The echocardiographic distinction between pericardial effusion and left-sided pleural effusion is made using the descending thoracic aorta (DTA) as a landmark. When imaged in the parasternal long-axis view, pericardial fluid tracks anterior to the DTA, with its superior (basal) extent limited by the reflection of the oblique sinus ( Figure 1 , Video 1 ; available at www.onlinejase.com ). By contrast, left pleural effusions track posterior to the DTA ( Figure 2 ). It is, however, important to note that the position of the DTA is not fixed. Although it is normally located posterior to the posterior atrioventricular groove, its position can be altered by tortuosity and left atrial enlargement. Such anatomic alterations could potentially limit the utility of the DTA as a landmark in distinguishing pericardial from left pleural effusions.
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