Pulmonary Arterial Sling This is a rare congenital anomaly in which the left pulmonary artery does not arise intrapericardially from the pulmonary trunk but takes its origin extrapericardially from the posterior aspect of the right pulmonary artery to the right side of the trachea (Fig 47.20A and B).75–78 The left pulmonary artery then makes a hairpin turn to the left and courses toward the hilum of the left lung through the space between the lower trachea and esophagus As a consequence, a vascular sling is formed around the right side of the trachea The arterial duct or its ligament connects the pulmonary trunk to the descending aorta on the left side of the trachea The sling is almost always associated with an abnormality of the airways The anomalous course of the left pulmonary artery may cause direct mechanical compression of an otherwise normal trachea, resulting in tracheomalacia However, an abnormality of the airways, occurs far more commonly as an associated lesion An abnormal branching pattern of the tracheobronchial tree is seen in up to 80% of patients with a pulmonary arterial sling.75–77 Typically the trachea bifurcates into the bronchi to the right and left lungs at a lower level than normal, with a wide angle between the right and left bronchi, producing an inverted T appearance In approximately 30% of the cases with a low inverted T bifurcation of the airways, the right upper lobar bronchus arises from the trachea slightly above the level of normal tracheal bifurcation The left pulmonary artery then forms a sling on the right side of the airway immediately above the lower bifurcation The low inverted T pattern of bifurcation has previously been described as a “bridging bronchus.”76 In this rather confusing concept, it has been considered that this part of the airway between the supposedly normal level of bifurcation and the lower bifurcation into the right and left bronchi is erroneously assumed to be the left main bronchus and that the bronchus to the entire right lung, or to the right middle and lower lobes, arises abnormally from the left main bronchus Therefore this bronchus was thought to create a bridge between the right and left lungs Almost all cases with the low inverted T bifurcation of the airways, with or without a separate origin of the right upper lobe bronchus, are associated with narrowing of a long segment of the lower airways above the bifurcation The narrowing is due to complete cartilaginous rings, with absence of the membranous part of the trachea posteriorly The narrowing may also extend into the main bronchi In addition, the right and, less frequently, the left bronchus may show bronchomalacia.10 One-third of patients with tracheal stenosis due to complete cartilaginous rings have an associated pulmonary arterial sling However, the presence of complete rings does not necessarily imply important stenosis, although the trachea is narrower than normal Imperforate anus is seen in 16% of cases with a low inverted T bifurcation but not in cases with a tracheal bifurcation at normal level.76 FIG 47.20 (A) Volume-rendered computed tomographic angiograms seen from the front and above (left) and from behind and above (right) show a pulmonary arterial sling with abnormal tracheobronchial branching and stenosis The left pulmonary artery (LPA) arises from the proximal right pulmonary artery (RPA), makes a hairpin turn around the airway in the mediastinum and courses to the hilum of the left lung The airway has two bifurcations in the mediastinum, with a narrow intermediary segment, characteristic of congenital stenosis due to complete cartilaginous rings (B) This diagram showing a pulmonary arterial sling also illustrates a leftsided arterial ligament connecting the distal LPA to the descending aorta (C) Chest radiographs are shown together with computed tomographic correlation of a pulmonary arterial sling The diagnosis can be entertained when careful observation shows the abnormality seen in the chest radiograph The arrows indicate the narrowed vertical segment of trachea between its upper and lower bifurcations MPA, Main pulmonary artery The left pulmonary artery is often relatively hypoplastic, being considerably smaller than the right pulmonary artery Stenosis of the right pulmonary artery, with hypoplasia of the right lung, has also been reported.79 Rarely, the sling is associated with agenesis of the right lung.80 Also rarely, the pulmonary artery to the left upper lobe may arise anomalously from the proximal right pulmonary artery, with normal pulmonary arterial supply to the left lower lobe In addition, the right upper lobe may have an aberrant supply from the anomalous left pulmonary artery The sling is reported to be associated with congenital cardiac disease in from 30% to 80% of cases.77,79,81,82 The most common reported defects are atrial septal defect, patency of the arterial duct, ventricular septal defect, tetralogy of Fallot, coarctation of the aorta, and persistence of the left superior caval vein The anomaly has also been reported in association with imperforate anus, congenital megacolon, biliary atresia, and genitourinary anomalies Rarely, an aberrant left pulmonary artery arising distally from the right pulmonary artery does not form a vascular sling but is associated with tracheal stenosis.83,84 A sling involving the right pulmonary artery has been