Pulmonic Stenosis is a congenital heart defect in which a malformation impedes the flow of blood through the heart. It is found in all breeds, though some (listed below) are thought to be at greater risk.

This illustration indicates a cause of pulmonic stenosis.
 

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

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

Various scientific studies have shown that this problem mostly affects the following breeds: English bulldogs, Mastiffs, Terriers, Spaniels, Boxers and miniature Schnauzers.