Pulmonary valve stenosis is when the pulmonary valve is thickened or can't open fully.

The heart pumps blood out of the right side of the heart, through the pulmonary valve, to the lungs. When this valve is not working properly, it can decrease the amount of blood going to the lungs for oxygen or increase the work the heart muscle has to do to maintain it. Blood can also back up into the heart. The condition can be mild to severe.

Heart Chambers and Valves
heart anatomy
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Blood Flow Through the Heart
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Pulmonary stenosis is caused by abnormal development of the heart valve before birth. In most cases, it is not known exactly why it happens.