The pericardium is a sac that is normally present around the heart. Normally, there is a tiny amount of fluid that is difficult to detect. Pericardial effusion is a condition in which an abnormally large amount of fluid collects in the pericardial sac that surrounds the dog's heart. The large amount of fluid surrounding the heart restricts the filling of the heart and ultimately decreases the hearts ability to pump blood to the rest of the body. This is called cardiac tamponade. Eventually the pressure outside the heart exceeds the pressure inside the heart, resulting in right sided congestive heart failure or fluid in the belly (ascites) or around the lungs. Signs of pericardial effusion include: weakness, vomiting, lethargy, collapse, coughing, abdominal distension, fainting, exercise intolerance, and difficulty breathing. Treatment of pericardial effusion involves placement of a needle into the sac to allow us to drain the fluid or pericardiocentesis, and reduce the pressure and improve the symptoms your pet is having. Most common causes of this fluid formation include cancer or neoplasia, idiopathic (we don’t know the cause), congestive heart failure, and some other less common causes are constrictive effusive pericardial disease and coagulopathy or a bleeding disorder.