Tetralogy of Fallot occurs in approximately 400 out of every million live births. This congenital heart condition causes mixing of oxygen-poor blood with oxygen-rich blood, which is then pumped out of the heart into the circulatory system of blood vessels.
The blood leaving the heart has less oxygen than is needed by the organs and tissues of the body, a condition called hypoxemia.
Chronic (ongoing, long-term) lack of oxygen causes cyanosis, a bluish color of the skin, lips, and membranes inside the mouth and nose.
The heart is made up of 4 chambers: 2 upper chambers called atria and 2 lower, larger chambers called ventricles. Each atrium is separated from its paired ventricle by a valve.
The heart has a left and a right side. The left and right sides of the heart are separated by a septum (wall). The right side of the heart receives oxygen-depleted or blue blood returning by veins (superior vena cava and inferior vena cava) from the body.
The blood flows from the right atrium through the tricuspid valve into the right ventricle, which pumps it through the pulmonic valve into the pulmonary artery, the main artery to the lungs.
In the lungs, the blood absorbs oxygen and then returns to the left atrium through the pulmonary veins.
From the left atrium, the blood is pumped through the mitral valve to the left ventricle. The left ventricle pumps the blood out of the heart into the circulatory system via a large artery known as the aorta.
The blood moves throughout the body, supplying oxygen and nutrients to organs and cells.
Organs cannot work properly if they do not receive enough oxygen-rich blood.
The 4 abnormalities (tetralogy) of the heart described by Fallot include the following:
Right ventricular hypertrophy: Right ventricular thickening, or hypertrophy, occurs in response to narrowing or obstruction at or below the pulmonic valve, due to an increase in right ventricular work and pressure.
Ventricular septal defect (VSD): This is a hole in the heart wall (septum) that separates the 2 ventricles. The hole is usually large and allows oxygen-poor blood in the right ventricle to pass through, mixing with oxygen-rich blood in the left ventricle. This poorly oxygenated blood is then pumped out of the left ventricle to the rest of the body. The body gets some oxygen, but not all that it needs. This lack of oxygen in the blood causes cyanosis.
Abnormal position of the aorta: The aorta, the main artery carrying blood out of the heart and into the circulatory system, exits the heart from a position overriding the right and left ventricles. (In the normal heart, the aorta exits from the left ventricle.)
Pulmonary valve stenosis (PS): The major issue with tetralogy of Fallot is the severity of pulmonary valve stenosis, since VSD is always present. If the stenosis is mild, minimal cyanosis occurs, since the oxygen-poor blood from the right ventricle can pass through the pulmonic valve to the lungs and less of it passes through the VSD. However, if the PS is moderate to severe, a smaller amount of blood reaches the lungs, since most is shunted right-to-left through the VSD.
Tetralogy of Fallot accounts for 10%-15% of all congenital (newborn) heart defects. Infants with this abnormality develop signs of the condition very early in life.