We may find both pericardial and pleural effusions in the parasternal long axis (PLAX) view and mistaking one for the other can lead to inappropriate management. The descending thoracic aorta is the anatomical landmark to differentiate between these two effusions. Fluid anterior to the descending aorta (toward the top of the screen) is pericardial effusion and the fluid at or posterior is likely pleural effusion. Here is a cross section of thorax demonstrating the relationship of the descending aorta to the left lung and pleura. Further, pleural effusion will most likely have atelectatic lung floating in the fluid (big clue!). Even if its not there, remember that pleural effusion follows the curvature of the chest wall while pericardial effusion wedges anterior to the aorta.

Here are some clips illustrating this concept.

Below is an example of left pleural effusion in a patient with malignancy that can be easily confused with pericardial effusion on quick beside exams. Pay attention to the location of descending aorta. Scanning in different views helps further. Also note fibrin stranding in the mid-axillary view suggestive of exudative pleural effusion.

Below is a nice example of moderate pericardial effusion along with left pleural effusion. Pericardial effusion appears to be distributed more anteriorly (next to RVOT [right ventricular outflow tract]) likely due to pressure effect of pleural effusion. M-mode image shows early diastolic collapse of RV (observe the EKG) suggestive of tamponade physiology. In this case, draining pleural effusion might resolve the tamponade.