Tetralogy of Fallot

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The tetralogy of Fallot is a congenital heart defect which classically has four anatomical components. It is the most common cyanotic heart defect and the most common cause of blue baby syndrome.

It was first described by Niels Stensen in 1672. In 1888 the French physician Etienne Fallot accurately detailed its four anatomical characteristics.

1 Anatomic morphology
2 Epidemiology and etiology
3 Pathophysiology
4 Symptoms
5 Treatment
6 External links

[edit] Anatomic morphology

As classically described, tetralogy of Fallot involves four 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. The degree of stenosis varies between individuals with TOF and is the primary cause of symptoms.
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 and may also be dilated. This causes a characteristic coeur-en-sabot (boot-shaped) appearance as seen by chest X-ray.

There is anatomic variation between the hearts of individuals with tetralogy of Fallot. The degree of right ventricular outflow tract obstruction varies between patients and is generally the primary determinant of clinical symptoms and disease progression. In addition, this condition is also sometimes associated with other anatomical analomies, such as:

- stenosis of the left pulmonary artery (in 40% of patients)
- a bicuspid pulmonary valve (in 40% of patients)
- right-sided aortic arch (in 25% of patients)
- coronary artery anomalies (in 10% of patients)
- an atrial septal defect, in which case the syndrome is sometimes called the pentalogy of Fallot
- an atrioventricular septal defect
- anomalous pulmonary venous return
- forked ribs and scoliosis

Tetralogy of fallot with pulmonary atresia (pseudotruncus arteriosus) is a severe variant in which there is complete obstruction of the right ventricular outflow tract and absence of the pulmonary trunk. In these individuals, there is complete right to left shunting of blood. The lungs are perfused via extensive 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.

[edit] Epidemiology and etiology

Tetralogy of Fallot occurs in approximately 3 to 6 per 10,000 births and represents 5-7% of congenital heart defects. 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.

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 progressive right heart failure arising from this combination, and can be minimized or even averted by early surgical repair.

[edit] 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 tets" in which the degree of obstruction in the right ventricular outflow tract 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.

Untreated tetralogy of Fallot, over the long term, results in progressive right ventricular hypertrophy and dilatation due to the increased resistance on the right ventricle. This progresses to right heart failure and death.

[edit] Symptoms

Symptoms include cyanosis from birth or developing in the first year of life caused by the lung’s inability to oxygenate sufficient blood for the system, difficulty in feeding, failure to gain weight, retarded growth and physical development, dyspnea on exertion, clubbing of the fingers and toes, and polycythemia.

[edit] Treatment

Tetralogy of Fallot is treated on two levels: with immediate emergency care for its aforementioned "hypoxic spells" or "tet spells" and with surgery for its long term progressive effects.

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.

[edit] Emergency management of tet spells

Consequential acute hypoxia 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 the 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.

[edit] Palliative surgery

The condition was initially thought untreatable until surgeon Alfred Blalock, cardiologist Helen B. Taussig, and lab assistant Vivien Thomas at Johns Hopkins University developed a shunt procedure, which involved joining the left subclavian artery leaving the heart to the left pulmonary artery leading to the lungs. This gave a large portion of the partially oxygenated blood leaving the heart a second chance at oxygenation, and greatly relieved symptoms in patients. The first Blalock-Taussig shunt surgery was performed on 15-month old Eileen Saxon on November 29, 1944.

The Pott shunt and the Waterson procedure are other shunt procedures which were developed for the same purpose.

[edit] Surgical repair

The Blalock-Taussig procedure was the first surgery to be performed on the great vessels surrounding the heart, and became historically important because it 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 at the University of Minnesota in 1954 on a 10-month boy. 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 <5% perioperative mortality. The surgery generally involves making incisions into the heart muscle, relieving the right ventricular outflow tract stenosis by careful resection of muscle, and repairing the VSD using a Gore-Tex or Dacron patch or a homograft. Additional reparative or reconstructive work is done on some patients, since the anatomy varies.

Patients who have undergone "total" repair of tetralogy of Fallot often have good or excellent cardiac function after the operation with some to no exercise intolerance and have the potential to lead normal lives. Surgical success greatly depends on the particular anatomy of the patient and the surgeon's skill and experience with this type of repair.

While current surgical techniques greatly improve 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 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 interventional procedures or 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.