However, 14 boys and girls aged 5 to 18 were still alive one year after beginning treatment with the erectile dysfunction drug, according to a report in the June 14 issue of Circulation. Viagra was also easier to use than existing therapies and caused minimal side effects.

Five of the children did die during the second year of treatment, noted Dr. Tilman Humpl, director of the Childhood Pulmonary Hypertension Clinic at the Hospital for Sick Children in Toronto. "But, so far, the other nine are alive and well," he added.

Increases Blood Flow

It's too early to tell exactly what role Viagra will play in treatment of the lung disease, Humpl said, because the number of children in the study was small and the trial did not include a control group, a standard requirement for judging the effectiveness of any treatment.

But because of the trial results, "we currently have around 30 patients on Viagra," Humpl said. "And we are getting the same sort of clinical response we saw in the trial."

Pulmonary arterial hypertension can be inherited, caused by a congenital heart defect or chronic disease of the heart or lung. Blood pressure in the lung arteries rises, causing a further narrowing of blood vessels, which pushes blood pressure even higher. The result is fatigue, dizziness, shortness of breath and eventually heart failure.

Viagra was tried as a treatment for the condition because it relaxes the smooth muscle of blood vessels, letting them expand to increase blood flow.

Better Quality of Life

In addition to extended survival, Viagra gave the children "a better quality of life," Humpl said. In a standard test of physical performance -- the average distance walked in six minutes -- treated children displayed an average increase of 508 feet. Resistance in lung arteries dropped by about 20 percent, as well, so the children literally breathed easier.

The treatment was initiated in Toronto by Dr. Ian Adatia, then director of the hospital's childhood hypertension clinic. He is now associate professor of pediatrics at the University of California, San Francisco.

The idea came to him, Adatia said, because "blood vessels in the penis and the lung act in the same way." Both are sensitive to nitric oxide, which acts to widen vessels.

Other drugs used to treat the condition act through nitric oxide. Viagra affects the activity of nitric oxide on blood vessels by altering activity of an enzyme called phosphodiestase. Because of its effect on blood vessels in the penis, "it was not a huge leap to think of it acting in the lung," Adatia said.

Further, controlled trials -- one of which has already begun -- should clarify Viagra's role in treating childhood pulmonary hypertension. "My bias is that it will find a place in combination therapy," Adatia said.

More information: Childhood pulmonary hypertension is explained by the National Heart, Lung, and Blood Institute.