Introduction
Congenitally corrected transposition of the great arteries is a congenital cardiac
anomaly in which there is both atrioventricular and ventriculoarterial discordant
connection. The condition can occur in situs solitus or inversus, and rarely exists
without other major cardiac anomalies. Anatomically corrected transposition
of the great arteries exists when the spatial arrangements of the great arteries are those
of congenitally corrected transposition of the great arteries, but there is concordant
atrioventricular and ventriculoarterial connections. In the latter condition, associated
cardiac defects can be surgically repaired.
Morphology and embryology.
The most common segmental arrangement is {S,L,L}. The right
atrium empties into a morphological left ventricle and then into the pulmonary artery. The
left atrium empties into a morphologic right ventricle and then into the aorta. The left
ventricle usually lies to the right and the right ventricle to the left, and the mitral
valve lies to the right and the tricuspid valve lies to the left. Compared with the
crossing of outflow tracts in a normal heart, the outflow tracts in patients with
"corrected" transposition are parallel and the ventricular septum lies in a more
anteroposterior position. The mirror-image pertains when there is situs inversus {IDD}.
The conduction system is an important consideration due to the high incidence of
arrhythmias and conduction defects, and due to it’s abnormal location anatomically.
The SA node is in its usual location at the right atrial-superior vena caval junction, and
the septum (the ventricles being reversed) depolarizes in the reverse of normal. The
atrioventricular node and bundle of His differs from the normal . Although a regular
(posterior) AV node is present in front of the coronary sinus ostium in the apex of the
triangle of Koch, the penetrating bundle of His rarely extends from it because of the
septal malalignment. Instead, there is a second anterior (superior) node located adjacent
to the right atrioventricular valve just beneath the right atrial appendage at the
junction it makes with the anterior horn of the limbus and the atrioventricular valve
ring. From this node the penetrating bundle of His usually arises, penetrating the fight
fibrous trigone which passes to lie immediately inferior (caudad) to the pulmonary annulus
in the anterior right ventricular free wall. From there it passes over the annulus and
descends away from it onto the anterior part of the outlet septum. The bundle descends for
some distance before branching, lying between the membranous and muscular portions of the
septum. Hence, in the presence of the typical perimembranous ventricular septal defect,
the bundle of His courses along the anterior margin of the ventricular septal defect on
the left ventricle (right) side in a subendocardial position and bifurcates at its
anteroinferior angle. The right branch penetrates across the crest of the muscular
septum to reach the left-sided right ventricle septal surface near the origin of the
papillary muscle of the conus and passes downward on the surface of the septal band to
reach the moderator band. The sheet-like left bundle continues downward on the left
ventricle septal surface from the branching bundle.
The coronary arteries are reversed so as to supply their appropriate ventricle,
and if not so, make the diagnosis of congenitally corrected transposition of the great
arteries suspect. Hence, the right-sided coronary artery gives rise to the anterior
descending branch, which follows the course of the septum and gives rise to the left
circumflex coronary artery, which encircles the mitral valve. Conversely, the left-sided
coronary artery follows the usually course of the right coronary artery.
Associated anomalies. Ventricular septal defects are the most common
associated cardiac lesion, and occur in 80% of cases. Although they may occur anywhere,
the typical ventricular septal defect is large and subpulmonary and is associated with
virtual absence of the membranous septum (perimembranous). The ventricular septal defect
is bounded by the pulmonary annulus superiorly, the mitral ring posteriorly, the outlet
septum anteriorly, and the muscular septum inferiorly. This perimembranous ventricular
septal defect therefore usually lies within the Y of the septal band and beneath the
outlet septum. The ventricular septal defect is not infrequently narrowed or nearly closed
by an aneurysm of the membranous septum or valvar excrescence from the left-sided
tricuspid valve.
Pulmonary outflow tract The pulmonary valve arises from the right-sided
morphologically left ventricle, and is wedged in between the mitral and tricuspid valve.
Pulmonary outflow tract obstruction is not uncommon and is usually subpulmonary, sometimes
associated with a valvar deformity. Subpulmonary stenosis may be due to a subvalvar
membrane that is adherent to the right-sided mitral valve, or to a frank aneurysmal
bulging of the membranous septum into the posterior part of the outflow tract with or
without a ventricular septal defect. Less severe obstruction is usually due to fibrous
tags (valvar excrescence) attached to the left ventricular-pulmonary artery junction
("annulus"), or membranous septum, or the right-sided mitral valve; or to valvar
excrescence projecting through a ventricular septal defect from the left-sided tricuspid
valve.
Tricuspid valve abnormalities are extremely common, occurring in perhaps as many
as 90% of cases. In most cases, there is leaflet dysplasia with abnormal, thickened
chordal attachments of septal and posterior leaflets., and in a minority there is a true
Ebstein-like deformity in which the septal leaflet is displaced downward and some of the
left ventricle appears atrialized.
Treatment
Repair of ventricular septal defect. The right atrium is opened and the
ventricular septal defect is viewed through the mitral valve. Improved exposure can be
achieved by either incising the base of the septal leaflet of the mitral valve near the
superior commissure and retracting the leaflet upwards, or alternatively, by repairing the
ventricular septal defect through the aorta. The subpulmonary tract is examined, and
unless the pulmonary valve itself is stenotic or there are valvar excrescence obstructing
the subvalvar area, little can be done to improve the obstruction short of placing a
valved extracardiac conduit. However, if the pulmonary blood flow has been large (Qp:Qs
> than 2.0 preoperatively), even a 50 mm Hg gradient does not necessarily indicate the
need for a conduit. With ablation of the left-to-right shunt by closure of the large
ventricular septal defect, the right-sided left ventricle pressures will usually fall
appreciably. The margins of the ventricular septal defect are studied. The location of the
anterior AV node and bundle of His arching over the subpulmonary outflow tract and passing
anterior to the ventricular septal defect are understood. The repair is made by keeping
the sutures on the left (right ventricle) side of the defect. |