What is Tetralogy of Fallot (TOF)?

What is Tetralogy of Fallot (TOF)?

Tetralogy of Fallot (TOF) is a congenital heart defect that affects infants and young children. It is caused by four heart defects that allow low-oxygen blood to flow from the right side of the heart and throughout the body.

The four defects are a result of a single error in the development of the heart: a part of the muscle that makes up the wall separating the two pumps of the heart (the ventricles) is misaligned. This combination of four issues (the tetralogy) includes:

  • Pulmonary stenosis – When the valve located between the lower right heart chamber and pulmonary artery narrows, limiting blood flow from the heart to the lungs
  • Right ventricular hypertrophy – The muscle of the right ventricle becomes thickened as the heart’s pumping action works harder than normal. Like any other muscle, it gets thicker and stronger when exercised. This can make the heart stiff and weak over time.
  • Ventricular septal defect (VSD) – A hole in the wall between the bottom heart chambers (left and right ventricles). This hole allows blood to cross from one side of the heart to the other, limiting blood flow and decreasing the body’s supply of oxygen-rich blood.
  • Overriding aorta – With malalignment of the muscle, the aorta (the main blood vessel going to the body) sits over the VSD, receiving blood from both of the pumps. This results in a mix of low-oxygen and high-oxygen blood from the right and left ventricles.

Signs & Symptoms of Tetralogy of Fallot


Tetralogy of Fallot (TOF) is an error in the formation of the heart that occurs before birth. The heart is fully formed by the tenth week of pregnancy, so TOF may be present even before many parents know they are pregnant.

In most cases of tetralogy of Fallot, there are no problems for the baby before birth. The baby is not breathing on its own at this point, so it is not yet necessary to pump blood to the lungs. Once born, most babies with tetralogy of Fallot will be diagnosed in the hospital or within a few weeks of age.

Common tetralogy of Fallot symptoms include:

  • Cyanosis - The baby’s lips, skin, and nail beds turn blue in color due to low levels of oxygen. If oxygen levels are too low, the baby will stay in the hospital for extra care.
  • Heart murmur
  • Rapid breathing or shortness of breath, particularly while playing or during feeding
  • Poor weight gain
  • Fainting
  • Nail deformities
  • Fatigue, problems with feeding, and irritability - These symptoms sometimes occur later, when the tetralogy of Fallot has not yet been repaired, and the pathway to the lungs is getting narrower and tighter. All are signs of lower oxygen levels.

Tet spells 

Babies and young children with tetralogy of Fallot may experience “tet spells” caused by a sudden drop of oxygen levels in the blood. Tet spell episodes can produce sudden, alarming symptoms, most commonly in infants 2 to 4 months old.

Symptoms of a tet spell can include:

  • Skin suddenly turns deep bluish in color
  • Breathing difficulties
  • Seizures
  • Fainting
  • Extreme weakness
  • Unexplained irritability

If your baby is exhibiting signs of a tet spell, seek medical attention immediately.

If your infant turns blue (cyanosis), turn the baby on their side and pull their knees up into their chest to increase blood flow to the lungs. Toddlers or older children can squat down to increase the blood flow to the lungs.

What Causes Tetralogy of Fallot?


The exact cause of tetralogy of Fallot is unknown. In some patients (about 15%), tetralogy of Fallot is linked to a chromosomal deletion, where a part of one of the genes is missing. It is signified as 22q11, meaning that the missing portion of the gene is from the 22nd chromosome. A patient can be tested for this chromosomal deletion at any time in life.

Other maternal medical factors can increase the risk of TOF, including:

  • Smoking during pregnancy
  • Alcohol use during pregnancy
  • Older maternal age (> 40)
  • Poor maternal diet during pregnancy
  • A family history of tetralogy of Fallot (particularly in a family with a known 22q11 deletion)

Tetralogy of Fallot Complications


Left untreated, only 3 of 4 babies will be alive at 1 year, only 3 in 10 at 10 years of age, only 1 in 10 at 20 years old, and less than 1 in 20 at age 40 years. 

Those that do survive have a limited quality of life, as the low oxygen levels keep them from being able to do exercise or strenuous work.

Other complications in unrepaired tetralogy of Fallot patients can include:

  • Sudden cardiac death, often related to life-threatening rhythm disturbances
  • Infections of the heart (endocarditis)
  • Blood clots that may cross through the VSD and cause a stroke
  • Infections of the brain (cerebral abscess)

Risk Factors

Risk Factors

There are certain environmental (in utero) and genetic risk factors that can contribute to the development of tetralogy of Fallot and other congenital heart defects:

  • Smoking during pregnancy
  • Drinking alcohol during pregnancy
  • Advanced maternal age (the mother is older than 40)
  • Detection of DiGeorge syndrome or Down syndrome in the baby
  • Poor diet during pregnancy
  • Mother takes certain medications suspected to cause developmental irregularities
  • Family history
  • A viral illness such as rubella during pregnancy
Get Care

Trust NewYork-Presbyterian for Tetralogy of Fallot Treatment

NewYork-Presbyterian is one of the leading hospitals in the nation for pediatric and adult cardiovascular care. We specialize in a range of heart conditions, including congenital heart diseases, aortic disease, pulmonary hypertension, and valvular heart disease.

Our top-notch cardiologists are familiar with the symptoms of tetralogy of Fallot. We can provide you and your child with an expert diagnosis and options for treatment. Reach out to us today for compassionate cardiac care.