Stenosis is the term for a valve that will not open properly. When a valve is working properly, it opens fully for all the needed blood to flow through. When a valve is stenotic, the opening is narrowed, the heart must pump harder, less blood gets through and the body as well as the heart suffer from a reduced supply of oxygen-rich blood. This can cause fatigue or a feeling of being out of breath. It may also cause the heart to work extra hard to get an adequate amount of blood circulating to the body.