Tetralogy of Fallot is a combination of four heart defects present at birth: a hole between the lower chambers of the heart, a narrowed pathway to the lungs, a mispositioned aorta, and a thickened right heart wall. It occurs in roughly 3.5 out of every 10,000 live births, making it one of the most common forms of cyanotic (blue-tinted skin) congenital heart disease. All four defects work together to reduce the amount of oxygen that reaches the body.
Ventricular Septal Defect (The Hole)
The first and most central defect is a hole in the wall, called the septum, that separates the heart’s two lower pumping chambers (ventricles). In a healthy heart, this wall keeps oxygen-rich blood on the left side completely separate from oxygen-poor blood on the right side. In tetralogy of Fallot, the hole is typically large enough that pressure in both ventricles equalizes, meaning blood can flow freely between them in either direction.
Which direction the blood actually flows depends on the severity of the second defect. When the pathway to the lungs is severely narrowed, blood takes the path of least resistance: it crosses through the hole from the right ventricle into the left and gets pumped out to the body without ever picking up oxygen from the lungs. This is what causes the bluish skin color many babies with this condition develop.
Pulmonary Stenosis (The Narrowing)
The second defect is a narrowing of the pulmonary valve and the artery that carries blood from the right ventricle to the lungs. This narrowing can range from mild to severe, and it’s the single biggest factor in determining how symptomatic a child will be. A mild narrowing may allow enough blood to reach the lungs, producing little or no visible cyanosis. A severe narrowing starves the lungs of blood flow and forces most of the oxygen-poor blood backward through the hole and out to the body.
The narrowing isn’t limited to the valve itself. The entire outflow pathway from the right ventricle can be underdeveloped, including the main pulmonary artery and its branches. This happens because a portion of the inner heart wall is displaced forward during fetal development, physically intruding into the space where blood needs to flow toward the lungs.
Overriding Aorta (The Shifted Vessel)
In a normal heart, the aorta (the body’s largest artery) connects exclusively to the left ventricle and carries only oxygen-rich blood to the rest of the body. In tetralogy of Fallot, the aorta is shifted to the right so that it sits directly over the hole between the ventricles. This means the aorta receives blood from both the left and right sides of the heart. Because the right side contains oxygen-poor blood, the result is a mixture of oxygenated and deoxygenated blood being sent throughout the body.
The overriding aorta and the ventricular septal defect are closely linked. The same displacement of internal heart tissue that creates the narrowed lung pathway also positions the aorta over the hole rather than squarely over the left ventricle.
Right Ventricular Hypertrophy (The Thickened Wall)
The fourth defect is an abnormal thickening of the muscular wall of the right ventricle. This develops because the right ventricle has to work much harder than normal to push blood through the narrowed pathway to the lungs. Over time, this extra workload causes the muscle to bulk up, similar to how a bicep grows when you repeatedly lift heavy weight.
Right ventricular hypertrophy is more of a consequence of the other three defects than an independent problem. It contributes to the characteristic “boot-shaped heart” visible on a chest X-ray: the thickened right ventricle tips the heart’s lower point upward, while the underdeveloped pulmonary artery narrows the upper silhouette.
How the Four Defects Work Together
Individually, each defect would cause its own set of problems. Together, they create a self-reinforcing cycle. The narrowed lung pathway raises pressure in the right ventricle. That pressure pushes oxygen-poor blood through the hole into the left ventricle and up through the mispositioned aorta. The right ventricle thickens in response to all this extra effort, which can further crowd the already narrow outflow tract to the lungs.
The severity of this cycle varies widely. Some babies turn visibly blue within hours of birth. Others, sometimes called “pink tets,” have only mild narrowing and may not show obvious cyanosis for weeks or months. The common thread is that all four structural problems are present from birth, even when symptoms are subtle.
Tet Spells: When Oxygen Drops Suddenly
Babies and toddlers with unrepaired tetralogy of Fallot can experience episodes called “tet spells,” where oxygen levels drop sharply. During a spell, the child’s skin turns a deeper blue, breathing becomes rapid and labored, and the child may become limp or irritable. These episodes are triggered by anything that increases the obstruction to the lungs or decreases resistance in the body’s blood vessels, such as crying, feeding, or straining during a bowel movement.
A classic behavior in older infants and toddlers is squatting during play. Pulling the knees up toward the chest compresses the large arteries in the legs, which raises overall blood pressure in the body. This makes it harder for blood to take the shortcut through the hole and easier for it to flow toward the lungs instead, improving oxygen levels. Parents and caregivers of babies too young to squat on their own are often taught to hold the child in a knee-to-chest position during a spell.
Surgical Repair and Long-Term Outlook
Tetralogy of Fallot is treated with open-heart surgery, typically performed before a baby’s first birthday. The surgeon closes the hole between the ventricles with a patch and widens the narrowed pathway to the lungs, which also addresses the overriding aorta and, over time, allows the right ventricle to thin back toward a more normal size.
Some newborns with very severe narrowing need a preliminary procedure first: a small tube is placed to create an alternate route for blood to reach the lungs, buying time until the baby is large enough for a full repair. Operative mortality for both the temporary procedure and the full repair is in the range of 6 to 7 percent in large surgical databases, and survival after repair has improved dramatically over the decades.
For children born with straightforward anatomy and no associated genetic conditions, 30-year survival after repair is around 95 percent. Most of these individuals lead active, largely normal lives, though they typically need periodic follow-up with a cardiologist because the pulmonary valve can develop leakiness over time, sometimes requiring a valve replacement later in adulthood. On average, a person with repaired tetralogy of Fallot undergoes about three total cardiac interventions over 30 years.
Genetic Connections
About 16 percent of people with tetralogy of Fallot carry a specific chromosomal deletion known as 22q11.2, which is also linked to DiGeorge syndrome. This genetic change affects the development of several body systems, not just the heart, and its presence can influence the complexity of the heart defect and the number of surgeries needed. Genetic testing is routinely offered after a diagnosis of tetralogy of Fallot because identifying this deletion early helps guide care for related issues like immune function and calcium regulation.