A two-pack-a-day smoker is likely to have a heart attack 11 years earlier than a non-smoker, a study concludes.

In addition, smokers who quit had their first heart attacks an average of three years later than those who continued to smoke, said Dr. Arthur Moss, a cardiologist at the University of Rochester in New York."There was a striking inverse relationship" between amount smoked and the age at which people had heart attacks, Moss reported Tuesday at the annual meeting of the American College of Cardiology.

Non-smokers had their first heart attacks at a median age of 62; two-pack-a-day smokers had their heart attacks at a median age of 51, Moss found.

When the numbers were calculated separately for men and women, the danger was higher for women, Moss said.

Men who smoked a half pack to a pack per day, for example, had their first heart attacks at a median age of 57, compared to 60 for non-smoking men.

Women who smoked the same amount advanced the median age at which they had heart attacks by four years, from 63 in non-smokers to 59. Women who smoked more than two packs a day had heart attacks at a median age of 51, Moss found.

Moss' findings suggest that smoking encourages the formation of blood clots, which can lead to heart attacks if they block the arteries that supply blood to the heart muscle.

In another study reported Tuesday, Dr. James Quillen of the University of Iowa said smoking tightens and shrinks coronary arteries by up to 38 percent for about 30 minutes after each cigarette. That constriction can contribute to heart attacks.

Moss said he undertook the study to give doctors ammunition when discussing the risks of smoking with their patients.