Tetralogy of Fallot

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 described in 1672 by Niels Stensen and in 1888 by the French physician Etienne Fallot, for whom it is named.^[1]

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Anatomic morphology

Primary four malformations

As classically described, tetralogy of Fallot involves four heart malformations which present together:

1. A ventricular septal defect (VSD): a hole between the two bottom chambers (ventricles) of the heart. The defect is centered around the 'outlet septum', the most superior aspect of the septum, and in the majority of cases is single and large. In some cases septal hypertrophy can narrow the margins of the defect. ^[2]
2. Pulmonic stenosis: Right ventricular outflow tract obstruction, a narrowing at (valvular stenosis) or just below (infundibular stenosis) the pulmonary valve. The stenosis is mostly the result of hypertrophy of the septoparietal trabeculae,^[2] however the deviated outlet septum is believed to play a role. The stenosis is the major cause of the malformations, with the other associated malformations acting as compensatory mechanisms to the pulmonic stenosis.^[3]. The degree of stenosis varies between individuals with TOF, and is the primary determinant of symptoms and severity. This malformation is infrequently described as sub-pulmonary stenosis or subpulmonary obstruction. ^[4]
3. Overriding aorta: defined as when the aortic valve is not restricted to the left ventricle, thus having biventricular connections. The aortic root can be moved anteriorly or override the septal defect, but it is still to the right of the root of the pulmonary artery. The degree of override is quite variable, being between 5-95% of the valve being connected to the right ventricle.^[2]
4. Right ventricular hypertrophy: The right ventricle is more muscular than normal, causing a characteristic coeur-en-sabot (boot-shaped) appearance as seen by chest X-ray. Due to the misarrangement of the external ventricular septum, the right ventricular wall increase in size to deal with the increased obstruction to the right outflow tract. This feature is now generally agreed to be a secondary anomaly, as the level of hypertrophy generally increases with age. ^[5]

Other variations

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 determines clinical symptoms and disease progression. Tetralogy of Fallot may present with other anatomical anomalies, including:

1. stenosis of the left pulmonary artery, in 40% of patients
2. a bicuspid pulmonary valve, in 40% of patients
3. right-sided aortic arch, in 25% of patients
4. coronary artery anomalies, in 10% of patients
5. an atrial septal defect, in which case the syndrome is sometimes called a pentalogy of Fallot
6. an atrioventricular septal defect
7. partially or totally anomalous pulmonary venous return
8. forked ribs and scoliosis

Tetralogy of fallot with pulmonary atresia or 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.

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.

Embryology studies show 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. Right ventricular hypertrophy results from this combination, which causes resistance to blood flow from the right ventricle.

Pathophysiology

Tetralogy of Fallot results in low oxygenation of blood due to mixing of oxygenated and deoxygenated blood in the left ventricle through the VSD and preferential flow of both oxygenated and deoxygenated blood from the ventricles through the aorta because of obstruction to flow through the pulmonary valve. This is known as a right-to-left shunt.

Children with tetralogy of Fallot may develop acute severe cyanosis or hypoxic "tet spells". The precise mechanism of these episodes is in doubt, but presumably results from an increase in resistance to blood flow to the lungs with increased preferential flow of desaturated blood to the body.

Symptoms

The primary symptom is low blood oxygen saturation with or without cyanosis from birth or developing in the first year of life. Without cyanosis, the baby is 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.

Tet spells are characterized by a sudden, marked increase in cyanosis, syncope, and may result in hypoxic brain injury and death.

Diagnosis

Often a simple chest x-ray is enough to determine the presence of this condition. The heart may present with a "boot-like" appearance, rather than the symmetric appearance of a normal heart. The most common method to diagnose this condition , or any other congenital heart defect, is with echocardiography. It is quick, involves no radiation and is very specific.

Treatment

Tetralogy of Fallot is treated on two levels: with immediate emergency care for hypoxic or "tet" spells and with corrective surgery.

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 morphine to reduce ventilatory drive and phenylephrine to increase blood pressure. Oxygen is ineffective in treating hypoxic spells because the underlying problem is lack of blood flow through the pulmonary circuit and not alveolar oxygenation. There are also simple procedures such as 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.^[6]

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 surgical procedure, which involved forming an anastomosis between the subclavian artery and the pulmonary artery (See movie "Something the Lord Made").^[7] It was actually Helen Taussig who convinced Alfred Blalock that the shunt was going to work. This redirected a large portion of the partially oxygenated blood leaving the heart for the body into the lungs, increasing flow through the pulmonary circuit, and greatly relieving symptoms in patients. The first Blalock-Thomas-Taussig shunt surgery was performed on 15-month old Eileen Saxon on November 29, 1944 with dramatic results.

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

Currently, Blalock-Thomas-Taussig shunts are not normally performed on infants with TOF except for severe variants such as TOF with pulmonary atresia.

Total surgical repair

The Blalock-Taussig procedure was the only surgical treatment until the first total repair was performed in 1954. Between 1944 and when total repair became available at major surgical centers in the early 1960s, many infants and children were treated with Blalock-Taussig procedures.

The total repair was performed by C. Walton Lillehei at the University of Minnesota in 1954 on a 10-month boy.^[8] Total repair initially carried a high mortality risk which 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 may be done on patients as required by their particular anatomy.

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

Prognosis

Untreated, tetralogy of Fallot results in progressive right ventricular hypertrophy and dilatation due to the increased resistance on the right ventricle. The dilated cardiomyopathy progresses to right heart failure, usually with accompanying 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 with repaired tetralogy of Fallot have the potential to lead normal lives with continued excellent cardiac function, with some considerations:

Current techniques for total surgical repair greatly improve the hemodynamic function of the heart with tetralogy of Fallot but do not provide a lifetime correction of the defect. Ninety percent of patients with total repair as infants develop a progressively leaky pulmonary valve as the heart grows to its adult size. Patients also may have some degree of residual right outflow stenosis and damage to the electrical system of the heart from surgical incisions, causing abnormalities as detected by EKG and/or arrhythmias.

Long-term follow up studies show that this patient population is at risk for sudden cardiac death and for heart failure. Therefore, lifetime follow-up care by an adult congenital cardiologist is recommended to monitor these risks and to recommend treatment, such as interventional procedures or re-operation, if it becomes necessary.

Antibiotic prophylaxis is indicated during dental treatment in order to prevent infective endocarditis.

See also

• Trilogy of Fallot

References