Like pulmonic stenosis, aortic stenosis is a congenital heart defect in which a malformation impedes the flow of blood through the heart. However, where pulmonic stenosis blocks the flow of blood from the right side of the heart to the lungs, aortic stenosis blocks the flow from the left side of the heart to the body, with more serious consequences. Aortic stenosis can be seen in all breeds, though some (listed below) are thought to be at greater risk.

This illustration indicates a cause of aortic stenosis.
 

The outgoing flow of blood, from the heart to body, is partially blocked by a malformed valve. This 'pinches' the flow and causes a restriction.
 

The effect of this partial obstruction is to force the heart to work harder to pump blood around the body. The extent to which a dog will be affected depends on the degree of narrowing (stenosis) of the valve area.
 

In the UK, Boxers are at greatest risk of aortic stenosis. It is also seen more commonly in Golden Retrievers and Rottweilers.