Cardiac tamponade (aka pericardial tamponade) is a type of pericardial effusion (i.e. abnormal accumulation of fluid in the pericardial cavity, around the heart), in which fluid, pus, blood, clots or gas, accumulates in the pericardium (sac in which the heart is enclosed), causing a slow or rapid compression of the heart. Because of the fluid build up in the pericardial space, faster than it can stretch, this causes pressure on the heart muscle, called tamponade. If the amount of fluid increases slowly (as in hypothyroidism), the pericardial sac can expand to contain 1L+ prior to tamponade occurring. If fluid effusion occurs rapidly (e.g. after trauma or myocardial rupture), as little as 100mL can cause tamponade.