Anatomy:
Left pulmonary artery sling (LPAS) describes the aberrant path of the left pulmonary artery, instead of arising from the main pulmonary artery, it takes its origin from the postero-superior aspect of the right pulmonary artery. The aberrant left pulmonary artery then courses between the trachea and oesophagus (retrotracheal) to the left hilum, causing tracheobronchial compression at the level of the distal trachea and carina. The trachea is caught between the main pulmonary artery abutting anteriorly and the left pulmonary artery bordering the posterior aspect. Wells et al classified LPAS in two types based on tracheobronchial anatomy, Type 1 (normal bronchus) and Type 2 with a bridging bronchus (Table 1).8,9 In type-1A LPAS, the carinal anatomy is normal and bifurcates into left and right mainstem bronchi, usually with mild airway compression (figure 1 b). In type-1B LPAS, the carinal position  is normal (T4/5 level) however the right upper lobe bronchus is tracheal (arising from the trachea) above the carina and supplies either a single subsegmental apical right upper lobe bronchus (RUL) or rarely the entire RUL (figure 1c 1d) . The eparterial tracheal RUL bronchus may be malacic and can also have a blind ending as a diverticulum.10 Type-II LPAS has an abnormal tracheobronchial tree without a well defined carina and is generally associated with tracheal stenosis distal to the tracheal RUL bronchus takeoff. This tracheal narrowing leads to a lower pseudo carina (T6) from which the left main bronchus originates (LMB), figure 2. Additionally the RML and RLL are supplied by a single bridging bronchus that crosses the midline from the left  (Type-IIA). Type-II B is characterised by a complete absence of the right bronchial tree, with the right lung being supplied entirely by a bridging bronchus from the left mainstem bronchus and mostly, the right lung is hypoplastic.11,12In some case series type-II lesions predominate.8 The narrowed tracheal section often contains complete tracheal rings (CTR) with an absent trachealis muscle.13
CTR have been reported in approximately two thirds of LPAS.3 This combination is often described as ‘sling-ring complex.5,6 The number of complete rings may vary from one or two rings (focal stenosis at the LPA site) to long segment tracheal stenosis involving the proximal trachea as well. CTR may extend distally into the bridging bronchi, creating significant surgical challenges for repair. The trachea may also taper from proximal to distally in a funnel shape as the “rat’s tail” trachea.14 CTR are associated with genes encoding cartilage signalling pathways including Sonic Hedgehog and Wnt.13 These genetic embryonal developmental pathways are well described in animal models but the  exact association of complete tracheal rings and LPA sling remains unknown in humans.9