Aortic stenosis refers to a condition that causes obstruction to blood flow between the left ventricle and the aorta. There are a variety of causes, including muscular obstruction below the aortic valve, obstruction at the valve itself, or aortic narrowing immediately above the valve.

The most common form of aortic stenosis is obstruction at the valve itself, referred to as aortic valvar stenosis.

The normal aortic valve consists of three thin and pliable valve leaflets. When the left ventricle ejects blood into the aorta, normal aortic valve leaflets spread apart easily and cause no obstruction to outflow of the blood from the heart.

Aortic stenosis occurs when abnormalities of the aortic valve lead to narrowing and obstruction between the left ventricle and the aorta.

The most common abnormality occurs when the aortic valve has only two (instead of three) leaflets. This is called a bicuspid aortic valve (or BAV). Often the valve leaflets are thickened and less pliable than normal, and the lines of separation between them (or "commissures") are fused together to a variable degree. When the aortic valve does not open freely, the left ventricle must work harder to eject blood into the aorta.

To compensate for this additional workload, the muscle of the left ventricle (the myocardium) gradually thickens to provide additional strength to left ventricular ejection. The increased left ventricular muscle, also known as hypertrophy, is rarely a problem itself. Instead, hypertrophy is an indication of significant valve obstruction.

When the aortic valve is severely obstructed, the left ventricular muscle may not be able to compensate satisfactorily. In these very severe, but fortunately rare, instances the left ventricle may fail to pump blood effectively and the patient may develop cardiac failure. If this occurs it is almost always in a newborn infant with very severe valve obstruction. Heart failure rarely occurs later in childhood.