pericardial effusion
A pericardial effusion is an abnormal build-up of fluid in the space between the two serous layers of the pericardium. Picture the heart sitting inside a water balloon; normally there are only a couple of teaspoons of lubricating fluid, but in an effusion that volume swells, sometimes to a litre or more.
Whether an effusion causes trouble depends as much on speed as on size. The pericardial sac stretches slowly, so a slow accumulation over weeks may reach a large volume while the heart still fills adequately. A rapid accumulation of even a few hundred millilitres, however, can sharply raise the pressure around the heart and threaten its filling.
Causes overlap with those of pericarditis: viral or idiopathic inflammation, autoimmune disease, kidney failure, low thyroid function, cancer, tuberculosis, and bleeding into the sac after trauma or procedures. The key clinical worry is progression toward cardiac tamponade, so echocardiography is used to estimate the size and, crucially, to look for signs that the heart is being squeezed.
A large, slowly forming effusion can be surprisingly well tolerated, while a small but rapidly forming one can be dangerous; it is the rate of filling against a stiff sac that matters most.