Retroesophageal left subclavian artery is an additional vascular abnormality found in 1/3 of dogs with PRAA. This develops when the left 7th intersegmental artery, being pulled forward along the dorsal aorta, fails to reach the left 4th arch before the latter separates from the dorsal aorta. The result is a retroesophageal left subclavian artery causing dorsal compression of the esophagus in addition to the constriction caused by the PRAA and left-sided ductus arteriosus or ligamentum arteriosum.

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Necropsy specimen showing PRAA and a retroesophageal left subclavian artery. Both the ligamentum and the retroesophageal left subclavian were ligated and divided previously to demonstrate the surgical procedures. They were reattached to photograph the demonstration.

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The ligamentum arteriosum is divided where it crosses from the aorta to the pulmonary artery.

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The left subclavian is divided close to the aorta and proximal to the vertebral artery. This relieves the forward tension on the aortic arch.

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The left subclavian retracts cranially and the aorta caudally after dividing the left subclavian. Division of the left subclavian allows the right aortic arch to move further away from the esophagus and helps to relieve the compression on the dorsal and medial aspect of the esophagus. The surgical approach to the ligamentum should always include careful examination of the dorsal aspect of the aorta to reveal a retroesophageal left subclavian. Distal blood flow dependent on the left subclavian is provided by collateral circulation via the vertebral artery.

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