What you are talking about sounds like a pericardial effusion
. This is where fluid fills the pericardium
(the sac that covers your heart).
This is a very dangerous condition because, as the pericardium fills with fluid, the pressure that the fluid creates compresses the heart and hampers it's ability to pump blood to the body effectively.
There are a number of medical conditions as well as blunt force and penetrating trauma
that can cause an effusion in varying degrees.
Of course, it is difficult to let you know what is going on without knowing the specifics; please talk in detail with your physician regarding your (or your loved one's) specific condition.