Chapter 77 Tetralogy of Fallot
PATHOPHYSIOLOGY
Tetralogy of Fallot (TOF) is a cyanotic congenital heart defect composed of four structural components: (1) ventricular septal defect (VSD); (2) pulmonic stenosis, which can occur at any level (infundibular, valvular, supravalvular) and causes obstruction of blood flow into the pulmonary arteries; (3) right ventricular hypertrophy; and (4) overriding of the aorta. In children with TOF, the diameter of the aorta is larger than normal, whereas the pulmonary artery is smaller than normal.
The pulmonic stenosis causes portions of the deoxygenated blood from the right ventricle to shunt across the VSD into the aorta (right to left shunt) and results in cyanosis. The degree of cyanosis is related to the severity of the pulmonic stenosis, with more severe cyanosis associated with more severe obstruction. The degree of cyanosis ranges from none (pink TOF) in children with minimal right to left shunting to profound in children with more significant right to left shunting (blue TOF).
INCIDENCE
1. TOF affects boys and girls equally.
2. Incidence is higher with older maternal age.
3. Few affected individuals survive beyond 20 years without surgery.
4. TOF accounts for 8% to 10% of congenital defects and is the most common cyanotic defect beyond the first week of life.
5. For patients with uncomplicated repair, the mortality rate is 2% to 3% during the first 2 years.
CLINICAL MANIFESTATIONS
1. Cyanosis—appears after neonatal period, although children with mild degree of right ventricular outflow obstruction may be acyanotic
2. Hypercyanotic spells during infancy, also known as “tet spells”
3. Clubbing—late sign seen in unrepaired older children
4. Initially normal blood pressure—can increase after several years of marked cyanosis and polycythemia in unrepaired older children
5. Preference for classic squatting position—decreases venous return from lower extremities and increases pulmonary blood flow and systemic arterial oxygenation during spells or after exercise; rarely seen now owing to earlier repair of the defect
6. Anemia (if severe hypoxia and polycythemia are present)—contributes to worsening of symptoms
7. Decreased exercise tolerance
8. Murmur (systolic ejection murmur at upper left sternal border)
COMPLICATIONS
Postoperative complications
1. Bleeding—especially prominent in children with polycythemia
3. Low cardiac output or heart failure
4. Pleural effusion (surgical complication related to multiple cardiac procedures)
5. Phrenic nerve damage (surgical complication related to multiple cardiac procedures)
6. Cerebral embolism or thrombosis—risk greater with polycythemia, anemia, or sepsis
7. Complications specific to Blalock-Taussig shunts: congestive heart failure (CHF) if the shunt is too large, persistent cyanosis if the shunt is too small; shunt occlusion
LABORATORY AND DIAGNOSTIC TESTS
1. Chest radiograph—indicates increase or decrease in pulmonary flow, size of heart, and borders
2. Electrocardiogram (ECG)—to monitor right ventricular hypertrophy due to the pressure overload of the right ventricle; may also find left ventricular hypertrophy in patients with increased pulmonary blood flow due to collaterals or large shunts
3. Oxygen saturation by pulse oximetry—to monitor cyanosis as a clinical indication of the degree of pulmonic stenosis
4. Hematocrit or hemoglobin level—to monitor oxygen-carrying capacity and viscosity of blood, and for the development of iron deficiency anemia
5. Echocardiogram—to evaluate anatomy, pressure gradient across the obstruction
6. Cardiac catheterization; most patients with TOF do not require cardiac catheterization unless there is concern regarding the pulmonary artery anatomy
Preoperative
Refer to Appendix D for normal values and ranges of laboratory and diagnostic tests.
MEDICAL AND SURGICAL MANAGEMENT
Several medications may be used to medically manage TOF. The specific medication or combination of medications used depends on the child’s phsyiology. Propranolol (Inderal) is a beta-blocker that may be used to prevent or treat hypercyanotic spells and to treat arrhythmias. Diuretics, such as furosemide, may be used to promote diuresis. Though most children with TOF do not develop CHF because the pulmonic stenosis limits pulmonary blood flow, diuretics may be particularly useful in children with large ventricular septal defects and minimal pulmonic stenosis. In these same children, digitalis may be prescribed to improve cardiac performance by improving contractility and decreasing venous pressures. Further, digitalis slows conduction through the atrioventricular node and can therefore be used to treat selected cardiac arrhythmias (rarely given before correction unless shunt is too large).
Hypercyanotic spells are a medical emergency. A child experiencing a hypercyanotic spell should be placed in a knee-chest position and be given 100% oxygen by face mask. The knee-chest position improves venous return to the heart and therefore improves pulmonary blood flow. Oxygen is a potent pulmonary vasodilator and will also improve pulmonary blood flow. There are several medications that may also be helpful. Morphine is a sedative that decreases irritability, suppresses the respiratory center, and abolishes hyperpnea. Children with hypercyanotic spells quickly become acidotic. Therefore sodium bicarbonate, a potent systemic alkalizer, may be used to treat acidosis.
