Infundibuloarterial Situs Equations and Analysis

Infundibuloarterial Situs Concordance and Discordance

• 1.

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

• 2.

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

Relationships between the great arteries that have infundibuloarterial situs concordance and hence normally related great arteries include:

• Solitus normally related great arteries (SNRGA):

• Inverted normally related great arteries (INRGA):

• Tetralogy of Fallot (TOF):

TOF is a hypoplastic, obstructive subpulmonary infundibulum and its sequelae.

• Absent subpulmonary infundibulum (AOSPI) and its sequelae, formerly misinterpreted as truncus arteriosus communis (TAC):

The third element in the segmental situs set, a horizontal dash, that is { , ̶L} indicates absence of a subarterial muscular infundibulum, both right-sided and left-sided: = 0R + ( ̶L). In Equation 27.6 there is an aortopulmonary window (APW) or aortopulmonary septal defect (APSD).

In Equation 27.7 , there is absence of the main pulmonary artery (MPA). Absence of the MPA is fatal to the idea of truncus arteriosus communis. If the MPA is absent, truncus arteriosus communis cannot be present because truncus arteriosus communis means a very large APW. Hence, if the MPA is absent, there can be no APW.

The symbol zero (0) means that there is no subarterial infundibular muscle. Zero means that there is subarterial fibrous tissue. By contrast, the symbol (–) means that the subarterial region is absent, with no muscle and no fibrous tissue. Consequently, the symbols zero and dash have very different meanings. Zero (0) means subarterial fibrous tissue, whereas dash (–) means no subarterial fibrous tissue or muscular tissue, that is, subarterial tissue is absent. For example, there is no subarterial infundibular muscular tissue and there is no subarterial fibrous tissue such as an intervalvar fibrosa.

TOF and AOSPI appear to be interrelated anomalies. TOF is hypoplasia of the subpulmonary infundibulum, whereas AOSPI is absence of the subpulmonary infundibulum (septum and free wall). The foregoing seven infundibular malformations ( Equations 27.1 to 27.7 ) all have infundibuloarterial situs concordance with essentially normally related great arteries in all except Equation 27.7 , in which the MPA is absent (type A2).

The following six equations have different anatomic types of infundibuloarterial situs discordance, resulting in different anatomic types of abnormally related great arteries.

• D-Transposition of the great arteries (D-TGA), physiologically uncorrected:

• L-TGA, physiologically corrected:

• Double-outlet right ventricle (DORV), Taussig-Bing type:

• Double-outlet left ventricle (DOLV), Paul type:

• Anatomically corrected malposition of the great arteries (ACMGA):

These are the most basic equations. Many more could be written to reflect variations in visceroatrial situs (situs inversus, and situs ambiguus) and variations in infundibular development (e.g., TGA {S,D,D} = 4R + 2L with PS), and associated malformations (e.g., ventricular septal defect [VSD]).

The Laws of the Great Arteries

Equations 27.1 to 27.15 illustrate the laws of the great arteries, which, in words, are as follows:

• 1.

  A well-developed subpulmonary infundibular free wall and absence of a subaortic infundibular free wall result in (cause) normally related great arteries, solitus ( Equation 27.1 ) and inversus or inverted ( Equation 27.2 ).

• 2.

  Underdevelopment of the subpulmonary infundibulum causes TOF with variable right ventricular outflow tract obstruction ( Equations 27.3, 27.4, and 27.5 ).

• 3.

  Absence of the subpulmonary infundibulum (septum and free wall) causes what we used to call truncus arteriosus communis (a widespread misinterpretation) ( Equations 27.6 and 27.7 ).

• 4.

  Isolated absence of the aortopulmonary septum, a rare anomaly that I have never seen in person ( Equation 27.8 ).

• 5.

  A well-developed subaortic infundibular free wall and absence of a subpulmonary infundibular free wall (or a relative small subpulmonary infundibular free wall) causes D-TGA and L-TGA ( Equations 27.9 and 27.10 ).

• 6.

  Bilaterally well-developed infundibular free walls, subaortic and subpulmonary, result in DORV of the Taussig-Bing type ( Equation 27.11 ).

• 7.

  Bilateral absence of the subarterial infundibular musculature, both subaortic and subpulmonary, results in DOLV of the Paul type ( Equation 27.12 ).

• 8.

  When the ventricular sinuses or inflow tracts loop in one direction, say to the right, and the subarterial infundibulum and the great arteries twist in the opposite direction, to the left, and the situs of the subarterial infundibulum is different from the situs of the great arteries, this results in ACMGA ( Equations 27.13 and 27.14 ). Because the ventricles and the great arteries twist in opposite directions, these abnormally related and malposed great arteries end up arising from the morphologically appropriate ventricles—aorta arising from the morphologically left ventricle (LV) and pulmonary artery originating from the morphologically right ventricle (RV). Hence, these malposed great arteries are anatomically corrected ( Equations 27.13 and 27.14 ), that is, with ventriculoarterial (VA) alignment concordance. In Equation 27.13 , the great arteries were also physiologically corrected. In Equation 27.14 , the systemic and pulmonary circulations were physiologically uncorrected.

Ventriculoarterial Alignment Concordance

Note that there are two very different anatomic types of VA alignment concordance: (1) with solitus and inversus normally related great arteries ( Equations 27.1 and 27.2 ); and (2) with anatomically corrected malposition of the great arteries (ACMGA), as in Equations 27.13 and 27.14 . ACMGA is rarely possible because the ventricles loop in one direction, and the infundibulum and great arteries twist in the opposite direction.

Normally related great arteries have infundibuloarterial situs concordance, as in Equations 27.1 and 27.2 . ACMGA has infundibuloarterial situs discordance, as in Equations 27.13 and 27.14 .

Note also normally and abnormally developed great arteries ( Equations 27.1 to 27.15 ) display variations in the development of the subarterial infundibular free walls . Variations in the aortopulmonary septum (spiral/straight) are secondary effects.

Morphogenetic Movements of the Great Arteries

The laws or rules governing the development of normal and abnormal great arteries are regarded as fundamental (earlier). Doing infundibuloarterial situs analysis and comparing the results with solitus normal ( Equation 27.1 ) or with inversus normal ( Equation 27.2 ) can be diagnostically helpful. But what is responsible for normal and abnormal morphogenetic movements of the great arteries?