Tetralogy of Fallot: pathophysiology and clinical management

Tetralogy of Fallot (TOF) is a very complex set of abnormalities. It arises from a maldevelopment of the right ventricular infundibulum. Fallot originally described the anatomy of this problem as consisting of right ventricular infundibular stenosis, aortic valve positioned in a way that would override the right ventricle, a subaortic ventricular septal defect, and right ventricular hypertrophy. In other words, TOF is actually not one problem but a collection of them that occur together. There is a wide variation that exists in the anatomic morphology that occurs with this particular problem, as well as the pathophysiology, the surgical methods of therapy, and the signs and symptoms that are clinically presented. A right to left shunting is also considered typical with this problem, and the main pathophysiology is generally dependent on the severity of the obstruction of the right ventricular outflow tract. Within the United States, approximately 10% of the cases of congenital heart disease that are seen come from TOF. The history of its morbidity and mortality, however, is variable. If patients that have TOF are not treated, approximately 25% of them will die before they reach their first birthday. By 4 years of age, 40% will die, by 10 years, 70% will die, and by 40 years of age, 95% will die. It can be seen from this that TOF is a serious condition that must be treated when still in infancy if the baby is to have any kind of normal life expectancy. Generally, the incidence of TOF is somewhat higher in male infants than in female infants, but the reason for this is apparently not known (www.emedicine.com,2004).

There is no specific and genetic abnormality that can be identified as a cause of TOF, but there is a family of cardiac malformations that are known as conotruncal abnormalities that appear to have a genetic etiology where this condition is concerned. Other causes include the ablation of the cells of the neural crest, as well as DiGeorge syndrome and abnormalities of the branchial arch. Also associated with TOF are maternal phenylketonuria birth defects, fetal carbamazepine syndrome, fetal alcohol syndrome, and fetal hydantoin syndrome. When attempting to diagnose TOF, other things that should be considered are pulmonary atresia, ventricular septal defects, pseudotruncus arteriosus, and ebstein malformation of the tricuspid valve (www.emedicine.com,2004).

It is possible that a small number of infants that are diagnosed with TOF may also have other ventricular septal defects or an abnormality in the way that the coronary arteries branch. Some also have a complete obstruction from the right ventricle, getting no flow from there at all. Usually, the flow of blood to the lungs is quite limited by the condition, as well. When the flow of blood to the lungs is found to be restricted, the combination of having an overriding aorta and the ventricular septal defect allows blood that is poor in oxygen to return to the right atrium and right ventricle, where it is then pumped out to the rest of the body, instead of the oxygen-rich blood that the body needs and should be receiving. This shunting can cause babies to look very blue and this occurs because the blood that is low on oxygen has a darker color, which can make the lips and skin appear to have a bluish cast to them. How serious this bluing, or cyanosis, is depends on the severity of the narrowing of both the right ventricular outflow tract and the pulmonary valve. The narrower the outflow tract, the more restricted is the blood flow to the lungs, and therefore the lower the oxygen level in the arteries and the more blood that is low on oxygen that is shunted to the aorta from the right ventricle. Treatment first requires determining whether the baby is getting enough oxygen, because this is critical to life and growth. Usually, infants must have surgery soon after birth, but those that have only mild cyanosis can often go home after only a few weeks. When children reach six months of age, the surgical correction usually takes place, and this correction is required, as the problem will not go away on its own, nor will it get better as the child grows (www.cincinnatichildrens.org, n.d.).