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Tetralogy of Fallot is a birth defect of the heart consisting of four abnormalities that results in insufficiently oxygenated blood pumped to the body. It is classified as a cyanotic heart defect because the condition leads to cyanosis, a bluish-purple coloration to the skin, and shortness of breath due to low oxygen levels in the blood. Surgery to repair the defects in the heart is usually performed between 3 and 5 years old. In more severe forms, surgery may be indicated earlier. In most cases the heart can be surgically corrected and the outcome is good.

There is a higher incidence of tetralogy of Fallot in children with Down syndrome (a common genetic disorder which results from having an extra 21st chromosome).

Tetralogy of Fallot is classified as a cyanotic heart defect because the condition causes insufficiently oxygenated blood to be pumped to the body, which leads to cyanosis (a bluish-purple coloration to the skin) and shortness of breath.

The classic form of Tetralogy includes 4 defects within the heart structures:

• Ventricular septal defect (hole between the right and left ventricles)
• Narrowing of the pulmonic outflow tract (tube that connects the heart with the lungs)
• An aorta (tube that carries oxygenated blood to the body) that arises from both ventricles, rather than exclusively from the left ventricle
• A thickened muscular wall of the right ventricle (right ventricular hypertrophy)

There is flow of deoxygenated (blue) blood into the general body circulation and decreased blood flow to the lungs.

At birth, infants may not show the signs of the cyanosis, but later may develop sudden frightening episodes of bluish skin from crying or feeding (called "Tet spells"). Tetralogy of Fallot occurs in approximately 5 out of 10,000 infants.

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