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New Intravascular Ultrasound (IVUS) Study of Underexpanded Stents, Thrombosis and Restenosis

intravascular ultrasound (IVUS)
A 2D (left) and 3D reconstruction (right) of a
coronary artery using intravascular ultrasound
(IVUS) -- the colored areas represents plaque

May 19, 2009 -- A study, published in the current issue of JACC Interventions, details 120 drug-eluting stent cases as viewed by intravascular ultrasound (IVUS) in order to examine characteristics that may lead to in-stent restenosis (ISR) or stent thrombosis.

Underexpansion of stents, both drug-eluting and bare metal, has been long identified as a significant predictor of adverse clinical events. Prominently discussed by Dr. Antonio Colombo in the early days of stenting, inadequate expansion of the stent struts is known to increase these problems.

This latest examination studied whether there was a difference in the type of underexpansion that caused thrombosis (blood clotting) versus in-stent restenosis (the growth of excess tissue inside the stent) in drug-eluting stents (DES). Dr. Akiko Maehara and a team from the Cardiovascular Research Foundation and Columbia University Medical Center in New York looked at 20 definite DES thrombosis patients, which represented all definite thromboses from 1,407 consecutive DES patients who underwent intravascular ultrasound imaging. These were compared to 50 risk-factor-balanced ISR patients with no evidence of stent thrombosis and 50 risk-factor-balanced "no-event" patients with neither thrombosis nor ISR.

Using IVUS allows the cardiologist to see not only the amount of blockage, as in a 2D angiogram, but the spatial and volumetric relationship of the blockage to the actual arterial wall in three dimensions. An issue with inadequate stent expansion is that, using angiography alone, the operator may not be sure that the stent struts are pressed up against the interior surface of the coronary artery. Additionally, by using a 3D real-time reconstruction of an IVUS "pull-back" (the right image above), the interventionalist can see immediately after stent implantation any eccentricities of the arterial segment and can ensure that full expansion has occurred. If the stent is not adequately expanded, the placement can be "touched-up" with a high-pressure balloon expansion in all or part of the stent. Incomplete expansion allows a space to exist between the stent struts and arterial wall, a space where thrombus can form and can also promote unwanted tissue growth which then blocks the stent.

85% of the thrombosis studied in this report occurred within 30 days of the stent procedure, pointing up the fact that inadequate placement, not the drug or polymer or other characteristics of the drug-eluting coating, was the prime predictor. The study concluded, however, that there is a difference between underexpanded stents that thrombose versus underexpanded stents that restenose: the underexpansion in DES that thrombose is more severe, more diffuse, and more often proximal in location. The researchers found that in cases of thrombosis, the proximal parts of many of the stents were inadequately expanded, possibly because stents are usually sized more for the center and distal ends of blockages where there is more disease.

The past few years have seen an increase in the the use of IVUS during procedures. Reports, such as this one, may give further momentum to the ability of this imaging modality to positively impact patient outcomes. In an accompanying editorial, Drs. Karan S. Bhalla and H. Vernon "Skip" Anderson of the University of Texas Health Science Center, Houston, summarize one of the main take-away messages of this study:

"Inadequate deployment is responsible for at least some of the adverse clinical events of thrombosis and restenosis that continue to haunt us. Adequate stent expansion throughout the entire stent length must be the goal. All of this might mean a greater use of IVUS to check for optimal stent expansion. The devotees of IVUS will continue to champion this approach, and for those who prefer IVUS but have been constrained by outside forces, the justification for IVUS has never been greater.... Only by taking the necessary time and paying close attention to detail will we be able to reduce the contribution that inadequate DES expansion makes to these insidious phenomena."     H. Vernon "Skip" Anderson
H. Vernon "Skip" Anderson, MD, FACC, FSCAI


Reported by Burt Cohen, May 19, 2009