Pulmonary stenosis is a condition where blood flow from the right ventricle to the pulmonary artery is obstructed by narrowing or stenosis at one or more points from the right ventricle to the pulmonary artery. In some cases, obstruction can be caused by thickened muscle below the pulmonary valve. In most cases related to Paxil exposure during pregnancy, the infant is found to have a narrowing of the valve itself, or narrowing of the pulmonary artery above the valve. This is called pulmonary valvar stenosis.

A normal pulmonary valve consists of three thin and pliable valve leaflets. When functioning correctly the pulmonary valve leaflets spread apart easily and cause no obstruction (blockage) to outflow of blood from the heart.

Pulmonary valve stenosis occurs when abnormalities of the pulmonary valve lead to narrowing and obstruction between the right ventricle and the pulmonary artery.

When the pulmonary valve is obstructed, the right ventricle must work harder to eject blood into the pulmonary artery. To compensate for this additional workload, the muscle of the right ventricle (the myocardium) gradually thickens to provide additional strength to right ventricular ejection.

In newborns with pulmonary stenosis, the right ventricle may not eject sufficient volume of blood flow into the pulmonary artery. When this occurs, blue or unoxygenated blood bypasses the right ventricle flowing from the right atrium to left atrium, through the foramen ovale. This is a communication or "hole" between these two chambers that is normally present in newborns. Newborns with critical pulmonary stenosis will have will have a blue discoloration of the lips and nailbeds due to lower oxygen levels in their blood.

A newborn with critical pulmonary stenosis requires immediate intervention to stay alive. This can be done either by balloon dilation of the valve or surgery.

There is a 20% risk of recurrence of significant pulmonary stenosis in newborns and young infants who undernet balloon dilation. In these patients a second balloon dilation may be performed, or in some cases open-heart surgery may be required.

Long-term follow-up with evaluation by a qualified cardiologist is required.