Tetralogy of Fallot in detail.

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Tetralogy of Fallot (TOF)

Overview

Tetralogy of Fallot is the most common cyanotic congenital heart disease, accounting for approximately 5% of all congenital cardiac malformations and occurring in about 420 per million live births. It was first described systematically by Etienne-Louis Fallot in 1888, though the anatomy had been noted earlier by Stensen (1672) and Hunter (1784).

The Four Components ("Tetrad")

All four defects arise from a single embryologic event: anterosuperior displacement (malalignment) of the infundibular (outlet) septum, which creates abnormal septation between the pulmonary trunk and aortic root.

Classic Tetralogy of Fallot - heart anatomy showing RA, RV, LA, LV, PT, and Ao with right-to-left shunt arrow

Classic Tetralogy of Fallot - Robbins & Kumar Basic Pathology

#	Component	Details
1	VSD	Large, unrestrictive, malaligned; near the membranous septum; aortic valve lies immediately over it
2	RVOT obstruction	Most often subpulmonary (infundibular/subpulmonic stenosis); may also involve pulmonary valve stenosis or complete atresia
3	Overriding aorta	Aortic valve straddles the VSD, receiving blood from both ventricles
4	RV hypertrophy	Secondary to the pressure load imposed by RVOT obstruction; RV wall may exceed LV thickness

The heart is enlarged and classically described as "boot-shaped" (coeur en sabot) due to RV hypertrophy, with a dilated proximal aorta and hypoplastic pulmonary trunk.

Braunwald's Heart Disease, p. 886
Robbins & Kumar Basic Pathology, p. 350

Embryology

The abnormality is an anterocephalad deviation of the outlet septum together with hypertrophy of the septoparietal trabeculations, causing subpulmonary stenosis. This single developmental error is responsible for all four anatomic components. Neural crest cell migration defects (chromosome 22q11 deletion - DiGeorge/velocardiofacial syndrome) are implicated in a significant proportion of cases.

Anatomy and Variants

Native Anatomy

Left: Native TOF anatomy. Right: Post-repair TOF with possible long-term complications. (From ESC Guidelines 2020, via Braunwald's Heart Disease)

Associated anomalies:

Right aortic arch: ~25% of cases
Anomalous coronary arteries: LAD originating from RCA crossing the RVOT (surgically important - may require RV-PA conduit)
ASD, second muscular VSD, or AVSD (especially in Down syndrome)
PA hypoplasia: reported in up to 50%

Important Anatomic Variants

TOF with Pulmonary Atresia + MAPCAs (~15% of all TOF): Extreme form with no direct heart-to-PA connection. Pulmonary blood flow via PDA and/or major aortopulmonary collateral arteries. Repair requires MAPCA unifocalization + VSD closure + RVOT conduit reconstruction.
TOF with Absent Pulmonary Valve: Marked stenosis of the pulmonary annulus with absent/rudimentary leaflets and severely aneurysmal pulmonary arteries that can compress airways at birth.
"Pink" TOF: Mild RVOT obstruction; flow is predominantly left-to-right, no cyanosis initially. Gradually evolves to cyanotic form by age 1-3 years as the RVOT obstruction worsens.

Hemodynamics and Pathophysiology

The severity of disease directly depends on the degree of RVOT obstruction:

Mild obstruction: Left-to-right shunt predominates (resembles isolated VSD), no cyanosis ("pink TOF")
Severe obstruction: Right-to-left shunting of deoxygenated blood across the VSD into the systemic circulation → cyanosis
The overriding aorta receives mixed blood from both ventricles
Pulmonary blood flow is decreased
Because RVOT obstruction protects the pulmonary vasculature from pressure/volume overload, pulmonary hypertension does NOT develop

The amount of right-to-left shunting depends on three interacting factors:

Severity of RVOT obstruction
Size of the VSD
Degree of systemic vascular resistance (SVR)

When SVR drops, more deoxygenated blood shunts right-to-left.

Tet Spells (Hypercyanotic/Hypoxic Spells)

Tet spells are potentially life-threatening events. Peak incidence is at 2-4 months of age.

Pathophysiologic mechanisms of a hypoxic (tet) spell - Rosen's Emergency Medicine

Mechanism - the vicious cycle:

Acute drop in SVR (crying, defecation, fever, hypovolemia, tachycardia)
↑ Right-to-left shunting across VSD
↓ PaO₂, ↑ PaCO₂, ↓ arterial pH
Respiratory center stimulation → hyperpnea (deep, rapid breathing)
↑ Negative intrathoracic pressure → ↑ systemic venous return to right heart
More blood in RV → more right-to-left shunting → worsening cyanosis (the cycle perpetuates itself)

Management of Tet Spells

Intervention	Details
Knee-to-chest position	↑ SVR by compressing femoral vessels → ↓ right-to-left shunt; also ↓ venous return
Supplemental O₂	Limited benefit alone but given routinely
Morphine	0.1-0.2 mg/kg IV/IM - reduces hyperpnea and calms the infant
Fentanyl	1 μg/kg IV/IM (alternative to morphine); 1.5-2 mcg/kg intranasal also used
IV fluids	Volume expansion to ↑ preload
Propranolol	Reduces HR and RVOT spasm; also used for chronic prophylaxis
Phenylephrine	↑ SVR, reduces right-to-left shunting
Sodium bicarbonate	Corrects metabolic acidosis
Ketamine	↑ SVR and sedation

Rosen's Emergency Medicine, p. 3210

Clinical Presentation

Neonatal/Infant:

Worsening cyanosis during crying, feeding, or agitation
Severe RVOT obstruction: profound cyanosis within first days of life; may require PGE₁ infusion to maintain pulmonary blood flow via PDA

Older children (if uncorrected or partially treated):

Cyanosis, clubbing of fingers/toes (from chronic hypoxemia)
Squatting after exertion (classic - increases SVR)
Dyspnea on exertion
Compensatory polycythemia (hyperviscosity, risk of stroke)

Physical examination findings:

Varying degrees of cyanosis
Systolic ejection murmur along the left sternal border (from RVOT obstruction - NOT from the VSD itself, as the large VSD is unrestrictive)
Single or diminished S₂
RV heave if significant RV hypertrophy

Diagnostics

Chest X-ray

Boot-shaped heart (coeur en sabot): RV hypertrophy + concave main pulmonary artery segment along the left heart border
Decreased pulmonary vascular markings (oligemic lung fields)
Normal heart size overall
Right aortic arch in ~25%

ECG

Right ventricular hypertrophy (dominant R in V1, right axis deviation)
"Pink" TOF may initially have normal ECG
Post-repair: complete right bundle branch block (RBBB) is very common

Echocardiography

First-line imaging tool for initial diagnosis and paediatric follow-up
Demonstrates all four components
Assesses severity of RVOT obstruction, size of VSD, degree of aortic override
Post-repair: screens for significant pulmonary regurgitation, RV dilatation, residual VSD, RVOT obstruction, tricuspid regurgitation
Large right atrial area is associated with sustained tachyarrhythmias

Cardiac MRI (CMR)

Gold standard for assessment of the right ventricle in repaired TOF
Accurately quantifies RV volumes, ejection fraction, and degree of pulmonary regurgitation (PR fraction ≥40% = significant)
Defines RVOT anatomy, aneurysms, and branch PA anatomy
Late gadolinium enhancement correlates with adverse prognosis
Phase-contrast velocity mapping quantifies PR and peak RVOT velocities

Cardiac CT (CCT)

Used for coronary artery anatomy before surgery
RVOT and PA dimensions for transcatheter pulmonary valve planning
Delineation of MAPCAs

Cardiac Catheterization

Rarely needed for diagnosis in asymptomatic patients
Preoperative MAPCA mapping in TOF with pulmonary atresia
Grainger & Allison's Diagnostic Radiology, p. 303

Surgical Management

Historical Milestones

1944: Alfred Blalock and Helen Taussig - first palliative Blalock-Taussig (BT) shunt
1954: C. Walton Lillehei - first successful complete intracardiac repair

Palliative Surgery (Staging)

Used when severe cyanosis due to very narrow RVOT or significant PA hypoplasia:

Modified Blalock-Taussig (BT) shunt: PTFE tube graft between innominate artery and right pulmonary artery, increasing pulmonary blood flow. Taken down at definitive repair.

Definitive Repair (Primary Complete Repair)

Now favoured as early single-stage repair (usually before 6 months of age, often in first 3-6 months):

VSD closure with a patch
RVOT relief: Resection of infundibular muscle bundles ± transannular patch (TAP) across pulmonary valve annulus to enlarge RVOT
Transannular patch is avoided where possible to preserve pulmonary valve competence (TAP causes pulmonary regurgitation)
RV-PA conduit repair for: TOF with pulmonary atresia, anomalous LAD from RCA crossing RVOT

Survival: With successful early primary repair, survival rates exceeding 90% beyond 40 years are expected for uncomplicated anatomy.

A 2024 meta-analysis (PMID 39444206) comparing primary vs. staged repair in symptomatic neonates confirmed that outcomes depend significantly on patient selection and institutional experience. A 2025 meta-analysis (PMID 39757359) on Del Nido vs. conventional blood cardioplegia during TOF repair found comparable outcomes.

Long-Term Complications (Post-Repair)

Repaired TOF patients constitute one of the largest groups of adult congenital heart disease (ACHD) patients. Residual abnormalities are almost universal.

Complication	Notes
Pulmonary regurgitation (PR)	Most common sequela, especially after transannular patch. Well tolerated for decades but leads to RV dilatation and dysfunction if untreated
RVOT obstruction	Residual or recurrent subpulmonary stenosis, RVOT aneurysm
RV dilatation and dysfunction	Driven by chronic severe PR
LV dysfunction	Less common; can occur
Arrhythmias	Atrial flutter/fibrillation, ventricular tachycardia (VT); large RA area predicts atrial arrhythmias; akinetic RVOT area length predicts VT
Sudden cardiac death (SCD)	Due to ventricular arrhythmia; inducible VT on EP study is prognostic
Residual VSD	From patch leakage
Aortic root dilatation + aortic regurgitation	Due to increased aortic flow in original anatomy
Branch PA stenosis	From surgical manipulation or shunt placement
Complete RBBB	Very common post-repair, related to ventriculotomy

Pulmonary Valve Replacement (PVR)

The key long-term intervention. Indicated when:

Symptomatic patients with significant PR
Asymptomatic patients when RV end-systolic volume indexed to BSA exceeds threshold values (ongoing research to define optimal timing)
Can be surgical or transcatheter (TPVI - e.g., Melody, SAPIEN valve)

BNP is predictive of mortality in these patients; cardiopulmonary exercise testing provides objective functional assessment.

Genetic and Associated Conditions

22q11.2 deletion (DiGeorge/velocardiofacial syndrome): Most common genetic association
Down syndrome (Trisomy 21): AVSD variant of TOF
JAG1 mutations (Alagille syndrome)
Isolated (sporadic) in many cases

Summary Table

Feature	Detail
Incidence	~420/million live births; 5% of all CHD
Most common cyanotic CHD	Beyond infancy (after TGA in neonates)
Core defect	Anterosuperior outlet septum displacement
Shunt direction	Right-to-left (after RVOT obstruction worsens)
Murmur	Systolic ejection murmur at left sternal border (RVOT obstruction)
CXR	Boot-shaped heart, decreased pulmonary markings
ECG	RVH, right axis deviation
Tet spell position	Knee-to-chest (squat)
Definitive Rx	Complete surgical repair (VSD closure + RVOT relief)
Gold standard post-repair imaging	Cardiac MRI
Key long-term concern	Pulmonary regurgitation → RV failure + arrhythmia

Sources:

Braunwald's Heart Disease, pp. 885-888 (ACHD, surgical repair, long-term outcomes)
Robbins & Kumar Basic Pathology, pp. 349-350 (pathology and morphology)
Rosen's Emergency Medicine, pp. 3209-3212 (tet spells, emergency management)
Grainger & Allison's Diagnostic Radiology, pp. 302-305 (imaging, CMR)

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