Pulmonary stenosis is a condition characterized by obstruction to blood flow from the right ventricle to the pulmonary artery.  This obstruction is caused by narrowing (stenosis) at one or more points from the right ventricle to the pulmonary artery.   Areas of potential narrowing include thickened muscle below the pulmonary valve, stenosis of the valve itself, or stenosis of the pulmonary artery above the valve. The most common form of pulmonary stenosis is obstruction at the valve itself, referred to as pulmonary valvar stenosis. 

The normal pulmonary valve consists of three thin and pliable valve leaflets. When the right ventricle ejects blood into the pulmonary artery, the normal pulmonary valve leaflets open easily and cause no obstruction to blood leaving the heart.  Most commonly with pulmonary valvar stenosis, the pulmonary valve leaflets are thickened and fused together along their separation lines (commissures). As a result, the leaflets become less pliable than normal, which contributes to the obstruction.   At times, the diameter of the pulmonary valve is small, or hypoplastic, resulting in obstruction. 

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 gradually thickens.  The thicker right ventricular muscle, known as hypertrophy, is rarely a problem in itself, but instead is an indication that significant valve obstruction exists.