Transpositions of the Great Arteries

With Kevin S. Roman

Transposition is a term to describe ventriculoarterial connection. It refers to discordant ventriculoarterial connection: the right ventricle connects to the aorta and the left ventricle to the pulmonary artery. Transposition occurs with various types of atrioventricular connections:
- • With concordant atrioventricular connection: complete transposition
- • With discordant atrioventricular connection: congenitally corrected transposition
- • With various forms of univentricular atrioventricular connections
- • With various forms of atrioventricular connections in the presence of right or left atrial isomerism
There are two classic types of transposition:
- • Complete transposition
- • Congenitally corrected transposition

Complete Transposition

Definition and Classification (Fig. 24.1)

Characterized by normal atria, normally related ventricles, normal atrioventricular connections, and abnormal discordant ventriculoarterial connections
- • Right atrium connects to morphologic right ventricle, which gives rise to the aorta
- • Left atrium connects to morphologic left ventricle, which gives rise to the pulmonary artery
Also called D-transposition, as it occurs with D-loop ventricles (the morphologically right ventricle on the right side of the morphologically left ventricle) and, in the majority of cases, with a right-sided and anterior aorta. It is a confusing term because of variations in ventricular loop and great arterial relationship.
Complete transposition as a simple form: about 50% of patients are without associated defects other than a patent foramen ovale or a small patent ductus arteriosus.
Associated lesions in 50%: ventricular septal defect, left ventricular outflow tract obstruction, and obstructive lesion of the aortic arch.

Fig. 24.1 Complete transposition. Asterisk indicates subpulmonary outflow tract, which is often narrowed in the complex form. Ao, ascending aorta; d, ventricular septal defect; LA, left atrium; LV, left ventricle; PA, pulmonary artery; RA, right atrium; RV, right ventricle.

Pathophysiology

Concordant atrioventricular and discordant ventriculoarterial connections result in two isolated parallel circulations:
- • Deoxygenated systemic venous blood returns to the right atrium, enters the right ventricle, and exits into the aorta, only to return to the right atrium.
- • Oxygenated pulmonary venous blood returns to the left atrium, enters the left ventricle, and exits into the pulmonary artery, only to return to the left atrium.
Survival depends on mixing between these circulations, which may be across a patent foramen ovale or atrial septal defect, additional ventricular septal defect, or patent ductus arteriosus.
As the pulmonary vascular resistance falls postnatally, there is increased pulmonary blood flow unless an associated pulmonary stenosis is present.

Clinical Manifestations

Varies according to the presence of associated defects
Soon after birth with closure of ductus arteriosus there is severe hypoxemia.
Cyanosis is the most common presentation and is often seen within the first hours of life.
Mild cyanosis with associated lesions that permit good mixing
Heart failure is seen with associated large ventricular septal defects.

Chest Radiographic Findings

Neonates often have normal heart size and pulmonary vascularity.
Increased pulmonary vasculature with an overcirculation pattern with decreasing pulmonary vascular resistance
Over time, heart size enlarges.
An oblong egg-on-side appearance due to rightward displacement of the right ventricular outflow tract and combined right atrial and left ventricular dilatation
Narrow vascular pedicle due to parallel course of anteroposteriorly related great arteries and small thymus as it atrophies with postnatal stress
Decreased pulmonary vasculature with associated severe pulmonary stenosis
Aortic arch usually left sided