Stent thrombosis is a rare event, but when it happens, it is quite dramatic

Dr Thaddeus Tolleson

It is thought that there is a 1% chance of this happening, but the latest research suggest that previous studies have been over-optimistic.

However, even taking this into account, doctors are still likely to opt for treatments involving stents, as they are significantly safer than alternatives.

Research has shown that the combination of using stents with balloon angioplasties appears to be safer and more effective than conventional bypass surgery.

Approximately 23,000 such operations are carried out in the UK each year.

Doctors have been advised by the National Institute for Clinical Excellence to make sure stents are positioned after balloon angioplasties.

Sicker patients

The researchers, from Duke University Medical Center in the US, said that they felt previous studies had been over-optimistic, because many patients at higher risk of clotting had been excluded.

They used data from studies which included these sicker patients, and found that the overall overall average rate of such clotting was 3.5%.

Those at highest risk were diabetic patients, the elderly, or those who had had a previous heart attack or angioplasty procedure.

Dr Thaddeus Tolleson, from Duke University, said: "Stent thrombosis is a rare event, but when it happens, it is quite dramatic.

'Catastrophic'

"They usually occur within the first two weeks of a stent placement and typically involve a large infarction (heart attack) or sudden death.

"They are infrequent but catastrophic."

The team suggests that doctors should be aware that clotting is a significant risk, particularly in some patients.

The reason why clotting occurs is related to the placement of the stent itself, which irritates the lining of the artery wall - and provokes an immune response.

This means that platelets - a part of the blood involved with the healing process - are directed to the "injury", where they can contribute to clots which are large enough to block even the enlarged artery.

The research was presented at the European Society of Cardiology annual conference.