So, you’ve attended a nephrology POCUS workshop and feel confident in your knowledge of cardiac sonographic anatomy. Below are a couple of Twitter polls asking to identify a structure posterior to the aorta in the PLAX plane (not necessarily the standard PLAX view that doesn’t show the cardiac apex and structures beyond the aortic root). A significant number of people got it wrong.

In a half-day or one-day workshop, we mostly cover high-yield topics like identifying major chambers and pathologies such as pericardial effusion and gross ventricular function. However, it’s also important to identify other structures you might notice incidentally as you scan.

Right Pulmonary Artery (RPA): This is the answer to the quiz. The RPA passes posterior to the aorta. Though not always seen in standard PLAX views, it can be visible when you rock the transducer more towards the right shoulder (i.e., away from the apex) or when there is pericardial effusion. It should not be mistaken for an abnormal structure (such as aortic aneurysm). Below illustration clarifies the anatomy.

Let’s consider the other options provided and their clinical significance:
Descending Thoracic Aorta: Seen in the oblique/transverse plane posterior to the left atrium, it cannot be adjacent to the aortic root. As discussed before, it is an important landmark to distinguish between left pleural and pericardial effusion on the PLAX view. Occasionally, an aortic dissection flap may also be seen on the PLAX view.

Coronary Sinus: This large venous structure is located on the inferior aspect of the left atrium, coursing within the left atrioventricular groove and opening into the right atrium. It’s barely noticeable in normal PLAX views but can be enlarged, sometimes enough to be confused with the descending aorta. This enlargement can occur in conditions like persistent left superior vena cava and chronic severe pulmonary hypertension.

Esophagus: This is another posterior structure that is typically not noticeable since it’s collapsed and not filled with fluid. However, when enlarged, as in a hiatus hernia or achalasia, it may compress the left atrium. Below is an example showing a hiatus hernia, which appears as a heterogeneous mediastinal posterior mass. When in doubt, have the patient swallow some soda, and you’ll see fluid/bubbles in the structure. Below is a nice example where the esophageal dilation became more apparent and severe after drinking soda. Case courtesy: CASE (Phila). 2018; 2(6): 285–292. In the first image (before soda US), RPA is seen posterior to the aorta.