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 ).
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 ).
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 ).
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.
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 ).
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:
MPA from the truncus arteriosus; and
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 )?
Absence of the subpulmonary infundibulum
Absence of the pulmonary valve
Absence of the MPA
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.
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).