Tetralogy of Fallot (TOF)

Why is this topic important?

Tetralogy of Fallot (TOF) is the commonest cyanotic congenital heart disease beyond the neonatal period and one of the most important congenital heart conditions tested in MRCPCH examinations.

Candidates are expected to:

Understand the four anatomical abnormalities
Recognise cyanosis and hypercyanotic ("Tet") spells
Interpret the characteristic murmur
Recognise the classic chest X-ray appearance
Understand initial and definitive management

TOF frequently appears in MRCPCH questions involving a cyanosed infant, a squatting toddler, or a child with hypercyanotic episodes.

Definition

Tetralogy of Fallot is a cyanotic congenital heart disease consisting of four anatomical abnormalities:

Ventricular Septal Defect (VSD)
Right Ventricular Outflow Tract Obstruction (Pulmonary Stenosis)
Overriding Aorta
Right Ventricular Hypertrophy

These abnormalities result in reduced pulmonary blood flow and right-to-left shunting causing cyanosis.

Key MRCPCH Facts

Most common cyanotic congenital heart disease presenting beyond the neonatal period.
Severity depends largely on the degree of pulmonary stenosis.
Cyanosis may not be present immediately after birth.
Hypercyanotic ("Tet") spells are a medical emergency.
Squatting improves symptoms in older children.
Chest X-ray classically shows a boot-shaped heart.
Definitive treatment is surgical repair.

Anatomy

The Four Components

1. Ventricular Septal Defect

Usually large and non-restrictive.

2. Pulmonary Stenosis

Obstruction to blood flow from the right ventricle to the lungs.

Most important determinant of severity.

3. Overriding Aorta

The aorta receives blood from both ventricles.

4. Right Ventricular Hypertrophy

Develops secondary to increased right ventricular pressure.

Pathophysiology

Normal Circulation

Blood flows:

Right ventricle → Pulmonary artery → Lungs

In TOF

Pulmonary stenosis obstructs blood flow to the lungs.

This causes:

Increased right ventricular pressure
Right-to-left shunting across the VSD
Decreased pulmonary blood flow

Deoxygenated blood enters the systemic circulation causing:

Cyanosis

The greater the pulmonary obstruction, the more severe the cyanosis.

Why Cyanosis Occurs

Because blood bypasses the lungs through:

Right ventricle → VSD → Aorta

without becoming fully oxygenated.

This is a fundamental MRCPCH concept.

Clinical Features

Infants

May present with:

Cyanosis
Murmur
Poor feeding
Failure to thrive

Older Children

May develop:

Exercise intolerance
Breathlessness
Squatting episodes

Examination Findings

Cyanosis

Severity varies.

Clubbing

May develop later.

Murmur

Typically:

Harsh ejection systolic murmur
Upper left sternal edge

Caused by pulmonary stenosis rather than the VSD.

Hypercyanotic ("Tet") Spells

What Are Tet Spells?

Sudden episodes of profound cyanosis caused by:

Increased right-to-left shunting
Reduced pulmonary blood flow

Triggers

Crying
Feeding
Exercise
Defecation
Pain

Features

Deep cyanosis
Tachypnoea
Irritability
Limpness
Syncope
Seizures (rare)

Importance

Tet spells are a medical emergency and frequently tested in MRCPCH.

Why Squatting Helps

Older children may instinctively squat.

Squatting:

Increases systemic vascular resistance
Reduces right-to-left shunting
Improves pulmonary blood flow
Improves oxygenation

This is a classic MRCPCH examination fact.

Differential Diagnosis

Cyanotic Congenital Heart Disease

Transposition of the Great Arteries
Tricuspid Atresia
Pulmonary Atresia
Total Anomalous Pulmonary Venous Drainage

Respiratory Causes

Severe bronchiolitis
Pneumonia

Investigations

Pulse Oximetry

Shows reduced oxygen saturation.

ECG

Typically demonstrates:

Right Ventricular Hypertrophy

A common examination finding.

Chest X-Ray

Classic appearance:

Boot-Shaped Heart

(Coeur en sabot)

Caused by:

Right ventricular hypertrophy
Upturned cardiac apex

One of the most famous radiological signs in paediatrics.

Echocardiography

Gold Standard Investigation

Demonstrates:

VSD
Pulmonary stenosis
Overriding aorta
Right ventricular hypertrophy

Management

Initial Management

Monitor:

Oxygen saturation
Feeding
Growth

Management of Tet Spells

Immediate Measures

Knee-chest position
Oxygen
Keep child calm

Medical Treatment

May include:

Morphine
Intravenous fluids
Beta-blockers

Definitive Treatment

Surgical Repair

Usually performed in infancy.

Aims to:

Close the VSD
Relieve right ventricular outflow obstruction

Outcome is generally excellent.

Complications

Short-Term

Tet spells
Hypoxic seizures
Failure to thrive

Long-Term

Arrhythmias
Pulmonary regurgitation
Right ventricular dysfunction
Residual obstruction

Most children now survive into adulthood.

Common Exam Traps

Trap 1

TOF is the commonest cyanotic congenital heart disease beyond the neonatal period.

Trap 2

Severity depends mainly on pulmonary stenosis.

Trap 3

The murmur is caused by pulmonary stenosis, not the VSD.

Trap 4

Tet spells are a medical emergency.

Trap 5

Squatting improves cyanosis.

Trap 6

Chest X-ray shows a boot-shaped heart.

Trap 7

Echocardiography is the gold standard investigation.

One Minute Revision

Four abnormalities: VSD, pulmonary stenosis, overriding aorta, RV hypertrophy.
Cyanotic congenital heart disease.
Right-to-left shunt.
Harsh ejection systolic murmur.
Tet spells = emergency.
Squatting improves symptoms.
Boot-shaped heart on chest X-ray.
Surgical repair is definitive treatment.

Suggested References

NICE Guidance
RCPCH Guidance
BNF for Children
Nelson Textbook of Pediatrics
Illustrated Textbook of Paediatrics
Park's Pediatric Cardiology for Practitioners
Moss & Adams' Heart Disease in Infants, Children and Adolescents

Disclaimer

These notes are intended for MRCPCH revision and educational purposes only. They do not replace local, national or institutional guidelines. Clinical decisions should always be based on current guidance, senior advice and individual patient circumstances.

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