Tetralogy of Fallot (TOF) is the congenital heart disease that Step exams love because it ties anatomy, embryology, physiology, and management into one classic cyanotic picture. If you can explain why these babies turn blue, why they squat, and how a VSD can paradoxically cause right-to-left shunting, you’re basically doing TOF at a “test-day” level.
What Is Tetralogy of Fallot?
Tetralogy of Fallot is a cyanotic congenital heart disease caused by anterosuperior displacement (malalignment) of the infundibular septum, leading to four linked abnormalities:
- Ventricular septal defect (VSD) (typically large)
- Right ventricular outflow tract obstruction (usually subpulmonic/infundibular pulmonary stenosis)
- Overriding aorta
- Right ventricular hypertrophy (RVH)
First Aid cross-reference
- First Aid Step 1: Cardiovascular → Congenital Heart Defects → Tetralogy of Fallot (cyanotic lesion; “boot-shaped heart”; squatting; hypercyanotic spells)
Pathophysiology: The “Why” Behind the Cyanosis
TOF’s cyanosis comes from right-to-left shunting across the VSD, but the key concept is:
The direction of shunting depends on relative resistance/pressure.
Core mechanism
- The VSD is usually large, so pressures in the ventricles can equalize.
- The “deciding factor” becomes how hard it is to push blood into the pulmonary circulation.
- With pulmonary stenosis / RV outflow obstruction, RV pressure rises, encouraging blood to go through the VSD into the aorta (right-to-left shunt) → cyanosis.
Why “overriding aorta” matters
An overriding aorta sits over the VSD and can receive blood from both ventricles, making it easier for deoxygenated blood to enter systemic circulation.
Why RVH happens
Chronic RV pressure overload from RV outflow obstruction → RV hypertrophy.
Embryology (High Yield)
TOF is classically due to abnormal neural crest cell migration affecting the conotruncal (outflow tract) septation.
Associations you should recognize
- 22q11 deletion (DiGeorge syndrome) → conotruncal defects including TOF
- Findings to pair: CATCH-22 (Cardiac defects, Abnormal facies, Thymic aplasia, Cleft palate, Hypocalcemia)
First Aid cross-reference
- First Aid Step 1: Genetics → 22q11 deletion; Cardio → Conotruncal defects
Clinical Presentation
Typical presentation
- Cyanosis (often after birth; may worsen with crying/feeding)
- Dyspnea and irritability
- “Tet spells” = hypercyanotic spells
- Episodes of sudden, profound cyanosis and hypoxia (often with crying, feeding, or exertion)
Why “tet spells” happen
During stress, systemic vascular resistance (SVR) can drop (or dynamic RV outflow obstruction can worsen), increasing right-to-left shunting → abrupt hypoxemia.
Squatting (classic Step association)
Older children may squat after exertion to improve symptoms.
Squatting increases SVR → more resistance to systemic outflow → reduces right-to-left shunt → promotes blood flow into pulmonary circulation → improved oxygenation.
Testable phrasing: increased SVR decreases R→L shunting in TOF.
Physical Exam Findings (Know These Cold)
Murmurs
- Harsh systolic ejection murmur at the left upper sternal border
- From pulmonary stenosis / RV outflow obstruction (not from the VSD)
- VSD in TOF is often large and nonrestrictive, so it may produce less prominent murmur than a small VSD would.
Other findings
- Central cyanosis, possible clubbing over time
- Single S2 may be heard if pulmonary component is soft/absent (varies)
Diagnosis
Initial clues
- Pulse oximetry: low O2 saturation
- Hypercyanotic spells clinically
Imaging and tests
Chest X-ray
- “Boot-shaped heart” (coeur en sabot) due to RV hypertrophy
- Often decreased pulmonary vascular markings (due to reduced pulmonary blood flow)
Echocardiography (diagnostic)
- Defines the VSD, degree of RV outflow obstruction, and overriding aorta
- Used for surgical planning
ECG
- Right axis deviation and RV hypertrophy patterns
Hyperoxia test (conceptual tie-in)
Cyanotic congenital heart disease (R→L shunt) typically shows limited improvement in PaO₂ with 100% O₂ compared with primary lung disease. This is a common board-style discriminator.
Treatment (Step-Relevant)
Acute management of a “tet spell”
Goal: reduce right-to-left shunting and improve pulmonary blood flow.
High-yield interventions:
- Knee-chest position (mimics squatting) → ↑ SVR
- Oxygen
- Morphine (calms, decreases catecholamines; can reduce infundibular spasm)
- IV fluids (increase preload; may reduce dynamic obstruction)
- Beta-blocker (e.g., propranolol) (reduces infundibular spasm; slows HR)
- Phenylephrine can be used to increase SVR in severe cases
Definitive management
- Surgical repair (timing depends on severity): close VSD and relieve RV outflow obstruction (e.g., resection of infundibular muscle, patch enlargement)
Prostaglandin E₁ (PGE₁): when it matters
In ductal-dependent cyanotic lesions, PGE₁ maintains PDA. TOF is not always ductal-dependent, but severe RV outflow obstruction can make pulmonary blood flow more dependent on a PDA—so PGE₁ may be used as a bridge in critically ill neonates.
PGE₁ adverse effect to memorize: apnea.
High-Yield Associations & Differentials
TOF vs Transposition of the Great Arteries (TGA)
| Feature | Tetralogy of Fallot | Transposition of Great Arteries |
|---|---|---|
| Core issue | RV outflow obstruction + VSD → R→L shunt | Parallel circulations (aorta from RV, PA from LV) |
| CXR | Boot-shaped heart | Egg-on-a-string |
| Response to squatting | Improves cyanosis (↑ SVR) | Not a classic feature |
| Initial lifesaving therapy | Manage tet spells; surgery | PGE₁ to keep PDA + balloon atrial septostomy |
TOF vs Truncus arteriosus
- Truncus: single arterial trunk, increased pulmonary blood flow early → CHF
- TOF: often decreased pulmonary blood flow → cyanosis, “tet spells”
Classic “Step” pairings
- TOF + 22q11 deletion
- Boot-shaped heart
- Tet spells triggered by crying/feeding
- Squatting/knee-chest increases SVR to reduce R→L shunt
- Murmur is from pulmonary stenosis, not necessarily the VSD
Rapid Review: Exam-Ready TOF Checklist
Must-know bullets
- Four defects: VSD, pulmonary stenosis (RVOT obstruction), overriding aorta, RVH
- Cyanosis due to R→L shunt across VSD driven by RVOT obstruction
- Harsh systolic ejection murmur at LUSB
- Boot-shaped heart on CXR; ↓ pulmonary markings
- Tet spells: treat with knee-chest, O₂, morphine, fluids, propranolol (± phenylephrine)
- Associated with 22q11 (DiGeorge) from neural crest migration defects
- Definitive treatment: surgical repair
Mini Vignette (Practice Pattern)
A 6-month-old becomes cyanotic and tachypneic during feeding. A harsh systolic murmur is heard at the left upper sternal border. The infant is soothed by flexing the knees to the chest.
Diagnosis: Tetralogy of Fallot with a hypercyanotic (“tet”) spell
Mechanism of improvement: Knee-chest → ↑ SVR → ↓ R→L shunt → ↑ pulmonary blood flow