Harvey's Garden

Tetralogy of Fallot

2 min read

Epidemiology

Etiology

  • Caused by anterosuperior displacement of the infundibular septum.
  • Associated with 22q11 deletion syndromes (DiGeorge) and Down syndrome.

Pathophysiology

  1. Pulmonary infundibular stenosis (Determine severity of cyanosis).
  2. Right ventricular hypertrophy (RVH) – concentric, due to pressure overload.
  3. Overriding aorta (sits over the VSD).
  4. VSD (Ventricular Septal Defect) – usually large and non-restrictive.

Clinical features

  • Tet spells: intermittent hypercyanotic, hypoxic episodes with a peak incidence at 2–4 months after birth
    • Associated with psychological and physical stress (e.g., crying, feeding, defecation)
    • Caused by either an increase in pulmonary vascular resistance (PVR) or a decrease in systemic vascular resistance (SVR) → low SVR:PVR ratio allows the deoxygenated right ventricular output to take the low-resistance route to the systemic circulation, leading to acute hypoxemia (“Tet spell”).
    • Spells occur suddenly and are potentially lethal.
  • Untreated children tend to squat
    • Squatting → ↑ SVR → ↓ right-to-left shunt → ↑ blood flow to pulmonary circulation → ↓ hypoxemia → relief of symptoms
  • Cardiac examination findings
    • Harsh crescendo-decrescendo systolic ejection murmur at the left upper sternal border
      • Caused by pulmonary stenosis, not the associated VSD
        • VSD is so large that no sound
    • Single S2
    • Possible RV heave and systolic thrill

Diagnostics

  • Increased hematocrit: secondary polycythemia

Treatment

  • Ductal-dependent CHDs: a group of CHDs that require the patent ductus arteriosus (PDA), which supplies either pulmonary or systemic circulation, to sustain life until surgery can be performed
    • PDA supplies systemic circulation in the following:
    • PDA supplies pulmonary circulation in the following:
      • Tricuspid atresia
    • Treatment: administration of prostaglandin E1 (PGE1) infusion
      • Example: IV alprostadil infusion
      • Mechanism of action: prevents the ductus arteriosus from closing, creating an intentional shunt to allow mixing of deoxygenated with oxygenated blood

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