Patients who had an additional stent implanted as emergency treatment for their stent thrombosis were at an increased risk for cardiac death or recurrent thrombosis, Dutch researchers said on Saturday.

Heart stents are tiny metal devices that are inserted into the arteries to keep them from clogging up with damaging plaque which can lead to heart attack. Companies manufacturing such devices in the U.S. include Boston Scientific Corp., Johnson & Johnson and Medtronic Inc. A fourth company, Abbot Laboratories Inc. is expecting approval to enter the market in the near future.

The researchers, led by Dr. Jochem Wouter van Werkum, a cardiologist at St. Antonius Hospital in Nieuwegein in the Netherlands, analyzed 437 patients with various kinds of stents who had stent thrombosis between January 2004 and February 2007. They found that 74 of the patients, or about one in six, experienced multiple episodes of stent thrombosis. Sixty-one patients had two episodes, 12 had three episodes and one patient had four episodes.

According to the findings of the “Dutch Stent Thrombosis Study,” patients who had a second stent implanted during emergency treatment for their first clot were 4.2 times as likely as other patients to endure another episode of stent thrombosis. Also, patients with a prior heart attack were 2.6 times as likely to have another episode and patients who had a clot long after the stent was implanted had 2.1 times the usual odds of another clot.

Dr. van Werkum and his colleagues concluded that the placement of a second stent during emergency treatment for the first episode was a strong predictor of second clots, and thus, should be avoided.

The study was released during the Society for Cardiovascular Angiography and Intervention’s annual conference, which opened Saturday and is being held alongside the American College of Cardiology’s annual meeting.