Tetralogy of Fallot, or TOF, is a congenital heart disease,
classically known as a combination of four defects that disrupt the normal blood flow in the heart.
In normal circulation, oxygen-poor blood from the body returns to the right side of
the heart where it is pumped into the pulmonary artery and to the lungs. After being oxygenated,
oxygen-rich blood from the lungs returns to the left side of the heart to be pumped
into the aorta and out to the body. Infants born with TOF are presented
with 4 major structural defects: - Narrowing of the pulmonary output
- Ventricular septal defect: an opening in the interventricular septum
- Displacement of the aorta: it now connects to both ventricles
and - Hypertrophy of the right ventricle
The 4 defects, however, are likely the result of a single incident during embryonic development
– the malalignment of the upper part of the ventricular septum, known as the conal septum,
with the rest of it. As the conal septum moves rightward and anteriorly, it creates an opening
in the septum; blocks the right ventricular outflow tract; and pulls the aorta over the
ventricular septum. Finally, because the output to the lungs is obstructed, the right ventricle
develops thicker muscle to push harder, resulting in right ventricular hypertrophy.
The cause of TOF remains largely unknown but several genetic disorders and prenatal
factors are thought to be associated with increased risks of this condition.
Connected ventricles in TOF allow blood to flow from one side to the other. The
direction of this flow, however, depends on the degree of pulmonary tract obstruction.
When the obstruction is minimal, the flow is left-to-right, because the left ventricular
pressure is usually higher. While some of the already oxygenated blood leaks back to the lungs,
most of it goes the usual route to the aorta, and the baby appears “pink” as normal. However, in the
long-term, if too much blood flows to the lungs, patients may develop congestive heart failure.
On the other hand, when pulmonary stenosis is severe, blood in the right ventricle has to escape
through the septal defect during ventricular contraction, and a right-to-left shunt results.
The mixed blood, which is low in oxygen, is then pumped into the aorta and to the body,
causing oxygen deprivation in body's tissues, or hypoxia. This may result in
a bluish skin color, known as cyanosis. The greater the pulmonary obstruction,
the more deoxygenated blood enters the systemic circulation, the more severe the symptoms.
Children with TOF may develop acute episodes of hypoxia, known as "tet spells", during activities
that demand more oxygen. These episodes are characterized by: shortness of breath,
increased cyanosis, loss of consciousness, and may result in hypoxic brain injury and death. Tet
spell is a medical emergency but simple procedures such as squatting and the knee chest position,
which increase systemic vascular resistance and therefore decrease right-to-left shunting,
can help to temporarily relieve symptoms. Diagnosis is by echocardiography and can
be done prenatally. Treatment is usually by repair surgery within the first year
of life. The surgery involves enlargement of the pulmonary tract and closure of the septal defect.