Retrocardiac space

The retrocardiac space refers to the anatomic region located posterior to the heart within the mediastinum. It is best appreciated on the lateral chest radiograph, where it corresponds to the area between the posterior border of the cardiac silhouette (primarily the left atrium and left ventricle) and the anterior surface of the thoracic vertebral bodies. On a normal lateral radiograph, this space should appear relatively lucent (dark), as it is occupied by aerated lung tissue of the left lower lobe.

Anatomic boundaries and contents

• Anteriorly: The posterior wall of the left atrium and, to a lesser extent, the left ventricle.

• Posteriorly: The thoracic spine and paravertebral structures, including the descending thoracic aorta and esophagus.

• Inferiorly: The posterior diaphragm.

• Contents: Normally, this space is occupied by aerated lung (left lower lobe), the descending aorta, the esophagus, and the oblique sinus of the pericardium a cul-de-sac located behind the left atrium, delineated by the pulmonary veins and the inferior vena cava.

Clinical significance

The retrocardiac space is a region frequently overlooked on frontal chest radiographs because pathology in this area can be obscured by the cardiac silhouette. As emphasized in the radiology literature, it is essential to "look behind the heart" on the frontal view, as abnormalities such as left lower lobe pneumonia, posterior mediastinal masses, and metastases may be located solely or partially in this region and would be missed without deliberate inspection. On the lateral radiograph, progressive opacification of the retrocardiac space (loss of the normal lucency over the lower thoracic vertebrae) is an important sign of left lower lobe pathology or left atrial enlargement. In neonates, the retrocardiac space can also serve as a site for retrocardiac pneumomediastinum, representing air trapped in a potential space in the posterior mediastinum behind the heart, typically as a complication of assisted ventilation.

On the lateral view, the retrocardiac space is also used as a reference for comparison with the retrosternal clear space anteriorly; in approximately 50% of normal individuals, the two regions have equal opacity, though the retrosternal region may appear more opaque, particularly in women.