6.3 Transposition of the Great Arteries (d-TGA & l-TGA)

Dextro-Transposition of the Great Arteries (d-TGA)

Dextro-Transposition of the Great Arteries (d-TGA) is a critical, ductal-dependent cyanotic congenital heart lesion. It is characterized by ventriculoarterial discordance. The morphological right ventricle gives rise to the aorta, and the morphological left ventricle gives rise to the pulmonary artery.

Normally, the systemic and pulmonary circulations operate in series. In d-TGA, they operate in parallel. Deoxygenated systemic venous blood returns to the right atrium, enters the right ventricle, and is pumped straight back out to the body via the aorta. Simultaneously, oxygenated blood returning from the lungs enters the left atrium, goes to the left ventricle, and is pumped right back to the lungs via the pulmonary artery. Without mixing between these two parallel circuits (via an ASD, VSD, or PDA), the condition is rapidly fatal upon birth.

Sonographic Diagnosis of d-TGA

A pivotal, life-saving sonographic principle is that the 4-chamber view is almost always perfectly normal in isolated d-TGA. The atria, ventricles, and AV valves are structurally sound and appropriately connected (atrioventricular concordance). The pathology lies entirely above the ventricles.

The diagnosis hinges exclusively on the outflow tract sweeps and the 3-vessel trachea (3VT) view:

1. Parallel Outflow Tracts: Normally, the left ventricular outflow tract (aorta) and right ventricular outflow tract (pulmonary artery) cross over each other at roughly a 70-degree angle. In d-TGA, a sweep from the 4-chamber view upward will reveal the two great arteries exiting the heart running completely parallel to one another in a "double barrel" or "shotgun" appearance.
2. Vessel Identification: The vessel arising from the anterior, morphological right ventricle can be traced up to form the aortic arch and give off head and neck vessels. The vessel arising from the posterior, morphological left ventricle will bifurcate almost immediately into the left and right pulmonary branches.
3. 3VT View: The normal "V" configuration (formed by the convergence of the aortic arch and ductus arteriosus) is obliterated. Because the aorta is displaced anteriorly and to the right, and the pulmonary artery is posterior, the ultrasound beam often cuts through them transversely, showing a single large vessel (the aorta) or a parallel, non-converging arrangement (an "I" sign).

Postnatal Management of d-TGA

Delivery must occur at a tertiary care center. Immediate survival depends on blood mixing.

• Prostaglandin E1 (PGE1) is infused immediately to keep the ductus arteriosus open.
• If mixing at the atrial level is inadequate (restrictive patent foramen ovale), an urgent Balloon Atrial Septostomy (Rashkind procedure) is performed in the catheterization lab to rip open the atrial septum and allow oxygenated blood into the systemic circulation. The definitive surgical fix is the Arterial Switch Operation (ASO).

Levo-Transposition of the Great Arteries (l-TGA)

Levo-Transposition (l-TGA), also known as Congenitally Corrected TGA (ccTGA), is a fascinating embryological anomaly resulting from L-looping of the primitive heart tube instead of the normal D-looping.

This creates a "double discordance":

1. Atrioventricular Discordance: The right atrium connects to the morphological left ventricle, and the left atrium connects to the morphological right ventricle.
2. Ventriculoarterial Discordance: The morphological left ventricle gives rise to the pulmonary artery, and the morphological right ventricle gives rise to the aorta.

Physiology and Sonography of l-TGA

Because of the double discordance, the blood flow pathway is physiologically "corrected." Deoxygenated blood from the right atrium goes to the left ventricle, which appropriately pumps it to the pulmonary artery. Oxygenated blood from the lungs enters the left atrium, goes to the right ventricle, and is pumped to the aorta. The child is not cyanotic at birth.

Sonographically, diagnosing l-TGA requires careful attention to ventricular morphology in the 4-chamber view:

• Inverted Ventricles: The morphologic right ventricle (identified by the moderator band, coarser trabeculations, and the tricuspid valve which inserts slightly lower/apical on the septum) is situated on the left side of the chest.
• Parallel Outflow Tracts: Similar to d-TGA, the outflow tracts run parallel. The aorta arises anteriorly and to the left (hence "levo").

While physiologically corrected, l-TGA is fraught with long-term complications. The morphological right ventricle and tricuspid valve are not designed to withstand high systemic pressures over a lifetime, inevitably leading to right ventricular failure and tricuspid regurgitation. There is also a high risk of complete heart block due to the abnormal position of the AV conduction bundle.