TETRALOGY OF FALLOT:
Tetralogy of Fallot (TOF) is the most common cyanotic congenital heart
defect after infancy. It is characterized by a large VSD, an aorta that overrides
ventricle, right ventricular hypertrophy and obstruction of the right ventricular
Etienne-Louis Arthur Fallot a French physician described it accurately in 1888.
Fallot’s pentology means Tetralogy of Fallot plus persistent foramen Ovale or
atrial septal defect. Fallot’s Trilogy means atrial septal defect plus pulmonary
stenosis with intact interventricular septum.
The right ventricular outflow tract obstruction is the most important
component of Fallot’s tetralogy. It is most often sub-valvular in location. But it can
also be valvular or supra valvular or a combination of these. The ventricular
septal defect is usually large and causes right to left shunting of blood in this
situation of elevated right ventricular pressure.
The peri-membraneous location of the ventricular septal defect is
responsible for the over-riding of aorta. The obstruction to the right ventricular
outflow leads to the development of right ventricular hypertrophy. The right
ventricular outflow obstruction also results in reduction to the pulmonary blood
Arterial desaturation develops because of the right to left shunting of
blood between the ventricles and hence patient develops cyanosis. The tetralogy
of Fallot generally results in low oxygenation of blood due to mixing of
oxygenated and deoxygenated blood in the left ventricle and preferential flow of
blood from the ventricles to the aorta because of obstruction to flow through the
pulmonary valve. Thus a mixture of blood from right and left ventricles enters the
Tetralogy of Fallot seldom presents with cyanosis at birth.
Symptoms usually appear at the end of first year of life. Some children with mild
Fallot’s tetralogy may never develop cyanosis because the pulmonary stenosis
may be very mild, that the pulmonary blood flow is not compromised and any
significance admixture of blood occurs in the ventricular levels. These cases are
called acyanotic Fallot.
The infants usually do not manifest cyanosis. They develop cyanosis only
when they are feeding or crying. This is because the sub-valvular obstruction to
the right ventricular outflow is dynamic in nature. It increases with adrenergic
stimulation and the flow across the valve decreases. This makes the child
increasingly cyanosed, breathless and even aponoeic. Some times loss of
consciousness or convulsions may occur. These are called Fallot’s spells.
The age, at which symptoms appear, depends upon the severity of right
ventricular obstruction. The main symptoms are cyanosis, cyanotic spells and
squatting and exertional dyspnoea. Slight bluish discoloration of the tongue and
lips may be evident on close observation of the child, in presence of large
ventricular shunting of blood.
Cyanotic spells usually occur after the age of three to six months, when
the child becomes increasingly active. It is characterized by sudden onset of
breathlessness, restlessness and increasing bluish discoloration of the skin and
mucous membranes. At the peak of the symptoms the child may loose
consciousness and develop convulsions. Sometimes the cyanotic spells can be
Squatting is the posture adopted by older children with Tetralogy of Fallot,
who have learned from experience that, it will relieve them from cyanosis. The
child stops playing and sits on ground in a knee close to cheat position. This
produces significant reduction in symptoms of breathless and cyanosis at least
After infancy the children become more and more cyanotic as the right
ventricular pressure increases and the right to left shunt also. These children do
not manifest much dyspnoea, but cyanosis is more apparent. These children
more often assume the squatting posture to get relief from the cyanosis.
The pulse is usually is not altered nor is the blood pressure. The jugular
venous pressure also is not elevated initially. Characteristically the child has a
silent precordium. The apex beat is not prominent. Depending on the degree of
right ventricular hypertrophy there may be left parasternal heave