Critical medicine topic-CMT 017
TETRALOGY OF FALLOT (TOF)
A CLINCAL PROFILE
Definition
Tetralogy
of Fallot (TOF) is the most common cyanotic congenital heart disease after the
first year of life, accounting for approximately 10% of all congenital heart
defects. It is characterized by four anatomical abnormalities that result
in reduced pulmonary blood flow and right-to-left shunting, leading to
cyanosis.
The
Four Cardinal Features (Tetralogy)
1.
Ventricular
Septal Defect (VSD)
o Usually a large, high membranous VSD located beneath
the right coronary cusp of the aortic valve.
o Allows free communication between the right and left
ventricles.
2.
Right
Ventricular Outflow Tract (RVOT) Obstruction
o Most commonly due to infundibular (subpulmonary)
stenosis.
o May also involve valvular, supravalvular, or
peripheral pulmonary artery stenosis.
o Severity determines the degree of cyanosis.
3.
Overriding
Aorta
o The aorta is displaced anteriorly and receives blood
from both ventricles.
o It overrides the ventricular septal defect.
4.
Right
Ventricular Hypertrophy
o Develops secondary to chronic pressure overload caused
by RV outflow obstruction.
Pathophysiology
The
clinical severity of TOF depends primarily on the degree of right
ventricular outflow tract obstruction.
- Mild obstruction
results in relatively normal pulmonary blood flow with minimal cyanosis.
- Severe obstruction
markedly reduces pulmonary blood flow.
- Blood preferentially
passes from the right ventricle through the VSD into the aorta
(right-to-left shunt).
- Deoxygenated blood
enters the systemic circulation, producing cyanosis.
- Chronic hypoxemia
stimulates erythropoietin production, leading to secondary polycythemia.
Types
of Pulmonary Outflow Obstruction
1.
Infundibular (Subpulmonary) Stenosis
- Present as the only
obstruction in approximately 50% of patients.
- Caused by narrowing
of the muscular outflow tract.
2.
Combined Infundibular and Valvular Stenosis
- Seen in
approximately 25% of patients.
- Produces more severe
obstruction.
3.
Supravalvular Pulmonary Stenosis
- May occur as an
associated lesion.
4.
Peripheral Pulmonary Artery Stenosis
- May involve one or
multiple pulmonary artery branches.
Hemodynamic
Consequences
- Increased right
ventricular pressure.
- Right-to-left
shunting through the VSD.
- Decreased pulmonary
blood flow.
- Systemic arterial
desaturation.
- Cyanosis.
- Secondary
erythrocytosis (polycythemia).
- Progressive right
ventricular hypertrophy.
Coronary
Artery Anomalies
Associated
coronary artery abnormalities are relatively common and are important during
surgical repair.
Examples
include:
- Left anterior
descending artery arising from the right coronary artery.
- A single right
coronary artery giving rise to a left coronary branch that crosses
anterior to the pulmonary trunk.
These
anomalies influence the surgical approach and must be identified
preoperatively.
Clinical
Features
Clinical
manifestations depend on the severity of pulmonary stenosis.
Mild
Obstruction
- Minimal cyanosis
- Mild exercise
intolerance
- Delayed presentation
Severe
Obstruction
- Marked cyanosis
- Dyspnea on exertion
- Cyanotic
("tet") spells
- Squatting episodes
in children
- Clubbing of fingers
and toes
- Secondary
polycythemia
- Poor growth and
exercise capacity
Key
Points
- Accounts for
approximately 10% of congenital heart disease.
- Most common cyanotic
congenital heart disease after infancy.
- Consists of VSD,
RVOT obstruction, overriding aorta, and RV hypertrophy.
- Severity depends
mainly on the extent of pulmonary outflow obstruction.
- Severe obstruction
causes right-to-left shunting, cyanosis, and polycythemia.
- Coronary artery
anomalies are common and are important during surgical correction.
Examination
Pearls
- Most important
determinant of symptoms:
Severity of right ventricular outflow tract obstruction.
- Cause of
cyanosis: Right-to-left shunting
through the VSD.
- Cause of
polycythemia: Chronic hypoxemia.
- Most common
obstruction: Infundibular
(subpulmonary) stenosis.
- Classic four defects: VSD, pulmonary stenosis, overriding aorta, and right ventricular hypertrophy.
DR.C.GANESAN
M.D.,
PROFESSOR
OF MEDICINE
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