Right-to-left shunts: tetralogy, transposition and others
- Lesion
These defects send blue blood straight to the body, so babies are cyanotic from early life.
What it is
Right-to-left shunts mix deoxygenated blood into the systemic circulation and cause early cyanosis, clubbing, polycythemia and paradoxical emboli. Tetralogy of Fallot results from anterior and superior displacement of the infundibular septum, producing a VSD, subpulmonary stenosis, an overriding aorta and right ventricular hypertrophy. In transposition of the great arteries the aorta arises from the right ventricle and the pulmonary artery from the left, giving two parallel circulations that need a mixing shunt to survive. Tricuspid atresia, persistent truncus arteriosus and total anomalous pulmonary venous return complete the classic ‘five T's’.
Structure
- Tetralogy: large VSD below the aortic valve, narrow right ventricular infundibulum, thick right ventricular wall, aorta astride the defect.
- Transposition: right ventricle markedly hypertrophied as it supports the systemic circulation; left ventricle thin.
The cyanotic lesions to know
| Lesion | Embryological fault | Survival depends on | Clue |
|---|---|---|---|
| Tetralogy of Fallot | Anterosuperior shift of the infundibular septum | Degree of pulmonary stenosis | Boot-shaped heart; squatting |
| Transposition of the great arteries | Aorticopulmonary septum does not spiral | A mixing shunt (VSD, ASD, PDA) | Severe cyanosis at birth; maternal diabetes |
| Tricuspid atresia | Absent tricuspid orifice | ASD plus VSD or PDA | Hypoplastic right ventricle, left axis on ECG |
| Persistent truncus arteriosus | Failure of truncal septation | Associated VSD; early pulmonary overload | 22q11.2 deletion |
| Total anomalous pulmonary venous return | Pulmonary veins fail to join the left atrium | An ASD or patent foramen ovale | Pulmonary venous obstruction in some |
Clinical notes
- Prostaglandin keeps the ductus open in duct-dependent cyanotic lesions until surgery.
- Ebstein anomaly (downward displacement of the tricuspid leaflets, linked to lithium) may also cause cyanosis through an ASD.
Test yourself
Think of the answer, then tap the question to see it.
What are the four features of tetralogy of Fallot?
Ventricular septal defect, right ventricular outflow obstruction (subpulmonary stenosis), overriding aorta and right ventricular hypertrophy.
Why is a shunt required to survive transposition of the great arteries?
The systemic and pulmonary circulations run in parallel; oxygenated blood reaches the body only through a VSD, ASD or PDA.
Why do children with tetralogy squat during spells?
Squatting raises systemic vascular resistance, decreasing right-to-left shunting and pushing more blood through the lungs.
Elsewhere in DurgiPedia
- Outflow tract: the spiral aorticopulmonary septum and neural crest
- Transposition of the great arteries
- Persistent truncus arteriosus
- Ebstein anomaly
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Tetralogy of Fallot (StatPearls (NCBI Bookshelf), 2025); Congenital heart disease: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.