(Note: It will be helpful to review normal heart function before reading this.)

The pulmonary valve opens to let blood flow from the right ventricle to the lungs. Narrowing of the pulmonary valve (valvar pulmonary stenosis) causes the right ventricle to pump harder to get blood past the blockage. If the stenosis is severe, especially in babies, some cyanosis (blueness) may occur. Older children usually have no symptoms.

Treatment is needed when the pressure in the right ventricle is high (even though there may be no symptoms). In most children the obstruction can be relieved during cardiac catheterization by balloon valvuloplasty. In this procedure, a special catheter containing a balloon is placed across the pulmonary valve. The balloon is inflated and the valve is stretched open. In other patients surgery may be needed. During surgery the valve can usually be opened so that it works well again.

The outlook after balloon valvuloplasty or surgery is favorable. Still, follow-up is needed to find out if the heart works normally. Children with pulmonary stenosis are at risk for an infection of the valve (endocarditis) before and after surgery. Your child should get antibiotics such as amoxicillin before dental work and certain surgeries to help prevent endocarditis. Good dental hygiene lowers the risk of endocarditis, too. Ask your pediatric cardiologist for more information about dental hygiene and preventing endocarditis.

Copied from the American Heart Association Site