Tetralogy of Fallot

Definition and Anatomy

• Most common cyanotic congenital heart disease.
• Primary embryonic defect: Anterior and cephalad deviation of infundibular (conal) septum.
• Four classic anatomic components:
  • Right ventricular outflow tract (RVOT) obstruction (infundibular, valvar, or supravalvar pulmonary stenosis).
  • Malalignment ventricular septal defect (VSD).
  • Dextroposition/overriding aorta.
  • Right ventricular hypertrophy (RVH).

Pathophysiology

• Unrestrictive VSD permitting equalized systolic pressures between right and left ventricles.
• Shunt direction and cyanosis severity dictated by RVOT obstruction severity.
• Mild RVOT obstruction: Balanced or left-to-right shunting, minimal cyanosis (“Pink TOF”).
• Severe RVOT obstruction: Right-to-left shunting, systemic hypoxemia, overt clinical cyanosis.
• Pulmonary blood flow inversely proportional to RVOT obstruction severity.
• Extreme obstruction: Ductus arteriosus patency critical for neonatal pulmonary blood flow.

Epidemiology and Genetics

• Incidence: 25 to 40 per 100,000 live births.
• Accounts for 5% to 10% of all congenital heart defects.
• Genetic association: 22q11.2 microdeletion (DiGeorge/Velocardiofacial syndrome) in approximately 13% to 40% of cases.
• Additional syndromic associations: Trisomy 21 (commonly with associated atrioventricular canal defects), Alagille syndrome, VACTERL association.
• Maternal risk factors: Untreated phenylketonuria, pregestational diabetes, retinoic acid exposure.

Clinical Features

• Cyanosis present from birth or appearing in early infancy (worsens with ductal closure).
• Severity of cyanosis inversely related to systolic murmur intensity.
• Effort intolerance and exertional dyspnea.
• Squatting behavior in older children (increases systemic vascular resistance, augmenting pulmonary blood flow).
• Delayed growth and puberty in unoperated patients.
• Digital clubbing in older, chronically hypoxemic patients.

Physical Examination

• Normal peripheral pulses and central venous pressures.
• Prominent left parasternal heave or right ventricular impulse.
• Palpable systolic thrill at mid/upper left sternal border.
• Normal first heart sound (S1).
• Single second heart sound (S2) or soft, delayed pulmonary component.
• Harsh, crescendo-decrescendo ejection systolic murmur at mid/upper left sternal border (originating from RVOT turbulence, not VSD).
• Murmur shortens and softens with progressive RVOT obstruction.

Associated Anomalies

• Right aortic arch (20% to 25% of cases).
• Coronary artery anomalies (e.g., Left anterior descending artery originating from right coronary artery in 5% to 10% of cases).
• Patent ductus arteriosus (PDA).
• Multiple VSDs.
• Atrioventricular septal defect (AVSD).
• Absent pulmonary valve syndrome.
• Major aortopulmonary collateral arteries (MAPCAs) (in pulmonary atresia variants).

Diagnostic Investigations

Modality	Key Findings
Electrocardiogram (ECG)	Right axis deviation (RAD).Right ventricular hypertrophy (RVH) (tall R wave in V1, deep S wave in V6).Upright T wave in V1.Right atrial enlargement (peaked P waves).
Chest Radiograph (CXR)	Normal overall cardiac size.”Boot-shaped” cardiac silhouette (Coeur en Sabot) due to upturned RV apex.Concave main pulmonary artery segment (“pulmonary bay”).Oligemic (decreased) pulmonary vascular markings.Right-sided aortic arch (indentation on right side of trachea).
Echocardiography	Diagnostic modality of choice.Anterior malalignment of conal septum.Overriding aorta with mitral-aortic fibrous continuity.Large malalignment VSD.Multilevel RVOT obstruction evaluation via continuous/pulsed wave Doppler.Assessment of pulmonary valve annulus and branch pulmonary arteries.
Cardiac CT / MRI / Angiography	Indicated for complex pulmonary artery anatomy evaluation.Delineation of coronary artery anomalies (LAD from RCA) crossing RVOT.Post-repair quantification of RV volumes, ejection fraction, and pulmonary regurgitation.

Management

Medical Management

• Intravenous Prostaglandin E1 (PGE1) infusion (0.01 - 0.1 mcg/kg/min) for severe neonatal RVOT obstruction (maintains ductal patency for pulmonary blood flow).
• Iron supplementation for optimal oxygen-carrying capacity (treatment/prevention of relative anemia in polycythemic patients).
• Intravenous hydration to prevent hyperviscosity complications.

Surgical Palliation

• Indicated for premature, low-birth-weight infants, or patients with unfavorable anatomy (e.g., severe pulmonary artery hypoplasia, complex coronary crossing RVOT).
• Modified Blalock-Taussig-Thomas (mBTT) shunt: Polytetrafluoroethylene (PTFE) tube graft anastomosis between subclavian artery and ipsilateral branch pulmonary artery.
• Transcatheter alternatives: RVOT stenting, balloon pulmonary valvuloplasty, or ductal stenting.

Complete Surgical Repair

• Elective timing typically between 3 and 12 months of age (commonly 4-6 months).
• Cardiopulmonary bypass utilization.
• Transatrial-transpulmonary approach preferred (avoids large right ventriculotomy).
• VSD patch closure.
• RVOT obstruction relief (infundibular muscle resection, pulmonary valvotomy).
• Transannular patch placement for hypoplastic pulmonary valve annulus.
• “Valve-sparing” strategies increasingly preferred to mitigate long-term pulmonary regurgitation.

Complications

Preoperative Complications	Postoperative Complications
Polycythemia and hyperviscosity syndrome	Pulmonary valve regurgitation (common post-transannular patch)
Cerebral thrombosis/stroke (exacerbated by dehydration and iron deficiency)	Right ventricular dilation and systolic/diastolic dysfunction
Brain abscess	Ventricular arrhythmias and sudden cardiac death
Infective endocarditis / endarteritis,	Right bundle branch block (RBBB),
Delayed puberty and growth failure	Residual RVOT obstruction or branch pulmonary artery stenosis
Congestive heart failure (rare, seen in “pink” TOF with large left-to-right shunt),	Atrial arrhythmias (e.g., atrial flutter, atrial fibrillation)
	Aortic root dilation and progressive aortic regurgitation