These patients may have a silent risk of future coronary events, such as a heart attack

Dr Peter Ruygrok, Green Lane Hospital, Auckland

However, blood vessels widened using angioplasty tend to be prone to re-clogging.

And the latest research, from a hospital in Auckland, New Zealand, reveals that in half of cases, when this happens, it will not be accompanied by warnings such as chest pain which may have originally alerted the patient to the problem.

Doctors who conducted the study say some patients may need routine follow-up tests to make sure this "silent" re-hardening of the artery has not happened.

Unaware of problem

Dr Peter Ruygrok, a consultant cardiologist at Green Lane Hospital in Auckland, and the lead author of the study, said: "These patients may have a silent risk of future coronary events, such as a heart attack.

"Yet, they believe they have had a successful treatment for their obstructive artery narrowing."

The re-clogging of the artery at the point of angioplasty is called "restenosis" by doctors.

It happens in between 10% and 40% of angioplasty patients.

The New Zealand team checked 2,690 patients six months after their initial treatment.

They found that 607 had some new blockage - their heart arteries had closed to only 50% of their normal width.

And in 55% of these, the patient had detected no symptoms suggestive of this new problem.

Men were more likely to suffer this kind of restenosis, although the team struggled to find any other factor which could help them pick out those at risk.

Dr Ruygrok said: "We were surprised there were not more predictive factors."

The research was published in the journal Circulation.