Things just mesh: making stents even better at keeping arteries open

Science News, Nov 24, 2001 by Damaris Christensen

Fatty deposits on the insides of blood vessels pile up less obviously than in a bulging belly or widening hips, yet in some ways they carry as much weight. These tiny products of the process called atherosclerosis can block blood flow and oxygen to the heart, the brain, or other parts of the body.

After using a variety of techniques to clear blocked arteries, physicians now commonly insert metal-mesh tubes called stents into arteries to hold them open and prevent loose bits of the fatty plaque from closing the vessel again or getting swept into the bloodstream, where they can obstruct other blood vessels.

Cardiologists credit stents with improving the safety of artery-clearing procedures and boosting the percentage of blood vessels that stay open afterward. Because of this success, physicians are now investigating whether stents can be used more widely. For example, the tubes might be inserted into blood vessels narrower than those treated today and into people so sick that they now receive major surgery to replace and bypass blocked vessels. What's more, stents may find a role in blood vessels other than those servicing the heart. With each potential application of stents, however, the risks and benefits must be reassessed.

Problems identified in current applications of the devices provide several obstacles to broadening their use. Some, stents trigger immune inflammation, and up to a third of them reclog--a process called restenosis--within a few months after their insertion. Restenosis is typically marked by chest pain, as blood flow is impaired and the heart protests a lack of oxygen. Doctors treat restenosis either by performing additional artery-clearing procedures or by open-heart surgery to bypass the blocked vessel. Such repeat operations are expensive, and recovery from them can be difficult for patients who are already ill.

"If we had the promise of a stent with a low risk of restenosis, we could treat difficult atherosclerotic lesions more confidently and probably send fewer people to bypass surgery," notes Alan Heldman of Johns Hopkins University School of Medicine in Baltimore.

To develop a more long-lasting treatment, scientists are coating existing stents with polymers or gels containing drugs that block cell growth. "This is an exciting time for the whole field," Heldman says.

Doctors attack atherosclerois to prevent it from triggering heart disease, one of the leading causes of death in the United States. Which treatment they choose depends on the severity of the artery clogging, often detected with angiograms, which are x-ray images of blood vessels. People with mild atherosclerosis may receive only medications designed to prevent blood clots and cholesterol-lowering drugs to block any further fatty buildup inside arteries. In people who have severe blockages in vessels serving the heart, cardiologists typically perform surgery to bypass the diseased arteries.

Since the early 1980s, physicians have used an intermediate procedure to treat people with moderate atherosclerosis. In this process, called angioplasty, a surgeon threads a tiny balloon inside an artery and inflates it, flattening the fatty deposits that narrow the artery. In the 1990s, doctors doing angioplasty started implanting a stent to improve the success of the procedure. For example, after Vice President Richard Cheney experienced chest pain in November 2000, he received a stent during angioplasty. Today, between 70 and 90 percent of people undergoing angioplasties get stents.

Recent studies have shown that the success rate for angioplasty is rising, even though physicians have been treating sicker and older patients than ever before. According to researchers, most of that success is due to the widespread use of stents (SN: 2/3/01, p. 72).

In response to such findings, doctors are starting to treat severe atherosclerosis with angioplasty and a stent instead of bypass surgery. In the April 12 NEW ENGLAND JOURNAL OF MEDICINE, an international team of researchers reported that each technique offers about the same degree of protection against death, stroke, and heart attack among people eligible for either procedure.

Angioplasty and a stent provided the less expensive approach but were more likely than bypass surgery to eventually lead to additional angioplasty or a follow-up round of bypass surgery, says study leader Patrick W. Serruys of the Academisch Ziekenhuis Rotterdam Dijkzigt in Rotterdam, the Netherlands. If restenosis in stents were less common, angioplasty with stents might become the preferred treatment for many people otherwise slated for open-heart surgery, he says.

"Some things get to be trendy--used widely just because they are new--but I don't think that's the case with stents," says David O. Williams of Brown University in Providence, R.I. "The more we look, the better [stents' performance] gets."

The current problems arise because a newly implanted stent doesn't stay bare for long. Eventually, the metal surface is covered by a thin layer of epithelial cells, which are the normal lining of blood vessels. The stent is thereby incorporated into artery walls, resembling steel bars embedded in concrete.