A popular procedure to relieve chest pain among heart patients by inserting small metal tubes — called stents — into blocked arteries may be useless for many of them and create a placebo effect, according to new research.

The tiny wire cages, which are used to open constricted arteries, are lifesaving for those suffering from a heart attack – but are most often used by angina sufferers who experience chest pain after exertion.

Heart disease is the leading killer in the US, where about 790,000 people have heart attacks every year, according to The New York Times.

More than half a million heart patients worldwide have stents inserted each year to relieve chest pain, according to the researchers. Other estimates are much higher.

Several companies — including Boston Scientific, Medtronic and Abbott Laboratories — sell the devices, which cost from $11,000 to $41,000 to insert at hospitals across the US.

The new study, published in the Lancet, shocked top cardiologists and raises questions about whether stents should be used as often — or at all — to treat chest pain.

“It’s a very humbling study for someone who puts in stents,” Dr. Brahmajee K. Nallamothu, a cardiologist at the University of Michigan, told The Times.

For the study, Dr. Justin E. Davies, a cardiologist at Imperial College London, and his colleagues used 200 people with a severely blocked coronary artery and acute chest pain — common reasons for inserting a stent.

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

The subjects then underwent a procedure: a real or fake insertion of a stent.

In both groups, doctors threaded a catheter through the groin or wrist of the patient and up to the blocked artery. Once the catheter reached the site, the doctor inserted a stent or, under the fake procedure, pulled out the catheter.

Neither the patients nor the experts who later assessed them knew who had received a stent.

Those who received stents experienced greatly improved blood flow through the previously blocked artery.

When the researchers tested the patients six weeks later, members of both groups reported less chest pain and they did better than before on treadmill tests.

But there was no real difference between the two groups, the scientists found. Those who underwent the fake procedure fared as well as those who got stents.

Cardiologists said one reason could be that atherosclerosis affects many blood vessels, and inserting a stent only into the largest blockage may not make much difference in the level of discomfort.

Those who report feeling better may only be experiencing a placebo effect from the procedure, the said.

Dr. David Maron, a cardiologist at Stanford University, described the study as“very well conducted” – but said it left some questions unanswered.

The subject had a severe blockage but only in one artery, he said, and they were assessed after only six weeks.

“We don’t know if the conclusions apply to people with more severe disease,” Maron said. “And we don’t know if the conclusions apply for a longer period of observation.”