Pulmonary sling

A pulmonary sling is a term commonly used to describe an anomalous or aberrant left pulmonary artery. 

It is thought to result from a failure of formation of the 6^th aortic arch

The term “sling” is best used when the proximal portion of the anomalous vessel impinges on the right main bronchus and causes obstructive emphysema of the entire right lung or the right middle and lower lobes, depending on the site of the compression.

In most cases, the barium esophagram characteristically shows a mass between the trachea and the esophagus, just above the level of the carina. Conventional radiographs obtained in neonates at birth may show foetal fluid retention or air, with a mediastinal shift usually to the left side. Repositioning of the artery usually cures this compression, because the underlying tracheobronchial tree is basically normal. 

The second type of anomalous left pulmonary artery, which often is fatal, is associated with long-segment tracheal stenosis. This type of tracheal stenosis is due to complete cartilage rings—that is,a rigid “O”-ring trachea without a parsmembranacea. Conventional radiographs usually show an absence of unilateral aeration disturbance; the main bronchi have a very horizontal course(ie, low T-shaped carina) that is detectable on high-kilovoltage filtered radiographs and on CT or MR images.The mortality rate is high in patients with this type of anomalous left pulmonary artery, who require tracheal reconstruction,because the stenosis is primary and not due to the vessel.

Modalities such as ultrafast electron beam or spiral CT and MR imaging depict the vascular anatomy well and obviate angiography. The success of reconstructive procedures in the rigid trachea can be studied by using three-dimensional CT techniques, including virtual bronchoscopy.