Tetrology of Fallot

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"Tetralogy of Fallot"[MeSH Terms]

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Here is a comprehensive overview of Tetralogy of Fallot (ToF).

Tetralogy of Fallot

Definition & Epidemiology

Tetralogy of Fallot is the most common cyanotic congenital heart disease, accounting for ~5% of all congenital cardiac malformations, with an incidence of approximately 420 per million live births.

The Four Cardinal Defects

All four features arise from a single embryologic defect: anterosuperior displacement of the infundibular (outlet) septum, causing unequal division of the truncus arteriosus.

#	Defect	Details
1	Right Ventricular Outflow Tract (RVOT) Obstruction	Usually subpulmonic (infundibular) stenosis; may be valvular stenosis or complete pulmonary atresia
2	Large, Unrestrictive VSD	Near the membranous septum; allows bidirectional shunting
3	Overriding Aorta	Aortic valve straddles the VSD, receiving blood from both ventricles
4	Right Ventricular Hypertrophy	Secondary to high pressure load from RVOT obstruction

Classic Tetralogy of Fallot — showing RV, LV, VSD, overriding aorta (Ao), and hypoplastic pulmonary trunk (PT)

Classic ToF: right-to-left shunting across the VSD. Ao = Aorta, PT = Pulmonary Trunk, RA/RV/LA/LV = cardiac chambers. (Robbins & Kumar Basic Pathology)

Pathophysiology

The physiologic result is decreased pulmonary blood flow + right-to-left shunting of deoxygenated blood across the VSD into the aorta.

Blood flow diagram in ToF — deoxygenated blood (blue) shunted through VSD past the RVOT obstruction (OB) into the overriding aorta (OAo), producing mixed oxygenation in the systemic circulation

Hemodynamics of ToF: severity of shunt depends on degree of RVOT obstruction, VSD size, and systemic vascular resistance (SVR). (Rosen's Emergency Medicine)

Severity is proportional to the degree of RVOT obstruction
Mild obstruction → left-to-right shunt only → "Pink Tet" (acyanotic)
Severe obstruction → early profound cyanosis
In pulmonary atresia: pulmonary flow entirely via PDA or bronchial collaterals

Morphology

Boot-shaped heart (coeur en sabot) — from RV hypertrophy elevating the cardiac apex
Proximal aorta dilated; pulmonary trunk hypoplastic
RV wall markedly hypertrophied (may exceed LV thickness)
Left-sided chambers are normal in size

Clinical Features

Feature	Details
Cyanosis	Worsens with crying, feeding, exertion; may be absent at birth
Systolic ejection murmur	Left sternal border (from RVOT obstruction, NOT the VSD)
Clubbing	Fingers and toes; from chronic hypoxemia
Polycythemia	Compensatory response to chronic hypoxia
Squatting	Older children squat to increase SVR and reduce R→L shunt
Chest X-ray	Boot-shaped heart, decreased pulmonary vascular markings, possible right-sided aortic arch (~25%)
ECG	RV hypertrophy, right axis deviation

Tet Spells (Hypercyanotic/Hypoxic Spells)

Peak incidence: 2–4 months of age

Trigger: Anything that suddenly ↓SVR (crying, defecation, fever) or causes tachycardia/hypovolemia

Vicious cycle:

↓SVR → ↑R→L shunt → ↓PaO₂, ↑PCO₂, ↓pH → hyperpnea → ↑venous return to RV → more shunting → worsening hypoxia

Clinical signs: Hyperpnea, prolonged crying, deepening cyanosis, decreased murmur intensity, possible limpness, seizures, stroke, or death.

Management of Tet Spells (Rosen's Box 165.8)

Knee-to-chest position (or squatting) — increases SVR, reduces R→L shunt
Supplemental oxygen (limited effect alone)
Morphine 0.1–0.2 mg/kg IV/IM (sedation, ↓catecholamines) — note risk of histamine-mediated vasodilation
Fentanyl 1 µg/kg IV/IM or 1.5–2 µg/kg intranasal (preferred over morphine)
Midazolam 0.2–0.3 mg/kg intranasal
Ketamine 1–2 mg/kg IV or 3–5 mg/kg IM — excellent (↑SVR + sedation)
NaHCO₃ 1 mEq/kg IV — if metabolic acidosis (pH <7.4)
Phenylephrine 0.01–0.02 mg/kg IV — α-agonist to ↑SVR
Propranolol 0.1–0.2 mg/kg IV — may reduce infundibular spasm
IV fluids — to ↑preload and pulmonary flow

Associated Defects

Right-sided aortic arch (~25%)
ASD
Anomalous coronary artery origin (left coronary from right coronary artery in ~5% — critical surgical relevance)
Pulmonary atresia with VSD = extreme form of ToF

Surgical Management

Palliative (staged)

Modified Blalock-Taussig (BT) shunt: subclavian artery → ipsilateral pulmonary artery anastomosis
Used when: severe cyanosis, very narrow RVOT, hypoplastic central pulmonary arteries

Definitive Repair

VSD closure + RVOT relief (infundibular resection ± transannular patch across pulmonary valve annulus)
Preferred: early single-stage repair in infancy
Surgical mortality: ~5–10% in uncomplicated cases within first 2 years
PGE₁ (prostaglandin E₁) infusion may be needed pre-operatively to maintain PDA patency in severely cyanotic neonates

Post-repair Residua (almost universal)

Pulmonary regurgitation (PR) — most common long-term problem
RV dilatation, RVOT aneurysm
Residual RVOT obstruction or VSD
Aortic root dilatation
Right bundle branch block (from right ventriculotomy)
Complete heart block, ventricular arrhythmias
CMR is gold standard for RV assessment post-repair; guides timing of pulmonary valve replacement

Pulmonary Valve Replacement

Surgical redo vs. transcatheter pulmonary valve replacement (TPVR) — both are options; recent 2025 systematic review (PMID 40122137) compared outcomes
Timing of intervention guided by RV volumes/function on CMR

Recent Evidence

Primary vs. staged repair in symptomatic neonates (Meta-analysis, Cardiol Young 2024; PMID 39444206): systematic review comparing outcomes of primary vs. staged surgical approach in symptomatic neonates.
Surgical vs. transcatheter pulmonary valve redo (Expert Rev Cardiovasc Ther 2025; PMID 40122137): systematic review of redo pulmonary valve options in repaired ToF patients.

Sources: Robbins & Kumar Basic Pathology | Rosen's Emergency Medicine | Grainger & Allison's Diagnostic Radiology | The Developing Human (Moore) | Braunwald's Heart Disease

How crying fever decreases SVR

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how does crying decrease systemic vascular resistance SVR physiology

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fever decreases SVR mechanism prostaglandin nitric oxide vasodilation physiology

This is an important clarification — crying and fever are two separate triggers, each decreasing SVR through different mechanisms. The textbooks (and Merck Manual) actually categorize them differently.

How Crying and Fever Each Affect SVR

First — A Clarification on the Textbook Statement

Looking back at Rosen's and the Merck Manual carefully, the triggers for tet spells are listed as:

↓ SVR triggers: Playing, kicking legs on awakening, vigorous physical activity
↓ O₂ saturation triggers: Crying, defecating
Other: Tachycardia, hypovolemia

Crying is not primarily a direct SVR-lowering stimulus — its main effect is different. Here's the full breakdown:

1. Crying → ↑ Intrathoracic Pressure → ↑ Venous Return → Worsens Shunting

Crying involves a Valsalva-like maneuver and forceful expiratory effort:

The child strains and holds breath during a cry, raising intrathoracic pressure transiently
This compresses pulmonary vasculature, increasing pulmonary vascular resistance (PVR) and reducing pulmonary blood flow
Simultaneously, crying causes agitation and catecholamine release (sympathetic activation), which:
- ↑ Heart rate → shortens RV filling time
- Causes infundibular (RVOT) spasm via β-adrenergic stimulation of infundibular muscle → worsens RVOT obstruction
- The increased venous return overwhelms the obstructed RV, forcing more blood across the VSD into the aorta

So crying's primary mechanism is ↑ RVOT obstruction + ↑ PVR, not ↓SVR. The result is the same — more R→L shunting — but via a different pathway.

This is why morphine helps tet spells: it suppresses the catecholamine surge and reduces the cry-driven agitation, breaking the infundibular spasm cycle.

2. Fever → ↓ SVR via Peripheral Vasodilation

Fever decreases SVR through direct vascular mechanisms:

Mediator	Mechanism
PGE₂ (prostaglandin E₂)	Released during fever via COX → arachidonic acid → PGH₂ → PGE₂; acts on vascular smooth muscle to cause vasodilation (same prostaglandins that cause the flushed, warm skin of fever)
Nitric Oxide (NO)	Cytokine-stimulated endothelial NO release → cGMP in smooth muscle → relaxation → vasodilation
Direct thermal effect	Heat itself causes smooth muscle relaxation in arterioles (metabolic vasodilation)
Increased metabolic demand	Fever raises tissue O₂ consumption → local metabolic byproducts (CO₂, lactate, adenosine) cause local arteriolar dilation → ↓ SVR

This is why febrile children with ToF are at risk: the systemic vasodilation from fever lowers the "wall" of SVR that normally keeps blood flowing left-to-right (or balanced), tipping the balance toward R→L shunting across the VSD.

Summary: Pathways to R→L Shunting

CRYING                          FEVER
  ↓                               ↓
Agitation + catecholamines     PGE₂, NO, heat
  ↓                               ↓
Infundibular spasm             Peripheral vasodilation
↑ RVOT obstruction             ↓ SVR
  ↓                               ↓
↑ RV pressure                  Pressure gradient shifts
  ↓                               ↓
  └──────────────────────────────┘
         ↑ Right-to-Left shunting across VSD
                   ↓
            ↑ Cyanosis (Tet Spell)

Both converge on the same endpoint but through mechanistically distinct pathways — which is why treatment targets both (knee-chest/phenylephrine for SVR; morphine/fentanyl for infundibular spasm/catecholamine surge).

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