Teratology of fallots

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"tetralogy of Fallot"[MeSH Terms] AND management

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Here is a comprehensive overview of Tetralogy of Fallot (TOF), drawing from Robbins Basic Pathology, Rosen's Emergency Medicine, and The Developing Human.

Tetralogy of Fallot (TOF)

TOF is the most common cause of cyanotic congenital heart disease beyond infancy, accounting for approximately 5-10% of all congenital cardiac malformations.

The Four Cardinal Features

All four defects stem from a single embryologic error: anterosuperior displacement of the infundibular (outlet/conus) septum, causing unequal division of the truncus arteriosus.

Feature	Description
1. Ventricular Septal Defect (VSD)	Large, unrestrictive, misaligned defect near the membranous septum
2. Right Ventricular Outflow Tract (RVOT) Obstruction	Subpulmonic (infundibular) stenosis; sometimes pulmonary valve stenosis or complete atresia
3. Overriding Aorta	Aortic valve straddles the VSD, receiving blood from both ventricles
4. Right Ventricular Hypertrophy (RVH)	Secondary to the high pressure load from RVOT obstruction

The degree of RVOT obstruction determines clinical severity. The first three features are structural; RVH is a secondary consequence.

Anatomical Diagram

Classic Tetralogy of Fallot - Robbins Pathology

Classic TOF: note the right-to-left shunt through the VSD into the overriding aorta (Ao); PT = pulmonary trunk (hypoplastic)

Hemodynamics

TOF shunting diagram - Rosen's Emergency Medicine

Deoxygenated blood (blue arrows) from the RV is shunted across the VSD into the overriding aorta (OAo) due to outflow obstruction (OB), bypassing the lungs - ROSEN's Emergency Medicine, p.3210

Decreased pulmonary blood flow due to RVOT obstruction
Right-to-left shunting across the VSD → deoxygenated blood enters systemic circulation
Clinical severity directly proportional to the degree of RVOT obstruction

Clinical Features

At presentation:

Cyanosis - typically not present at birth; worsens within weeks to months
"Pink Tet" - mild RVOT obstruction causes only left-to-right shunt with no cyanosis initially
Cyanosis worsens during crying and feeding (infants), or physical exertion (older children)
Systolic ejection murmur along the left sternal border
Chronic hypoxemia leads to compensatory polycythemia and finger/toe clubbing

Tet Spells (Hypercyanotic/Hypoxic Spells)

A life-threatening complication; peak incidence at 2-4 months of age.

Trigger: Any event that suddenly lowers systemic vascular resistance (SVR) - crying, defecation, hypovolemia, tachycardia.

Vicious cycle:

SVR falls → increased R-to-L shunt across VSD
↓ PaO₂, ↑ PCO₂, ↓ arterial pH
Stimulates respiratory center → hyperpnea
Increased venous return to RV → more shunting → worsening hypoxia

Management of Tet Spells (Box 165.8 - Rosen's):

Intervention	Details
Knee-to-chest position	Increases SVR, reduces R-to-L shunt
Supplemental oxygen	Limited value alone
Morphine	0.1-0.2 mg/kg IV or IM - blunts hyperpnea drive
Fentanyl	1 mcg/kg IV/IM (or 1.5-2 mcg/kg intranasal) as alternative
IV fluids	Correct hypovolemia
Phenylephrine	Increases SVR
Beta-blockers (propranolol)	Reduce heart rate and RVOT spasm

Investigations

Test	Finding
Chest X-ray	"Boot-shaped" heart (coeur en sabot) due to RVH; concave pulmonary artery segment; decreased pulmonary vascular markings
ECG	Right ventricular hypertrophy; right axis deviation
Echocardiography	Diagnostic - defines all four components
Right-sided aortic arch	Present in ~25% of cases

Morphology Summary (Robbins)

Enlarged, boot-shaped heart due to RVH
Proximal aorta is dilated; pulmonary trunk is hypoplastic
Left-sided chambers are normal size
In pulmonary atresia (extreme form): blood reaches lungs only via patent ductus arteriosus or dilated bronchial arteries

Associated Defects

TOF is frequently associated with:

Right-sided aortic arch (~25%)
Atrial septal defect (ASD)
Anomalous origin of the left coronary artery

Treatment

Prostaglandin E1 (PGE1) infusion in severe neonatal cases - maintains PDA to preserve pulmonary blood flow
Surgical repair is definitive: VSD closure + RVOT reconstruction
Staged vs. primary repair in symptomatic neonates is an active area of research - recent meta-analysis (Bashir et al., 2026) of 4,628 neonates found outcomes data favoring primary repair in most centers
In pulmonary atresia variant: initial shunt palliation (e.g., Blalock-Taussig shunt) may be needed before definitive correction
Long-term: pulmonary regurgitation after repair often requires eventual pulmonary valve replacement (surgical or transcatheter)

Key memory hook: "PROVE" - Pulmonic stenosis, RVH, Overriding aorta, VSD, all from one embryologic defect (infundibular septal maldevelopment).

Sources: Robbins & Kumar Basic Pathology; ROSEN's Emergency Medicine; The Developing Human, Clinically Oriented Embryology

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