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), the most common cyanotic congenital heart disease.

Tetralogy of Fallot (ToF)

Definition & Epidemiology

Tetralogy of Fallot is the most common cyanotic congenital heart disease beyond infancy, accounting for approximately 5% of all congenital cardiac malformations, with an incidence of ~420 per million live births. The offspring of a mother with ToF has a 5-10% chance of inheriting the condition.

The Four Cardinal Defects

All four features arise from a single embryological defect: anterosuperior displacement (malalignment) of the infundibular (outlet/conus) septum, leading to unequal division of the truncus arteriosus.

#	Defect	Details
1	VSD	Large, unrestrictive, malalignment type; near the membranous septum
2	Right ventricular outflow tract (RVOT) obstruction	Most commonly subpulmonic/infundibular stenosis; may also be valvular or involve pulmonary atresia
3	Overriding aorta	Aortic valve straddles the VSD, receiving blood from both ventricles
4	Right ventricular hypertrophy (RVH)	Secondary to the pressure load from RVOT obstruction

Tetralogy of Fallot vs Normal heart anatomy showing the septal defect, overriding aorta, and small pulmonary artery

Fig: Tetralogy of Fallot (left) vs Normal (right) - note the overriding aorta, septal defect, and small pulmonary artery. (Creasy & Resnik's Maternal-Fetal Medicine)

Pathophysiology

The combination of RVOT obstruction and a large VSD results in decreased pulmonary blood flow and right-to-left shunting of deoxygenated blood across the VSD into the systemic circulation.

Diagram showing right-to-left shunting in ToF through the VSD, with deoxygenated blood (blue) shunting across to the overriding aorta (OAo)

Fig: Right-to-left shunting in ToF. Deoxygenated blood from the RV shunts across the VSD into the overriding aorta (OAo). The amount depends on the degree of RVOT obstruction (OB), VSD size, and systemic vascular resistance (SVR). (Rosen's Emergency Medicine)

Clinical Features

Cyanosis

Degree of cyanosis is directly proportional to the degree of RVOT obstruction
"Pink Tet": mild RVOT obstruction → little or no cyanosis at birth
Severe RVOT obstruction: profound cyanosis in the first days of life; may require prostaglandin E1 (PGE1) infusion to maintain ductal patency and pulmonary blood flow
Chronic hypoxemia leads to compensatory polycythemia and clubbing of fingers and toes

Physical Exam

Varying degrees of cyanosis
Systolic ejection murmur along the left sternal border (from RVOT obstruction - the VSD itself is usually silent because it's large and unrestrictive)

Associated Anomalies

Right-sided aortic arch (25%)
ASD
Anomalous origin of the left coronary artery
Pulmonary atresia with VSD (extreme form of ToF; all RV output exits via the aorta)

Investigations

Chest X-Ray

Boot-shaped heart ("coeur en sabot") - due to RVH causing upward elevation of the cardiac apex and a concave main pulmonary artery segment on the left heart border
Decreased pulmonary vascular markings
Right-sided aortic arch in ~25%

ECG

Right ventricular hypertrophy
Right axis deviation

Echocardiography

Primary diagnostic modality in children
Demonstrates all four anatomical components
Shows large VSD and overriding aorta on parasternal long-axis view

Cardiac MRI

Gold standard for assessing the right ventricle in repaired ToF
Quantifies pulmonary regurgitation, RVOT anatomy, RV volumes and function

Tet Spells (Hypercyanotic/Hypoxic Spells)

A potentially life-threatening complication, with peak incidence at 2-4 months of age.

Trigger: Any acute drop in SVR (crying, defecation, feeding), hypovolemia, or tachycardia → massive right-to-left shunt → hypoxia → hyperpnea → increased venous return → more shunting (vicious cycle).

Flowchart of the pathophysiological cycle of a hypoxic tet spell, starting from acute decrease in SVR

Fig: Pathophysiologic mechanisms of a hypoxic (tet) spell - a self-perpetuating cycle. (Rosen's Emergency Medicine)

Management of Tet Spells

Knee-to-chest position (squatting equivalent) - increases SVR, reduces right-to-left shunt
Supplemental oxygen (limited benefit alone)
Morphine 0.1-0.2 mg/kg IV/IM (reduces hyperpnea and sympathetic drive)
Fentanyl 1 mcg/kg IV/IM as alternative; or 1.5-2 mcg/kg intranasally
Midazolam 0.2-0.3 mg/kg intranasally
Sodium bicarbonate 1 mEq/kg IV if acidosis confirmed
Ketamine 1-2 mg/kg IV or 3-5 mg/kg IM (increases SVR)
Propranolol 0.1-0.2 mg/kg (reduces heart rate and infundibular spasm)
Phenylephrine 0.01-0.02 mg/kg IV (direct vasoconstrictor, increases SVR)

(Rosen's Emergency Medicine, Box 165.8)

Surgical Management

Definitive Repair (Complete/Primary Repair)

VSD closure (patch closure)
Relief of RVOT obstruction - typically by placement of a transannular patch (across the pulmonary valve annulus to enlarge the RVOT)
Preferred as early single-stage surgery in most centers
Surgical mortality in the modern era is very low

Recent meta-analyses (PMID 39444206, PMID 40659877) comparing primary vs. staged repair in symptomatic neonates show primary repair is generally favored, though staged repair remains indicated in select cases.

Staged Repair (Palliation First)

Indicated when:

Very narrow RVOT or significant central pulmonary artery hypoplasia
Severe cyanosis in a very small/sick neonate

Modified Blalock-Taussig (BT) shunt: Anastomosis between the innominate artery and the right pulmonary artery - provides interim pulmonary blood flow until definitive repair is feasible. The shunt is taken down during subsequent definitive repair.

Post-Repair Residua and Long-Term Follow-Up

Repair is not curative - residual abnormalities are almost universal:

Residual Problem	Notes
Pulmonary regurgitation (PR)	Most common; from transannular patch; leads to progressive RV dilation
RVOT aneurysm	Can develop at patch site
Residual RVOT obstruction	May require re-intervention
Arrhythmias and conduction disease	Common; risk of sudden death
Residual ASD or VSD
Tricuspid regurgitation
Aortic root dilatation

Pulmonary valve replacement (PVR) is often required later in life due to progressive RV dilation from pulmonary regurgitation. A 2025 systematic review (PMID 40122137) compared surgical vs. transcatheter pulmonary valve replacement in ToF patients. CMR imaging is the gold standard for monitoring RV function/volumes and timing PVR.

Pregnancy in ToF

Per ACC/AHA guidelines, women with repaired ToF should undergo evaluation by an adult CHD specialist before pregnancy. In the absence of severe pulmonary hypertension, severe cardiomyopathy, or severe restrictive lung disease, pregnancy is generally well tolerated. Arrhythmia and pulmonary regurgitation are the main concerns.

Sources: Robbins & Kumar Basic Pathology | Rosen's Emergency Medicine | Creasy & Resnik's Maternal-Fetal Medicine | The Developing Human (Moore & Persaud) | Grainger & Allison's Diagnostic Radiology

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