On chest X-ray, how do I distinguish a pericardial effusion from cardiac enlargement?

As you no doubt know, both the diagnoses you listed give rise to an enlarged cardiac shadow (i.e. the cardio-thoracic ratio is> 0.5).  However:

1. In a pericardial effusion, the heart enlargement must be generalised; in the former case it doesn’t need to be.


2. In a pericardial effusion, the lung fields should appear clear, whereas in cardiac failure, the lung fields should show changes, such as ‘bat wing’ infiltrates, upper lobe blood diversion and pleural effusions.


3. Less reliably, pericardial effusions can bring forth two other changes. The cardiac shadow may cover both hila, and the superior vena cava may be enlarged. Neither of these two features are seen with cardiac failure. Superior vena caval enlargement is suggested by a whiteness to the right of the right tracheal border greater then 2-3 mm, but there are other causes of an increased shadow here.


4. Much more reliably, but much harder to see, is the dorsal displacement of the epicardial fat pad.  When you do see this, it is highly specific for a pericardial effusion.

You can see all these features in the X-ray below.  The pericardial fat pad is indicated on the lateral X-ray by the black arrows. Of course, you don't diagnose a pericardial effusion purely on a chest X-ray - if the X-ray is suggestive, you order an echocardiogram.