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Infundibuloarterial Situs Equations: How Normally and Abnormally Related Great Arteries Are Built and the Importance of Infundibuloarterial Situs Concordance and Discordance
Now that we have considered tricuspid valve anomalies (see Chapter 13 ), mitral valve anomalies (see Chapter 14 ), and common atrioventricular (AV) canal (see Chapter 11 ), we are ready to consider how normally related and abnormally related great arteries are aligned and connected with the underlying ventricles, ventricular septum, AV canal, and AV valves.
In other words, this chapter is about normally and abnormally related great arteries, all of them . These anomalies are also widely known as conotruncal malformations, that is, anomalies of the conus arteriosus (infundibulum) and truncus arteriosus (great arteries). A new, quantitative, symbolic anatomic approach is taken to the understanding of the embryology (morphogenesis) and anatomy of the infundibuloarterial malformations, using infundibuloarterial equations.
Solitus Normally Related Great Arteries
Solitus normally related great arteries (SNRGAs) means the usual or ordinary type of normally related great arteries, as opposed to the inverted or mirror-image type of normally related great arteries. ( Solitus, a, um = accustomed, usual, habitual, ordinary, in Latin.)
Following the developmental stages of the cardiogenic crescent and the straight heart tube, a solitus (noninverted) heart normally loops in a rightward direction forming a dextro or D-loop ( Fig. 15.1 ).
Dextro is a combining form meaning right, or relationship to the right ( dexter, dextra, dextrum, right, on the right side, Latin). Thus, D-loop means a heart tube that has looped to the right (see Fig. 15.1 )
The effect of D-loop formation is to make the developing semilunar valves to lie approximately side by side, aortic valve (indicated by coronary ostia) to the right and pulmonary valve (no coronary ostia) to the left (see Fig. 15.1 , dashed lines ).
The preconal mesoderm and the conal myocardium are indicated by parallel, finely hatched lines (see Fig. 15.1 ). Early after D-loop formation, a bilateral (subaortic and subpulmonary) conus is present. More specifically, both subaortic and subpulmonary conal free walls are present (see Fig. 15.1 ). Consequently, neither the developing aortic valve nor the developing pulmonary valve is in direct fibrous continuity with either AV valve (the tricuspid valve has three leaflets, and the mitral valve has two leaflets; see Fig. 15.1 ). Also, both developing great arteries are above the developing right ventricle (RV); that is, the double-outlet right ventricle (DORV) is present at this stage.
Key to Symbolic Anatomy
Development of the muscular subarterial conal free walls may be divided into five grades.
Grade 0 (0) means that the subarterial conal free wall musculature is absent, typically permitting semilunar (aortic or pulmonary)-to-AV (mitral, or tricuspid, or common AV) valvar direct fibrous continuity via an intervalvar fibrosa. Typically, there is no ventriculoarterial (VA) outflow tract obstruction (stenosis or atresia), but this feature is a variable.
Grade 1 (1) means that a very small amount of subarterial conal free wall musculature is present, preventing direct semilunar-AV fibrous continuity. Often, a very small amount of unexpanded, or poorly expanded subarterial conal free wall musculature is associated with total VA outflow tract obstruction (atresia).
Grade 2 (2) development of the subarterial conal free wall means that a small amount of subarterial conal free wall musculature is present, preventing semilunar-AV fibrous continuity, and associated with severe VA outflow tract stenosis.
Grade 3 (3) development of the subarterial conal free wall musculature means that a mildly to moderately subnormal development of the subarterial conal free wall is present. Frequently, this mildly to moderately subnormal conal free wall growth and expansion is associated with mild to moderate VA outflow tract stenosis.
Grade 4 (4) growth and expansion of the subarterial conal free wall is normal development that prevents semilunar-AV fibrous continuity and typically is not associated with VA outflow tract obstruction.
D-loop formation typically carries the developing aortic valve and the subaortic conal free wall to the right (R) relative to the developing pulmonary valve and the subpulmonary conal free wall, which lies to the left (L) (see Fig. 15.1 ).
L-loop formation characteristically carries the developing aortic valve and the subaortic conal free wall to the left (L) relative to the developing pulmonary valve and the subpulmonary conal free wall, which lies to the right (R) ( Fig. 15.2 ).
In more complex situations, such as TGA {S,D,L}, TGA {S,D,L} means transposition of the great arteries (TGA) with the segmental anatomic set ({}) of situs solitus (S) of the viscera and atria, ventricular D-loop (D), and L-TGA—the aortic valve and subaortic conal free wall lie to the left (L) of the pulmonary valve and subpulmonary conal free wall, which are located to the right (R).
Similarly, in TGA {S,D,A}, the transposed aortic valve and the subaortic conal free wall are directly anterior (A) relative to the transposed pulmonary valve and subpulmonary conal free wall, which are directly posterior (P) to the aortic valve and subaortic conal free wall.
Thus, we have the segmental anatomic diagnosis, such as
TGA {S,D, A } indicates not only the spatial relationships of the semilunar valves, but also the spatial relationships of the subarterial conal free walls, which usually are right-left (R-L) but occasionally can be directly anteroposterior (AP).
The combination of the degree of development and the relative spatial location of each subarterial conal free wall makes possible the infundibuloarterial (conotruncal) equations. These equations in turn describe the embryonic morphogenesis and the anatomy of normally and abnormally related great arteries, as will be seen.
SNRGAs result from complete right-left asymmetry in the development of the subarterial conal free walls. The right-sided subaortic infundibular free wall normally undergoes complete resorption (disappearance), and simultaneously the left-sided subpulmonary conal free wall undergoes good expansile growth. Both subarterial conal free wall resorption and growth are thought to be under genetic control, which is now undergoing intense investigation.
With SNRGA (see Fig. 15.1 ), resorption of the right-sided subaortic conal free wall permits the developing aortic valve to move inferiorly, posteriorly, and leftward and to pass mostly through the interventricular foramen and to come into direct fibrous continuity with the developing mitral valve, above the morphologically left ventricle (LV).
Simultaneously, the left-sided subpulmonary conal free wall undergoes good growth and expansion, elevating the pulmonary valve superiorly, anteriorly, and rightward, above the morphologically RV. The growth and expansion of the subpulmonary conus elevates the pulmonary valve and artery superiorly, getting the pulmonary valve and main pulmonary artery (MPA) out of the way, well above the interventricular foramen, making it possible for the aortic valve and the proximal ascending aorta to be switched from above the RV to above the LV.
SNRGAs show how the embryonic aortic switch procedure normally is done. There is only one way of doing it right—with solitus and inversus isomers (see Figs. 15.1 and 15.2 , respectively). There are many ways of doing the embryonic aortic switch wrong, and they all result in a conotruncal malformation, as will be seen.
In words, this equation means that after D-loop formation, SNRGAs result from complete resorption of the right-sided subaortic conal free wall plus good development and expansion of the left-sided subpulmonary conal free wall.
Inverted Normally Related Great Arteries
Inverted normally related great arteries (INRGAs) result from a mirror-image of the “recipe” for SNRGA (see Fig. 15.2 ):
In words, this equation means that after L-loop formation, INRGAs result from good development and expansion of the right-sided subpulmonary conal free wall plus involution of the left-sided subaortic conal free wall (permitting aortic-mitral continuity).
Transposition of the Great Arteries
TGA results from reversed right-left asymmetry in the development of the subarterial conal free walls. Typical D-TGA has good development of the right-sided subaortic conal free wall and resorption of the left-sided subpulmonary conal free wall (see Fig. 15.1 ). Consequently, the D-transposed aorta is elevated superiorly and anteriorly above the RV, and the pulmonary valve and MPA move inferiorly and posteriorly, passing mostly through the interventricular foramen, and the pulmonary valve typically comes into direct fibrous continuity with the mitral valve, above the LV. So in D-TGA, the wrong great artery gets switched from RV to LV: the MPA gets switched instead of the aorta.
The foregoing morphogenesis and pathologic anatomy of typical D-TGA may be expressed in the following equation:
In words, this equation means that after D-loop formation, typical D-TGA results from and is characterized by good development of the right-sided subaortic conal free wall and by involution of the left-sided subpulmonary conal free wall (permitting pulmonary-mitral fibrous continuity). Note that typical D-TGA is characterized by R-L reversal of the subarterial conal free wall development when compared with that of SNRGA: SNRGA {S,D,S} = 0R + 4L.
Thus, typical D-TGA results from conal free wall inversion (R-L reversal). In typical D-TGA, this is why the wrong great artery gets switched. Typical L-TGA (see Fig. 15.2 ) is a mirror-image of typical D-TGA (see Fig. 15.1 ):
In words, this equation means that after L-loop formation (see Fig. 15.2 ), the equation or “recipe” for typical L-TGA is right-sided subpulmonary conal free wall resorption and an arterial switch of the MPA through the interventricular foramen from the left-sided RV into the right-sided LV with pulmonary-to-mitral fibrous continuity and good development of the left-sided subaortic conal free wall with elevation of the left-sided aorta superiorly and anteriorly above the left-sided RV.
In typical L-TGA with a left-sided subaortic muscular conus and no right-sided subpulmonary muscular conus, there is R-L reversal of conal free wall development compared with what is normal for a ventricular L-loop (see Fig. 15.2 ):
whereas
The conus in typical L-TGA has sometimes been described as “inverted for situs inversus.” However, the definition of inversion breaks down in visceroatrial situs inversus. As the equation for typical L-TGA indicates, the conal anatomy in typical L-TGA appears the same as that for SNRGAs!
, and
Consequently, we have avoided conal inversion as a general definition of TGA. Conal inversion is accurate in visceroatrial situs solitus. But, in visceroatrial situs inversus, “conal inversion for situs inversus” does not work well because the conal equation for L-TGA is noninverted, the same as in SNRGA.
Instead, it seems clearer to say that TGA results from reversed R-L conal free wall asymmetry, bearing in mind what is normal for the situs of the semilunar valves that is present.
Situs in anatomy means the pattern of anatomic organization, that is, solitus (noninverted) or inversus (a mirror-image of solitus).
In L-TGA {S,L,L}, the great arteries are inverted with aorta to the left and pulmonary artery to the right, as the great arterial symbol in the segmental anatomic set indicates:
So the question is: What type of subarterial conus is normal for inverted (L-positioned) great arteries? The answer is the type of conus found with INRGAs:
The type of conus typically found in L-TGA is:
Therefore, in L-TGA (which is an inverted transposition), the conal situs is R-L reversed compared with the normal conal situs for INRGAs arteries: 0R + 4L (in L-TGA) compared with 4R + 0L (in INRGA). So, typical L-TGA {S,L,L} has infundibuloarterial (conotruncal) situs discordance (oppositeness). These inverted great arteries have a noninverted conal connector. Consequently, the great arteries are abnormally connected and hence are abnormally aligned with the underlying ventricles and AV junction and are abnormally positioned in space (abnormally related).
Infundibuloarterial (conotruncal) anomalies are malformations of right-left conal free wall laterality, as D- and L-TGA exemplify.
A structurally normal heart is characterized by segmental situs uniformity . The segmental situs anatomy of the solitus normal heart is {S,D,S}. The three main cardiac segments {atria, ventricles, great arteries} are all in situs solitus. So too are the two connecting segments: the AV canal or junction and the infundibulum or conus arteriosus. The solitus normal heart displays segmental situs uniformity. So, too, does the inverted normal heart: {I,L,I}. The three main cardiac segments, {I,L,I}, and the two connecting cardiac segments, the AV valves and the conus arteriosus, are all inverted. Consequently, there is AV concordance and VA concordance, as in situs inversus totalis, because there is segmental situs uniformity: all segments are in situs inversus.
But complex congenital heart disease is characterized by segmental situs nonuniformity . All of the cardiac segments do not have the same pattern of anatomic organization (situs), either all solitus, or all inversus. Instead, complex congenital heart disease is characterized by a segmental situs “salad” (mixture), as the infundibuloarterial equations make very clear.
D-TGA typically is characterized by infundibuloarterial situs discordance (oppositeness). D-TGA {S,D,D} has noninverted great arteries. The aorta is to the right of the pulmonary artery, as the segmental anatomy indicates: TGA {S,D, D} .
What kind of subarterial conus is normal for noninverted (D-positioned) great arteries? The kind of conus that is present in the solitus normal heart. What’s that?
What kind of conus is found in typical D-TGA?
So, as we can see, the pattern of anatomic organization of the conus in typical D-TGA (4R + 0L) is R-L reversed compared with the solitus normal conus (0R + 4L).
Consequently, TGA (both D- and L-) is characterized by infundibuloarterial situs discordance, whereas normally related great arteries (solitus and inversus) have infundibuloarterial situs concordance.
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