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Key Questions in CONGENITAL CARDIAC SURGERY
Subaortic outflow tract
A
Superior AV cushion
20
Inferior AV cushion
Subaortic outflow tract
B
Inferior AV cushion
Superior AV cushion
Left lateral cushion
Left lateral cushion
Figure 15. A) Atrioventricular septal defect with separate valvar orifices for
the left and right ventricles (‘ostium primum defect’); and B) a normal heart in
mouse embryos sacrificed at embryonic day 15.5. Both short-axis images are
viewed from the ventricular apex looking towards the base. The mouse with
the atrioventricular septal defect has maintained the trifoliate configuration of
the left half of the common atrioventricular junction despite the commitment
of the aortic root to the left ventricle, whereas the normal mouse, subsequent
to closure of the embryonic interventricular communication, has formed a
bifoliate left valve. The black dotted line shows the line of fusion between the
superior and inferior atrioventricular cushions in the normal mouse, whereas
the left ventricular components of the cushions have remained unfused in the
mouse with the atrioventricular septal defect.
1 Congenital cardiac anatomy
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Superior bridging leaflet
A
Left mural leaflet
Superior bridging leaflet
B
Right anterosuperior leaflet
Right
inferior
leaflet
Inferior bridging leaflet
Right anterosuperior leaflet
21
Right
inferior
leaflet
Left mural leaflet
Inferior bridging leaflet
Figure 16. The images show the commonality of the atrioventricular
junction in hearts with an atrioventricular septal defect, when the junction
is guarded by: A) a common valve; or B) when the bridging leaflets have
themselves fused to produce separate valvar orifices for the right and left
ventricles (star), and are additionally fused to the crest of the muscular
ventricular septum. The area previously considered to represent a ‘cleft’
in the mitral valve (double-headed arrow in B) can be seen to be the
zone of apposition between the left ventricular components of the
bridging leaflets (compare with double-headed arrow in A). The dotted
white lines shown the location of the crest of the muscular ventricular
septum.
Key Questions in CONGENITAL CARDIAC SURGERY
9 What is the origin of the membranous septum?
The normal ventricular septum is almost exclusively a muscular entity,
•
with a small part within the left ventricular outflow tract that is fibrous
(known as the membranous septum).
Most often, it is crossed on its right ventricular aspect by the hinge of
•
the septal leaflet of the tricuspid valve, which divides it into
atrioventricular and interventricular components (Figure 17).
Membranous septum
Aorta
22
Atrioventricular
component
Interventricular
component
Hinge of TV
Muscular septum
Mitral
valve
Left ventricular
outflow tract
Figure 17. ‘Four-chamber’ section through the
aortic root, with the non-coronary sinus and its
leaflet removed. The hinge of the septal leaflet of
the tricuspid valve (TV) crosses the membranous
septum, dividing it into its atrioventricular and
interventricular components.
During normal development, the expansion of the atrioventricular canal
•
allows the connection of the developing right atrium with the right
ventricle.
During this process, the ventricular outflow tract, which is separating
•
to form the aortic and pulmonary roots, remains supported above the
cavity of the right ventricle.
In order to close the ventricular septum, the developing aortic root, still
•
supported above the cavity of the right ventricle, needs to come into
continuity with the cavity of the left ventricle.
This is achieved by creating a shelf within the cavity of the right
•
ventricle by fusion of the proximal parts of the cushions dividing the
1 Congenital cardiac anatomy
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outflow tract into the aortic and pulmonary components. At the same
time, this process begins to transfer the larger part of the persisting
interventricular communication into the outflow tract for the left
ventricle (Figure 18).
Aortic root
A
Left ventricle
Tertiary
interventricular
communication
Aortic root
B
Tubercles of
AV cushions
Secondary
interventricular
communication
Left ventricular
outflow tract
Figure 18. Process of transfer of the aortic root to the left ventricle in the
developing mouse heart: A) embryonic day 13.5, where the greater part of
the embryonic interventricular communication is remodelling so as to
connect the aortic root with the left ventricle. This can be considered to
represent the secondary interventricular communication. A persisting third
component of the communication is then seen between the rightward part
of the aortic root and the right ventricle; B) embryonic day 14.5, where this
communication is closed by fusion of so-called tubercles derived from the
superior and inferior atrioventricular (AV) cushions. This produces the
membranous part of the ventricular septum and commits the secondary
interventricular communication to the outflow tract for the left ventricle.
23
Key Questions in CONGENITAL CARDIAC SURGERY
This part of the embryonic interventricular communication can be
•
considered as a secondary channel, since part of the initial, or primary,
interventricular communication has already been remodelled to form
the inlet of the right ventricle.
Subsequent to the remodelling of the secondary embryonic
•
interventricular communication to become the subaortic outflow tract,
a persisting third part of the communication remains as a channel
between the cavities of the right and left ventricles.
It is this channel that is then closed by growth of so-called tubercles
•
from the atrioventricular cushions to produce the membranous part of
the ventricular septum.
When initially closed, the septal leaflet of the tricuspid valve has yet to
•
delaminate from the surface of the ventricular septum. It is only
subsequent to the delamination of this leaflet that it becomes possible
to recognise the atrioventricular and interventricular components of the
membranous septum.
10 Is there a muscular outlet septum in the normal heart?
24
There is no muscular septal component interposed between the
•
ventricular outflow tracts in the normal heart.
Moreover, by virtue of the wedging of the aortic root between the
•
mitral valve and the ventricular septum, the inlet of the right ventricle
is separated from the outlet, rather than the inlet, of the left ventricle.
This is despite the fact that the proximal parts of the outflow cushions
•
fuse to produce a shelf in the roof of the right ventricle. This process
provides a channel between the developing aortic root and the cavity
of the left ventricle (Figure 19).
As part of this process, the shell of these proximal cushions achieves
•
a myocardial phenotype. At the same time, however, the core of the
cushions attenuates. The disappearance of the core of the cushions
then produces an extracavitary area that interposes between the
muscularising shell and the developing sinuses of the aortic root.
The muscularising shell of the proximal cushions, therefore, becomes
•
the freestanding infundibular sleeve. This lifts the pulmonary root
away from the base of the ventricular mass.
The formation of the freestanding infundibular sleeve makes it
•
possible to remove the entirety of the pulmonary root and use it as an
autograft in the Ross procedure. This would not be possible in the
normal heart had there been formation of a muscular outlet septum.
1 Congenital cardiac anatomy
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A
Developing
aortic root
Muscularising
parietal cushion
B
Right ventricle
Secondary
interventricular
communication
Septal cushion
Pulmonary root
25
Aortic root
Right ventricle
Right atrium
Closing interventricular foramen
Figure 19. Stages in development of the freestanding infundibular sleeve
of the right ventricle: A) fusion of the proximal cushions at embryonic day
13.5. This process builds a shelf in the roof of the right ventricle, committing
the aortic root to the left ventricle through the secondary interventricular
communication; B) oblique subcostal equivalent section at embryonic day
14.5, at the stage of closure of the tertiary interventricular communication.
The shell of the muscularising
subpulmonary infundibulum (black dashed line). The core of the cushion
mass (white star with red borders) is attenuating to produce an
extracavitary area between the developing infundibular sleeve and the
forming sinuses of the aortic root.
proximal cushion mass is transforming into the
Key Questions in CONGENITAL CARDIAC SURGERY
11 Is a ventricular septal defect (VSD) the same as an
interventricular communication?
During the development of the heart, there is extensive remoulding of
•
the initial embryonic interventricular communication.
The first, or primary, interventricular communication is the portal for
•
passage of all the blood entering the developing left ventricle
ultimately to reach the undivided outflow tract.
The defects originating from a primary interventricular
•
communication include:
a) defects in the setting of double-inlet left ventricle with double-
outlet from the right ventricle;
b) defects seen with straddling and overriding of the tricuspid
valve.
The secondary interventricular communication is the embryonic
•
interventricular channel underneath both outflow tracts where, during
the normal expansion of the atrioventricular canal and formation of
26
the right ventricular inlet, they remain supported above the cavity of
the developing right ventricle. Defects originating from the secondary
interventricular communication are found between the ventricles in
the setting of double-outlet right ventricle.
The tertiary interventricular communication is the route by which the
•
rightward margin of the developing aortic root remains in
communication with the cavity of the right ventricle, during the
process of connection of the aortic root with the left ventricular cavity,
with the secondary interventricular communication remoulded to
become the left ventricular outflow tract.
The tertiary interventricular communication is eventually closed
•
during normal development by formation of the membranous part of
the septum from the so-called tubercles of the atrioventricular
cushions. Failure to close the tertiary communication results in
production of a perimembranous ventricular septal defect.
It follows that the channels found in the setting of double-outlet right
•
ventricle, double-inlet left ventricle with DORV and perimembranous
VSD represent the different stages of the remoulding of the
embryonic interventricular communication and therefore cannot be
considered all the same ventricular septal defects.
12 Describe the defects that open into the inlet of the right
ventricle
There are four phenotypically different defects that open to the inlet
•
of the right ventricle.
The location of the atrioventricular conduction axis differs markedly in
•
all of them.
1 Congenital cardiac anatomy
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It is now well established that the ‘septum of the atrioventricular
•
canal’ is the muscularising vestibular spine.
The formation of this structure during normal development, along
•
with its fusion with the atrioventricular cushions, divides the initial
atrioventricular canal into the orifices of the tricuspid and mitral
valves.
One type of inlet defect results from a process of malalignment of the
•
muscular component of the interventricular septum, by virtue of
incomplete expansion of the atrioventricular canal, relative to the
atrial septum.
This defect is also perimembranous, since it is characterised by
•
fibrous continuity between the leaflets of the mitral and tricuspid
valves. It is found in the setting of straddling and overriding of the
tricuspid valve. Because of the septal malalignment, the
atrioventricular conduction axis arises from an anomalous
posteroinferior atrioventricular node (Figure 20).
Triangle of Koch in atrial septum
Straddling tricuspid valve
Malaligned muscular septum
Figure 20. Right ventricular inlet defect found in
the setting of straddling and overriding of the
tricuspid valve. It is a perimembranous defect with
malalignment between the atrial septum and the
muscular ventricular septum. The atrioventricular
conduction axis (red dashed line) no longer
originates from the regular atrioventricular node
(red star with white borders). Instead, it takes
origin from an anomalous inferior node (white star
with red borders).
27
Key Questions in CONGENITAL CARDIAC SURGERY
edial paillary muscle
pex of triangle of Koch
A
A
M
Muscular inlet defect
B
Perimembranous inlet defect
28
Triangle of Koch deviated inferiorly
Figure 21. Macroscopic images demonstrating the arrangement of the
atrioventricular conduction axis (red dashed line) in the setting of defects
opening to the inlet of the right ventricle with: A) exclusively muscular
borders; and B) perimembranous. The perimembranous defect shown in B)
differs from the one shown in Figure 20 because of the presence of
alignment between the atrial septum and the inferior part of the muscular
ventricular septum.
1 Congenital cardiac anatomy
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A second type of inlet defect is where the opening into the inlet of the
•
right ventricle exclusively has muscular margins. In these instances,
the conduction axis courses in cranial fashion relative to the defect.
A third type is the one represented by perimembranous defects
•
extending to open to the inlet of the right ventricle. They are
characterised by a fibrous continuity between the leaflets of the mitral
and tricuspid valves forming their posterior margin but with alignment
between the atrial and muscular ventricular septal structure.
In this defect, the conduction axis is deviated inferiorly but it
•
continues to arise from a regular atrioventricular node at the apex of
the triangle of Koch. The triangle is itself deviated inferiorly due to the
hypoplasia of the inferior component of the muscular ventricular
septum (Figure 21).
The fourth type is an atrioventricular septal defect opening directly to the
•
inlet of the right ventricle with the potential only for ventricular shunting.
This occurs when the bridging leaflets of the common atrioventricular
valve abut against the leading edge of the atrial septum during
ventricular systole. In this instance, the atrioventricular conduction axis
will arise from an inferiorly deviated atrioventricular node.
29
13 What is the phenotypic feature of a supracristal
ventricular septal defect?
For some time, ventricular septal defects were categorised on the
•
basis of being supracristal or infracristal.
Examination of the defects distinguished in this fashion shows a lack of
•
logic, since rather than the defects opening in different fashion to the
right ventricle, it is the structure nominated as the ‘crista’ that varies.
The majority of supracristal defects open into the right ventricle
•
between the limbs of the septomarginal trabeculation (SMT), also
known as the septal band (Figure 22A).
In this setting, the feature nominated as the ‘crista’ is the myocardial
•
structure produced by fusion of the caudal limb of the septal
trabeculation with the ventriculo-infundibular fold, the latter structure
being derived from the inner heart curvature.
The feature nominated as the ‘crista’ in an infracristal defect is the
•
supraventricular crest. This is formed by the freestanding
subpulmonary muscular infundibulum, along with the right ventricular
margin of the inner heart curvature, which is known as the ventriculoinfundibular fold (Figure 22B).
The differences between the appearance of the two defects is now
•
well explained on the basis of the development of the normal right
ventricular outflow tract.
The normal supraventricular crest is formed in its larger part by
•
muscularisation of the proximal outflow cushions, which subsequently
become the freestanding subpulmonary infundibulum (Figure 23A).
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