Pulmonary Valve Stenosis is a condition in which the pulmonary valve is narrowed. The narrowing or stenosis is caused by valve leaflets that are thickened and fused. This condition prevents the valve from opening completely during systole. Systole is the phase of the heartbeat in which the ventricles contract and the blood flows into the pulmonary artery. The result is that the right ventricular pressure increases above normal. A narrow valve can be opened by balloon angioplasty.