Infundibular situs

A solitus infundibulum has absence of the right-sided normally subaortic infundibular free wall musculature and well-developed left-sided normally subpulmonary infundibular free wall musculature ( Figure 1 ).

Normal and abnormal anatomy and development of the solitus (noninverted) heart, presented diagrammatically. (Top row) The straight heart tube normally loops convexly to the right forming a dextro- or D-loop, seen from the front. (Second row) The great arteries and the infundibulum, seen from the front. Infundibular or conal musculature indicated by hatching. (Third row) The semilunar valves, the infundibulum, and the AV valves, viewed from below. Broken lines indicate that the Ao and the PA are not 2 separate structures at these early stages; they are so depicted for clarity of understanding. The aortic valve is indicated by the coronary ostia. The pulmonary valve has no coronary ostia. The mitral valve (MV) is bicuspid, and the tricuspid valve (TV) is tricuspid. (Bottom row) Column descriptions. Rightmost column : At the straight tube stage, the AoV is believed to be anterior (ventral) relative to the developing PV. First column to the left : D-loop formation carries the developing AoV and PV through approximately 90° of dextrorotation relative to the sagittal plane. Second column to the left : SNRGA. Note that the solitus (noninverted) development of the conus (infundibulum) is characterized by complete R-L asymmetry with resorption of the right-sided subaortic conal free wall, making possible AoV-MV direct fibrous continuity and simultaneous growth of the left-sided subpulmonary conal free wall. Third column to the left : D-TGA with right-sided subaortic conus and no left-sided subpulmonary conus, permitting PV-MV direct fibrous continuity. The conal connector with D-TGA is inverted; it is a mirror image of the conus with SNRGA. Fourth column to the left : A bilateral conus occurs more frequently with DORV {S,D,D} in which the great arteries are usually in D-malposition (D-MGA). A bilaterally well-developed conus is in situs ambiguus. Why? Because the left-sided subpulmonary conus is like the normal solitus conus, and the right-sided subaortic conus is like the inverted conus. Fifth column to the right : A rare case of D-TGA is shown with a bilaterally deficient conus that permitted AoV-TV and PV-MV direct fibrous continuity. A = undivided atria; Ant = anterior; BC = bulbus cordis; Inf = inferior; Lt = left; Post = posterior; Rt = right; Sup = superior.

Modified and reproduced with permission from Van Praagh and Van Praagh.

An inversus infundibulum has absence of the left-sided infundibular free wall musculature and well-developed right-sided infundibular free wall musculature ( Figure 2 ).

Inverted anatomy and development of the inverted heart. This is essentially a mirror image of Figure 1 . Note that L-loop formation carried the Ao and the PA through approximately 90° of levorotation relative to the sagittal plane. The completely asymmetrical conal development of INRGA carries the semilunar valves through their final approximately 60° of levorotation to 150° rotation to the left relative to the sagittal plane, which is typical of INRGA (column 2 to the right) . The conal connector in L-TGA is a mirror image of the conus in INRGA and is the same as the conus in SNRGA ( Figure 1 ). A = undivided atria; Ant = anterior; BC = bulbus cordis; Inf = inferior; Lt = left; Post = posterior; Rt = right; Sup = superior.

Modified and reproduced with permission from Van Praagh and Van Praagh.

The infundibulum can have an ambiguous situs pattern—neither solitus nor inversus. Infundibular free wall musculature can be bilaterally present (both subaortic and subpulmonary) or bilaterally absent or very deficient (neither subaortic nor subpulmonary) ( Figure 1 ).

Situs of the great arteries

When the aortic valve (AoV) and the ascending aorta (Ao) are to the right of the pulmonary valve and the main pulmonary artery (MPA), the great arteries are in situs solitus, the noninverted pattern ( Figures 1, 3–5 ).

Types of human heart in terms of segmental anatomy. The cardiac diagrams are viewed from below, similar to a subxiphoid 2-dimensional echocardiogram. The cardiac structures are named morphologically, not positionally: RA = morphologically right atrium; LA = morphologically left atrium; RV = morphologically right ventricle; LV = morphologically left ventricle; Inf = infundibulum or conus arteriosus; aortic valve, identified by coronary ostia; and pulmonary valve identified by absence of coronary ostia. The 3 main cardiac segments are the atria, the ventricles, and the great arteries. The 2 connecting cardiac segments are the AV canal or junction and the infundibulum or conus arteriosus. The 3 main cardiac segments may be expressed as elements of segmental situs sets. Braces {} mean “the set of.” The 3 main cardiac segments are listed sequentially, in blood flow order: {atria, ventricles, great arteries}. The atria may be in situs solitus {S,-,-}, in situs inversus {I,-,-}, or in situs ambiguus {A,-,-} which is not included in this figure. The ventricular situs may be D-loop or situs solitus {-,D,-} or L-loop or situs inversus {-,L,-}. When normally related, the great arteries may be in situs solitus {-,-,S} or in situs inversus {-,-,I}. The main cardiac segments may be aligned with each other in many different ways, but they do not connect with each other tissue to tissue because of the interposition of the connecting segments. Consequently, we speak of segmental alignments, not connections. The AV alignments are concordant in column 1 , {S,D,-}; discordant in column 2 , {S,L,-}; concordant in column 3 , {I,L,-}; and discordant in column 4 , {I,D,-}. The ventriculoarterial (VA) alignments are normal in rows 1 to 4 : solitus normal {-,-,S} or inverted normal {-,-,I}. In rows 5 to 8 , the VA alignments are abnormal, although in 1 of them, ACMGA, row 6 , the VA alignments are concordant, although abnormal, because the ventricular segment has looped in 1 direction, and the infundibuloarterial cardiac segment has twisted in the opposite direction. For example, in ACM {S,D,L}, the ventricles looped to the right, but the conotruncus twisted to the left (row 6, column 1) . The key to normally related great arteries is infundibuloarterial situs concordance. The cause of abnormally related great arteries is infundibuloarterial situs discordance or ambiguity.

Reproduced with permission from Foran et al.

Normal development of the semilunar valves, presented diagrammatically, seen from above. The semilunar valvar endocardial cushions of the TA are numbered conventionally. Normally, truncal valvar cushions 1 and 3 fuse, dividing the quadricuspid truncal valve into the aortic valve anteriorly (A) and the pulmonary valve posteriorly (P). Both aortic and the pulmonary valves have 2 septal leaflets (1 and 3), adjacent to the aorticopulmonary septum that separates the aortic and pulmonary valves. Both aortic and pulmonary valves also normally have nonseptal (or intercalated) leaflets remote from the aorticopulmonary septum (aortic leaflet 4 and pulmonary leaflet 2).

Reproduced with permission from Van Praagh and Vlad.

(A) Morphologically right ventricle. (B) Morphologically left ventricle. Is this RV a D-loop (solitus or noninverted) RV? The answer is yes and no. Components 1, 2, and 3 are right handed (D-loop or solitus). However, component 4 is in situs solitus only if the great arteries are solitus normally related, as they are here. However, when D-TGA is present, component 4 is inverted. Therefore, in D-TGA, components 1, 2, and 3 are solitus, but component 4 is inverted. Therefore, in D-TGA, the RV is partly solitus and partly inverted. When DORV or DOLV is present, components 1, 2, and 3 are solitus, but component 4 can be ambiguous, neither solitus nor inversus. Hence, the RV in DORV and in DOLV may not be all in situs solitus because of the situs ambiguity of component 4. In (A) and (B) , component 1 is the AV canal or junction (AV septum and AV valves). Component 2 is the ventricular sinus. Component 3, in (A) is the septal band, the proximal or apical part of the infundibular (conal) septum. Component 3, in (B) , is the smooth, nontrabeculated proximal conal septum. The RV component 3 typically is continuous with the LV component 3. Component 4 is the distal or subarterial part of the infundibulum (or conus), consisting of the infundibular septum and the infundibular free wall which may be well developed, poorly developed, or absent. Because of the composite nature of the RV and the LV, an analytic approach is required for their understanding, component by component.

Reproduced with permission from Van Praagh et al.

When the AoV and the ascending Ao are to the left of the pulmonary valve and the MPA, the great arteries are in situs inversus ( Figures 2 and 3 ).

When the AoV is directly anterior (ventral) to the pulmonary valve—neither to the right nor to the left, the great arteries are in situs ambiguus—neither solitus nor inversus.

Grading infundibular development

It is helpful to grade infundibular muscular free wall development from absent (grade 0) to well developed (grade 4).

Infundibular arterial situs equations

Equation 1 says: solitus normally related great arteries (SNRGA) with a segmental anatomic situs set of solitus atria, D-loop (solitus) ventricles, and SNRGA equals grade 0 (absent) development of the right-sided subaortic infundibular free wall plus grade 4 (normal) development of the left-sided subpulmonary infundibular free wall.

Which great artery is right sided and which is left sided are indicated by the great arterial symbol in the segmental anatomy: {S,D,S}. Diagrams also help: Figure 1 (third column from the right) , Figure 3 (row 1, column 1) , Figures 4 and 5 .

Equation 2 says: inverted normally related great arteries (INRGA) with a segmental anatomic situs set of inverted atria, L-loop (inverted) ventricles, and INRGA equals grade 4 (normal) right-sided development of the subpulmonary infundibular free wall myocardium plus grade 0 (absence) of the left-sided subaortic infundibular free wall myocardium. The normal inverted infundibulum is shown in Figure 2 (third column from the right) and in Figure 3 (row 1, column 3) .

Equations 1 and 2 are the frames of reference concerning the solitus normal infundibulum and the inversus normal infundibulum, respectively.

In both anatomic types of normal, the infundibular situs and the great arterial situs are both the same. Both are solitus in equation 1 , and both are inverted in equation 2 .

Thus, normally related great arteries, solitus and inversus, are characterized by infundibuloarterial (IA) situs concordance (sameness).

Transposition of the great arteries

Equation 3 says: transposition of the great arteries (TGA) with the segmental anatomic situs set of solitus atria, D-loop (solitus) ventricles, and D-TGA (solitus TGA) equals a well-developed right-sided subaortic infundibular free wall plus absence of a left-sided subpulmonary infundibular free wall. D-TGA ( equation 3 ) is typical physiologically uncorrected TGA.

Typical D-TGA is depicted diagrammatically in Figure 1 (third column from the right) and in Figure 3 (row 5, column 1) .

Equation 4 says: TGA with the segmental anatomic situs set of solitus atria, L-loop (inverted) ventricles, and L-TGA (inverted TGA) equals absence of right-sided subpulmonary infundibular free wall myocardium plus a well-developed left-sided subaortic infundibular free wall. Equation 4 is typical physiologically corrected TGA.

Typical L-TGA is shown diagrammatically in Figure 2 (second column from the right) and in Figure 3 (row 5, column 2) .

Typical D-TGA ( equation 3 ) has an inverted infundibulum (4R + 0L), the same as in equation 2 , the inverted normal heart. In typical D-TGA, the great arteries are in situs solitus . Therefore, IA situs analysis in typical D-TGA is inversus-solitus, that is, discordant.

Typical L-TGA ( equation 4 ) has the reverse: a solitus infundibulum (0R + 4L), the same as in the solitus normal heart ( equation 1 ), and inverted great arteries. Therefore, IA situs analysis is solitus-inversus, that is, discordant.

Double-outlet right and left ventricles

Equation 5 represents the Taussig-Bing type of DORV, and equation 6 represents the Paul type of DOLV.

In the Taussig-Bing type of double-outlet right ventricle (DORV), both the right-sided subaortic and the left-sided subpulmonary infundibular free walls are well developed, as equation 5 indicates.

In the Paul type of double-outlet left ventricle (DOLV), there is bilateral absence of subarterial infundibular myocardium, as equation 6 indicates.

IA situs analysis in the Taussig-Bing type of DORV ( equation 5 ): ambiguus-solitus (discordant).

IA situs analysis in the Paul type of DOLV ( equation 6 ): ambiguus-solitus (discordant).

Remember that situs ambiguus is not a specific type of situs. Ambiguus means uncertain or unknown.

Anatomically corrected malposition of the great arteries

Anatomically corrected malposition of the great arteries (ACMGA) is characterized by malposition of the great arteries in which each great artery nonetheless arises above the morphologically appropriate ventricle—Ao above the morphologically left ventricle (LV), and pulmonary artery (PA) above the morphologically right ventricle (RV). This is malposition of the great arteries (Ao anterior and PA posterior), with ventriculoarterial alignment concordance. That is why this rare anomaly is called anatomically corrected malposition of the great arteries.

But how can that happen?

In all well-documented cases of ACMGA, to the best of my knowledge, the ventricles loop in 1 direction, say to the right, and the IA segment twists to the left, as in ACMGA {S,D,L} ( Figure 3 , row 6, column 1 ).

TOF and truncus arteriosus (TA) are 2 important IA anomalies that have IA situs concordance. In TOF, the great arteries are almost normally related. There is aortic-mitral fibrous continuity, but it is often tenuous (not as tight as normal), and hence, there is aortic overriding. The PA relates to the RV, but the PA is abnormally leftward, posterior, and inferior. The mild abnormality in the relation between the great arteries in typical TOF is due to hypoplasia of the subpulmonary infundibulum that results in subnormal semilunar dextrorotation. TOF is a subnormality: in the caliber of the PA outflow tract and in the degree of semilunar dextrorotation.

TOF may be represented by the following 3 equations:

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