Question: What causes Tetralogy of Fallot ?
Answer: Doctors often don't know what causes tetralogy of Fallot and other congenital heart defects. Some conditions or factors that occur during pregnancy may raise your risk of having a child who has tetralogy of Fallot. These conditions and factors include: German measles (rubella) and some other viral illnesses Poor nutrition Alcohol use Age (being older than 40) Diabetes Heredity may play a role in causing tetralogy of Fallot. An adult who has tetralogy of Fallot may be more likely than other people to have a baby with the condition. Children who have certain genetic disorders, such as Down syndrome and DiGeorge syndrome, often have congenital heart defects, including tetralogy of Fallot. Researchers continue to search for the causes of tetralogy of Fallot and other congenital heart defects.
Source: NHLBI

Question: What are the symptoms of Tetralogy of Fallot ?
Answer: Cyanosis is an important sign of tetralogy of Fallot. Cyanosis is a bluish tint to the skin, lips, and fingernails. Low oxygen levels in the blood cause cyanosis. Babies who have unrepaired tetralogy of Fallot sometimes have "tet spells." These spells happen in response to an activity like crying or having a bowel movement. A tet spell occurs when the oxygen level in the blood suddenly drops. This causes the baby to become very blue. The baby also may: Have a hard time breathing Become very tired and limp Not respond to a parent's voice or touch Become very fussy Pass out In years past, when tetralogy of Fallot wasn't treated in infancy, children would get very tired during exercise and could faint. Now, doctors repair tetralogy of Fallot in infancy to prevent these symptoms. Another common sign of tetralogy of Fallot is a heart murmur. A heart murmur is an extra or unusual sound that doctors might hear while listening to the heart. The sound occurs because the heart defect causes abnormal blood flow through the heart. However, not all heart murmurs are signs of congenital heart defects. Many healthy children have heart murmurs. Babies who have tetralogy of Fallot may tire easily while feeding. Thus, they may not gain weight or grow as quickly as children who have healthy hearts. Also, normal growth depends on a normal workload for the heart and normal flow of oxygen-rich blood to all parts of the body. Children who have tetralogy of Fallot also may have clubbing. Clubbing is the widening or rounding of the skin or bone around the tips of the fingers.
Source: NHLBI