What Determines Whether the Great Arteries Are Normally or Abnormally Related?

The situs, or pattern of anatomic organization, of the subarterial infundibulum and of the great arteries and the degree of development of the subarterial infundibulum largely determine whether the great arteries are normally or abnormally related. There are 2 types of situs: solitus (normal) and inversus (a mirror image of solitus). Situs ambiguus means that the pattern of anatomic organization is uncertain or unknown. Infundibular development varies from absent, to atretic, to severely stenotic, to mildly or moderately stenotic; great arteries are solitus normally related or inversus normally related, respectively. When the situs of the subarterial infundibulum and the situs of the great arteries are discordant (different), then the great arteries are abnormally related. Equations indicating the situs of the infundibulum and the situs of the great arteries show whether infundibuloarterial (IA) situs concordance or discordance is present. Many types of IA anomalies typically have IA situs discordance, including transposition of the great arteries, double-outlet right ventricle, double-outlet left ventricle, and anatomically corrected malposition of the great arteries. However, tetralogy of Fallot and truncus arteriosus typically have IA situs concordance, with hypoplasia or atresia of the subpulmonary infundibulum. The relation between the great arteries in tetralogy of Fallot and in truncus arteriosus is almost normal. The IA equations demonstrate the infundibular situs, the great arterial situs, the IA situs concordance or discordance, and the degree of development of the infundibulum. The infundibular situs and the great arterial situs are the formulas or "recipes" for each of the abnormal types of conotruncal malformation.