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Morphology and embryology
Complete transposition of the great arteries is one of the most common types of
cyanotic congenital heart disease, and is defined as a concordant connection at the
atrioventricular level and a discordant one at the ventriculoarterial level. The term
"complete" was traditionally used to differentiate transposition of the great
arteries from double outlet right ventricle, the latter being regarded as an incomplete
form of transposition of former. The atria in transposition of the great arteries can be
virtually normal, or have any type of interatrial communication. The atrioventricular
junction is also relatively normal, as are the course and disposition of the conduction
tissues[387]. The pulmonary trunk is in fibrous continuity with the atrioventricular
valves, although they may not be wedged in between the atrioventricular valves as much as
the aortic valve in concordant ventriculoarterial connection. With very few exceptions and
some subtleties, the ventricular morphology is normal.
The anatomy of the coronary arteries[387], which was previously of academic
interest only, has gained new importance due to the arterial switch operation. When
considering the coronary arteries, it is necessary to take into account separately their
origin from the aortic sinuses, their course relative to the pedicles of the arterial
trunks, and the origin and course of the artery to the sinus node. The epicardial course
of the arteries within the ventricular mass is virtually normal, with the left anterior
descending coronary artery within the anterior interventricular groove and the right and
circumflex arteries within their respective atrioventricular grooves. The origin of the
coronary arteries are virtually always from the two sinuses facing the pulmonary trunk,
named the right and left facing sinuses when observed from the non-facing coronary
sinus[386]. Any of the three coronary arteries may arise from either of the two facing
sinuses. Most (~60%) cases of transposition of the great arteries have normal coronary
arterial anatomy, in which the left anterior descending and left circumflex coronary
arteries branch from the left main coronary artery which arises from the right facing
sinus, while the right coronary artery arises from the left facing sinus. The most
common variation (~15%) is for the left circumflex coronary artery and right coronary
artery to arise from the left facing sinus, the left circumflex coronary artery then
running behind the pulmonary artery and through the transverse sinus, while the left
anterior descending coronary artery arises normally from the right facing sinus. Almost
any combination is possible, although particularly important variations are those in which
one of the coronary arteries crosses in front of the subaortic infundibulum after
originating from the sinus, and those in which an artery, or a common stem, runs between
the aorta and the pulmonary trunk. Experience has shown that almost any pattern of
coronary anatomy can be transferred successfully, although it is more difficult to avoid
distortion if all the coronary arteries arise from a single sinus by one, two or three
orifices.
The course of the coronary artery to the sinus node is also of importance in
order to decrease the incidence of postoperative rhythm disturbances. The usual course of
the artery is through the interatrial groove, having originated from the proximal part of
either the right coronary artery or left circumflex coronary artery. It can also burrow
deeply through the atrial wall as it ascends, placing it at risk should procedures be
carried out on the superior rim of the fossa ovalis. In a small proportion of cases,
perhaps 10%, the artery may cross the lateral wall of the right atrial appendage or the
dome of the left atrium, and be at risk during atriotomy or Blalock-Hanlon septectomy.
Associated malformations
The most commonly associated malformations are those of the ventricular septum and of
the left ventricular outflow tract. The term "simple transposition" is used to
differentiate between transposition of the great arteries with coexistent atrial septal
defect, ventricular septal defect, patent arterial duct, or pulmonary stenosis, from
transposition of the great arteries with more complicated anomalies such as common,
straddling, stenotic, or atretic atrioventricular valves, or hypoplasia or absence of
ventricular chambers, the latter cases of which are referred to either by the dominant
lesion, as in, for example, tricuspid atresia with discordant atrioventricular connection,
or simply as complex transposition.
Ventricular septal defect. An intraventricular communication can exist at any of
the basic sites expected for hearts with concordant ventriculoarterial connections. Thus
the defect can be perimembranous, (that is, it abuts directly on an area of the fibrous
trigone, which in this case is adjoined by the tricuspid, mitral and usually pulmonary
valve), it can have exclusive muscular borders, or rarely, may be subarterial. The
location of the ventricular septal defect in transposition of the great arteries is
dependent on the alignment of the ventricular septum. Approximately 79% of hearts with
transposition of the great arteries have a normally aligned ventricular septum, 18% have
anterior alignment, and 3% have posterior malalignment. In hearts with a normally aligned
ventricular septum, approximately half of the ventricular septal defects are
perimembranous and half are trabecular, while in hearts with a malaligned ventricular
septum approximately 75% of the resultant subpulmonary ventricular septal defect are
perimembranous, the remainder being trabecular[282].
Left ventricular outflow tract obstruction. Approximately 20% of (older) infants
with transposition of the great arteries with intact ventricular septum have left
ventricular outflow tract obstruction. The majority of these obstructions are dynamic, and
may be due to leftward septal deviation from elevated right ventricle pressures ± an
elevated Qp:Qs. This type of dynamic left ventricular outflow tract obstruction often gets
worse after balloon atrial septostomy as better interatrial mixing is produced, and is
reliably relieved by the arterial switch operation, although some patients may develop
septal hypertrophy and persistent left ventricular outflow tract obstruction following the
arterial switch operation. Less commonly, left ventricular outflow tract obstruction is
due to a discrete fibrous diaphragm below the pulmonary valve cusps, fibrous tissue tags
arising from mitral valve apparatus or membranous septum, or from valvar pulmonary
stenosis. Approximately 20% of patients with transposition of the great arteries and ventricular
septal defect have left ventricular outflow tract obstruction. The obstruction is
usually more severe than in patients with transposition of the great arteries with intact
ventricular septum, and is thought to be due to posterior malalignment of the outlet
septum, which leads to a long-segment, tunnel-like obstruction.
Almost any other cardiac lesion can coexist with transposition of the great arteries.
Among the more important and common are coarctation of the aorta, which is commonly
associated with a restrictive subaortic obstruction, and straddling and overriding of
either of the atrioventricular valves.
Hemodynamics
In transposition of the great arteries the pulmonary artery and systemic circulations
are arranged in parallel rather than in series, with the aorta arising from the right
ventricle and the pulmonary artery from the left ventricle. Survival depends upon mixing
between the two circulations, which can occur at the atrial level through a patent foramen
ovale, atrial septal defect, or following balloon atrial septostomy, at the ventricular
level through a perimembranous, muscular or malalignment ventricular septal defect (if one
is present), at the great arterial level through a patent arterial duct, or through
bronchial collateral vessels.
In transposition of the great arteries with intact ventricular septum mixing
initially occurs through the foramen ovale and arterial duct. As pulmonary vascular
resistance declines in the postnatal period, aortic to pulmonary artery shunting at the
arterial duct level greatly increases, resulting in increased pulmonary blood flow and
return to the left atrium. Because of the parallel circulations, the degree of shunting at
the arterial level must be matched by an equal amount of left to right atrial shunting. As
the arterial duct constricts and eventually closes over the first few days of life, there
is less aortic to pulmonary artery shunting, and arterial hypoxemia increases. Mixing is
then solely dependent on bi-directional mixing at the atrial level. However, as less
pulmonary blood flow results in lower left atrium pressure, the patent foramen ovale
becomes restrictive and profound hypoxemia results. Effective palliation is predicated
upon maintenance of an adequate atrial-level shunt, which suffices in almost all cases,
but if not, also upon the maintenance of ductal patency.
In transposition of the great arteries with a nonrestrictive ventricular septal
defect or patent arterial duct, profound hypoxemia is uncommon in the neonatal
period. In the presence of a nonrestrictive ventricular septal defect, there is
bi-directional shunting at the ventricular level, which becomes increasingly left to right
as pulmonary vascular resistance drops. The increased pulmonary blood flow results in
increased return to the left atrium and left ventricle, facilitating intracardiac mixing
at both ventricular and atrial levels (the latter if an adequate patent foramen ovale
exists). In the presence of a large patent arterial duct and an intact septum, the degree
of right to left shunting that occurs at the arterial level must be matched by an equal
amount of left to right atrial shunting. Usually the left atrium is quite compliant, the
patent foramen ovale adequate, and there is almost exclusive left to right atrial
shunting. Due to the increased pulmonary blood flow, patients with a either a
nonrestrictive ventricular septal defect or large patent arterial duct have a relatively
high SaO2, usually in the 75 - 85% range. The marked volume overload, however, eventually
leads to congestive cardiac failure, which is the usual presenting symptom in these
patients, or if prolonged, the development of pulmonary vascular obstructive disease.
In transposition of the great arteries with a ventricular septal defect and pulmonary
stenosis or pulmonary atresia the level of hypoxemia depends upon the degree of
obstruction to pulmonary blood flow, and the amount of pulmonary blood flow derived from
collateral vessels. The hemodynamics in infants with minimal pulmonary stenosis resemble
those with a large ventricular septal defect, while those with severe stenosis or
pulmonary atresia present with a clinical picture similar to other infants with diminished
pulmonary blood flow and hemodynamically may be indistinguishable from patients with
tetralogy of Fallot.
Clinical presentation & management
The clinical picture of infants with transposition of the great arteries ranges from
extreme cyanosis without congestive failure to minimal cyanosis with profound congestive
failure and circulatory collapse, depending upon the degree of mixing between the systemic
and pulmonary circulations and the presence or absence of associated malformations.
Infants with transposition of the great arteries and intact ventricular septum
almost invariably present with cyanosis during the first week of life. Otherwise
healthy newborns with cyanosis should be considered an emergency, since complete closure
of the arterial duct leading to hemodynamic collapse may occur rapidly. Neonates with
transposition of the great arteries and ventricular septal defect more commonly
present later on in the first month of life with signs of congestive failure.
Classically, these infants have difficulty feeding and subsequent failure to thrive, along
with tachypnea and excessive perspiration, especially with feeding. Infants with
transposition of the great arteries, ventricular septal defect and pulmonary
stenosis present variably depending on the degree of pulmonary stenosis. If the
pulmonary stenosis is severe, pulmonary blood flow is restrictive, hypoxemia profound, and
the infant presents in the first week of life. If the pulmonary stenosis is only mild,
then pulmonary blood flow may be excessive and infants present with congestive failure
somewhat later in life.
The chest film in transposition of the great arteries and intact ventricular
septum is frequently characteristic, but may be normal. Typically the superior
mediastinum is narrow due to the anterior-posterior relationship of the great vessels and
due to reduced thymic tissue. Cardiac size is initially normal but soon enlarges with the
cardiac apex shifted to the left and inferiorly, giving the typical oval-shaped or
egg-on-side pattern. In patients with a transposition of the great arteries and ventricular
septal defect there is almost invariably increased pulmonary vascular markings. In
those with transposition of the great arteries, ventricular septal defect and pulmonary
stenosis, the size of the heart and the degree of pulmonary vascular markings depends
upon the degree of right ventricular outflow tract obstruction and pulmonary blood flow.
Patients with severe obstruction may have a normal cardiac size with decreased pulmonary
markings, whereas those with only mild right ventricular outflow tract obstruction may
have an enlarged cardiac silhouette and increased vascular markings.
Cardiac echocardiography is a sensitive and specific tool in the diagnosis and
evaluation of an infant with transposition of the great arteries and has supplemented, and
in may cases replaced, other diagnostic modalities, including cardiac cineangiography. The
goals of echocardiography are
- to define the ventriculoarterial connections and coronary arteries,
- to assess for the presence of atrial-, ventricular-, and great arterial-level shunts,
- to define the adequacy of intra-atrial communication,
- to assess the subpulmonary area, with particular emphasis on defining any dynamic
obstruction in transposition with an intact ventricular septum, and
- to assess for the presence of long-tunnel stenosis occasionally present in transposition
with ventricular septal defect.
Selective angiocardiograms in the right and left ventricles and frequently in the great
vessels are important to the understanding of the ventriculoarterial connections,
atrioventricular and semilunar valves, and associated lesions, including defects of the
ventricular septum, right or left ventricular outflow obstruction, and coarctation of the
aorta.
D-Transposition With Ventricular Septal
Defect
In the majority of patients with D-transposition of the great arteries, as in those
with normally related great arteries, perimembranous, malalignment, atrioventricular, and
midmuscular ventricular septal defects are exposed through the right atrium. Techniques
for closure of ventricular septal defects are discussed in the section of ventricular
septal defects. Some of the perimembranous or malalignment defects can also be closed
through the anterior semilunar valve; for some malalignment ventricular septal defects
coexisting with multiple muscular defects, a combined transatrial and trans-neoaortic
approach may be necessary. A right ventriculotomy is only very rarely indicated in closure
of a ventricular septal defect, those with infundibular or subarterial ventricular septal
defects or those with anterior muscular defects. A right ventricular incision may also be
necessary in patients with right ventricular infundibular hypoplasia and a ventricular
septal defect (often associated with hypoplasia or interruption of the aortic arch).
Apical muscular defects are closed with a left ventriculotomy, or preferably, in the
catheterization laboratory with a clamshell device, followed within hours by an arterial
switch operation and closure of any additional ventricular septal defects. Alternatively,
a clamshell device can be positioned under direct vision in the operating room. Excellent
visualization of the seating of the device is obtained when both great arteries have been
divided.
In approximately 10% of patients the sternum is not closed primarily; instead, the
sternotomy is covered with a Silastic sheet without reapproximation of the bone edges,
followed by secondary closure after surgery. Most of these patients will have had
significant myocardial edema, often following intraoperative revision of the coronary
anastomosis or closure of a residual ventricular septal defect and prolonged
cardiopulmonary bypass.
Techniques for simultaneous repair of other associated defects (e.g., coarctation,
interrupted aortic arch, and hypoplastic aortic arch) in conjunction with the arterial
switch operation are discussed in sections dealing with those lesions. Patients with
D-transposition of the great arteries and hypoplasia or coarctation of the aortic arch
have a strong likelihood of having subvalvar obstruction of the right ventricular outflow
tract. This obstruction may not be apparent preoperatively with an open ventricular
septal defect and a patent arterial duct when less than full cardiac output is crossing
the right ventricular outflow tract, but it may result in right ventricular hypertension
and obstruction after repair. Liberal use of a patch on the right ventricular outflow
tract is recommended in these cases. In the occasional case in which D-transposition of
the great arteries with a ventricular septal defect is accompanied by extreme diffuse
aortic hypoplasia, including the aortic valve, a Damus-Kaye-Stansel approach may be
required.
Why the arterial switch operation is preferred
Primarily, there is increasing concern about late results of the atrial or physiologic
type of repair: Although the hospital mortality rate for both the Mustard and the Senning
operations is low, the long-term outcome of these procedures is affected by a number of
late complications, the most important being a high incidence of atrial dysrhythmias (more
than 50% within 10 years) and a less clearly established incidence of late right
ventricular (systemic ventricular) dysfunction (approximately 10%). A variety of
theoretical considerations support the assumption that the left ventricle is more suitable
than the right to serve the systemic circulation. The left ventricle (with its cylindric
shape, its concentric contraction pattern, and both the inlet and the outlet orifices
situated in close proximity) seems ideally adapted to work as a pressure pump, whereas the
right ventricle (with its crescent-shaped cavity, its large internal surface
area-to-volume ratio, its bellows-like contraction pattern, and its more separated inlet
and outlet segments) seems better suited to serve as a low-pressure-volume pumping
chamber. Also, the left ventricle has two coronary arteries (left anterior descending and
left circumflex), while the right ventricle has one (right coronary). Developmentally, the
stratum compactum of the myocardium is thicker in the left ventricle than in the right,
and phylogenetically, the left ventricular sinus is derived from the primitive ventricle
at the stage of ventricular looping (the "original pump" of our phylum
Chordata), while the conus and much of the right ventricle are derived from the bulbus
cordis. Furthermore, the papillary muscles of the right ventricle are small and numerous,
originating both from the septum and from the right ventricular free wall, in contrast to
the two papillary muscles of the left ventricle. This architecture allows the tricuspid
valve to be pulled apart as the right ventricle dilates, leading to tricuspid
regurgitation. The arterial switch operation recruits the left ventricle as a systemic
pump, and because atrial manipulation is essentially limited to closure of an atrial
communication, it is also anticipated that after the arterial switch, atrial dysrhythmias
should be significantly reduced.
For D-transposition of the great arteries with an intact ventricular septum the
arterial switch operation is the procedure of choice and is carried out during the
neonatal period when the left ventricle is still "prepared" to support the
systemic circulation by the intrauterine physiology. Ideally, the repair should be
performed within the first weeks of life. Later, there is increasing likelihood that the
left ventricle will be unable to accommodate the increased workload. A number of
circumstances can arise that cause postponement of surgery beyond the "safe"
period for an arterial switch. For example, a neonate may be seriously ill with
necrotizing enterocolitis, renal or hepatic failure, or a hemorrhage in the central
nervous system. Also, the neonate may be geographically distant from a center offering the
arterial switch operation, or occasionally a patient with D-transposition of the great
arteries and a ventricular septal defect awaiting "elective" repair may
experience spontaneous partial closure of the ventricular septal defect, resulting in a
low left ventricular pressure.
Because of these possibilities, some empiric criteria for predicting postoperative left
ventricular performance have been developed. Before the age of 2 weeks, every patient with
D-transposition of the great arteries and an intact ventricular septum is repaired,
regardless of preoperative left ventricular pressure measurements. Laboratory studies in
rats have demonstrated surprisingly rapid induction (within 48 hours) of the genes
responsible for the isozyme adaptation of the myocardial myosin, actin, and tropomyosin in
response to an acute pressure load. Furthermore, various proto-oncogenes involved in the
regulation of cell growth accumulate in rat cardiac cells within 1 hour of an acute
pressure load. Stress protein HSP70 is also seen at high levels within 2 to 3 hours of an
acute pressure load. Coincident to these developments has been further refinement of
echocardiographic techniques to assess left ventricular mass and volume more accurately.
Therefore, a successful arterial switch operation can be performed 7 to 10 days after
preliminary pulmonary artery banding and placement of a modified Blalock-Taussig shunt.
In patients who have D-transposition of the great arteries with a ventricular septal
defect, repair is recommend shortly after the diagnosis is made. Although the left
ventricle remains "prepared" when a large septal defect maintains systemic left
ventricular pressure, a delay in surgery may result in pulmonary vascular obstructive
disease, failure to thrive, or pulmonary infections, and, in some cases partial closure of
the ventricular septal defect results in an "unprepared" left ventricle. There
is therefore no advantage in delaying corrective surgery. Palliative pulmonary artery
banding is indicated only in patients with multiple muscular defects, to allow time for
growth and spontaneous closure of the less surgically accessible ventricular septal
defects. Also, trans-catheter closure of the muscular ventricular septal defects (avoiding
a ventriculotomy) can currently be accomplished when infants reach a weight of 5 kg.
Reoperations after Mustard and Senning Operations.
Senning introduced the physiological repair transposition of the great arteries in 1958
and Mustard published his experience with the atrial switch in 1964. The Mustard operation
soon became the operation of choice, and survival rates of over 90% were reported. The
original Mustard operation was a two-stage correction and included a Blalock-Hanlon atrial
septectomy followed by an atrial switch operation in the second or third year of
life. Balloon atrial septostomy was introduced by Rashkind and Miller in 1966,
considerably improving the survival of infants with transposition of the great arteries,
and led to early balloon atrial septostomy followed by a Mustard operation in the first
year of life as the preferred operative management. Brom and Quagebeur reintroduced the
Senning operation in 1975 and thereafter it became the procedure of choice. Both
operations offer excellent early, but only good mid-term results, and as such, only a few
centers are currently performing the atrial switch operation as first-line therapy
for transposition of the great arteries[218]. The Mustard operation consists of an atrial
septectomy and placement of a baffle that directs caval blood to the mitral valve, thereby
leaving the pulmonary veins to drain into the tricuspid valve. The Senning operation
utilized a flap the atrial septum as the inferior wall of the caval tunnel, and a flap of
lateral atrial wall as the superior wall of the caval tunnel in order to achieve the same
goal. A not insignificant number of patients who are 2 - 3 decades out of the atrial
switch operation are returning for management of complications associated with these
operations.
Complications following atrial switch operations which may require operative therapy
include systemic or pulmonary venous obstruction, baffle leaks, tricuspid valve
incompetence, and right ventricular dysfunction. Complications such as residual or
recurrent ventricular septal defect or left ventricular outflow tract obstruction are
treated as in isolated ventricular septal defect or left ventricular outflow tract
obstruction. Other complications in which operative therapy has not been completely
defined include left ventricular dysfunction and arrhythmias. These complications are
generally diagnosed by echocardiography, and confirmed by cardiac catheterization.
Systemic venous obstruction much more commonly follows the Mustard operation as
compared to the Senning operation, affects the superior vena cava much more commonly than
the inferior vena cava, and is thought to be primarily related to the shape of the baffle
used in the Mustard operation. superior vena cava obstruction may be clinical silent, or
can be associated with facial edema, pleural effusions, chylothorax, venous collateral on
the chest wall, increased head circumference, or delayed closure of the fontanelles[379].
inferior vena cava obstruction is associated with hepatomegaly, protein losing
enteropathy, ascites or leg edema. Asymptomatic isolated superior vena cava obstruction
may not need treatment, however, all inferior vena cava obstructions and symptomatic
superior vena cava obstructions require either balloon dilatation or operative revision.
Pulmonary venous obstruction is rare after the Senning operation and infrequent
following the Mustard operation, but is a serious complication which requires prompt
attention. Tachypnea, dyspnea, cough, fatigue and decreasing exercise tolerance are the
common symptoms. Mild cyanosis may be present and pulmonary venous congestion or
interstitial pulmonary edema may be seen on the chest film. Urgent reoperation is
generally indicated, although interventional balloon dilatation has been attempted
successfully.
Significant Baffle leaks are unusual, although small baffle leaks can be found
in approximately 20% of studied patients following the Mustard operation, and only
infrequently following the Senning operation. Indications for repair are similar to those
for atrial septal defect with a left-to-right shunt. If there is a coexisting inferior
vena cava or superior vena cava obstruction, a baffle leak above the site of the
obstruction may cause considerable right-to-left shunting, and is repaired at the time of
the relief of the venous obstruction.
Tricuspid regurgitation is more commonly seen in patients after repair of
transposition of the great arteries/ventricular septal defect and may be due to
morphologic abnormalities of the tricuspid valve, damage to the valve at the time of
repair, or to right ventricle failure and/or arrhythmias. Symptoms include progressive
exertional dyspnea, a cough and fatigue, and the chest film often shows cardiomegaly. Mild
to moderate tricuspid regurgitation is usually well tolerated. Severe tricuspid
regurgitation may be treated by tricuspid repair or replacement, but requires an arterial
switch operation or transplantation if due to severe right ventricle failure.
Right ventricular failure not uncommonly follows atrial switch operations, and
may be due to the fact that the morphological right ventricle is not capable of handling
the systemic workload over many years. As in tricuspid regurgitation, right ventricle
dysfunction and failure is more common in patients in whom a ventricular septal defect was
repaired at the time of the atrial switch operation, and particularly in those in whom the
ventricular septal defect was repaired through a right ventriculotomy. Patients who
develop right ventricle failure can be treated by pulmonary artery banding followed by an
arterial switch operation, or alternatively, by cardiac transplantation.
The incidence of serious arrhythmias appears to have decreased in recent years,
but remains a significant source of long term morbidity and mortality. The incidence of
arrhythmias appears to increase over time, the percentage of patients who remain in normal
sinus rhythm being 72%, 56%, and 50% at 1, 5, and 10 years following the Mustard
procedure. The more common arrhythmias include atrial fibrillation or flutter,
atrioventricular dissociation, sick sinus syndrome, junctional arrhythmia, and episodes of
supraventricular tachycardia. Benign arrhythmias do not require treatment.
Supraventricular tachyarrhythmia is treated medically, while bradyarrhythmias may require
pacemaker insertion. |