vascular ring pulmonary artery sling anomalous origin of the left pulmonary artery from the posterior aspect of the right pulmonary artery, with the anomalous left pulmonary artery coursing over the right mainstem bronchus and then from right to left, posterior to the trachea or carina and anterior to the esophagus, to reach the hilum of the left lung, which compresses the lower trachea and right mainstem bronchus pulmonary arterial sling csmith MP:0010467 2010-09-29T04:22:55Z MPheno.ontology