CHICAGO, Dec. 27 (AP) - Patients being treated for heart attacks involving narrowed arteries and clots that reduce blood flow to the heart are often given overdoses of powerful blood-thinning drugs in the emergency room, increasing their risk of serious bleeding, a study has found.

Excessive bleeding occurred at catheter sites, from existing stomach ulcers and in the brain, where it was particularly dangerous, said Dr. Karen Alexander, a researcher at Duke University and the lead author of the study, which is to be published Wednesday in The Journal of the American Medical Association.

Of 30,136 heart patients treated last year at 387 hospitals in the United States, 42 percent were given excessive doses of blood thinners. Those given extra amounts of two blood thinners - low molecular weight heparin and glycoprotein IIb/IIIa blockers, which are sometimes called super-aspirin - had about a 30 percent greater chance of major bleeding than those given the recommended dose.

The study suggests that 15 percent of the bleeding episodes were from overdoses and might have been avoidable.

Heparin, low molecular weight heparin and the glycoprotein blockers, which are injected or given intravenously, are effective at helping to prevent clots and further heart damage.

Dr. Alexander said determining the correct dose could be tricky in an emergency, when quick treatment is essential.

Dosages are computed according to age, sex, weight and kidney function, and sometimes require a calculator.

But when faced with a patient just brought in on a stretcher, doctors ask how much the person weighs, or they estimate, Dr. Alexander said.

Study participants who got overdoses had slightly longer hospital stays and higher death rates than those who received the recommended doses, but Dr. Alexander said that might have been because they were sicker.

She said more research was needed.

Dr. Robert Bonow, a former president of the American Heart Association, said, "Even giving the right drug at the right dose increases the risk of bleeding," and giving too low a dose could increase patients' risk for clot-related damage.

A separate study found that having a sibling with heart disease might be a bigger predictor of a person's own risks than a parent's health history.

Family history, genes and lifestyle factors are known contributors to heart disease, and it may be that siblings are more similar to each other than to their parents when it comes to lifestyle, said Dr. Bonow, who was not involved with either study.