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Tetralogy of Fallot

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Overview

Tetralogy of Fallot (TOF) is one of the most common forms of cyanotic congenital heart disease (CHD) and is characterised by the presence of 4 classic findings:

  • Ventricular septal defect (VSD)
  • An aorta overriding the VSD:
    • The aorta is positioned over the VSD receiving blood from both the left and right ventricles
  • Pulmonary stenosis and right ventricular outflow tract obstruction
  • Right ventricular hypertrophy:
    • Pulmonary stenosis increases resistance to blood flow through the right ventricular outflow tract, leading to right ventricular hypertrophy
    • This can worsen the outflow obstruction
    • Eventually, blood can flow through the aorta due to right ventricular outflow tract obstruction, causing a right-to-left shunt

Epidemiology

Presentation

The degree of right ventricular outflow tract obstruction determines the severity of cyanosis and clinical features. Some features include:

  • Hypercyanotic episodes (tet spells):
    • Acute episodes characterised by cyanosis, shortness of breath, irritability, and reduced consciousness due to occlusion of the right ventricular outflow tract
    • May be triggered by pain, fever, dehydration, or anxiety
    • Children may squat to increase systemic vascular resistance to increase pressure on the left side of the heart, temporarily reversing the right-to-left shunt.
  • Ejection systolic murmur:
    • Suggestive of pulmonary stenosis
  • Shock due to severe hypercyanotic episode or right ventricular outflow obstruction

Investigations

Investigations include:

  • ECG:
    • May show right ventricular hypertrophy
  • Chest x-ray:
    • May show a boot-shaped heart, but can be normal
  • Echocardiography – diagnostic test of choice:
    • Shows defects seen in TOF
  • Hyper-oxygenation testing:
    • May be used to determine if the cyanosis is due to lung disease or circulation
    • Arterial blood gases are measured while the patient breathes room air, then after the patient has breathed 100% oxygen for at least 10 minutes
    • If the cause of hypoxaemia is pulmonary, then PaO2 increases
    • If the cause of hypoxaemia is cardiac, then there is no significant increase in PaO2
    • In TOF, the cause of hypoxaemia is cardiac, therefore, there is no significant increase in PaO2

Management

Overview

Management involves surgical repair. The techniques used vary depending on the patient’s age and anatomy.

Complications

Overview

Complications include:

  • Tet spells
  • Paradoxical emboli due to the VSD
    • Thrombi forming in the venous circulation can cross the VSD into the left side of the heart which can embolise to the cerebral vasculature causing stroke
  • Arrhythmia
  • Congestive heart failure
  • Sudden cardiac death

Prognosis

  • The prognosis depends on how early TOF is identified and how early interventions are carried out
  • If untreated, TOF can carry a mortality of 50% in the first three years of life

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