Tetralogy of Fallot

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}} In medicine, the tetralogy of Fallot (described by Etienne Fallot, 1850 - 1911, Marseille) is a significant and complex congenital heart defect. It is the most common cyanotic heart defect.

The term blue baby syndrome is sometimes applied to the tetralogy of Fallot, but is less specific and includes other conditions.

1 Four malformations
2 Pathophysiology
3 Treatment
4 External links

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Four malformations

As classically described, it involves four different heart malformations:

A ventricular septal defect (VSD): a hole between the two bottom chambers (ventricles) of the heart.
Pulmonic stenosis: Right ventricular outflow tract obstruction, a narrowing at or just below the pulmonary valve.
Overriding aorta: The aorta is positioned over the VSD instead of in the left ventricle.
Right ventricular hypertrophy (RVH): The right ventricle is more muscular than normal.

However, our current understanding of the embryology of this disease is that it is a result of anterior malalignment of the conal septum, resulting in the clinical combination of a VSD, pulmonary stenosis, and an overriding aorta. The development of right ventricular hypertrophy is a result of a longstanding, untreated disease.

Pseudotruncus arteriosus is a particularly severe variant of the tetralogy of Fallot, in which there is complete obstruction of the right ventricular outflow tract. In these individuals, there is complete right to left shunting of blood. The lungs are perfused via collaterals from the systemic arteries. These individuals are severely cyanotic and will have a continuous murmur on physical exam due to the collateral circulation to the lungs.

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Pathophysiology

The tetralogy of Fallot generally results in low oxygenation of blood due to mixing of oxygenated and deoxygenated blood in the left ventricle and preferential flow of blood from the ventricles to the aorta because of obstruction to flow through the pulmonary valve. This is known as a right-to-left shunt. It is often evidenced by a bluish tint to the baby's skin (cyanosis). However there are "pink Fallots" in which the degree of obstruction in the pulmonary tract (right ventricular outflow, pulmonary valve and pulmonary arteries) is low. Blood flows preferentially from the ventricles to the lungs and only minimal desaturation occurs in the systemic circulation because of mixing of saturated and desaturated blood in the ventricles. This degree of desaturation may be undetectable to the eye and requires a pulse oximeter to identify it.

Even children who are generally not too deeply cyanosed (blue) may develop acute severe cyanosis or hypoxic "tet spells". The precise mechanism of spelling is in doubt but certainly this is a dangerous event and presumably results from an increase in resistance to blood flow to the lungs with increased preferential flow of desaturated blood to the body. Such spells may be treated with beta-blockers such as propranolol, but acute episodes may require rapid intervention with oxygen, morphine (to reduce ventilatory drive) and phenylephrine (to increase blood pressure). There are also simple procedures such as knee-chest position which reduces systemic venous return (to reduce the right-to-left shunting), increases systemic vascular resistance (and hence blood pressure) and provides a calming effect when the procedure is performed by the parent.

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Treatment

The condition was initially thought untreatable until cardiologist Helen B. Taussig suggested a shunt procedure, which involved joining an artery leaving the heart to an artery leading to the lungs, in an attempt to give the blood a second chance at oxygenation. The surgical procedure was designed and developed (using lab dogs) by Vivien Thomas, who then taught the process to Dr. Alfred Blalock, who performed the first surgery on Eileen Saxon on November 29, 1944. This first blue baby operation came to be known as the Blalock-Taussig shunt; there is some historical dispute over where priority and credit should be given, as Blalock, Thomas, and Taussig have each given different accounts which are difficult to confirm. Note that prior to the first surgery on a human being, Viven Thomas performed the operation at least two hundred times in the lab, and Thomas himself taught Blalock how to perform the operation.

The Blalock-Taussig procedure was the first surgery to be performed on the great vessels surrounding the heart, and demonstrated to the medical community that heart surgery was indeed possible. The first total repair of tetralogy of Fallot was performed by C. Walton Lillehei in 1954 on a 10-month boy, who is still alive today and a physician. Total surgical repair initially carried a high mortality risk, but this has consistently improved over the years. Surgery is now often carried out in infants 1 year of age or younger with a roughly 5% average perioperative mortality.

Patients who have undergone "total" repair of tetralogy of Fallot often have good or excellent cardiac function after the operation with little exercise intolerance, and have the potential to lead normal lives. In reality, while the surgery greatly improves the hemodynamic function of the defective heart, it does not completely correct the defect. Patients with repaired tetralogy of Fallot often have a leaky pulmonary valve, some degree of residual right outflow tract stenosis, and damage to the electrical system of the heart from the surgical incisions. Long-term follow up studies show that these patients are at risk for sudden cardiac death and congestive heart failure. Therefore, long-term follow-up care by a cardiologist with expertise in congenital heart disease is recommended to monitor these risks and to recommend any action, such as re-operation, if it becomes necessary.

As with patients that have undergone any heart surgery, antibiotic prophylaxis is indicated during dental treatment in order to prevent infective endocarditis.

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