Stent thrombosis

May 11 2007

Over the last year, it's been rare to utter the word "stent" without at least thinking "thrombosis," an uncommon but serious complication that's been on the minds of interventional cardiologists and patients alike.

At the 30th Annual Scientific Sessions of the Society for Cardiovascular Angiography and Interventions (SCAI), an international panel of experts shared their perspective on both the causes of stent thrombosis, or blood clot formation, and the latest research on its prevention.

The symposium is the most recent of several proactive steps SCAI has taken in addressing concerns about stent thrombosis. Others include testimony before the Food and Drug Administration Circulatory System Devices Advisory Panel and release of a clinical alert advising physicians on practical steps for reducing the risk of this potentially life-threatening late complication.

"The issue of stent thrombosis, especially late stent thrombosis after placement of a drug-eluting stent, is critically important," said Gregory J. Dehmer, M.D., FSCAI, a co-moderator of the session and SCAI'S president. "Interest continues to be extremely high on this timely topic."

Drug-eluting stents are metallic mesh tubes that prop open narrowed arteries in the heart while slowly releasing a medication to prevent the build-up of scar tissue inside the stent. These tiny devices have been very successful in preventing renarrowing, or restenosis, of the coronary arteries, reducing its rate by 40-60 percent compared to bare metal stents. However, several recent analyses that tracked patient outcomes for four to five years after stent placement showed that blood clots were slightly more likely to form inside drug-eluting stents than inside bare metal stents. It is not yet clear how large the difference in risk is, but available data suggest that each year drug-eluting stents hike the risk of late stent thrombosis by about 0.2 percent over bare metal stents. When this occurs, the patient may suffer a heart attack or even die.

The annual meeting symposium will address several key topics related to stent thrombosis, including subacute thrombosis, occurring within a few days of stent implantation; late thrombosis, occurring months or years after stent implantation; and comparisons among drug-eluting stents.

In addition, speakers will focus on the possible causes of stent thrombosis. One common culprit is early discontinuation of double anticlotting medication, typically consisting of aspirin and clopidogrel. A substantial number of patients stop taking anticlotting medication before they should. Today, patients who are treated with drug-eluting stents are advised to continue anticlotting medications for at least 1 year. If it appears they cannot continue therapy for that long, many interventional cardiologists are choosing to implant a bare metal stent instead.

A far less common suspect in stent thrombosis is stent fracture. When stent struts separate, they leave a gap that exposes the artery and becomes the focal point for the build-up of scar tissue or even a blood clot. The risk of stent fracture is greatest at the point where stents overlap or where there is repeated bending of the stent with each cardiac contraction.

"As we have begun to extend stent use to very complex lesions, it has led to rare complications. One of them is stent fracture," said Jeffrey Popma, M.D., FSCAI, director of invasive cardiovascular services for Caritas Christi Health Care System, and director of the cardiac catheterization laboratory at St. Elizabeth's Medical Center, both in Boston. "Stent fracture is a rare but real cause of stent thrombosis."

Research is adding new pieces to the puzzle all the time, Dr. Dehmer said. For example, pathology studies suggest that in some patients, the artery may not heal properly after implantation of a drug-eluting stent, perhaps the result of an abnormal reaction to the polymer that binds the therapeutic drug to the stent's surface.

Ongoing clinical trials will also add clarity to the frequency of stent thrombosis, by carefully documenting this complication as it happens, rather than after the fact.

"We need a much better understanding of the true incidence and the causes of very late stent thrombosis," Dr. Dehmer said. "Then we can come up with better strategies to solve this problem."