Harvard Heart Letter

Ask the doctor: What is pericardial effusion?

Q. My doctor told me I have pericardial effusion. I know it has something to do with fluid in the heart. Can you tell me more?

A. Pericardial effusion is the medical term for a buildup of fluid inside the sac that surrounds the heart. This sac, called the pericardium, protects the heart, helps hold it in shape, and prevents it from expanding too much when blood volume increases.

There usually isn't any fluid between the pericardium and the heart muscle. Infection, a heart attack, heart or other surgery, injury, kidney failure, an underactive thyroid, and many other conditions can cause fluid to accumulate in the sac. This buildup can cause pain, interfere with heart function, or go entirely unnoticed.

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