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.