Left pulmonary artery (LPA) sling is a relatively rare congenital vascular anomaly where the LPA arises from the posterior aspect of the proximal right pulmonary artery (RPA). In this anomaly, the LPA courses over the right main bronchus and then posteriorly between the trachea and esophagus before reaching the left hilum. This is the only vascular anomaly that results in a vascular structure lying between the trachea and esophagus. The course of the LPA forms a partial sling around the trachea, which can become completely encircled in the presence of a patent ductus arteriosus or ligamentum. As a result, upwards of 60% of slings have tracheal stenosis, tracheobronchomalacia, or complete tracheal rings. Due to the high incidence of tracheal stenosis, this complex is commonly referred to as a "ring-sling" complex. Between 40-60% of LPA slings are associated with other congenital heart disease lesions.
Embryologically, normally the distal branch pulmonary arteries arise from the lung buds and join to the proximal branch pulmonary arteries which arise from sixth aortic arches. Current theories suggest that if the proximal left sixth aortic arch regresses or fails to develop normal connections to the left lung bud, a "collateral" vessel to left lung develops from RPA and results in an LPA sling.
Diagnosis can be made solely by echocardiogram but often CT scan is used to confirm the diagnosis and assess for tracheal pathology. Evaluation of the branch pulmonary arteries for the presence of an LPA sling is best established in the suprasternal notch and parasternal short-axis views.
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