of stenosis (false positives). Consequently, new velocity thresholds must be established for use in stented carotid arteries.

"It is interesting that this difference has not been observed in stented renal or peripheral arteries, and the standard velocity criteria appear to apply in these situations. The utility of the specific velocity thresholds described in this study for 50% or greater stenosis in stented carotid arteries will be determined by further experience and prospective validation. However, as carotid stenting becomes more prevalent, it is important that sonographers and physicians associated with vascular laboratories be aware of this important issue, he concluded.