Most of the children had hypoplastic left heart syndrome, which involves a poorly functioning small ventricle that leaves children weak and blue in color. Other children had floppy or imperfect heart valves, and one had Kawasaki disease, which causes inflammation in coronary arteries. Many were facing multiple surgeries, and three-quarters already had shunts in their hearts to keep their blood flowing properly.

The children were divided into a treatment group and a placebo group. Those in the treatment group received one of four doses of clopidogrel, ranging from .01 to .20 milligrams per kilogram of body weight per day over a period of one to four weeks.

The study found that the optimal dose for infants and toddlers up to 24 months was 0.2 milligrams per day.

Adults with an average weight of 75 kilograms -- or 165 pounds -- typically receive 75 milligrams per day of clopidogrel. When extrapolated, that would predict an optimal dose of about 1 milligram per day for children under age 2.

The findings "show that you can't simply extrapolate from what you do in adults and apply it to children," Li said.

The study was published in the Jan. 29 issue of Circulation.

More information

The Nemours Foundation has more about congenital heart defects.