Pericardial diseases are those that affect the protective sac around the heart. They cause it to harden and/or fill with fluid. The heart then no longer has the space in which to expand and contract properly, and loses its ability to pump efficiently. This condition is sometimes caused by a heart tumour.

The heart is enclosed within a protective sac, illustrated in part by the blue line in the illustration.
 

If this should harden, or fill with fluid, it will prevent the heart muscles from expanding and contracting correctly.
 

The effect is much the same as that of myocardial disease (failure of the heart muscle itself), except that pericardial disease may often be treated.

Although pericardial disease can affect any dog, it is more commonly seen in certain large breeds, such as Golden Retrievers.