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Key Questions in CONGENITAL CARDIAC SURGERY
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Figure 8. Combinations of atrial arrangement and ventricular topology
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that produce the various biventricular atrioventricular connections. When
the atrial appendages are isomeric, the atrioventricular connections are
always mixed. In this setting, therefore, it is always necessary to specify
topology so as to provide a complete description.
5 What is the origin of the pulmonary veins?
The atrial component of the developing heart tube is attached to the
•
pharyngeal mesenchyme through the dorsal mesocardium (Figure 3).
At this stage, neither the lungs nor any boundaries between the atrial
component of the heart tube and the systemic venous tributaries
have been formed.
Once the venous valves have formed, it becomes possible to recognise
•
the boundaries of the systemic venous sinus. Associated with this, a
strand of tissue forms in the midline of the pharyngeal mesenchyme.
Subsequent canalisation of this mid-pharyngeal strand provides a
•
connection between the developing intraparenchymal pulmonary veins
and the left atrium, with the newly formed channel opening into the left
atrium between the folds showing the dorsal mesocardial connection.
This newly formed pulmonary vein opens into the left atrium through a
•
myocardial portal with atrial characteristics. The walls at the site of
connection are different from the walls of the systemic venous tributaries.
The initial opening of the pulmonary vein is adjacent to the left
•
atrioventricular junction. Only at a much later stage does it become
possible to recognise separate openings for the right and left
pulmonary veins. Following formation of the separate pulmonary
1 Congenital cardiac anatomy
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venous orifices, the superior interatrial fold develops, which provides
the superior rim of the oval fossa.
The walls of the pulmonary veins possess a myocardial phenotype
•
and have never had any relationship with the walls of the systemic
venous sinus (Figure 9).
Superior caval vein
A
Venous valve
Right atrium
eмг~е=ЙгДкуз=Ф=`~кеЙЦбЙ=лн~ЦЙ=NQ
Superior caval vein
B
Venous valve
Outflow tract
Left superior
caval vein
Left superior
caval vein
Left atrium
Dorsal mesocardium
Coloured to show NKX 2.5
11
Opening of pulmonary vein
eмг~е=ЙгДкуз=Ф=`~кеЙЦбЙ=лн~ЦЙ=NQ
Coloured to show TBX18
Figure 9. Human embryo sections (at Carnegie stage 14)
demonstrating: A) myocardium (using staining of the gene
NKX 2.5); and B) walls of the systemic venous tributaries,
derived from the sinus venosus (using staining of the gene
TBX18). The pulmonary vein opens into the myocardium,
with no relationship to the derivatives of the systemic
venous sinus.
collaboration with Professor Antoon F. M. Moorman and Dr.
Aleksander Sizarov, Academic Medical Center, Amsterdam, The
Netherlands, and are reproduced with their permission.
The original images were prepared in
Key Questions in CONGENITAL CARDIAC SURGERY
6 Describe the development of the atrial septum
The first signs of atrial septation is the formation of a ridge in the
•
roof of the atrial component of the heart tube. As it grows towards
the orifice of the atrioventricular canal, this first septum (septum
primum) carries a cap of mesenchyme on its leading edge.
The atrioventricular canal itself is divided into right and left parts by
•
the fusion within its cavity of the superior and inferior
atrioventricular cushions. As the primary atrial septum grows
towards the cushions, the space between the mesenchymal cap on
its leading edge and the cushions represents the primary atrial
foramen (foramen primum).
With ongoing growth of the primary septum and its mesenchymal
•
cap towards the cushions, there is a concomitant decrease in the
size of the primary foramen, which is eventually closed by fusion of
the cap with the cushions.
By the time the primary foramen has been obliterated, however, the
•
upper edge of the primary septum has broken away from the atrial
12
roof to form the secondary atrial foramen (foramen secundum,
Figure 10).
As the primary foramen closes, a new mesenchymal structure
•
develops from the dorsal margin of the mesocardium, known as the
vestibular spine.
With ongoing development, the vestibular spine and the
•
mesenchymal cap muscularise, forming the inferoanterior buttress
of the oval fossa (fossa ovale).
The primary septum itself then forms the floor of the newly formed
•
oval fossa. Its superior rim is formed by an infolding of the atrial
roof between the attachments of the pulmonary veins to the left
atrium and the caval veins to the right atrium.
The oval foramen (foramen ovale) is the space between the
•
infolded superior interatrial fold and the cranial margin of the
primary septum, with the primary septum itself forming a flap valve
over the foramen (Figure 11).
1 Congenital cardiac anatomy
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Mesenchymal cap
A
Right atrium
Atrioventricular canal
Venous valves
B
Primary septum
Left atrium
Developing LV
Developing RV
Secondary foramen
13
Primary septum
Primary foramen
Mesenchymal cap
Superior AV cushion
Figure 10. Initial stages of development of the
atrial septum in the human heart: A) growth of the
primary septum, with its mesenchymal cap, from
the roof of the atrial component of the heart tube;
B) growth of the primary septum towards the
atrioventricular (AV) cushions, with its upper part
breaking down to form the secondary atrial
foramen. The primary foramen is the space
between the mesenchymal cap and the atrial
margins of the atrioventricular cushions. LV = left
ventricle; RV = right ventricle.
Key Questions in CONGENITAL CARDIAC SURGERY
Vestibular spine
A
rimary septum
P
esenchymal cap
M
Primary foramen
14
Inferior atrioventricular cushion
Venous valves
Interventricular communication
Pulmonary veins
B
Left SCV
Right atrium
Oval fossa
Right
ventricle
Primary septum
Inferoanterior
buttress
Figure 11. Later stages of development of the
atrial septum in a mouse heart: A) growth of the
vestibular spine; B) completion of atrial septation
with infolding of the superior atrial roof that forms
the cranial margin of the oval fossa (white arrow
with black borders). The mesenchymal cap and
the vestibular spine have muscularised to form the
inferoanterior buttress of the oval fossa. SCV =
superior caval vein.
1 Congenital cardiac anatomy
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7 What is the difference between an atrial septal defect
and an interatrial communication?
The normal atrial septum has two components:
•
a) floor of the oval fossa, which is derived from the primary atrial
septum;
b) anteroinferior buttress, which is formed by muscularisation of
the mesenchymal cap carried on the primary septum and the
vestibular spine.
True atrial septal defects are produced either because of deficiencies
•
or fenestrations of the floor of the oval fossa, or because of
inappropriate formation of the anteroinferior buttress, including:
a) oval fossa (ostium secundum) defects — which represent
deficiencies or fenestrations of the floor of the fossa resulting
from inappropriate formation of the primary atrial septum. They
should be distinguished from persistent patency of the oval
foramen (PFO), when the flap valve of the fossa overlaps but
fails to fuse with the anterosuperior interatrial fold. Such
persistent patency is found in around 25% of normal
individuals;
b) vestibular defects — which are much rarer and secondary to
improper fusion of the muscularising components of the
anteroinferior buttress of the oval fossa (Figure 12).
15
Interatrial communications represent lesions outside the confines of
•
the atrial septum, including:
a) sinus venosus defects — which are the commonest of these
extraseptal communications and are the consequence of an
anomalous connection of one or more pulmonary veins to a
caval vein, with the pulmonary veins retaining their left atrial
connection. Sinus venosus defects can be found in the
openings of either the superior or inferior caval veins. On
occasion, they can be distant from the atrial orifice of the
systemic venous channel (Figure 13);
b) coronary sinus defect — which is the rarest interatrial
communication outside the confines of the atrial septum. This
lesion permits shunting across the right atrial orifice of the
sinus because of fenestration, or absence, of the walls that
usually interpose between the cavities of the coronary sinus, or
a persistent left superior caval vein, and the left atrium;
c) ostium primum defect — which is an atrioventricular rather
than an atrial septal defect.
Key Questions in CONGENITAL CARDIAC SURGERY
16
Deficient flap valve
A
B
Flap valve
Perforated flap valve
Vestibular defect
Coronary sinus
Anterosuperior border
Right atrium
Left atrium
Figure 12. Atrial septal defects: A) true defects
within the oval fossa (yellow oval) and a vestibular
defect within the anteroinferior buttress; B)
persistent patency of the oval foramen, with the
flap valve of sufficient size to cover the margins of
the fossa (double-headed white arrow) but fails to
fuse with the anterosuperior margin, leaving a
potential interatrial communication (doubleheaded white arrow with red borders). Shunting
across the communication is only possible when
right atrial pressure exceeds that in the left atrium.
1 Congenital cardiac anatomy
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Anomalously connected
pulmonary vein
A
Normal arrangement
of the atrial septum
B
LSCV
Venovenous conduit
Oval fossa
17
Opening of the
coronary sinus
Figure 13. Interatrial communications outside
the atrial septum, including: A) sinus venosus
defect, in proximity to the opening of the superior
caval vein; and B) coronary sinus defect, which is
caused by failure of formation of the walls that
normally separate a left superior caval vein
(LSCV) or coronary sinus, from the cavity of the left
atrium (red dotted lines).
Key Questions in CONGENITAL CARDIAC SURGERY
8 What are the phenotypic features of an atrioventricular
The commonality of the atrioventricular junction, along with the
•
trifoliate configuration of the left atrioventricular valve are the
fundamental phenotypic features of an atrioventricular septal defect
(AVSD), which is also known as an atrioventricular canal defect.
During gestation, the bridging leaflets of the common atrioventricular
•
valve, as typically seen in the atrioventricular septal defect, are
derived from the superior and inferior atrioventricular cushions,
which when fused separate the atrioventricular canal into the right
and left atrioventricular orifices (Figure 14).
In the past, the left component of the common atrioventricular valve
•
or junction was usually described as a ‘cleft mitral valve’, which is
incorrect. This is because the left half of the persisting common
atrioventricular junction is guarded by a trifoliate valve (Figure 14B)
rather than a bifoliate valve, even when the common atrioventricular
junction itself is divided into separate valvar orifices for the right and
left ventricles, as in the case of an AVSD without ventricular septal
defect (Figure 15).
The building blocks of the left half of the common atrioventricular
•
valve are the same as those producing the aortic leaflet (or anterior
leaflet) of the mitral valve in normal hearts.
In AVSD, the commonality of the atrioventricular junction and the
•
trifoliate configuration of the left atrioventricular valve are present
irrespective of the level of shunting across the septal defect, hence
above the valve plane, below the valve plane or both.
Comparable arrangements are also found when there is a common
•
valve guarding the common junction, or when the bridging leaflets
are themselves fused to produce separate right and left valvar
orifices for the right and left ventricles (Figure 16).
In most cases, the bridging leaflets float more or less freely within the
•
atrioventricular septal defect. This permits shunting at both atrial and
ventricular levels.
When the bridging leaflets are themselves fused to the crest of the
•
muscular ventricular septum, the arrangement confines the potential
for shunting across the atrioventricular septal defect at atrial level,
producing a so-called ‘ostium primum’ defect.
Less frequently, the bridging leaflets can be attached to the leading
•
edge of the atrial septum, thus producing an atrioventricular septal
defect with exclusively ventricular shunting. In this latter setting, the
heart retains the common atrioventricular junction, along with the
trifoliate configuration of the left atrioventricular valve.
In rare circumstances, the atrioventricular septal defect can close
•
spontaneously. This produces the arrangement of a heart with intact
septal structures, but with a common atrioventricular junction and a
trifoliate left atrioventricular valve.
1 Congenital cardiac anatomy
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A
Outflow cushions
Right lateral
cushion
Subaortic outflow tract
B
Right ventricular
leaflets
Superior AV cushion
Left lateral
cushion
Inferior AV cushion
Superior bridging leaflet
19
Inferior bridging leaflet
Left mural leaflet
Figure 14. A) Atrioventricular cushions as seen in a developing mouse
embryo sacrificed at embryonic day 12, just prior to fusion of the superior
and inferior cushions that lie edge-to-edge within the atrioventricular canal.
B) Atrioventricular septal defect with the common atrioventricular junction
guarded by a common atrioventricular valve. There is an obvious similarity
between the arrangements of the bridging leaflets and the superior and
inferior atrioventricular (AV) cushions. The ‘trifoliate’ arrangement of
apposition of the left ventricular cushions in the developing mouse heart
(white dotted lines) parallels the configuration of the leaflets that guard the
left half of the common atrioventricular junction in the specimen with
deficient atrioventricular septation. In the developing mouse, however, the
outflow tract remains undivided, whereas the subaortic root is committed to
the left ventricle in the specimen with an atrioventricular septal defect. Both
images are viewed from the ventricular apex looking towards the base of
the ventricular mass.
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