The aorta is the major artery which carries "red blood" (high in oxygen) from the left side of the heart to the rest of the body. The aortic valve, located at the beginning of the aorta, opens and closes to regulate blood flow into the aorta. The normal aortic valve has three parts (leaflets), is flexible, and allows blood to flow only in one direction. When the normal aortic valve is open, blood flows easily into the aorta; when closed, no blood leaks back out of the aorta.

Stenosis means narrowing. Subaortic stenosis (or subvalvular aortic stenosis) is a narrowing of the area below the aortic valve. This may vary from a thin layer of extra tissue to large bundles of heart muscle. The left ventricle works harder to pump blood past the narrow region into the aorta. With time, the extra work stresses the left ventricle causing it to enlarge and fail.

Many children with subaortic stenosis have no symptoms. If the stenosis is mild, surgery may not be needed. However, the narrowing tends to progress as the child grows older. If the stenosis is very severe, infants develop rapid heart rate, rapid breathing, poor feeding, and heart failure. Older children with severe stenosis may tire easily, have chest pain, or fainting. They may need to restrict their physical activities. The age and type of surgical repair or treatment vary according to the severity of stenosis and symptoms it causes the child.

Children with subaortic stenosis require penicillin (also called SBE prophylaxis) prior to any dental work or surgery on the mouth, bowel, or bladder. This helps prevent the uncommon, but possible occurrence of bacterial infection affecting the heart near the defect or the heart valves.