[[Image:ECG000028.jpg|thumb|Electrical alternans on the ECG]]
In case of tamponade, fluid collects in the pericardium.
As the pericardium is stiff, the heart is compressed, resulting in filling difficulties. This is a potentially life-threatening situation and should be treated with pericardiocentesis, which is drainage of the fluid. Tamponade can be the result of pericarditis or myocarditis. Also, after a myocardial infarction a tamponade may develop; this is called Dresslers' Syndrome. In case of cancer,increased pericardial fluid may develop. This is usually caused by pericarditis carcinomatosis, meaning that the cancer has spread to the pericardium
The ECG shows: