Tetralogy of Fallot (TOF)
The most common cyanotic congenital heart defect — four abnormalities, one condition, and a very treatable diagnosis.
What is Tetralogy of Fallot?
Tetralogy of Fallot (TOF) is a combination of four heart defects that are present together from birth. It was described by French physician Étienne-Louis Fallot in the 19th century, though first recognised by British physician Thomas Peacock in 1846.
“Tetralogy” means four — and all four components arise from a single developmental error: abnormal forward and leftward shift of the outlet (infundibular) septum during fetal heart development. This single event creates all four defects simultaneously.
TOF is the most common cause of cyanotic (“blue baby”) congenital heart disease, accounting for 5–10% of all CHD.
The Four Components
Narrowing of the passage from the right ventricle to the pulmonary artery (infundibular stenosis and/or pulmonary valve stenosis). This reduces blood flow to the lungs and is the primary cause of cyanosis. The degree of obstruction determines how blue the child is.
A large hole in the ventricular septum, typically perimembranous with malalignment. In TOF, this defect is always large and non-restrictive — pressures in both ventricles are equal. This allows deoxygenated blood to pass into the aorta.
The right ventricle muscle thickens (hypertrophies) in response to the high outflow resistance. This is a consequence of the outflow obstruction, not an independent defect. The RV works much harder than normal because its outlet is narrowed.
The aorta is abnormally positioned — it sits over the VSD rather than entirely over the left ventricle. This means the aorta receives blood from both ventricles. The degree of “override” varies from mild to severe (up to 50% or more).
Anatomy of Tetralogy of Fallot — The Four Defects
| The Four Defects of TOF | |
|---|---|
| ① RV Outflow Obstruction → Reduced blood to lungs → Cyanosis ② Large VSD |
③ RV Hypertrophy → RV pumps against obstruction → Muscle thickening ④ Overriding Aorta |
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Result: Cyanotic CHD — “Blue Baby” at birth or with crying/feeding Characteristic: Right ventricular hypertrophy on ECG | “Boot-shaped” heart on CXR |
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TOF is characterised by the combination of VSD, RVOTO, RV hypertrophy, and overriding aorta. The degree of RVOTO determines severity of cyanosis.
Why is the baby blue? Normally, the right ventricle sends all blood to the lungs to pick up oxygen. In TOF, the pulmonary outlet is narrowed — so instead, deoxygenated blood takes the “easier” path through the VSD and into the aorta, bypassing the lungs. This dark, oxygen-poor blood circulates through the body, turning the skin, lips, and fingernails blue (cyanosis).
The Spectrum — “Pink” to “Blue” Tetralogy
Not all TOF is the same. The degree of right ventricular outflow obstruction determines how blue the child is:
- “Pink Tetralogy”: When obstruction is mild, blood still preferentially flows left-to-right (as in VSD). The child may have minimal or no cyanosis — sometimes only detected on echocardiogram.
- Moderate obstruction: Mixed flow — some blood reaches the lungs, some bypasses them. Mild-moderate cyanosis at rest, worsening with activity.
- Severe obstruction / Pulmonary atresia: Virtually no blood flows to the lungs through the normal route. Survival depends on the ductus arteriosus (a fetal vessel that normally closes after birth) or on collateral vessels called MAPCAs (Major Aortopulmonary Collateral Arteries).
“Tet Spells” — The Emergency
Hypoxic (“Tet”) Spell — What Happens and What To Do
| 🔄 What Happens in a Tet Spell | 🚨 Emergency Management |
|---|---|
| TRIGGER: Crying, feeding, straining, fever ↓ Infundibular spasm → Outflow tract contracts ↓ Less blood reaches lungs ↓ More blood through VSD to aorta ↓ Severe CYANOSIS Pallor, limpness, LOC risk |
1. 🧸 Knee-chest position (increases SVR) 2. 💤 Calm the child, reduce oxygen demand 3. 💊 Morphine 0.1 mg/kg SC/IV 4. 💊 Propranolol (prevents recurrence) 5. 🩸 IV fluid bolus (increases preload) 6. 🏥 Urgent surgical repair after stabilisation |
Tet spells are acute episodes of severe cyanosis due to infundibular spasm. Immediate knee-chest positioning and morphine are first-line treatment.
Why do children with TOF squat? Older children with unrepaired TOF instinctively squat when they feel short of breath or turn blue. Squatting increases resistance in the leg arteries (systemic vascular resistance), which raises the pressure that must be overcome for blood to shunt right-to-left through the VSD. The result: more blood flows to the lungs instead, improving oxygenation. This was one of the earliest recognized features of TOF — and a clue to the underlying physiology.
Diagnosis
| Investigation | Findings in TOF |
|---|---|
| Physical Exam | Cyanosis (variable), loud systolic ejection murmur at upper-left sternal border (from RVOT obstruction), soft or single S2, digital clubbing (in older unrepaired cases) |
| Pulse Oximetry | SpO₂ typically 70–90% at rest in moderate-severe TOF; may be near-normal in pink TOF |
| ECG | Right axis deviation, right ventricular hypertrophy (tall R waves in V1), normal in neonates initially |
| Chest X-Ray | Classic “coeur en sabot” (boot-shaped heart) — upturned cardiac apex, concave pulmonary artery segment, oligaemic (dark) lung fields; right aortic arch in ~25% |
| Echocardiography | Gold standard — defines all four components, measures RVOT gradient, identifies coronary anomalies, assesses pulmonary artery size; guides surgical planning |
| CT/MRI Angiography | Defines pulmonary artery anatomy in detail, identifies MAPCAs, coronary course — essential for complex cases and pre-surgical planning |
| Cardiac Catheterisation | Measures pressures; defines MAPCAs and pulmonary vascular anatomy when echo is insufficient; less often needed with modern imaging |
Treatment — Staged or Complete Repair
TOF Treatment Pathway
| TOF Diagnosed Cyanosis + murmur → Echo confirmation |
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| ▼ | |
| Immediate: PGE₁ Infusion (if severe neonatal cyanosis) Keeps ductus arteriosus open → maintains lung blood flow |
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| ▼ | |
| ✅ Good anatomy Complete Repair (3–6 months) VSD patch + RVOT relief On bypass, <3% mortality in good centres |
⚠️ Unsuitable anatomy BT Shunt (palliation) Subclavian → PA Staged repair later |
TOF repair is ideally performed at 3–6 months before the RV becomes too hypertrophied. Complete repair gives excellent long-term outcomes.
Complete Repair — What Happens in Surgery: The surgeon closes the VSD with a patch (stopping the right-to-left shunt), and widens the RVOT obstruction — either by removing muscle (infundibulectomy), enlarging the pulmonary valve, or placing a “transannular patch” across the valve ring. The result: blood now flows normally to the lungs, oxygen levels normalise, and cyanosis resolves. Over 90% of patients survive long-term after repair.
Frequently Asked Questions
🔑 Key Takeaways
• TOF = four defects caused by one developmental error: RVOT obstruction, large VSD, RV hypertrophy, overriding aorta.
• It is the most common cyanotic CHD — the degree of cyanosis depends on severity of RVOT obstruction.
• Tet spells are acute cyanotic episodes requiring immediate knee-chest positioning and emergency care.
• Complete surgical repair at 3–6 months is the standard of care with excellent long-term outcomes.
• Screen for 22q11 deletion — affects 25% and has broader systemic implications.
• Lifelong cardiology follow-up is needed — pulmonary valve replacement may be required later in life.
