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Stent Study
Shows Risks
new analysis conducted by Duke Medical Center cardiologists
concludes that the incidence of potentially life-threatening clots
forming inside stents—tiny mesh girders designed to prop open
newly cleared arteries—is almost three times higher than previously
reported.
The rate is much higher, they report, because earlier
clinical trials that pegged the rate at about 1 percent enrolled a
highly selective group of patients at high-volume academic centers.
They say these patients are not truly representative of the mix of
patients routinely treated by cardiologists in their practices.
In the past decade, the use of stents has taken off, and
it is estimated that up to 80 percent of the nearly 600,000 angioplasty
procedures performed each year in the United States also involve the
placement of a stent. The major problem encountered by cardiologists
using stents is the formation of a blood clot within the stent. Various
agents have been developed to keep platelets in the blood from clumping
together, which has reduced the rates of clotting, or thrombosis,
and increased the success rates of stent procedures.
“The conventional wisdom has been that in this modern
era of anti-platelet therapy, the risk of stent thrombosis has been
around 1 percent,” says Thaddeus Tolleson, a cardiology fellow
at the Duke Clinical Research Institute (DCRI). “When we looked
at a different set of data, which we felt was more representative
of what cardiologists actually see every day, the rate of stent thrombosis
approached 3.5 percent.
“Stent thrombosis is a rare event, but when it happens,
it is quite dramatic. They usually occur within the first two weeks
of a stent placement and typically involve a large infarction or sudden
death. They are infrequent but catastrophic.”
Tolleson prepared the results of the DCRI analysis for
presentation in September at the annual meeting of the European Society
of Cardiology.
In the early trials of anti-platelet therapy during angioplasty,
sicker patients with more severe coronary artery disease were typically
excluded. For this reason, the Duke researchers combined the data
from two clinical trials involving anti-platelet therapies designed
expressly for these patients with acute coronary syndromes.
Specifically, they used data gathered from the SYMPHONY
(Sibrafiban versus aspirin to Yield Maximum Protection from ischemic
Heart events post-acute coronary syndromes) and Second SYMPHONY trials.
Both trials, which compared the effectiveness of aspirin to a new
class of “super aspirin” to prevent recurrent heart attacks,
enrolled 15,904 patients at 716 hospitals in thirty-five countries.
Of those patients, 4,641 went on to receive an angioplasty procedure
followed by a stent placement.
Using a group of patients who were diagnosed with acute
coronary syndromes, the overall rate of stent thrombosis was 3.5 percent—2.1
percent in the SYMPHONY trial and 4.7 percent in Second SYMPHONY,
Tolleson says. “These are the types of patients who were not
included in earlier trials, but who can make up a significant percentage
of a cardiologist’s practice.”
The data from the two trials showed that those who are
at the highest risk for having stent thrombosis are patients with
diabetes, who are elderly, who are female, or those who have had a
prior heart attack or angioplasty procedure.
Ironically, it is the actual placement of the stent itself
that seems to ultimately lead to thrombosis. When the stent is expanded
within the artery, it inevitably causes irritation of the endothelial
lining of the artery. The body responds to this as it would to any
other injury—platelets arrive at the scene to initiate the healing
process, which usually takes two to four weeks. However, in some patients,
this natural response can provoke clots large enough to block the
stent.
“We will still continue to put stents into our patients
with acute coronary syndromes, but as a result of this study, we will
do it with a higher awareness that thrombosis is not as rare as we
once thought,” says Kristin Newby, a cardiologist at the DCRI
and senior author of the study. “Will these findings change practice?
Not immediately, but it should stimulate further studies on new treatments—both
before and after stent placement—to lower these rates.”
The new approaches could include giving patients anti-platelet
agents for longer periods either before or after the procedure, as
well as the development of a new class of stents impregnated with
anti-clotting agents or even radiation, which has been shown to stem
the proliferation of endothelial cells.
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