Tetralogy of Fallot consists of 4 abnormalities:
- A large hole between the 2 pumping chambers of the heart
- Narrowing of the outlet of the right heart associated with a small, abnormally formed outlet valve (pulmonary valve)
- Excess muscle in the right side of the heart
- The aorta is partly connected to (over-rides) the right side of the heart (should only be connected to the left side of the heart)
Tetralogy of Fallot results in a murmur and blueness (oxygen saturations <90%) and worsened blueness with crying/agitation (blue “spells"). Sometimes there can be abnormalities in the development of the lung arteries causing them to be very small and the lungs may be supplied by blood from abnormal arteries called collaterals. Tetralogy of Fallot can be diagnosed before birth on screening ultrasounds assisted by fetal echocardiography but often is diagnosed in the newborn or early infant period. The most common signs are reduced oxygen levels on routine newborn screening and a heart murmur detected when listening to the chest.
Surgical repair involves open heart surgery and use of the heart-lung machine. The hole between the 2 pumping chambers of the heart is closed with a patch, muscle from the outlet of the right heart that is causing narrowing is excised, the lung valve is also removed if it is contributing to the narrowing, and the lung artery and outlet of the right heart are enlarged with a patch. Almost all babies and infants who undergo surgical repair of Tetralogy of Fallot early in life will require further heart surgery later in life to replace the pulmonary (lung) valve. Once the pulmonary valve has been removed during the initial repair, leakage of blood back into the heart from the lungs will cause the right heart to eventually stretch and enlarge increasing the risk of heart failure and abnormal heart rhythms. This is corrected by implantation of an adult-size pulmonary valve in teenage years.
Some babies are born with either a very small right heart or a very small left heart and have essentially only one functional pumping chamber. Other babies are born with two pumping chambers with holes between them and abnormal connections to arteries and veins that are so complex that the two pumping chambers cannot be separated and must function as a single pumping chamber. This single pumping chamber will be required to push blood to the body through the aorta whereas alternative methods can be used to get blood to the lungs without a pump. Because red (oxygenated) and blue (deoxygenated) blood inevitably mix within the heart before being pumped to the body in all these abnormalities, the baby is blue (oxygen saturations <90%).
Abnormalities associated with a small right heart include pulmonary atresia with intact ventricular septum, tricuspid atresia and double inlet left ventricle.
Abnormalities associated with a small left heart include hypoplastic left heart syndrome (HLHS), double outlet right ventricle with mitral atresia and right ventricle dominant unbalanced atrioventricular septal defect.
Babies with any of these abnormalities usually require a sequence of three operations in childhood.
- The first operation usually occurs within the first few days or weeks of life depending on the underlying abnormality and can be simple (creation of a systemic to pulmonary artery shunt, banding of the pulmonary artery) or very complex (Norwood procedure).
- The second operation usually occurs around 3-6 months of age and involves connecting the superior vena cava (large vein draining blue blood from the head and upper body towards the heart) to the right lung artery to provide a stable form of blood flow to the lungs that grows with the child.
- The last operation usually occurs around 4-5 years of age and involves connecting the inferior vena cava (large vein draining blue blood from the lower body towards the heart) to the lung arteries using a Gore-Tex tube. At this stage, all the blue blood from the body returns directly to the lung arteries and bypasses the heart. Therefore, after this final operation, the child will be much pinker with oxygen saturations >90%. This final plumbing arrangement is known as the Fontan circulation or pathway.