Pleural Effusion

Pericardial or Pleural?

Differentiating pericardial from pleural effusion relies on their relationship to the descending thoracic aorta and the left atrium. Pericardial fluid accumulates within the pericardial sac and therefore tracks anterior to the descending aorta, typically behind the left ventricle and around the right heart border, but generally does not extend behind the left atrium. In contrast, pleural effusion is located outside the pericardial reflection and appears posterior to the descending aorta, often extending superiorly toward the lung fields. In the parasternal long-axis view, increased imaging depth helps visualize the pericardial reflection: if the pericardial echo lies between the aorta and the echo-free space, the collection is pericardial; if the echo-free space is posterior to the aorta without separation of the aorta from the posterior left ventricular wall, it is pleural. When both are present, two distinct fluid spaces are separated by a bright pericardial line. Multiple views should always be used to confirm the anatomy and avoid misinterpretation.

Key message: Pericardial fluid tracks posterior to the left atrium in the oblique sinus of the pericardium, anterior to the descending aorta