SAN ANTONIO - Scientists say that patients who had smoked just before the onset of a heart attack had bigger blood clots in their coronary arteries than those who hadn't smoked.

The results come from the same researchers who last year showed that eating a large, heavy meal raises the risk of a heart attack over the next two hours. Now, said Dr. Murray A. Mittleman, director of cardiovascular epidemiology at Beth Israel Deaconess Hospital in Boston, evidence is growing that cigarettes can trigger heart attacks as well.

``So now we know not only that smoking chronically increases your risk, but this seems to indicate there is also an abrupt short-term risk with each cigarette smoked,'' said Mittleman, who headed the study.

``That's an important implication for people who are thinking about quitting. What this implies is that if you quit smoking, there should be a health benefit in the very short term.''

The study was presented last week at the American Heart Association's Annual Conference on Cardiovascular Disease and Epidemiology in San Antonio.

In their study, researchers evaluated the angiograms of 902 heart attack patients.

An angiogram is a diagnostic tool that locates and measures the size of a clot before a patient undergoes angioplasty, in which a catheter is used to unblock a coronary artery.

Blood clots were significantly larger in patients who had recently smoked, even though some smokers had fewer overall risk factors for heart disease, researchers said.

``It appears the effect (of a cigarette) diminishes in time in terms of how big a clot forms,'' Mittleman said. ``After about six hours since your last cigarette, it doesn't seem to have any effect.''

The finding doesn't shock doctors, but should be a jolt for smokers, said Dr. Rose Marie Robertson, president of the American Heart Association.

``This might make it easier for people to visualize what's actually happening in their coronary arteries,'' said Robertson, a professor of medicine at Vanderbilt University in Nashville, Tenn.

Over the long term, smoking damages the protective lining inside blood vessels, making them more susceptible to plaque formation.

More immediately, Mittleman said, smoking causes blood vessels to constrict and stimulates platelets, the blood cells that form clots.

``Now we have evidence that smoking a cigarette increases your body's tendency to form a bigger clot,'' he added.

Robertson said she hopes the study will motivate smokers to ask their doctors for help.

``We have many better ways to help people quit now,'' she said. ``And maybe those better ways, plus this way of visualizing what's really going on in your arteries, might be an additional help to people.''

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