Define Transposition of the Great Arteries. Describe its clinical features, investigations, management and prognosis.

Definition

• In Transposition of the Great Arteries (TGA), the aorta arises from the right ventricle and the pulmonary artery from the left ventricle (ventriculo-arterial discordance).
• It is the commonest cyanotic CHD to present in the newborn period.

Haemodynamics

• Two parallel circulations are set up — deoxygenated blood recirculates to the body and oxygenated blood to the lungs.
• Survival depends on mixing between the two circuits through a PDA, ASD or VSD.
• As the ductus closes, cyanosis worsens dramatically.

Clinical Features

• Severe central cyanosis from birth / the first days of life, often without much respiratory distress ('happy blue baby').
• Cyanosis is unresponsive to oxygen; a murmur may be absent unless there is an associated VSD.
• Progressive hypoxia, acidosis and heart failure if mixing is inadequate.

Investigations

• Chest X-ray — an 'egg-on-side' (egg-on-a-string) cardiac silhouette with a narrow pedicle.
• Echocardiography — diagnostic; defines the anatomy and mixing sites.

Management

• Prostaglandin E1 infusion — to keep the ductus arteriosus open and maintain mixing (immediate life-saving step).
• Balloon atrial septostomy (Rashkind procedure) — to improve atrial-level mixing.
• Arterial switch operation — the definitive surgical correction, done in the neonatal period.

WHY It Is an Emergency

• In simple TGA the two circulations run in parallel and are incompatible with life unless the deoxygenated and oxygenated blood can mix.
• As the ductus closes, cyanosis and acidosis worsen rapidly — hence the immediate need for prostaglandin and, often, balloon atrial septostomy.

Prognosis

Untreated TGA has very high early mortality; with prompt stabilisation and the arterial switch operation, long-term outcomes are now excellent.

Clinical Clue

• Suspect TGA in a newborn with severe cyanosis that does not improve with oxygen and with relatively little respiratory distress.
• The chest X-ray shows an 'egg-on-side' heart; urgent echocardiography confirms it and prostaglandin is started immediately.

Prostaglandin E1 keeps the duct open until surgery.