Tetralogy of Fallot
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Tetralogy of Fallot
Tetralogy of Fallot (TOF) is a congenital heart defect which is classically understood to involve four anatomical abnormalities (although only three of them are always present). It is the most common cyanotic heart defect, and the most common cause of blue baby syndrome.
Signs and symptoms
Tetralogy of Fallot results in low oxygenation of blood due to the mixing of oxygenated and deoxygenated blood in the left ventricle via the VSD and preferential flow of the mixed blood from both ventricles through the aorta because of the obstruction to flow through the pulmonary valve. This is known as a right-to-left shunt. The primary symptom is low blood oxygen saturation with or without cyanosis from birth or developing in the first year of life. If the baby is not cyanotic then it is sometimes referred to as a "pink tet." Other symptoms include a heart murmur which may range from almost imperceptible to very loud, difficulty in feeding, failure to gain weight, retarded growth and physical development, dyspnea on exertion, clubbing of the fingers and toes, and polycythemia.
Children with tetralogy of Fallot may develop "tet spells." The precise mechanism of these episodes is in doubt, but presumably results from a transient increase in resistance to blood flow to the lungs with increased preferential flow of desaturated blood to the body. Tet spells are characterized by a sudden, marked increase in cyanosis followed by syncope, and may result in hypoxic brain injury and death. Older children will often squat during a tet spell, which increases systemic vascular resistance and allows for a temporary reversal of the shunt.
Its cause is thought to be due to environmental or genetic factors or a combination. It is associated with chromosome 22 deletions and DiGeorge syndrome.
It occurs slightly more often in males than in females.
Tetralogy of Fallot with pulmonary atresia (pseudotruncus arteriosus) is a severe variant in which there is complete obstruction (atresia) of the right ventricular outflow tract, causing an absence of the pulmonary trunk during embryonic development. In these individuals, blood shunts completely from the right ventricle to the left where it is pumped only through the aorta. The lungs are perfused via extensive collaterals from the systemic arteries, and sometimes also via the ductus arteriosus.
Congenital heart defects are now diagnosed with echocardiography, which is quick, involves no radiation, is very specific, and can be done prenatally.
Before more sophisticated techniques became available, chest x-ray was the definitive method of diagnosis. The abnormal "coeur-en-sabot" (boot-like) appearance of a heart with tetralogy of Fallot is classically visible via chest x-ray, although most infants with tetralogy may not show this finding. Absence of interstitial lung markings are another classic finding in tetralogy.
Emergency management of tet spells
Prior to corrective surgery, children with tetralogy of Fallot may be prone to consequential acute hypoxia (tet spells), characterized by sudden cyanosis and syncope. These may be treated with beta-blockers such as propranolol, but acute episodes may require rapid intervention with morphine to reduce ventilatory drive and a vasopressor such as epinephrine, phenylephrine, or norepinephrine to increase blood pressure. Oxygen (100%) is effective in treating spells because it is a potent pulmonary vasodilator and systemic vasoconstrictor. This allows more blood flow to the lungs. There are also simple procedures such as squatting and the knee chest position which increases aortic wave reflection, increasing pressure on the left side of the heart, decreasing the right to left shunt thus decreasing the amount of deoxygenated blood entering the systemic circulation.
Tetralogy of Fallot occurs in approximately 400 per million live births.
Untreated, Tetralogy of Fallot rapidly results in progressive right ventricular hypertrophy due to the increased resistance on the right ventricle. This progresses to heart failure (dilated cardiomyopathy) which begins in the right heart and often leads to left heart failure. Actuarial survival for untreated Tetralogy of Fallot is approximately 75% after the first year of life, 60% by four years, 30% by ten years, and 5% by forty years.
Patients who have undergone total surgical repair of Tetralogy of Fallot have improved hemodynamics and often have good to excellent cardiac function after the operation with some to no exercise intolerance (New York Heart Association Class I-II). Surgical success and long-term outcome greatly depend on the particular anatomy of the patient and the surgeon's skill and experience with this type of repair.
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