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 or stenosis at one or more of several points from the right ventricle to the pulmonary artery. It includes obstruction from thickened muscle below the pulmonary valve, narrowing of the valve itself, or narrowing 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 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.
Most commonly with pulmonary valvar stenosis, the pulmonary valve leaflets are thickened and fused together along their separation lines (commissures).
When the tissue is thickened, the leaflets become less pliable than normal, which contributes to the obstruction. At times, the diameter of the pulmonary valve itself is small or hypoplastic.
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.
The increased right ventricular muscle, known as hypertrophy, is rarely a problem in itself, but instead is an indication that significant valve obstruction exists.
When the pulmonary valve is severely obstructed, especially in newborns with critical degrees of pulmonary stenosis, the right ventricle cannot eject sufficient volume of blood flow into the pulmonary artery.
In these instances, blue blood bypasses the right ventricle flowing from the right atrium to left atrium, through the foramen ovale, a communication or "hole" between these two chambers that is normally present in newborns. Newborns with critical pulmonary stenosis therefore will have cyanosis (blue discoloration of the lips and nail beds) due to lower oxygen levels in their blood.
Right ventricular failure rarely occurs with pulmonary valve stenosis.