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