Cardiac tamponade is pressure on the heart that occurs when blood or fluid builds up in the space between the heart muscle (myocardium) and the outer covering sac of the heart (pericardium). This prevents the heart ventricles from expanding fully. The excess pressure from the fluid prevents the heart from working properly. As a result, the body does not get enough blood.
Cardiac tamponade is an emergency condition that needs to be treated in the hospital; the fluid around the heart must be drained as quickly as possible.