The heart is surrounded by a membrane known as pericardium. Inflammation of this sac like membrane is known as pericarditis.
The pericardium is made up of two layers; a visceral layer (epicardium) that is tightly adherent to the heart muscle and an outer parietal layer. The inner layer is thin and soft while the outer layer is thick, fibrous and rather tough. There is a small space between these two layers called pericardial space which contains small amount of fluid, known as pericardial fluid. This serous fluid is secreted by inner thin layer of the heart sac. This fluid keeps the surface of membranous sac and heart moist.
In case of inflammation of either of the layers, the natural response of the affected membrane is to produce more serum fluid, resulting in excess fluid in the pericardial space. Since pericardium is inelastic and does not distend easily, the heart and nearby organs are compressed with the buildup of the fluid, resulting in further inflammation and swelling.
With heart becoming compressed, it starts losing its normal diastolic movement within the sac since there is no more space due to fluid accumulation. This situation leads to a major complication known as cardiac tamponade — a syndrome of increased pericardial pressure resulting in reduced diastolic expansion of the heart. Cardiac tamponade can rapidly lead to cardiac dysfunction, reduced cardiac output and heart failure. Some dogs progress to hemorrhagic pericarditis which is blood in the heart sac. This can also result in life-threatening buildup of fluid in the heart sac and tamponade.
Right sided congestive heart failure is the most common symptom of pericarditis. Other common causes include poor appetite, weakness, abdominal fluid, breathing difficulty, weak pulse, increased heart rate and collapse.