THE safety of a common heart implant has been called into question by cardiologists amid fears that in some cases it poses a long-term risk of causing blood clots.

The drug-eluting stents have been hailed as a major advance intreating heart problems and are meant to save lives by propping open diseased blood vessels.

They also release a drug to stop the body's healing mechanism producing new tissue that grows round the implant and blocks blood supply. However, new research suggests they actually increase the chances of a clot forming several years later.

Experts stressed the vast majority of stent patients were not at risk but, with up to six million people worldwide now carrying the devices inside them, it still means thousands could be affected.

The findings, presented at the World Congress of Cardiology in Barcelona, have led to calls for patients to be kept on anti-clotting drugs for much longer.

Researchers at the University Hospital of Geneva re-examined the findings from previous trials. When the data was pooled, it revealed an increase in the risk of death or heart attack in patients given a drugeluting stents.

Dr Edoardo Camenzind, who led the research, said that the risk was increased by up to 38per cent compared with older, bare metal stents. Doctors give anticoagulants like aspirin for up to a year after the device is implanted. The latest findings suggest patients may be at risk from clots for much longer than this.

An estimated 90,000 patients a year in Britain are fitted with a drug-eluting stent because of heart disease. They are around three times more expensive than the older metal devices and are coated with a medicine designed to halt tissue growth.

The risks appear to increase the longer the patient has the implant, suggesting new tissue does eventually cover the implant.

Christopher Allman, a spokesman for Johnson & Johnson, who make one of the drug-eluting stents, admitted last night: "A recent concern with drug-eluting stents is that their use may factor into the incidence of the formation of blood clots at least one year after therapy.

"Although the incidence is rare, any observation is of concern. . . it is an important clinical challenge that we continue to investigate."

Boston Scientific has also confirmed there is a problem with its drug-coated devices but stressed there was no evidence they caused more deaths or heart attacks.

The Medicines and Healthcare Products Agency said it was monitoring the new stents. It added: "Reports to date do not indicate that drug-eluting stents are less safe than others."

British experts said patients may need to stay on clot-busting drugs. Dr Martin Godfrey, from the British Cardiac Society, said: "Drug-eluting stents were a major breakthrough, but it looks as if patients should be on anti-coagulants for longer than they are now."

Dr Peter Weissberg, from the British Heart Foundation, urged cardiologists to use stents only when heart disease symptoms could not be controlled by drugs.