The impotence drug Viagra, originally developed to treat Heart disease, may help children with a serious heart-lung condition - pulmonary arterial hypertension - to walk further and breathe more easily, US researchers said on Monday.

The small study involving the drug, known generically as sildenafil, must be repeated in a larger group to be accepted, but it suggests the easy-to-take daily pill could be an alternative to current cumbersome treatments, the researchers write in the journal Circulation.

“Sildenafil compared favorably to the drugs used now and had far fewer side effects,” said Ian Adatia, an associate professor of pediatrics at the University of California San Francisco Children’s Hospital, who led the study.

“Untreated, children usually die within one year of diagnosis,” said Adatia, who conducted the study while working at Toronto’s Hospital for Sick Children in Canada.

“And even with the best therapy - continual intravenous infusion of the drug prostacyclin that helps lower the pressure in the pulmonary arteries - few patients live five years past diagnosis.”

In childhood pulmonary arterial hypertension, the blood pressure in the arteries that supply the lungs is extremely high. The small blood vessels in the lungs steadily narrow and their walls thicken, so they carry less blood.

This causes pressure to build as the blood backs up.

Eventually, the heart labors to work properly and goes into chronic heart failure, which causes patients to feel tired, dizzy and short of breath.

With prostacyclin treatment patients must always carry an infusion pump, and their parents must mix the drug daily. Side effects include jaw and muscle pain and facial flushing, Adatia said.

Sildenafil works by relaxing the smooth muscle of blood vessels, expanding them and increasing blood flow. When tested on young men for development as a heart drug, doctors noticed the side effect that the drug is now used for - it caused erections.

Adatia’s team gave varying doses of Viagra to 14 children aged 5 to 18 with pulmonary arterial hypertension for one year.

After a year the children could walk considerably farther, blood resistance dropped and the patients reported they could breathe more easily.

“Very importantly, the side effects were minimal, the drug was very easy to take, and there were no changes in liver or kidney function that can happen with other drugs,” Adatia said.