Tetralogy of Fallot consists of 4 features: a large ventricular septal defect, right ventricular outflow tract and pulmonary valve obstruction, right ventricular hypertrophy, and over-riding of the aorta. Symptoms include cyanosis, dyspnea with feeding, poor growth, and tet spells (sudden, potentially lethal episodes of severe cyanosis). A harsh systolic murmur at the left upper sternal border with a single 2nd heart sound (S2) is common. Diagnosis is by echocardiography or cardiac catheterization. Definitive treatment is surgical repair.
Tetralogy of Fallot (see Fig. 6: Congenital Cardiovascular Anomalies: Tetralogy of Fallot.) accounts for 7 to 10% of congenital heart anomalies. Associated anomalies include right aortic arch (25%), abnormal coronary artery anatomy (5%), stenosis of the pulmonary artery branches, presence of aorticopulmonary collateral vessels, patent ductus arteriosus, complete atrioventricular septal defect, atrial septal defect, additional muscular ventricular septal defects (VSDs), and aortic valve regurgitation.
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Pathophysiology
The VSD is typically large; thus, systolic pressures in the right and left ventricles (and in the aorta) are the same. Pathophysiology depends on the degree of right ventricular outflow obstruction. A mild obstruction may result in a left-to-right shunt through the VSD; a severe obstruction causes a right-to-left shunt, resulting in low systemic arterial saturation (cyanosis) that is unresponsive to supplemental O2.
In some children with tetralogy of Fallot, most often those several months up to 2 yr of age, sudden episodes of profound cyanosis and hypoxia (tet spell) may occur, which may be lethal. A spell may be triggered by any event that slightly decreases O2 saturation (eg, crying, defecating) or that suddenly decreases systemic vascular resistance (eg, playing, kicking legs when awakening) or by sudden onset of tachycardia or hypovolemia. The mechanism of a tet spell remains uncertain, but several factors are probably important in causing an increase in right to left shunting and a fall in arterial saturation. Factors include an increase in right ventricular outflow tract obstruction and a decrease in systemic resistance—a vicious circle caused by the initial fall in arterial Po2, which stimulates the respiratory center and causes hyperpnea and increased adrenergic tone. The increased circulating catecholamines then stimulate increased contractility, which increases outflow tract obstruction.
Symptoms and Signs
Neonates with severe right ventricular outflow obstruction (or atresia) have severe cyanosis and dyspnea with feeding with poor weight gain. But those with mild obstruction may not have cyanosis at rest.
Tet spells may be precipitated by activity and are characterized by paroxysms of hyperpnea (rapid and deep respirations), irritability and prolonged crying, increasing cyanosis, and decreasing intensity of the heart murmur. The spells occur most often in young infants; peak incidence is age 2 to 4 mo. A severe spell may lead to limpness, seizures, and occasionally death. During play, some toddlers may intermittently squat, a position that increases systemic vascular resistance and aortic pressure, which decreases right to left ventricular shunting and thus raises arterial O2 saturation.
Auscultation detects a harsh grade 3 to 5/6 systolic ejection murmur at the left mid and upper sternal border. The murmur in tetralogy is always due to the pulmonary stenosis; the VSD is silent because it is large and has no pressure gradient. The 2nd heart sound is often single because the pulmonary component is markedly reduced. A prominent right ventricular impulse and a systolic thrill may be present.
Diagnosis
Diagnosis is suggested by history and clinical examination, supported by chest x-ray and ECG, and established by 2-dimensional echocardiography with color flow and Doppler studies. Chest x-ray shows a boot-shaped heart with a concave main pulmonary artery segment and diminished pulmonary vascular markings. A right aortic arch is present in 25%. ECG shows right ventricular hypertrophy and may also show right atrial hypertrophy. Cardiac catheterization is rarely needed, unless there is suspicion of a coronary anomaly that might affect the surgical approach (eg, anterior descending arising from the right coronary artery) that cannot be clarified with echocardiography.
Treatment
Neonates with severe cyanosis may be palliated with an infusion of prostaglandin E1 (0.05 to 0.1 μg/kg/min IV) to open the ductus arteriosus.
Tet spells: Tet spells are treated by placing infants in a knee-chest position (older children usually squat spontaneously and do not develop tet spells), establishing a calm environment, and giving O2. If the spell persists, options (roughly in order of preference) include morphine
0.1 to 0.2 mg/kg IV or IM, IV fluids for volume expansion, NaHCO3 1 mEq/kg IV, and propranolol
starting at 0.02 to 0.05 mg/kg, titrated up to 0.1 to 0.2 mg/kg IV if needed for effect. If these measures do not control the spell, systemic BP can be increased with ketamine
0.5 to 3 mg/kg IV or 2 to 3 mg/kg IM ( ketamine
also has a beneficial sedating effect) or phenylephrine
starting at 5 μg/kg and titrating up to 20μg/kg IV for effect. Ultimately, if the preceding steps do not relieve the spell or if the infant is rapidly deteriorating, intubation with muscle paralysis and general anesthesia may be necessary. Propranolol
0.25 to 1 mg/kg po q 6 h may prevent recurrences, but most experts feel that even one significant spell indicates the need for expeditious surgical repair.
0.1 to 0.2 mg/kg IV or IM, IV fluids for volume expansion, NaHCO3 1 mEq/kg IV, and propranolol
starting at 0.02 to 0.05 mg/kg, titrated up to 0.1 to 0.2 mg/kg IV if needed for effect. If these measures do not control the spell, systemic BP can be increased with ketamine
0.5 to 3 mg/kg IV or 2 to 3 mg/kg IM ( ketamine
also has a beneficial sedating effect) or phenylephrine
starting at 5 μg/kg and titrating up to 20μg/kg IV for effect. Ultimately, if the preceding steps do not relieve the spell or if the infant is rapidly deteriorating, intubation with muscle paralysis and general anesthesia may be necessary. Propranolol
0.25 to 1 mg/kg po q 6 h may prevent recurrences, but most experts feel that even one significant spell indicates the need for expeditious surgical repair.
Definitive management: Complete repair consists of patch closure of the VSD, widening of the right ventricular outflow tract with muscle resection and pulmonary valvuloplasty, and a limited patch across the pulmonic annulus or main pulmonary artery if necessary. Surgery is usually done electively at age 3 to 6 mo but can be done at any time if symptoms are present.
In neonates and very small infants with complex anatomy, initial palliation may be preferred to complete repair; the usual procedure is a modified Blalock-Taussig shunt, in which the subclavian artery is connected to the ipsilateral pulmonary artery with a synthetic graft.
Perioperative mortality rate for complete repair is < 5% for uncomplicated tetralogy of Fallot. For untreated patients, survival rates are 55% at 5 yr and 30% at 10 yr.
Endocarditis prophylaxis is recommended preoperatively but is required only for the first 6 mo after repair unless there is a residual defect adjacent to a surgical patch or prosthetic material.
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