Abnormality of the valve dividing the right ventricle of the heart and the pulmonary artery, which normally prevents blood flowing back into the heart from the lungs during the diastole (relaxed) phase in the heartbeat. A valve can be constricted (stenosis) or leak (insufficiency), but the latter is rare in the pulmonary valve. Pulmonary stenosis is almost always congenital. The consequence is that the right ventricle can pump too little blood to the lungs, causing symptoms such as cyanosis and shortness of breath. Medication is pointless; the only effective treatment is surgery to widen the valve.