Excerpt from Tetralogy of Fallot

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Background

Tetralogy of Fallot is the most common type of cyanotic congenital heart disease. It consists of a right ventricular outflow tract obstruction, a malalignment ventricular septal defect, an overriding aorta, and right ventricular hypertrophy.

Patient Education: For excellent patient education resources, visit eMedicine's Heart Center. Also, see eMedicine's patient education article Tetralogy of Fallot.

Pathophysiology

Anterior and superior displacement of the crista supraventricularis causes narrowing of the developing right ventricular outflow tract. A ventricular septal defect and an overriding aorta are the consequences of the septal displacement. The large ventricular septal defect equalizes the right and left ventricular pressures, resulting in persistent right ventricular hypertrophy. Right-to-left shunting varies with the degree of pulmonary outflow tract obstruction, but it is progressive.

Frequency

United States

Congenital heart disease is present in 1% of neonates. Of these neonates, tetralogy of Fallot is the most common cause of cyanotic congenital heart disease and is responsible for as many as 10% of all cases of congenital heart disease.

Mortality/Morbidity

The mortality rates for patients who are untreated for tetralogy of Fallot are progressive with the patient's age.

Mortality Rates for Untreated Tetralogy of Fallot

Surgical treatment has greatly improved the prognosis of tetralogy of Fallot because of the introduction of palliative shunts and corrective surgery.

Morbidity is a consequence of hypoxia and right-to-left shunting. Chronic hypoxia that is caused by such shunting is associated with decreased neurologic function. Episodes of acute hypoxia from infundibular spasm are life threatening. Polycythemia that is associated with chronic hypoxia causes hypercoagulability and thrombosis. Right-to-left shunting that bypasses the filtering of the pulmonary capillaries is associated with a higher incidence of systemic infection such as a brain abscess.

Sex

Tetralogy of Fallot has a slight male predominance.

Age

Tetralogy of Fallot usually becomes clinically apparent in infants.

Anatomy

Tetralogy of Fallot includes the following:

• Right ventricular outflow tract obstruction
• Ventricular septal defect
• Overriding aorta
• Right ventricular hypertrophy

Infundibular stenosis

Infundibular stenosis is the classic form of right ventricular outflow tract obstruction; however, 75% of affected children have a combination of infundibular and pulmonary valve stenosis. Pulmonary artery supravalvular stenosis and branch pulmonary artery hypoplasia or atresia are frequently associated with tetralogy of Fallot; they are a consequence of decreased pulmonary flow during development. A branch pulmonary artery, usually the left, can originate from a patent ductus arteriosus. The branch pulmonary arteries may be nonconfluent or completely absent

Branch pulmonary arteries

The size and number of the branch pulmonary arteries are critical to management of tetralogy of Fallot. Children with hypoplastic pulmonary arteries, particularly those with associated pulmonary artery atresia, are not candidates for immediate corrective surgery.

As Kirklin and Barratt-Boyes reported, the McGoon ratio, which is the result of the combined branch pulmonary arterial diameters divided by the descending aorta diameter, and the Nakata index, which is the combined cross-sectional area of the branch pulmonary arteries per square meter, are used to determine if the branch pulmonary arteries are large enough to permit corrective surgery.¹ Children with small McGoon ratios are poor candidates for complete surgical repair because of the inability of the small pulmonary arteries to adequately handle the increased blood flow after complete repair. Typically, a systemic-pulmonary artery shunt is placed in children with small pulmonary arteries until the arteries grow to a sufficient size to allow corrective surgery.

The branch pulmonary arteries can be completely separate from each other or nonconfluent. Children with nonconfluent branch pulmonary arteries require bilateral palliative shunts. Children in whom a pulmonary artery is absent are not candidates for conventional systemic-pulmonary artery shunt placement or corrective surgery. Unifocalization, or the creation of a neopulmonary artery from large systemic collateral vessels, can be performed in children without branch pulmonary arteries.

Vascular anomalies

A right-sided aortic arch is present in 25% of children with tetralogy of Fallot. Of these children, 90% have mirror-image branching, and 10% have an aberrant left subclavian artery. Among children with combined congenital heart disease and a mirror-image, right-sided aortic arch, 90% have tetralogy of Fallot. Of the children with congenital heart disease and an aberrant left subclavian artery, 71% have tetralogy of Fallot.

Coronary arterial anomalies are common when a single right or left coronary artery is present, as is the case in 4% of children. The anterior descending artery and prominent conal branches may originate from the right coronary artery. The relationship of these vessels to the infundibulum influences surgical management.

Postoperative complications

Common postoperative complications include residual pulmonary outflow tract stenosis, branch pulmonary arterial stenosis or occlusion, and shunt stenosis or occlusion. Pulmonary regurgitation is almost universal in corrected tetralogy of Fallot and results in right ventriculomegaly. Long-term pulmonary regurgitation reduces cardiac function.

Clinical Details

Infants may initially be acyanotic and typically have progressive cyanosis in their first months of life. In fact, the ventricular septal defect may act as a left-to-right shunt with pulmonary overcirculation. However, the right ventricular tract stenosis is progressive and leads to the progressive cyanosis that is usually identified when the neonate is several weeks old. Those with severe right ventricular outflow tract obstruction present earlier. The pulmonary infundibulum is prone to spasm, which causes acute episodes of cyanosis, referred to as "tet spells." Activity stimulates these episodes, and squatting may relieve them. In children with a right-sided aortic arch, the large ascending aorta causes tracheal compression that can result in feeding or respiratory difficulty. Polycythemia that is associated with cyanosis may lead to pulmonary thrombosis or cerebral thrombosis.

Preferred Examination

Echocardiographic findings are diagnostic for tetralogy of Fallot in infants and young children, and echocardiography may be the only examination required before surgery. Conventional radiography, magnetic resonance imaging (MRI), and angiography are also helpful for complete preoperative evaluation. Angiography has been used for preoperative evaluation of the coronary arteries and peripheral pulmonary circulation; however, intracardiac catheterization may stimulate pulmonary outflow tract spasm. Noninvasive peripheral pulmonary arterial evaluation is possible with MRI.

In the postoperative evaluation, echocardiography and MRI are preferred. The choice of imaging examinations depends on their availability and clinical preference.

Limitations of Techniques

Echocardiographic findings are diagnostic of intracardiac abnormalities, but echocardiography is limited in the evaluation of the peripheral pulmonary arteries and collateral flow. If the confluence or presence of the branch pulmonary arteries is in doubt after echocardiography, perform MRI or angiography.

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