What Prevents and What Permits the Embryonic Great Arterial Switch?

Basic Principles

• 1.

  When the situs (or pattern of anatomic organization) of the subarterial infundibulum and the situs of the great arteries are the same (concordant), the great arteries are normally related.

• 2.

  When the situs of the subarterial infundibulum and the situs of the great arteries are different (discordant), the great arteries are abnormally related.

• 3.

  A well-developed subarterial muscular infundibulum prevents the embryonic great arterial switch from above the morphologically right ventricle (RV) to above the morphologically left ventricle (LV).

• 4.

  Absence of a subarterial muscular infundibulum permits an embryonic great arterial switch from above the RV to above the LV. So the subarterial infundibulum is the embryonic great arterial “switch master.”

Segmental situs equations, including the situs of the subarterial infundibulum and the situs of the great arteries, help clarify infundibuloarterial situs comparison, which in turn determines whether the great arteries are normally or abnormally related.

Synergy also helps to explain the normal and abnormal relationships between the great arteries, as we shall see.

Types of Situs

In anatomy, there are two types of situs, or patterns of anatomic organization:

• 1.

  situs solitus, the normal or usual pattern; and

• 2.

  situs inversus, a mirror-image, characterized by right-left reversal, but without superoinferior or anteroposterior change.

Situs ambiguus is not a third type of situs. It means that the pattern of anatomic organization is ambiguous or uncertain and is therefore not diagnosed. Situs ambiguus is different from situs solitus and situs inversus. There is more than one anatomic type of situs ambiguus, as we shall see.

Grading Infundibular Development

The development of the subarterial infundibular free wall is highly variable and may be graded as follows: