Tetralogy of Fallot (TOF)

1. The Four Components ("Tetrad")

2. Pathophysiology

• Decreased pulmonary blood flow + right-to-left shunting across the VSD
• Severity of cyanosis is directly proportional to the degree of RVOT obstruction
• If RVOT obstruction is mild: left-to-right shunt only ("pink TOF" or acyanotic TOF) - behaves like an isolated VSD
• As the child grows, the RVOT narrows further relative to a fixed cardiac mass, worsening stenosis and cyanosis progressively
• The RVOT obstruction protects the lungs from hypertension - pulmonary hypertension does NOT develop

3. Gross Morphology (Robbins)

• Heart is enlarged and "boot-shaped" due to RV hypertrophy
• Proximal aorta is dilated; pulmonary trunk is hypoplastic
• VSD is usually large, near the membranous septum
• In cases with complete pulmonary atresia: PDA or bronchial arteries are the only route to the lungs

4. Associated Anomalies

• Right aortic arch - ~25% of cases
• Anomalous coronary artery - most often the LAD arising from the RCA, crossing the RVOT (surgically important, may require an RV-PA conduit)
• ASD ("pentalogy of Fallot")
• Down syndrome - may be associated with AVSD
• TOF with pulmonary atresia + MAPCAs (~15%) - most severe form; requires unifocalization surgery
• TOF with absent pulmonary valve - rudimentary/absent leaflets, massively dilated PAs causing bronchial compression at birth

5. Clinical Features

Presentation

• Infants: Cyanosis worsening with crying/feeding; may present in neonatal period if RVOT is severe (PGE1 required to maintain ductal patency)
• Older children: Exertional cyanosis, squatting (increases SVR, reduces R→L shunt)
• Pink TOF may not manifest cyanosis until 1-3 years of age

Examination

• Cyanosis and clubbing (chronic hypoxemia)
• Systolic ejection murmur along the left sternal border (from RVOT obstruction - NOT from the VSD, which is large and unrestrictive)
• Single S2 (due to soft/inaudible P2)
• Polycythemia from compensatory erythropoiesis

ECG

• Right ventricular hypertrophy and right axis deviation

Chest X-ray

• "Boot-shaped heart" (coeur en sabot) - RV hypertrophy + concave pulmonary bay
• Decreased pulmonary vascular markings
• Right aortic arch may be visible in 25%

6. Hypoxic ("Tet") Spells

Management of a Tet Spell (Rosen's Emergency Medicine, Box 165.8)

7. Diagnosis & Imaging

8. Treatment

Surgical Repair

• Goal: VSD closure + relief of RVOT obstruction
• Timing: Early primary repair is preferred - usually within the first 6 months of life
• Technique: Transannular patch placed across the pulmonary valve annulus to enlarge the RVOT (most common approach); this intentionally creates pulmonary regurgitation (PR)
• Staged approach: If severe cyanosis with very narrow RVOT or hypoplastic central PAs - a modified Blalock-Taussig (BT) shunt (innominate artery → right PA) is placed first as palliation, then taken down at definitive repair
• TOF with pulmonary atresia/MAPCAs: Unifocalization of MAPCAs + VSD closure + RVOT conduit reconstruction

Long-term (Repaired TOF - ACHD patients)

• Pulmonary regurgitation (PR) - the dominant long-term problem from transannular patch; leads to progressive RV dilatation and dysfunction
• RVOT aneurysm
• Residual RVOT obstruction / PA stenosis
• Residual VSD
• Aortic root dilatation + aortic regurgitation
• Arrhythmias (sustained VT/VF - risk of sudden death; atrial tachyarrhythmias)
• Complete right bundle branch block (on ECG)

9. Prognosis

• Life expectancy with uncomplicated anatomy + early primary repair + good biventricular function is excellent
• Untreated TOF - some patients survive into adult life (degree dependent on RVOT severity)
• Repaired TOF patients constitute one of the largest groups of adults with congenital heart disease (ACHD) and require lifelong follow-up

Recent Evidence (PubMed, 2024-2026)

• A 2024 meta-analysis (PMID 39444206) compared primary vs. staged repair in symptomatic neonates with TOF, informing the current trend toward early primary repair when anatomy allows.
• A 2025 meta-analysis (PMID 39757359) of RCTs found Del Nido cardioplegia may have advantages over conventional blood cardioplegia in TOF repair.
• A 2024 systematic review (PMID 38558254) highlights the prognostic value of echocardiographic deformation imaging (strain) in adult CHD including repaired TOF.

The 4 Components of Tetralogy of Fallot — Simplified

Think of it like a plumbing problem in a house

Component 1 — VSD (Hole in the Wall)

"The dividing wall has a big hole in it"

• The wall (septum) between the right and left ventricles didn't close properly
• It's large and unrestrictive — blood freely crosses from one side to the other
• Because the right side is under high pressure (due to the blocked outlet), blood flows right → left (backwards), mixing deoxygenated blood into the body's circulation
• Result: blue blood gets into the body

Component 2 — RVOT Obstruction (Blocked Exit)

"The right ventricle's exit door is too narrow"

• The right ventricle pumps blood to the lungs via the pulmonary artery
• In TOF, that outflow tract is narrowed (stenosed) — most often just below the pulmonary valve (called infundibular/subpulmonary stenosis)
• Less blood reaches the lungs → less blood gets oxygenated
• This is the most important component — it determines how blue the baby is
• Mild narrowing = "pink TOF" (almost no cyanosis)
• Severe narrowing = deep cyanosis from birth

Component 3 — Overriding Aorta (Aorta Sitting Over the Hole)

"The body's main pipe is positioned over the hole, stealing from both sides"

• Normally the aorta sits entirely over the left ventricle
• In TOF, the aorta shifts and sits straddling the VSD — it receives blood from both ventricles
• So even without considering the shunt, the aorta is already receiving a mix of oxygenated (left) and deoxygenated (right) blood
• Result: mixed, partially blue blood goes to the whole body

Component 4 — RV Hypertrophy (Muscle Thickening)

"The right ventricle bulks up because it's working against a blocked exit"

• This is not a primary defect — it's a consequence of components 1-3
• The right ventricle has to push hard against the narrow outflow tract → it thickens its walls (hypertrophies) over time, just like any muscle under strain
• A hypertrophied RV is stiffer and less compliant, worsening the problem long-term

How They All Connect — One Sentence

The outlet septum shifts → blocks the right exit (RVOT obstruction) → right pressure builds up → blood takes the shortcut through the hole (VSD) → crosses into the aorta which is already sitting over that hole (overriding aorta) → right ventricle keeps straining and thickens (RV hypertrophy).

Visual Summary

Right Ventricle
      |
      |——[NARROW EXIT]——→ Pulmonary Artery (less blood to lungs)
      |
      |——[BIG HOLE / VSD]——→ → → → → ↓
                                        Overriding Aorta → Body (blue blood)
Left Ventricle ———————————————————→ ↑

Memory Hook — "PROVE"

Tetralogy of Fallot — Components (Professional)

Unifying Embryologic Basis

Component 1 — Ventricular Septal Defect (VSD)

Anatomic Basis

• Large and unrestrictive (non-restrictive to flow)
• Perimembranous in ~80% of cases - located adjacent to the tricuspid valve in the vicinity of the membranous interventricular septum
• Subarterial/doubly committed in a subset of cases

Hemodynamic Role

• PVR > SVR (due to RVOT obstruction): right-to-left shunting → cyanosis
• PVR < SVR (mild RVOT obstruction / "pink TOF"): left-to-right shunting → behaves like isolated VSD

Component 2 — Right Ventricular Outflow Tract (RVOT) Obstruction

Anatomic Basis

Clinical Significance

• Severe RVOT obstruction → profound neonatal cyanosis, PGE1 dependence
• Mild RVOT obstruction → "pink TOF," acyanotic at birth, progressive cyanosis by 1-3 years
• Worsens with growth: as the child grows, the infundibular muscle hypertrophies progressively, and the fixed RVOT fails to expand proportionately → increasing stenosis

Component 3 — Overriding Aorta

Anatomic Basis

• Mild override (~25% from RV): classic TOF
• Moderate override (~50% from RV): typical
• Severe override (>75% from RV): approaches double-outlet right ventricle (DORV) - there is a morphological continuum between severe TOF with aortic override and DORV with subaortic VSD (Lev classification)

Hemodynamic Role

Component 4 — Right Ventricular Hypertrophy (RVH)

Anatomic Basis

Morphologic Features

• RV wall thickness may equal or exceed that of the LV (normally the LV is 3x thicker)
• The heart adopts a classic "boot-shaped" (coeur en sabot) silhouette on CXR: RV hypertrophy elevates and upturns the cardiac apex; the hypoplastic pulmonary trunk creates a concavity at the left upper cardiac border
• RV cavity may be reduced in severe infundibular hypertrophy

Functional Consequences

• Progressive infundibular hypertrophy worsens RVOT obstruction over time (vicious cycle)
• A hypertrophied, stiff RV is susceptible to diastolic dysfunction and arrhythmia long-term
• After repair, residual RVH and stiffness (restrictive RV physiology) may persist for years

Hierarchical Relationship of the Four Components

Infundibular Septum Displaced Anterosuperiorly
              │
              ├──→ [1] RVOT Obstruction (direct mechanical narrowing)
              │
              ├──→ [3] Overriding Aorta (aortic root rotates over VSD)
              │
              ├──→ [2] VSD (failure of septal fusion at malalignment site)
              │
              └──→ [4] RV Hypertrophy (pressure response — SECONDARY)

• Tet spells - the physiology and exact management steps
• Surgical repair - what the surgeon actually does and why
• Post-repair complications - pulmonary regurgitation, arrhythmias, RV dysfunction (very high-yield for exams)
• Variants - TOF with pulmonary atresia, absent pulmonary valve, MAPCAs
• Radiology / ECG findings in detail
• Genetics - 22q11 deletion (DiGeorge), JAG1, NKX2-5 associations