Absence of the Subpulmonary Infundibulum With Its Sequelae Has Been Misinterpreted as Common Aortopulmonary Trunk

As classically conceived, truncus arteriosus communis (common aortopulmonary trunk) seems not to exist.

The classical embryologic conception is that the truncoconal septum has failed to grow downward from the aortic arch 4–pulmonary arch 6 junction. This results in a failure of septation (or separation) of the ascending aorta and main pulmonary artery, of the aortic and pulmonary valves, and of the subarterial pulmonary and aortic outflow tracts.

The anatomic definition of so-called “truncus arteriosus communis” is as follows: one great artery that arises from the base of the heart and gives rise to the coronary arteries (at least one), to the pulmonary branches (at last one), and to the systemic arteries.

Classification. In 1965, based on a study of 57 autopsied cases, we endeavored to classify “truncus arteriosus communis” (“TAC”) ( Fig. 21.1 ). We used this classification in later studies ( Fig. 21.2 ).

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Absence of the subpulmonary infundibulum and associated anomalies. Type A means that a subarterial ventricular septal defect is present. Type A1 indicates that a small main pulmonary artery (MPA) is present with a low aortopulmonary (AP) septal defect and a remnant of the AP septum. Type A2 means that the MPA is absent and that both pulmonary artery branches arise from the aortic sac on the dorsal surface of the ascending aorta. Type A3 is the same as type A2, except that one pulmonary branch is absent, the left in this case. Collateral arteries supply the lung without a pulmonary artery branch. Type A4 has a hypoplastic ascending aorta, an interrupted aortic arch, and a large MPA, large pulmonary arterial branches, and a large ductus arteriosus. We used to think, as did many others, that such hearts had truncus arteriosus communis. But now we have realized that truncus arteriosus, as classically conceived, probably does not exist. See the text.

Reproduced with permission from Calder L, Van Praagh R, Van Praagh S, et al. Truncus arteriosus communis: clinical, angiocardiographic, and pathologic findings in 100 patients, Am Heart J 1976;92:23.

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What is the subpulmonary infundibulum that appears to be absent in so-called truncus arteriosus communis? Perhaps the best way to make this clear is to review the segmental composition of the normal human heart. (A) is a diagram of the morphologically right ventricle (RV). (B) is a diagram of the morphologically left ventricle (LV). The normal human heart is composed of four segments, i.e., developmental and anatomic components: segment 1 is the atrioventricular canal or junction; segment 2 is the ventricular sinus, or body, or inflow tract, coarsely trabeculated on the RV side and finely trabeculated on the LV side; segment 3 is the proximal or apical part of the infundibulum or outflow tract, on the RV side known as the septal band that apically is continuous with the moderator band, and on the LV side is the smooth nontrabeculated part of the upper LV septal surface; and segment 4 is the subpulmonary part of the infundibulum or outflow tract, which has both septal and free-wall parts bilaterally. This is the subarterial infundibulum that is absent in “truncus arteriosus communis:” segment 4.

Reproduced with permission from Van Praagh R. What determines whether the great arteries are normally or abnormally related? Am J Cardiol 2016;118:1390.

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In “TAC” type A1, type A indicates that a subarterial ventricular septal defect (VSD) is present, and type 1 indicates the presence of an aortopulmonary septal remnant, and a small low aortopulmonary septal defect (or anteroposterior [AP] window) ( Fig. 21.1 ). Note also that there is a large aorta and a relatively small main pulmonary artery.

The subpulmonary infundibulum, normally above the septal band, is absent ( Fig. 21.3 ).

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Absence of the subpulmonary infundibulum type A1. This is the heart of a 4.75-month-old boy. A ventricular septal defect (VSD) is located between the aortic valve above and the septal band (SB) and the ventricular septum below. Normally the subpulmonary infundibulum is located beneath the pulmonary valve and above the septal band. But in this case, the pulmonary valve is absent and there is nothing above the septal band. The three aortic leaflets are moderately thick: right coronary leaflet (RC), the noncoronary leaflet (NC), and the left coronary leaflet (LC). The ascending aorta (Ao) with a right aortic arch is large. The main pulmonary artery (MPA) is relatively small. The left pulmonary artery (LPA) and the right pulmonary artery ( RPA) are also seen. The aortopulmonary septum (A-P Sept) is defective. The ascending Ao and the MPA are not completely separated because of the presence of a low AP window. From the right ventricular aspect, the inferior rim of the subaortic VSD is formed by the top of the septal band that extends from its left anterior division (LA Div) to its right posterior division (RP Div). ML, Muscle of Lancisi, or muscle of Lushka, or papillary muscle of the conus. Absence of the subpulmonary infundibulum is thought to be developmentally responsible for absence of the pulmonary valve, for the low AP window, and for the VSD.

Reproduced with permission from Van Praagh R, Van Praagh S. The anatomy of common aorticopulmonary trunk [truncus arteriosus communis] and its embryologic implications, a study of 57 necropsy cases, Am J Cardiol 1965;16:406.

If the classical nonseptation hypothesis were correct, one would expect to find an infundibular septal defect (not subpulmonary infundibular absence). The pulmonary valve also was absent, also not consistent with a failure of septation ( Fig. 21.3 ). The semilunar valve had three moderately thick leaflets, indistinguishable from an aortic valve (also inconsistent with the nonseptation embryologic hypothesis).

In “TAC” type A1, the appearance of the right ventricle (RV) outflow tract ( Fig. 21.3 ) was very similar to the appearance of the RV outflow tract in tetralogy of Fallot (TOF) with subpulmonary infundibular atresia ( Fig. 21.4 ). In fact, we could not reliably tell one from the other, unless we looked at the great arteries. One is comparing infundibular stenosis and atresia in TOF ( Fig. 21.4 ) with infundibular absence in TAC type A1 ( Fig. 21.3 ).

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Tetralogy of Fallot (TOF): (A) with severe infundibular stenosis, and (B) with infundibular atresia, opened right ventricular views. The subpulmonary infundibulum in TOF is present, but hypoplastic in A and hence hemodynamically stenotic, or in B even more hypoplastic with no lumen and hence atretic. But in Fig. 21.3 , the infundibulum is absent. However, the appearance of the infundibulum in TOF with subpulmonary infundibular atresia ( Fig. 21.4B ), and in these cases with infundibular absence ( Fig. 21.3 ), is very similar. This is why we think these patients with infundibular absence (types A1, A2, and A3, in Fig. 21.1 ) are closely related to tetralogy of Fallot ( Fig. 21.4 ).

Reproduced with permission from Van Praagh R, Van Praagh S. The anatomy of common aorticopulmonary trunk [truncus arteriosus communis] and its embryologic implications, a study of 57 necropsy cases, Am J Cardiol 1965;16:406.

This extreme similarity between “TAC” type A1 and TOF-atresia strongly suggests that these two anomalies are closely related developmentally and anatomically.

“TAC” type A2 ( Fig. 21.1 and Fig. 21.5 ) has no aortopulmonary septal remnant, and both pulmonary artery branches arise from the posterior surface of the arterial trunk.

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Absence of the infundibulum type A2. This was the heart of a 1-year-old boy. Note that there is no suggestion of a subarterial infundibulum beneath the three-leaflet aortic valve, or above the septal band, or elsewhere. The unmigrated right pulmonary artery (RPA) and the unmigrated left pulmonary artery (LPA) both arise from the aortic sac on the dorsal surface of the ascending aorta. The main pulmonary artery (MPA) is absent. That is thought to be why the RPA and LPA have not migrated from the aortic sac. Normally, the RPA and the LPA migrate from the aortic sac to the MPA. But when the MPA is absent, the RPA and the LPA have nowhere to migrate to. Absence of the subpulmonary infundibulum is considered developmentally responsible for absence of the pulmonary valve, absence of the main pulmonary artery, and nonmigration of the pulmonary artery branches. What is the basic difference between type A1 and type A2? Hypothesis: Both have absence of the subpulmonary infundibulum. Type A1 has an anteroposterior (AP) window, facilitating blood flow in the MPA. Type A2 did not have an AP window. Hence, no blood flow in the MPA, leading to hypoplasia, then atresia, then involution (disappearance) of the MPA. Or the MPA may never have formed.

Reproduced with permission from Van Praagh R, Van Praagh S. The anatomy of common aorticopulmonary trunk [truncus arteriosus communis] and its embryologic implications, a study of 57 necropsy cases, Am J Cardiol 1965;16:406.

Note that again the subpulmonary infundibulum beneath the semilunar valve and above the septal band is absent .

In “TAC” type A1 ( Fig. 21.3 ), note that the width of the arterial trunk below the origins of the pulmonary artery branches is wider than above the pulmonary artery branches. In “TAC” type A2 ( Fig. 21.5 ), the width of the arterial trunk below and above the origins of the pulmonary artery branches is the same. In “TAC” type A1 ( Fig. 21.3 ), the arterial trunk is wider below the pulmonary arteries than above them because in type A1, a small main pulmonary artery is present, whereas in type A2, there is no anatomic evidence to support the presence of a main pulmonary artery.

Nonetheless, in type A2, it has long been suspected that because the pulmonary artery branches both arise from the arterial trunk, the main pulmonary artery “must” be there too, despite the absence of supporting anatomic evidence.

In the early 2000s, Dr. Alfredo Vizcaino from the Department of Cardiology of the Hospital Infantil de Mexico, in Mexico City, brought us a fascinating case in consultation ( Fig. 21.6 ) that we published in 2002. The heart was normal, except that the right pulmonary artery (RPA) and the left pulmonary artery (LPA) arose from a single origin (the aortic sac) on the dorsal surface of the ascending thoracic aorta. The other rare finding was a normal sized main pulmonary artery (MPA), with no branches, that emptied through a patent ductus arteriosus into the descending thoracic aorta ( Fig. 21.6 ).

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Origin of the right pulmonary artery (RPA) and the left pulmonary artery (LPA) from the aortic sac of the ascending aorta, and origin of a nonbranching main pulmonary artery (MPA) from the right ventricle (RV). This rare anomaly shows that initially, the RPA and the LPA normally arise from the aortic sac, which we called the common pulmonary artery (CPA). When the ventral sixth aortic arches develop by 26 to 30 days of age in utero, the RPA and LPA then migrate out on the sixth arches and become confluent with the MPA. But if the sixth aortic arches fail to develop, then the RPA and the LPA cannot migrate to the MPA. This results in unmigrated RPA and LPA still arising from the aortic sac, and an MPA with no branches. The circulation in this case of Vizcaino is similar to that in the absent infundibulum type A2 patient ( Fig. 21.5 ), with one important difference: In this case ( Fig. 21.6 ), the pulmonary blood flow was normal (no pulmonary outflow tract obstruction, and no VSD). But in the infundibular absence type A2 case ( Fig. 21.5 ), there was no qP (pulmonary blood flow) because the MPA was absent. Dr. Vizcaino’s case ( Fig. 21.6 ) made it possible to understand the anatomy of absent infundibulum type A2 ( Fig. 21.5 ). In the case of Vizcaino, the segmental anatomy was normal, i.e., {S,D,S}. The great arteries were solitus normally related. But the associated anomalies resulted in a rare form of partial transposition of the circulations. Because the MPA had no pulmonary artery branches and because there was preductal coarctation of the aorta, the lower body was perfused with unoxygenated systemic venous blood. The RPA and the LPA in Vizcaino’s case perfused the lungs with oxygenated blood (as in typical D-transposition of the great arteries). The upper body was perfused with oxygenated blood. Differential cyanosis was not described (upper versus lower body). The danger of giving a baby with type A4 anomalies oxygen merits mention. Oxygen tends to make the ductus arteriosus close, depriving the lower body of adequate circulation. The left ventricle (LV) would not have been able to alleviate this situation because of the coexistence of preductal coarctation.

Reproduced with permission from Vizcaino A, Campbell J, Litovsky S, Van Praagh R. Single origin of right and left pulmonary artery branches from ascending aorta with nonbranching main pulmonary artery: relevance to a new understanding of truncus arteriosus, Pediatr Cardiol 2002;23:230.

Beitzke and Shinebourne published such a case in 1980, and Aotsuka and colleagues published a second such case in 1990. But it was Vizcaino’s case, the third known example of this anomaly, and the first that we had examined personally, that really opened our eyes.

We realized that the normal pulmonary artery and its right and left branches have a bipartite origin:

• 1.

  MPA from the truncus arteriosus; and

• 2.

  both right and left pulmonary artery branches from the aortic sac on the dorsal surface of the ascending thoracic aorta ( Fig. 21.6 ).

The RPA and the LPA arise initially from the aortic sac of the ascending aorta at the 4-mm stage (24 to 26 days after conception in utero), before the sixth embryonic arterial arches have completely formed.

Completion of both right and left sixth embryonic arches can occur as early as the 5-mm stage (26 to 28 days of age), and usually has occurred by the 6-mm stage (28 to 30 days of age).

As soon as both right and left sixth arches have been completely formed, normally they enlarge considerably and both pulmonary artery branches migrate out on to the sixth arches and join the MPA part of the truncus arteriosus.

But if the ventral part of both sixth arches fails to form, this prevents migration of both the RPA and the LPA.

Another hypothesis is that there may be abnormal growth of the wall of the aortic sac, preventing formation of the sixth arches.

It has long been known embryologically that the RPA and the LPA initially arise from the aortic sac on the posterior aspect of the ascending aorta. The anatomic finding of origin of both pulmonary arterial branches from the posterior surface of the ascending aorta with a nonbranching MPA proves that the aforementioned embryologic understanding is correct.

In Vizcaino’s case, why did the right and left pulmonary artery branches remain in their original unmigrated origins from the aortic sac?

We hypothesize that the embryonic ventral sixth arches failed to develop. Vizcaino’s patient was a 2-day-old female infant, with a birth weight of 2200 grams, who had visceral heterotaxy with polysplenia and multiple congenital anomalies. For whatever reason, the right and left pulmonary artery branches did remain in their initial, unmigrated positions, both arising from the aortic sac ( Fig. 21.6 ).

In Vizcaino’s patient, why was the unbranched MPA of such good, normal size?

Because the heart was otherwise normally formed. There was no pulmonary outflow tract stenosis and no VSD ( Fig. 21.6 ).

Given that we think that the diagnosis of “TAC” type A2 is wrong, what is the correct diagnosis ( Fig. 21.5 )?

Revised diagnosis:

• Absence of the subpulmonary infundibulum

• Absence of the pulmonary valve

• Absence of the MPA

• Subaortic VSD

• Origin of unmigrated pulmonary artery branches from aortic sac

• Absent ductus arteriosus

This case, too, is like a tetralogy of Fallot, only worse. (That’s what I mean by extreme tetralogy of Fallot.) The pulmonary artery branches are in their original unmigrated positions. Since the MPA was absent, the pulmonary artery branches had nowhere to migrate to.

“TAC” type A2 ( Fig. 21.5 ) is a solitary aortic trunk (truncus aorticus solitarius), not a common aortopulmonary trunk, because the subpulmonary infundibulum, the pulmonary valve, the MPA, and the ductus arteriosus are all absent in this 1-month-old boy.

What is so-called “TAC” type A3 ( Fig. 21.1 )?

It is the same as type A2, except that one pulmonary artery branch is missing. So, type A3 is a solitary aortic trunk (not a common aortopulmonary trunk), with absence of a pulmonary artery branch. A patent ductus arteriosus supplied the left lung in one case. The patient was a 3-week-old boy. He had a pulmonary leaflet remnant in a quadricuspid semilunar valve with right coronary, left coronary, noncoronary, and pulmonary leaflets. We thought that aortic stenosis and insufficiency were probable. There was right ureterovesical stenosis with right hydroureter and right hydronephrosis.

In so-called “TAC” type A4 ( Fig. 21.7 ), a large MPA arises from the base of the heart, the pulmonary arterial branches are large, and the ductus arteriosus is large, but closing. The ascending aorta is small, about the caliber of the RPA. The ascending aorta arises well above the semilunar valve, at about the height of the RPA. The ascending aorta looks like a small branch arising from the right side of the MPA. The isthmus of the aortic arch is interrupted (absent).

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Absence of the subpulmonary infundibulum type A4. A large main pulmonary artery arises from the base of the heart. The subarterial ventricular septal defect (VSD) is well seen immediately beneath the semilunar valve that has three leaflets: the left coronary leaflet, a combined noncoronary and right coronary leaflet (with a poorly developed noncoronary–right coronary commissure), and a pulmonary leaflet. The semilunar leaflets are markedly thickened, fleshy, soft, and polypoid. Note that there is no subpulmonary infundibulum between the semilunar valve above and the left anterior division (LA Div) of the septal band (SB) below. The right pulmonary artery (RPA) and the left pulmonary artery (LPA) are enlarged. The small ascending aorta (Ao) arises well above the semilunar valve. The Ao is about the same size as the RPA. The aortic arch is interrupted. The ductus arteriosus is large and appeared to be closing in this 5-day-old boy. The right coronary ostium is high, above the poorly developed right coronary–noncoronary commissure. Absence of the subpulmonary infundibulum is associated with both types of reciprocal development of the great arteries: (a) large aorta, and small or absent main pulmonary artery and ductus arteriosus in types A1, A2, and A3; and (b) large main pulmonary artery and ductus arteriosus, with small or interrupted aortic arch in type A4, exemplified by this case.

Reproduced with permission from Van Praagh R, Van Praagh S. The anatomy of common aorticopulmonary trunk [truncus arteriosus communis] and its embryologic implications, a study of 57 necropsy cases, Am J Cardiol 1965;16:406.

A large VSD is present, between the semilunar valve above and the septal band and ventricular septum below. Since the right posterior division of the septal band is prominent, the VSD is not confluent with the tricuspid valve; that is, the VSD is not paramembranous.

The subpulmonary infundibulum is absent. The semilunar valve is tricuspid, and the leaflets are markedly thick, fleshy, soft, and polypoid.

Is this a common aortopulmonary trunk? I would say no for the following reasons:

• 1.

  The subpulmonary infundibulum is absent. If the failure-of-septation hypothesis were correct, an infundibular septal defect “should” be present; but it is not.

• 2.

  The semilunar valve is tricuspid. It does not appear to be in common with another semilunar valve, as is required by the failure-of-fusion hypothesis.

• 3.

  The great artery that arises from the base of this heart looks like a large MPA. It does not look like a common aortopulmonary trunk. Instead, it looks like a large MPA, from which the small ascending aorta arises at about the level of the pulmonary artery branches, well above the semilunar valve.