Gradual accumulation of pericardial fluid may fail to produce clinical signs or symptoms for an extended period of time. However, rapid accumulation of as little as 100–200 mL of fluid can cause a haemodynamically significant compression of the heart, which severely impedes diastolic filling, resulting in pericardial tamponade. Despite the fact that the left ventricular contractility is normal, the stroke volume is decreased because of the diminished end diastolic volume, resulting in decreased cardiac output.

Because acute tamponade may occur with small effusions, clinically important pericardial enlargement may be difficult to detect on CXR. Subtle changes in cardiac contour may only be detectable by comparison with previous studies. If there is decreased pulmonary vascularity despite the cardiac enlargement or if the SVC and azygos veins are dilated, tamponade may be suspected. 

Echocardiographic demonstration of pericardial effusion and the clinical findings are usually sufficient to make the diagnosis of tamponade. CT and MRI are frequently useful for determining the cause of the effusion, such as haemorrhage, neoplastic involvement, inflammation caused by tuberculosis, or other infectious processes.

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