With pericarditis, “peri” means “around,” card means “the heart”, and itis means “inflamed”. So pericarditis means the pericardial layer of tissue that covers the heart has inflammation. Acute pericarditis generally lasts just a few weeks, whereas chronic pericarditis lasts longer, usually more than 6 months. People who develop pericarditis are also at risk of also developing a pericardial effusion - that’s when the inflammation causes fluid to accumulate around the heart.
The pericardium is a pouch or cavity that the heart sits inside of. The outer layer of this pouch is the fibrous pericardium and it helps keep the heart in place within the chest cavity. The inner layer of the pouch is the serous pericardium that includes the pericardial cavity, and is filled with a small amount of fluid that lets the heart slip around as it beats. The cells of the serous pericardium secrete and reabsorb the fluid, so usually there’s no more than 50 milliliters of fluid in the pericardial cavity at one time - that’s about as much as a shot glass.
Now, the cause of acute pericarditis is usually idiopathic, meaning that we don’t know what causes it. When the cause is identified, it’s usually a viral infection, like Coxsackie B virus. Another cause is Dressler syndrome which occurs several weeks after a myocardial infarction, or heart attack. Basically, when heart cells die in a myocardial infarction, it leads to massive inflammation that also involves the serous pericardium. Another cause of pericarditis, called uremic pericarditis, is when blood levels of urea, a nitrogen waste product, get really high usually due to kidney problems. The high levels of urea irritate the serous pericardium, making it secrete a thick pericardial fluid that’s full of fibrin strands and white blood cells. This gives the wall of the serous pericardium a “buttered bread” appearance.
Pericarditis can also be seen in autoimmune diseases, like rheumatoid arthritis, scleroderma, or systemic lupus erythematosus, because the immune system attacks its own tissues, including the pericardium. Other causes of pericarditis include cancer, radiation therapy in the chest, and even medications like penicillin, and certain anticonvulsants.
Regardless of the cause, inflammation in the pericardium means that fluid as well as immune cells start moving from tiny blood vessels in the fibrous and serous pericardium into the tissue or interstitium of those layers, making the layer itself a bit thicker and more boggy - think of how a piece of dry flat pasta gets cooked and thickens up as it soaks up fluid. Now, a pericardial effusion can also develop. That’s when pericardial fluid begins to pool in the pericardial space, because the serous pericardium can’t remove it as quickly as it comes in.
If a lot of fluid starts to collect in the pericardial space - in other words, if that pericardial effusion gets really big, then it can start putting pressure on the heart itself, preventing it from fully stretching out or relaxing between contractions. This can lead to tamponade physiology which is where the cardiac chambers can’t fill with blood properly, causing a decrease in cardiac output - which can be a medical emergency. When the inflammation persists for weeks to months, the process is called chronic pericarditis. In chronic pericarditis, immune cells initiate fibrosis of the serous pericardium which can produce a inelastic shell around the heart making it hard for the ventricles to expand—it’s like the heart is being squeezed by a boa constrictor. Over time, it becomes harder for the heart to relax or expand, and the stroke volume - the amount of blood the heart squirts out with each heartbeat goes down, and to compensate the heart rate goes up. This is similar to tamponade physiology but happens more gradually and is a result of a change in the composition of the serous pericardium, rather than a fluid collection around the serous pericardium.