Embryologic Development:
The proximal portion of the pulmonary artery is derived from the ventral sixth aortic arch and the distal part is derived from the post branchial vessels, which develop from the capillary plexus surrounding each lung bud. Normally, the left post branchial vessels connect to the left sixth branchial arch to form the left pulmonary artery, and the right post branchial vessels take the vascular supply from the right sixth branchial arch to make a right pulmonary artery.14 A left pulmonary artery sling develops when the left post branchial component of the pulmonary artery fails to connect to the left sixth aortic arch, instead connects to the neighbouring right post-branchial component of the pulmonary artery, which in turn connects to the right sixth aortic arch.15 LPAS is much more common than right pulmonary artery sling. The likely explanation is the “space-available” theory in the development of embryonic foregut mesoderm. The embryonic structures including respiratory diverticulum, lung buds, bronchial buds, sixth branchial arch and left common cardinal vein, all compete for the same space. In the setting of LPA sling, during embryogenesis, there might be an early branching of the right upper bronchus, leaving a wider space around the lower developing primitive trachea causing left post bronchial pulmonary vessel to approach the right ventral sixth branchial arch caudally resulting in LPA sling.
Furthermore, during development, the left sixth branchial arch may get vascular supply from a nearby right bronchial artery. Compression of the primitive right lung structures and flow competition from the left post-branchial primitive pulmonary vessel (future LPA) which causes the sling can result in hypoplasia of the right lung and associated tracheal stenosis. The right pulmonary artery sling is possible only in an isomeric setting with an associated left lung hypoplasia.16,17 Many associated congenital heart defects in LPAS can be explained on the basis of space availability theory. Since there will be an extra space available due to abnormal origin of the LPA during the development, this would leave roomier left side in the splanchnic mesenchyme for the unrestricted growth of the primitive left sixth arch and the left common cardinal vein resulting in increased occurrence of patent ductus arteriosus and left superior vena cava respectively.18
Epidemiology:
LPA slings are quite rare, accounting for less than 5% of vascular rings. The prevalence of LPAS in a large screening study using echocardiography has found a prevalence of 59 per million school-aged children.19 Cases are reported in patients with Down syndrome, associated oesophageal and lower gastrointestinal anomalies and VACTERL spectrum (vertebral anomalies, imperforate anus, cardiac anomalies, tracheoesophageal fistula, renal and limb anomalies.20-22 There is evidence of genetic risk factors in LPA sling in identical twins and in patients with trisomy 18.23,24 Other genetic syndromes associated with LPAS include Holt-Oram and Kartagener syndrome.24