The new study, published in the Lancet, stunned leading cardiologists by countering decades of clinical experience. The findings raise questions about whether stents should be used so often — or at all — to treat chest pain.

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A procedure used to relieve chest pain in hundreds of thousands of heart patients each year is useless for many of them, researchers reported Wednesday.

Their study focused on the insertion of stents, tiny wire cages, to open blocked arteries. The devices are lifesaving when used to open arteries in patients in the throes of a heart attack.

But they are most often used in patients who have a blocked artery and chest pain that occurs, for example, walking up a hill or going up stairs.

The new study, published in the Lancet, stunned leading cardiologists by countering decades of clinical experience. The findings raise questions about whether stents should be used so often — or at all — to treat chest pain.

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Dr. William E. Boden, a cardiologist and professor of medicine at Boston University School of Medicine, called the results “unbelievable.”

Dr. David Maron, a cardiologist at Stanford University, praised the new study as “very well conducted” but said that it left some questions unanswered. The participants had a profound blockage but only in one artery, he noted, and they were assessed after just six weeks.

For the study, Dr. Justin E. Davies, a cardiologist at Imperial College London, and his colleagues recruited 200 patients with a profoundly blocked coronary artery and chest pain severe enough to limit physical activity — common reasons for inserting a stent.

All were treated for six weeks with drugs to reduce the risk of a heart attack, such as aspirin, a statin and a blood pressure drug, as well as medications that relieve chest pain by slowing the heart or opening blood vessels.

Then the subjects had a procedure: a real or fake insertion of a stent. This is one of the few studies in cardiology in which a sham procedure was given to controls who were then compared to patients receiving the actual treatment.

In both groups, doctors threaded a catheter through the groin or wrist of the patient and, with X-ray guidance, up to the blocked artery. Once the catheter reached the blockage, the doctor inserted a stent or, if the patient was getting the sham procedure, simply pulled the catheter out.

Neither the patients nor the researchers assessing them afterward knew who had received a stent. Following the procedure, both groups of patients took powerful drugs to prevent blood clots.

The stents did what they were supposed to do in patients who received them. Blood flow through the previously blocked artery was greatly improved.

When the researchers tested the patients six weeks later, both groups said they had less chest pain, and they did better than before on treadmill tests. But there was no real difference between the patients, the researchers found. Those who got the sham procedure did just as well as those who got stents.

Cardiologists said one reason might be that atherosclerosis affects many blood vessels, and stenting only the largest blockage may not make much difference in a patient’s discomfort. Those who report feeling better may only be experiencing a placebo effect from the procedure.