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Key Questions in CONGENITAL CARDIAC SURGERY
50
Third arch artery
A
Pulmonary
arch arteries
Ventral
protrusion
B
Fourth arch arteries
Third arch artery
Distal extent of
outflow cushions
Intrapericardial
pulmonary trunk
Intrapericardial
aorta
Developing right
ventricle
Distal outflow cushions
Figure 33. Stages of formation of the aortopulmonary septum in the
developing mouse: A) early stage, in which the arteries of the 3rd and 4th
pharyngeal arches are bilaterally symmetrical, and separated from the
arteries of the pulmonary arches by the dorsal wall of the aortic sac (white
star with red borders). The white arrows with red borders show the junction
between the distal outflow tract and the aortic sac at the margins of the
pericardial cavity; B) the dorsal wall of the aortic
cavity of the distal outflow tract, separating the intrapericardial components
of the developing aorta and pulmonary trunk. The white arrows with red
borders continue to show the margins of the pericardial cavity. The
protrusion has become the aortopulmonary septum. At this stage, the
space between the protrusion and the edges of the major outflow cushions
sac has protruded into the
1 Congenital cardiac anatomy
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(double-headed white arrow) is an aortopulmonary foramen. It is closed at
the later stage to separate the intrapericardial pulmonary arteries. The
major cushions fuse to separate the intermediate and proximal parts of the
outflow tract.
24 Describe the embryological origin of vascular rings
Multiple arrangements of the extrapericardial branches of the aorta
•
are known to encircle the trachea-oesophageal pedicle and produce
‘dysphagia lusorum’.
All of these patterns are well explained on the basis of the
•
hypothetical double arch as proposed by Edwards (Figure 34).
Right
arch
Subclavian artery
Common carotid artery
Arterial duct
Descending
aorta
Aorta
Left
arch
Subclavian artery
Common carotid artery
Arterial duct
Pulmonary
trunk
Figure 34. Hypothetical double aortic arch (as hypothesised by
Edwards), which encircles the tracheo-oesophageal pedicle and unites
posteriorly to form a neutral descending aorta. Each arch gives rise to a
common carotid artery and a subclavian artery from its cranial surface,
and an arterial duct from its dorsal surface. The various vascular rings,
along with isolation of the brachiocephalic arteries, are all explained on
the basis of attenuation and disappearance of the different components
of the double arch, including the arterial ducts.
51
Key Questions in CONGENITAL CARDIAC SURGERY
Examination of the evolution of the arteries extending through the
•
pharyngeal arches of the developing embryo provides validation for
the correctness of the hypothesis as put forward by Edwards.
Although six sets of pharyngeal arch arteries tend to be shown in the
•
classical diagram based on the investigation of Rathke, carried out in
the 19th century, in fact it is only the arteries of the 3rd, 4th and
pulmonary arches that provide the vessels as shown by Edwards in
the hypothetical model (Figure 34).
During early development, these arteries are bilaterally symmetrical
•
and encircle the developing tracheo-oesophageal pedicle.
With ongoing normal development, the right-sided components of
•
these bilateral symmetrical primordiums undergo attenuation and the
components encircling the pedicle to join the descending aorta
disappear.
During the stages of attenuation, nonetheless, a pattern can be
•
visualised that provides the validation of the concept advanced by
Edwards (Figure 35).
52
Right third
arch artery
Right fourth
arch artery
Regressing right
pulmonary arch artery
Aorta
Pulmonary
arteries
Right horn
of aortic sac
Left horn of
aortic sac
Pulmonary trunk
Figure 35. Reconstruction of the developing arteries of the pharyngeal arches
in the mouse at embryonic day 12.5. There is clear bilateral symmetry of the
arteries of the 3rd and 4th arches, which both join the right- and left-sided
descending aortas. The arteries of the pulmonary arch are also bilateral at this
stage, although the right-sided artery is beginning to regress. The pattern is
remarkably similar to the hypothetical double arch as proposed by Edwards.
Image created and reproduced with permission by Dr. Simon Bamforth, Newcastle
University, UK.
Left third
arch artery
Left fourth
arch artery
Left pulmonary
arch artery
1 Congenital cardiac anatomy
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25 What is the origin of the arterial duct?
The arterial duct (also known as the ‘ductus arteriosus’) is an integral
•
part of the foetal circulation. Its presence ensures that the
deoxygenated systemic venous return of the foetus, having passed
through the right ventricle, is returned to the placenta, thus
bypassing the lungs during foetal life.
From a developmental standpoint, the arterial duct is derived from
•
the artery of the pharyngeal arch previously described as being ‘6th’.
We now know that there is never a 5th arch. The so-called ‘6th arch’,
therefore, is better described as being the pulmonary arch.
During early development, the arteries of the pharyngeal arches are
•
bilaterally symmetrical.
By the time that the arteries of the pulmonary arches are
•
recognisable, however, the arteries of the 1st and 2nd arches have
effectively disappeared, becoming incorporated into the arteries
supplying the head and face. It is possible, nonetheless, to recognise
symmetrical arteries extending through the 3rd, 4th and pulmonary
arches (Figure 36).
With ongoing development, the right-sided components of the
•
bilateral symmetrical arrangement largely disappear. This leaves the
left pulmonary arch artery as the arterial duct, with the left 4th arch
artery forming the transverse component of the aortic arch (Figure
37).
When first formed, the subclavian arteries, derived from the 7th
•
cervical intersegmental arteries, take their origin from the descending
aorta. It is only later in development that these arteries migrate
cranially so as to arise from the transverse aortic arch. They cross the
insertion of the duct to the descending aorta during this process. The
isthmus of the aortic arch is the component between the origin of the
left subclavian artery and the junction with the arterial duct. It cannot
be defined until the left subclavian artery achieves its definitive
position.
The right and left pulmonary arteries develop within the pharyngeal
•
mesenchyme, taking their origin from the caudal component of the
aortic sac, which gives rise initially to the bilaterally symmetrical
arteries of the pulmonary arches.
53
Key Questions in CONGENITAL CARDIAC SURGERY
Cranial dorsal aortas
Regressing
second arch
arteries
Third arch
arteries
Fourth
arch
arteries
Pulmonary
54
Dorsal aortas
arteries
Figure 36. Arrangement of the arteries of the pharyngeal arches in the
developing mouse at embryonic day 10.5. The arteries of the 3rd, 4th and
pulmonary arches are bilaterally symmetrical. They extend through the
pharyngeal mesenchyme before merging dorsally to form the
descending aorta. The 7th cervical intersegmental arteries, which will
become the subclavian arteries, arise from the dorsal aorta at this early
stage of development.
Simon Bamforth, Newcastle University, UK.
Image created and reproduced with permission by Dr.
Pulmonary
arch arteries
7th segmental
arteries
Pulmonary
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valve
1 Congenital cardiac anatomy
Transverse aortic arch
Intrapericardial
pulmonary trunk
Aortic valve
Right and left
pulmonary
arteries
Arterial duct
Figure 37. Reconstruction of the cavities of the right (blue) and left
(brown) ventricles, along with the extent of the pericardial cavity (green)
in the developing mouse at the end of embryonic day 12.5. The artery of
the left pulmonary pharyngeal arch has become the arterial duct, while
the artery of the left 4th arch has become the transverse aortic arch. Note
that the right and left pulmonary arteries develop within the pharyngeal
mesenchyma, taking their origin from the caudal component of the
aortic sac. The developing subclavian artery (white arrow with red
borders) still retains its origin from the descending aorta at this stage of
development.
Bamforth, Newcastle University, UK.
Image created and reproduced with permission by Dr. Simon
26 Is there an artery of the 5th pharyngeal arch?
It is frequent to find aberrant arterial channels in patients with
•
congenitally malformed hearts interpreted in terms of ‘5th arch
arteries’.
The classical diagram for the developing arteries of the pharyngeal
•
arches does, indeed, illustrate six pairs of bilaterally symmetrical
channels (Figure 38).
55
Key Questions in CONGENITAL CARDIAC SURGERY
1-4
?5 ?5
6
6
Aortic
56
sac
Figure 38. Classical ‘Rathke’ diagram showing the developing arteries
of the pharyngeal arches. The purported 5th arch arteries are shown
with dotted lines, since their existence remains contentious. The 7th
cervical intersegmental arteries (white arrows with red borders)
originate from the descending aorta (white star with red borders) and
later in development become the subclavian arteries.
To the best of the authors’ knowledge, there is but a solitary example
•
thus far identified of an attenuating artery of the 5th arch, enclosed
within its own segment of pharyngeal mesenchyme (Figure 39).
Arteries extending between the aortic sac and the descending aorta
•
at the site of the postulated 5th pharyngeal arch are exceedingly rare.
Left third
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arch artery
Left horn of
aortic sac
1 Congenital cardiac anatomy
Left fourth
arch artery
Collateral
channel
57
Left pulmonary
arch artery
Figure 39. Reconstruction from the left side of the pharyngeal region
of a development human embryo at Carnegie stage 15. It demonstrates
attenuation of the left-sided artery that initially coursed between the
aortic sac and the descending aorta parallel to the arteries of the left
4th and left pulmonary arches. This arterial channel was embedded
within its own segment of pharyngeal mesenchyme. Although initially
interpreted as a ‘5th arch artery’, we now consider it better described
as a persisting collateral channel.
Collateral channels extending between the posterior terminations of
•
the arteries of the 4th and pulmonary arches, in contrast, are found
in up to half of all developing mouse embryos, and in a comparable
number of human embryos.
The majority of vascular channels interpreted as persistence of the
•
hypothetical artery of the 5th pharyngeal arch is better explained on
the basis of presence of such collateral channels, or else as
remodelling of the walls of the aortic sac.
Key Questions in CONGENITAL CARDIAC SURGERY
27 Describe the anatomy of the sinus node and atrial
conduction
To this day, no evidence has ever been presented to show that
•
postulated internodal atrial conducting tracts or ‘specialised
internodal pathways’ extending through the atrial myocardium to join
the sinus and atrioventricular nodes have been identified and isolated
from the remainder of the atrial myocardium.
Instead, histological examination has demonstrated the sinus node to
•
be a well-defined anatomical entity.
The sinus node can be recognised as occupying the epicardial
•
aspect of the terminal groove at the superior cavoatrial junction,
usually located inferior to the crest of the right atrial appendage. Its
borders are well demarcated, with no extensions of nodal
cardiomyocytes identifiable as extending into the adjacent atrial
myocardium (Figure 40).
58
A B
Figure 40. A) Operative image demonstrating the superior cavoatrial
junction, with terminal groove (yellow dotted line) and anticipated
location of the sinus node (yellow oval). B) Histological section (taken
from the site of the yellow solid line) demonstrating the sinoatrial node
aggregated around a prominent artery. It has a discrete boundary (blue
dashed line) from the adjacent myocardium of the terminal crest and
superior caval vein. There are no insulated tracts identifiable extending
from the node into the adjacent atrial wall.
The specialised cardiomyocytes activate the adjacent atrial
•
myocardium at the margins of the node. The prominent myocardial
bundles within the right atrium, such as the terminal crest,
1 Congenital cardiac anatomy
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Bachman’s bundle and the margins of the oval fossa, then serve to
conduct the impulse generated within the sinus node towards the
atrioventricular node, at the apex of the triangle of Koch.
The pathways within the major myocardial atrial bundles are all
•
composed of ordinary working atrial cardiomyocytes. There is
nothing ‘specialised’, either histologically or electrophysiologically,
regarding these pathways.
Rather, the preferential conduction through the major pathways is
•
dictated by the parallel alignment of the working cardiomyocytes
making up the bundles.
28 Describe the anatomical location of the
atrioventricular conduction axis
Although the specialised conduction tissues themselves are invisible
•
to the cardiac surgeon, the landmarks regarding their anatomical
disposition are now sufficiently robust to permit recognition in all
patients with concordant atrioventricular connections.
The atrioventricular node is located at the apex of the triangle of
•
Koch. The atrial border of the triangle is demarcated by the extension
from the Eustachian valve into the myocardium separating the inferior
border of the oval fossa from the orifice of the coronary sinus. The
ventricular border is formed by the attachment of the septal leaflet of
the tricuspid valve. These borders come together at the site of the
atrioventricular component of the membranous septum, which forms
the apex of the triangle.
The atrioventricular bundle, or bundle of His, passes through the
•
atrioventricular component of the membranous septum to reach the
crest of the muscular ventricular septum. It branches on the crest of
the septum, or just below it, into the right and left bundle branches.
The right bundle branch then courses through the muscular
ventricular septum, emerging on the right ventricular surface in
relation to the medial papillary muscle, also known as the muscle of
Lancisi, or the conal papillary muscle. A line drawn from the apex of
the triangle of Koch to the medial papillary muscle shows the
anticipated course of the atrioventricular conduction axis (Figure 41).
Providing the surgeon keeps all operative manoeuvres outside the
•
boundaries of the triangle of Koch, no damage will be inflicted to the
atrioventricular node.
The landmarks indicating the site of the axis remain consistent in all
•
hearts with deficient ventricular septation when the atrioventricular
connections are concordant.
The connecting atrioventricular node does not occupy the apex of the
•
triangle of Koch in the setting of malalignment between the atrial and
59
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