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Key Questions in CONGENITAL CARDIAC SURGERY
40
A
Right atrium
Absent AV connection
B
Aorta
Subpulmonary infundibulum
Incomplete right ventricle
Undivided outflow tract
Developing right atrium
Absent connection
Embryonic IVC
Right ventricular apical component
Figure 28. Images demonstrating the commonest variant of tricuspid
atresia, where: A) the right atrioventricular (AV) connection is absent; and
B) the atrioventricular canal remains supported exclusively by the
developing left ventricle. The developing right ventricle at this early stage
receives its blood through the embryonic interventricular communication
(IVC), although it already possesses its apical component.
1 Congenital cardiac anatomy
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morphological left ventricle, and the developing right ventricle gives
rise to the entirety of the developing outflow tract (Figure 28).
The differences in morphology between the two phenotypic variants
•
of tricuspid atresia serve to illustrate the differences between the
hypoplastic right ventricle found in the setting of the imperforate
valve, and the incomplete ventricle found in the absence of the right
atrioventricular connection. Both variants, however, produce
functionally univentricular hearts.
19 What is the developmental background of common
arterial trunk?
The phenotypic feature of common arterial trunk is the commonality
•
of the ventriculo-arterial junction, rather than the presence of a
common intrapericardial arterial component or ‘truncus’.
The essential feature of common arterial trunk, therefore, is
•
persistence of the undivided embryonic ‘conus’, specifically the
intermediate and proximal components of the outflow tract, rather
than lack of division of its distal component (Figure 29).
When the term ‘persistent truncus arteriosus’ was first introduced, it
•
was used to describe the distal part of the developing outflow tract,
which is separated by the development of a protrusion from the
dorsal wall of the aortic sac to produce the intrapericardial arterial
trunks.
It is the manner of separation of the distal part of the outflow tract
•
that, traditionally, has been used to subcategorise the examples of
common arterial trunk.
In the initial approach, four types were described by Collett and
•
Edwards according to the origin of the pulmonary arteries. Type IV
was subsequently recognised to be a solitary arterial trunk, rather
than a common trunk, since it was defined on the basis of absence
of the intrapericardial pulmonary arteries.
In the revised version proposed by Van Praagh and Van Praagh,
•
Types I and II of the Collett and Edwards classification were unified
as Types 1 and 2. The Van Praaghs then introduced new Types 3 and
4, to account for variants with discontinuous pulmonary arteries and
hypoplasia of the aortic component of the trunk, respectively.
At the same time, the Van Praaghs pointed out that a simpler
•
approach was to separate the variants according to the dominance
of the aortic as opposed to the pulmonary component of the common
arterial trunk.
The concept of aortic as opposed to pulmonary dominance provides
•
a much simpler categorisation. It is pulmonary dominance that is
41
Key Questions in CONGENITAL CARDIAC SURGERY
A
Systemic
arteries
Coronary
artery
Pulmonary
arteries
Common
VA
junction
Common
AV
junction
LSCA
Arterial duct
42
Hypoplastic
aortic
component
B
Dominant
pulmonary
component
Common
VA
junction
Figure 29. Embryological images demonstrating two embryonic mice
genetically modified so as to produce common
basis of a trunk that gives rise directly to the systemic, pulmonary and coronary
arteries: A) following knock-out of the Tbx1 gene, there has been no separation
of the distal outflow tract, so that the trunk has an aortic dominance; B) following
disruption of the Furin enzyme, the aortopulmonary septum (white arrow) has
grown so as to divide the distal outflow tract disproportionately in favour of the
pulmonary component of the trunk. The aortic arch is interrupted (white star with
red borders), with the descending aorta fed through the persistently patent
arterial duct. The left subclavian artery (LSCA) arises from the descending aorta.
AV = atrioventricular; VA = ventriculo-arterial.
arterial trunk, defined on the
1 Congenital cardiac anatomy
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accompanied by interruption of the aortic arch or severe coarctation.
When there is aortic dominance, it then becomes necessary to
describe with precision the origin of the pulmonary arteries.
In the majority of cases, the pulmonary arteries arise in close
•
proximity to each other from the posterior or leftward component of
the aortic dominant trunk, meaning that frequently they are described
as ‘Type 1½’ using traditional classifications.
The origin of the pulmonary arteries, however, can involve one of the
•
sinuses of the truncal root, whilst the right and left pulmonary arteries
can cross as they extend towards the mediastinum.
20 What is transposition?
Transposition literally means ‘placed across’ and it refers to the
•
placement of the arterial roots across the ventricular septum, such
that they arise from morphologically inappropriate ventricles.
Irrespective of the atrioventricular connections, which can be
•
concordant, discordant, mixed in the setting of isomeric atrial
appendages, or univentricular, the transposition is always ‘complete’
in the sense that each of the arterial trunks is completely placed
across the septum.
The commonest variant is now simply described as ‘transposition’. In
•
most instances, it is usually qualified to include ‘of the great arteries’.
The latter part, however, is redundant.
The discordant ventriculo-arterial connections can be found with
•
various relationships of the arterial trunks and with varied infundibular
morphology. The combination of discordant ventriculo-arterial
connections with concordant atrioventricular connections can itself
be found in usual or mirror-imaged variants.
Most usually, the aortic root is found anteriorly and to the right. It can,
•
nonetheless, be found in a leftward location. The latter relationship is
the rule when transposition is found in its mirror-imaged variant. It is
inappropriate, therefore, to use ‘d-transposition’ as being the default
option for transposition.
The presence of a ventricular septal defect, or obstruction of the
•
ventricular outflow tracts, are the associated malformations of
greatest clinical significance.
The developmental defect underscoring the production of the
•
discordant ventriculo-arterial connections is the fusion of the major
outflow cushions in the outflow tract in a straight as opposed to their
normal spiralling fashion (Figure 30).
The fusion of the outflow cushions in a straight as opposed to
•
spiralling fashion serves to join the rightward component of the
43
Key Questions in CONGENITAL CARDIAC SURGERY
Aorta Pulmonary trunk
A
Right
ventricle
Spiralling
cushions
44
Aorta
B
Pulmonary trunk
Parallel cushions
Right ventricle
Figure 30. Embryological images demonstrating the arrangement of the
outflow cushions in a: A) control mouse; and B) mouse with knock-out of the
Ptx1 gene. Ongoing development in the mice with the knock-out gene
produces either transposition or double-outlet right ventricle with
subpulmonary interventricular communication. It is the fusion of the outflow
cushions in a straight as opposed to spiralling fashion that underscores
these changes.
1 Congenital cardiac anatomy
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proximal outflow tract to the aortic component of the aortic sac, and
the leftward component to the pulmonary component. It is then the
leftward component of the outflow tract, which is closest to the
embryonic interventricular communication that is transferred to the
developing left ventricle, thus producing the discordant ventriculoarterial connections.
21 How can transposition be congenitally corrected?
When discordant ventriculo-arterial connections (transposition) are
•
associated with concordant atrioventricular connections, the
systemic and pulmonary circulations are arranged in parallel, rather
than in series.
Connections across the atrioventricular junctions, in a discordant
•
fashion as opposed to concordant, serve to ‘correct’ the discordant
ventriculo-arterial connections. By virtue of the double discordance,
the circulations are then again in series, even though the arterial
trunks are supported by morphologically inappropriate ventricles.
This occurs as a consequence of looping of the ventricular
•
component of the heart tube in a mirror-imaged fashion compared to
normal.
In the usual situation, the ventricular component of the heart tube
•
loops to the right. Expansion of the atrioventricular canal then places
the developing right atrium in communication with the developing
right ventricle, hence ensuring the formation of concordant
atrioventricular connections.
If the ventricular component of the heart tube should loop to the left,
•
in contrast, expansion of the atrioventricular canal will similarly take
place in a leftward direction. This will serve to connect the
developing left atrium to the right ventricle, which is derived from the
outlet component of the heart tube. It will be positioned in a left-sided
position subsequent to leftward ventricular looping.
In the setting of a mirror-imaged atrial arrangement, it will be
•
rightward looping of the ventricular component of the heart tube that
will produce discordant, rather than concordant, atrioventricular
connections.
Almost always, the formation of discordant atrioventricular
•
connections sets the scene for associated formation of discordant
ventriculo-arterial connections. On occasion, nonetheless, the
ventriculo-arterial connection can be that of a double-outlet right
ventricle. On rare occasions, furthermore, the ventriculo-arterial
connections can be concordant. The latter combination will produce
the haemodynamic picture of regular transposition.
45
Key Questions in CONGENITAL CARDIAC SURGERY
22 What is the classification system used to describe the
origin of the coronary arteries?
Almost always, the major coronary arteries arise from one or other,
•
but usually both, of the sinuses of the aortic root that are adjacent to
the pulmonary root.
Endothelial stems grow out from the developing aortic root, which
•
occurs during the separation of the intermediate part of the outflow
tract into its aortic and pulmonary components. The stems initially
develop distal to the myocardial turret that surrounds the
intermediate part of the outflow tract. They are incorporated into the
sinuses, as the sinuses themselves expand proximally relative to the
myocardial boundary of the developing outflow tract.
The major coronary arteries themselves are formed within the
•
atrioventricular and interventricular grooves, producing in this way
the right, anterior interventricular and circumflex arteries.
With normal development, the endothelial stem growing from the
•
leftward of the two aortic valvar sinuses formed adjacent to the
pulmonary root joins with the anterior interventricular and circumflex
46
arteries. The stem from the rightward adjacent sinus joins with the
right coronary artery.
According to the Leiden classification, as viewed by an observer
•
standing upright (figuratively speaking) in the sinus that is not
adjacent to the pulmonary trunk, one of the sinuses will be to their
right hand. This sinus is conventionally nominated as #1. The other
sinus will be to the left hand of the observer and it is nominated as
#2 (Figure 31).
This arrangement of the aortic valvar sinuses, as seen by the
•
observer standing in the non-adjacent sinus, will remain constant
irrespective of the relationship of the arterial roots to each other. On
this basis, the arrangement of the right, circumflex and anterior
interventricular arteries can be described in constant fashion in terms
of their origin from either the right-handed (#1) or left-handed (#2)
aortic sinuses. According to their relationship to the arterial pedicles,
each of the arteries can arise from either of the adjacent valvar
sinuses.
In the setting of transposition, the commonest pattern is for the main
•
stem of the left coronary artery to arise from sinus #1, and to divide
into the anterior interventricular and circumflex arteries. The right
coronary artery then takes its origin from sinus #2.
Any arrangement is possible, including the crossing of coronary
•
arteries so as to arise from seemingly inappropriate sinuses. The
arteries can also cross between the arterial roots with an interarterial
course, usually with an intramural location within the wall of the aortic
A
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Right
coronary
artery
1 Congenital cardiac anatomy
Left-hand
sinus #2
Left
coronary
artery
Anterior
interventricular
artery
Circumflex
artery
Anterior
interventricular
artery
B
Right
coronary
artery
Right-hand
sinus #1
Left-hand
sinus #2
Right-hand
sinus #1
Figure 31. Classification of the origin of the coronary arteries. The images
show how the origin of the coronary arteries can be described in terms of
the view obtained by the surgeon ‘standing upright’ in the non-adjacent
sinus and looking towards the adjacent aortic
the relationship of the arterial trunks, one sinus will always be to the right
hand of the surgeon, which is then nominated as #1. The other sinus will be
to the left hand, and it is nominated as #2. The arrangements shown are as
seen in the: A) normal heart (1R,2LCx); and B) commonest variant of
transposition (1LCx,2R).
valvar sinuses. Irrespective of
47
Key Questions in CONGENITAL CARDIAC SURGERY
root. In most instances, nonetheless, the coronary arteries will arise
from their closest sinus according to the relationship of the arterial
roots.
23 What is the aortopulmonary septum?
In the postnatal heart, each of the aortic and pulmonary pathways
•
can be considered as possessing a ventricular outflow tract, an
arterial root, and then intrapericardial and extrapericardial arterial
components.
At present, it is usual to assess the developing outflow tract in terms
•
of its components labelled as the ‘truncus’ and ‘conus’, along with
an extrapericardial aortic sac. If we presume that the ‘conus’ gives
rise to the ventricular outflow tracts and the ‘truncus’ to the
intrapericardial arterial trunks, with the extrapericardial arterial
pathways derived from the aortic sac, however, the current
categorisation makes no provision for the arterial roots.
A better approach considers the outflow tract as possessing
•
48
proximal, intermediate and distal components, with the cavity of the
distal component becoming continuous with the cavity of the aortic
sac at the margins of the pericardial cavity.
The intermediate component can be recognised in anatomic terms by
•
the appearance within its lumen of the intercalated swellings. The
distal ends of these swellings, along with the major outflow cushions,
will cavitate to produce the leaflets of the arterial valves, with the
arterial roots forming within the intermediate component of the
outflow tract (Figure 32).
Earlier in development, the aortic sac is the manifold that gives rise
•
to the arteries of the pharyngeal arches, which will themselves
transform into the extrapericardial components of the aortic and
pulmonary pathways. The arteries of the 3rd and 4th arches will form
the systemic pathways, while the arteries of the pulmonary arches
will give rise to the right and left pulmonary arteries, which develop
within the pharyngeal mesenchyme. At this stage, it is the dorsal wall
of the aortic sac that represents the aortopulmonary septum (Figure
33).
With ongoing development, the dorsal wall of the aortic sac grows
•
and protrudes into the cavity of the distal outflow tract, separating it
into the intrapericardial components of the aorta and the pulmonary
trunk. The protrusion is the aortopulmonary septum.
Distal
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1 Congenital cardiac anatomy
Ventral
protrusion
Intermediate
Proximal
Right ventricle
Fused
distal
cushions
Unfused
proximal
cushions
Figure 32. Long axis of the developing outflow tract of
the mouse heart, subsequent to the formation of the socalled intercalated swellings (white stars with red
borders). The proximal part will become separated into
the ventricular outflow tracts, with the intermediate part
being transformed into the arterial roots, and the distal
part forming the intrapericardial arterial trunks. The
dotted white line shows the site of fusion between the
aortopulmonary septum and the major cushions within
the outflow tract. This obliterates the embryonic
aortopulmonary foramen.
49
The protrusion eventually fuses with the distal ends of the major
•
outflow cushions to obliterate the embryonic aortopulmonary
foramen. The major cushions themselves fuse with each other so as
to separate, the arterial roots distally, and the right and left ventricular
outflow tracts proximally.
Failure of fusion of the protrusion with the ends of the major outflow
•
cushions results in persistence of an aortopulmonary window.
Failure of fusion of the major outflow cushions themselves produces
•
a common arterial trunk.
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