Tetralogy of Fallot (TOF)

What is TOF?

TOF is the most common cyanotic congenital heart disease beyond infancy. It accounts for about 5% of all congenital cardiac malformations. The name comes from the French physician Étienne-Louis Arthur Fallot, who described the four defects in 1888.

The 4 Defects (The "Tetralogy")

All four defects arise from a single embryological cause: anterosuperior displacement of the infundibular septum (the part of the ventricular wall that separates the outflow tracts).

#	Defect	Description
1	VSD (Ventricular Septal Defect)	Large, unrestrictive hole in the interventricular septum
2	RVOTO (Right Ventricular Outflow Tract Obstruction)	Narrowing below/at/above the pulmonary valve (subpulmonic stenosis most common)
3	Overriding Aorta	The aorta straddles the VSD, receiving blood from BOTH ventricles
4	RVH (Right Ventricular Hypertrophy)	Secondary to the high pressure from the RVOTO

Mnemonic: "PROVE" - Pulmonary stenosis, Right ventricular hypertrophy, Overriding aorta, VSD (some use "HOPS": Hypertrophy, Overriding aorta, Pulmonary stenosis, Septal defect)

Here is a diagram showing the anatomy:

TOF Anatomy - RV in blue, showing the overriding aorta (Ao), hypoplastic pulmonary artery (PA), VSD, and enlarged RV

Pathophysiology

The key problem is decreased pulmonary blood flow and right-to-left shunting of deoxygenated blood.

TOF Shunting Diagram - deoxygenated blood (blue arrows) bypasses lungs via VSD into the overriding aorta (OAo), with RVOT obstruction (OB)

Deoxygenated blood in the RV cannot easily get to the lungs (blocked by RVOTO)
Blood is instead shunted right-to-left across the VSD into the left ventricle and the overriding aorta
Up to 75% of venous blood can bypass the lungs entirely
Result: systemic hypoxemia and cyanosis

Clinical Features

Degree of cyanosis depends on the severity of RVOTO:

Mild RVOTO = "Pink TOF" (acyanotic) - may not present until age 1-3 years
Severe RVOTO = Profound cyanosis within first days of life; may need PGE1 infusion to keep the PDA open

Signs and symptoms:

Cyanosis (bluish skin, lips)
Systolic ejection murmur at the left sternal border (from RVOTO, NOT the VSD)
Chronic hypoxemia leads to compensatory polycythemia
Clubbing of fingers and toes
Squatting in older children (increases SVR, reduces R-to-L shunt)

Investigations

Chest X-ray

"Boot-shaped heart" (coeur en sabot): due to RVH causing upward tilting of the cardiac apex + concave main pulmonary artery segment
Decreased pulmonary vascular markings (less blood reaching lungs)
Right-sided aortic arch in 25% of cases

ECG

Right axis deviation
Right ventricular hypertrophy (tall R in V1, deep S in V6)

Echocardiography (Echo)

Confirms all four defects
Shows overriding aorta, VSD, RVOTO severity
CW Doppler across pulmonic valve: high velocity consistent with severe stenosis

The "Tet Spell" (Hypercyanotic/Hypoxic Spell)

This is a medical emergency that occurs most commonly in infants aged 2-4 months.

Trigger: Anything that suddenly drops systemic vascular resistance (SVR) - crying, defecation, feeding, tachycardia, hypovolemia

Vicious cycle:

Drop in SVR → More R-to-L shunting across VSD → Hypoxemia + hypercapnia + acidosis → Hyperpnea → Increased venous return to RV → More shunting... (cycle worsens)

Clinical signs: Hyperpnea, prolonged crying, worsening cyanosis, limpness. Severe spells can cause seizures, stroke, or death. Notably, the murmur decreases during a tet spell (less blood going through the RVOT).

Management of Tet Spell

Priority	Intervention	Mechanism
1st	Knee-to-chest position (infants) / Squatting (children)	Increases SVR, reduces R-to-L shunt
2nd	Supplemental O2	Limited benefit alone
3rd	Morphine 0.1-0.2 mg/kg IV/IM	Calms child, reduces catecholamine surge, lowers respiratory rate
Alt. opioid	Fentanyl 1 µg/kg IV/IM or 1.5-2 µg/kg intranasal	No histamine release risk
Sedation	Midazolam 0.2-0.3 mg/kg intranasal
Acidosis	Sodium bicarbonate 1 mEq/kg IV if pH <7.4	Breaks hypoxemia-acidosis cycle
Refractory	Ketamine 1-2 mg/kg IV or 3-5 mg/kg IM	Analgesia + increases SVR
Refractory	Phenylephrine 0.01-0.02 mg/kg IV	Alpha-agonist, raises SVR
Refractory	Propranolol 0.1-0.2 mg/kg IV	May reduce infundibular spasm
Refractory	IV fluid bolus	Increases preload/pulmonary flow

Treatment / Surgery

Palliative (temporary)

Modified Blalock-Taussig (BT) shunt: anastomosis between the subclavian artery and ipsilateral pulmonary artery - increases pulmonary blood flow

Definitive Surgical Repair

Close the VSD (patch)
Relieve the RVOTO (resect infundibular tissue, possibly patch the RVOT/pulmonary valve annulus)
Mortality: 5-10% for corrective surgery (when done early)

Long-term Issues After Repair

The major late complication is pulmonary regurgitation (from disrupting the pulmonary valve during repair)
Leads to progressive RV dilation and dysfunction
May eventually require pulmonary valve replacement
Serial echo monitoring (and cardiac MRI for RV volumes) is essential

Associated Defects

Right-sided aortic arch (25%)
ASD (atrial septal defect)
Anomalous origin of the left coronary artery
Patent ductus arteriosus (PDA)

Key High-Yield Points

Fact	Detail
Most common cyanotic CHD beyond infancy	TOF
Embryologic cause	Anterosuperior displacement of infundibular septum
CXR appearance	Boot-shaped heart + decreased pulmonary markings
Murmur character	Ejection systolic murmur at left sternal border (from RVOTO)
Tet spell position	Knee-to-chest (infants), squatting (older children)
Palliative surgery	Modified Blalock-Taussig shunt
Long-term post-repair complication	Pulmonary regurgitation
Untreated prognosis	Average life expectancy only 3-4 years without surgery

Sources: Rosen's Emergency Medicine, Robbins & Kumar Basic Pathology, Guyton & Hall Medical Physiology, Textbook of Clinical Echocardiography (Braunwald's Heart Disease)

Would you like me to go deeper into any particular aspect - such as embryology, surgical techniques, echocardiographic findings, or post-repair complications?

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