Transposition of the great arteries

Transposition of the great arteries (TGA) is one of the commonest cyanotic congenital cardiac anomalies and can account for up to 7% of all congenital cardiac anomalies ¹. 

Pathology

Occurs as a result of ventriculo-atrial discordance with the aorta arising from the right ventricle and the pulmonary trunk from the left ventricle. It can be sub divided into either a D or L type transposition depending on the positional relationship of the aortic valve with the pulmonary valve. The article focuses on the D type transposition.

An isolated TGA is incompatible with life at birth without one of the following additional anomalies (which are a common occurrence ²).

• atrial septal defect (ASD) : uncommon  
• ventricular septal defect (VSD) : ≈ 35 % 
• patent ductus arteriosus (PDA) : unstable due closure following birth
• patent foramen ovale (PFO) : unstable 

Unstable associations account for 60 - 65 % of occurrences.

Radiographic features

Plain film

A frontal chest radiograph classically shows cardiomegaly with a cardiac contours classically described as appearing like an "egg on a string" ¹. There is often an apparent narrowing of the superior mediastinum as result of the aortic and pulmonary arterial configuration.

Echocardiography/ Ultrasound 

Allows direct visualization of anomalous anatomy with the aorta and pulmonary trunk lying in parallel with absence of crossing (best seen on the base view of the foetal heart) ⁴. 

Contrast CT / CTA

Allows direct visualisation of anomalous great vessel anatomy. Cardiac gated cine CT can additionally assess function. 

MRI 

Direct visualisation of anomlaous anatomy. SSFP cine sequences can additionally assess flow dynamics. 

Associations

 ~ 90% of TGA's occur as an isolated finding and extra cardiac syndromic associations are generally rare. associations have been described with 

• maternal diabetes
• coronary arterial anomalies 

Treatment

Initially TGAs were treated with atrial switch operations, such as a Mustard repair or Senning repair, which have been superseded by arterial switch procedures ⁵.