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Tetralogy of Fallot

Tetralogy of Fallot is a cardiovascular disorder consisting of the combination of (1) obstruction to right ventricular outflow (pulmonary stenosis), (2) ventricular septal defect (VSD), (3) dextroposition of the aorta with septal override, and (4) right ventricular hypertrophy. Obstruction to pulmonary arterial blood flow is usually at both the right ventricular infundibulum (subpulmonic area) and pulmonary valve. The main pulmonary artery is often smaller than usual, and there may be various degrees of branch pulmonary artery stenoses as well.

CLINICAL MANIFESTATIONS

Infants with mild degrees of right ventricular outflow obstruction may initially present with congestive heart failure caused by a ventricular level left-to-right shunt. Often cyanosis is not present at birth, but with increasing hypertrophy of the right ventricular infundibulum and growth, cyanosis occurs later in the 1st yr of life. It is most prominent in the mucous membranes of the lips and mouth, and in the fingernails and toenails. In infants with severe degrees of right ventricular obstruction, cyanosis is noted immediately in the neonatal period. In these infants pulmonary blood flow may be dependent on flow through the ductus arteriosus. When the ductus begins to close in the 1st few hours or days of life, severe cyanosis and circulatory collapse may occur. Older children with long-standing cyanosis may have extreme cyanosis, with a dusky blue skin surface, gray sclerae with engorged blood vessels (suggesting mild conjunctivitis), and clubbing of the fingers and toes. The extracardiac manifestations of long-standing cyanotic congenital heart disease.

Dyspnea occurs on exertion. Infants and toddlers will play actively for a short time and then sit or lie down. Older children may be able to walk a block or so before stopping to rest. Characteristically, children assume a squatting position for the relief of dyspnea due to physical effort; the child is usually able to resume physical activity within a few minutes. These findings occur most often in patients with significant cyanosis at rest.

Paroxysmal hypercyanotic attacks (hypoxic, "blue," or "tet" spells) are a particular problem during the first 2 yr of life. The infant becomes hyperpneic and restless, cyanosis increases, gasping respirations ensue, and syncope may follow. The spell occurs most frequently in the morning upon first awakening or following episodes of vigorous crying. Temporary disappearance or decrease in intensity of the systolic murmur is usual as flow across the right ventricular outflow tract diminishes. The spells may last from a few minutes to a few hours but are rarely fatal. Short episodes are followed by generalized weakness and sleep. Severe spells may progress to unconsciousness and, occasionally, to convulsions or hemiparesis. The onset is usually spontaneous and unpredictable. Spells are associated with a reduction of an already compromised pulmonary blood flow, which when prolonged results in severe systemic hypoxia and metabolic acidosis. Infants who are only mildly cyanotic at rest are often more prone to develop hypoxic spells because they have not developed the homeostatic mechanisms to tolerate rapid lowering of arterial oxygen saturation, for example, polycythemia.

Depending on the frequency and severity of hypercyanotic attacks, one or more of the following procedures should be instituted in sequence: (1) placement of the infant on the abdomen in the knee-chest position, making certain that there is no constricting clothing; (2) administration of oxygen; and (3) injection of morphine subcutaneously in a dose not in excess of 0.2 mg/kg. Calming the infant, while holding the child in a knee-chest position, may abort progression of an early spell. Premature attempts to obtain blood tests may cause further agitation and be counterproductive.

Since metabolic acidosis develops when the arterial PO2 is below 40 mm Hg, rapid correction (within several minutes) with intravenous administration of sodium bicarbonate is necessary if the spell is unusually severe and there is lack of response to the foregoing therapy. Recovery from the spell is usually rapid once the pH has returned to normal. Repeated blood pH measurements may be necessary because rapid recurrence of acidosis may occur. beta-Adrenergic blockade by intravenous administration of propranolol (0.1 to a maximum of 0.2 mg/kg) has been used successfully in some patients with severe spells, especially spells accompanied by tachycardia. Drugs that increase systemic vascular resistance, such as intravenous methoxamine or phenylephrine, will improve right ventricular outflow, decrease the right-to-left shunt, and thus improve the symptoms, but their use has been limited and should not be allowed to delay needed surgery.

Growth and development may be delayed in patients with severe untreated tetralogy of Fallot. Stature and nutritional status are usually below average for age. Puberty is delayed in unoperated patients.

The pulse is usually normal, as are the venous and arterial pressures. The left anterior hemithorax may bulge anteriorly due to right ventricular hypertrophy. The heart is usually normal in size, and there is a substernal right ventricular impulse. In 50% of cases a systolic thrill is felt along the left sternal border in the 3rd and 4th parasternal spaces. The systolic murmur is frequently loud and harsh; it may be transmitted widely, especially to the lungs, but is most intense at the left sternal border. The murmur may be either ejection or holosystolic and may be preceded by a click. The murmur is caused by turbulence through the right ventricular outflow tract. It tends to become louder, longer, and harsher as the severity of pulmonary stenosis increases from mild to moderate; however, it can actually become less prominent with severe obstruction, especially during a hypercyanotic spell. The 2nd heart sound is either single or the pulmonic component is soft. Infrequently a continuous murmur may be audible.

DIAGNOSIS

Roentgenographically, the typical configuration as seen in the anteroposterior view consists of a narrow base, concavity of the left heart border in the area usually occupied by the pulmonary artery, and normal heart size. The hypertrophied right ventricle causes the rounded apical shadow to be uptilted so that it is situated higher above the diaphragm than normal. The cardiac silhouette has been likened to that of a boot or wooden shoe (coeur en sabot). The hilar areas and lung fields are relatively clear, because of diminished pulmonary blood flow and/or the small size of the pulmonary arteries. The aorta is usually large, and in about 20% of instances the aorta arches to the right instead of to the left; this results in an indentation of the leftward-positioned air-filled tracheobronchial shadow in the anteroposterior view or may be confirmed by displacement of the barium-filled esophagus to the left.

The electrocardiogram demonstrates right axis deviation and evidence of right ventricular hypertrophy. The latter is found in the right precordial chest leads, where the configuration of the QRS complex is Rs, R, qR, qRs, or rsR' and the T wave may be positive. The P wave is tall and peaked, or sometimes bifid </1377640/222/11280.html#F381006">(see Fig. 381-6) .

Two-dimensional echocardiography establishes the diagnosis and provides information as to the extent of aortic over-ride of the septum, the location and degree of the right ventricular outflow tract obstruction, the size of the proximal branch pulmonary arteries, and the side of the aortic arch. The echo is also useful in determining whether a patent ductus arteriosus is supplying a portion of the pulmonary blood flow. It may obviate the need for catheterization.

Cardiac catheterization demonstrates systolic pressure in the right ventricle equal to systemic pressure, with a marked decrease in pressure as the catheter enters the pulmonary artery or, in some cases, the infundibular chamber beyond the obstruction. The mean pulmonary arterial pressure is commonly 5-10 mm Hg; the right atrial pressure is usually normal. The level of arterial oxygen saturation depends on the magnitude of the right-to-left shunt; in a moderately cyanotic patient at rest it is usually 75-85%. In the absence of a left-to-right shunt, samples of blood from the venae cavae, right atrium, right ventricle, and pulmonary artery will be similar in oxygen content.

Selective right ventriculography best demonstrates the anatomy of tetralogy of Fallot. The contrast medium outlines the heavily trabeculated right ventricle. The infundibular stenosis varies in length, width, contour, and distensibility. The pulmonary valve is usually thickened, and the annulus may be small. Among patients with pulmonary atresia and VSD, the anatomy of the pulmonary vessels may be extremely complex. Complete and accurate information regarding the anatomy of the pulmonary arteries is very important in evaluating these children as surgical candidates.

Left ventriculography demonstrates the size of the left ventricle, the position of the VSD, and the over-riding aorta; it also confirms mitral-aortic continuity, ruling out double-outlet right ventricle. Aortography or coronary arteriography will outline the course of the coronary arteries. In 5-10% of patients with tetralogy of Fallot, an aberrant major coronary artery crosses over the right ventricular outflow tract; this artery must be not be cut during surgical repair. Delineation of normal coronary arteries by angiography is most important when considering surgery in young infants who may need a patch across the pulmonary valve annulus. Echocardiography can in many cases delineate the coronary artery anatomy.

PROGNOSIS AND COMPLICATIONS

Patients with tetralogy of Fallot prior to correction are susceptible to several serious complications. Cerebral thromboses, usually occurring in the cerebral veins or dural sinuses and occasionally in the cerebral arteries, are more common in the presence of extreme polycythemia. They may also be precipitated by dehydration. Thromboses occur most often in patients under the age of 2 yr. These patients may have iron deficiency anemia, frequently with hemoglobin and hematocrit levels in the normal range. Therapy consists of adequate hydration and supportive measures. Phlebotomy and volume replacement with fresh frozen plasma are indicated in the extremely polycythemic patient. Heparin is of little value and is contraindicated in hemorrhagic cerebral infarction. Physical therapy should be instituted as early as possible.

Brain abscess is less common than cerebral vascular events. Patients are usually over the age of 2 yr. The onset of the illness is often insidious with low-grade fever and/or a gradual change in behavior. In some patients there is an acute onset of symptoms, which may develop after a recent history of headache, nausea, and vomiting. Epileptiform seizures may occur; localized neurologic signs depend on the site and size of the abscess, and the presence of increased intracranial pressure. The sedimentation rate and white blood cell count are usually elevated. Computed tomography (CT), magnetic resonance imaging (MRI), or ultrasonography confirms the diagnosis. Massive antibiotic therapy may help to keep the infection localized, but surgical drainage of the abscess is almost always necessary </1377640/222/15390.html#C5545578">(see Chapter 554) .

Bacterial endocarditis occurs in unoperated patients in the right ventricular infundibulum or on the pulmonic, aortic, or, rarely, tricuspid valves. Endocarditis may complicate palliative shunts or, among patients with corrective surgery, any residual pulmonic stenosis or residual VSD. Antibiotic prophylaxis is essential prior to and after dental and certain surgical procedures associated with a high incidence of bacteremia.

Congestive heart failure is not a usual feature of patients with tetralogy of Fallot. It may occur, however, in the young infant with "pink" or acyanotic tetralogy of Fallot. As the degree of pulmonary obstruction worsens with age, the symptoms of heart failure resolve and eventually the patient develops cyanosis, often by 6-12 mo of age. These patients are at increased risk for hypercyanotic spells at this time.

ASSOCIATED CARDIOVASCULAR ANOMALIES

An associated PDA may be present and defects in the atrial septum are occasionally seen. A right aortic arch occurs in approximately 20% of cases of tetralogy of Fallot, and other anomalies of the pulmonary arteries and aortic arch may also be seen. Persistence of a left superior vena cava draining into the coronary sinus may be noted. Multiple ventricular septal defects occasionally are present and must be diagnosed prior to corrective surgery. Tetralogy may also occur with atrioventricular septal defects, often associated with Down syndrome.

Congenital absence of the pulmonary valve produces a distinct syndrome, usually marked by signs of upper airway obstruction; cyanosis may be mild, the heart is large and hyperdynamic, and a loud to-and-fro murmur is present. Marked aneurysmal dilatation of the main and branch pulmonary arteries results in compression of the bronchi and produces stridorous or wheezing respirations and recurrent pneumonias. If the airway obstruction is severe, reconstruction of the trachea at the time of corrective cardiac surgery may be required to alleviate symptoms.

Absence of a branch pulmonary artery, most often the left, should be suspected if the roentgenographic appearance of the pulmonary vasculature differs on the two sides; absence of a pulmonary artery will often be associated with hypoplasia of the affected lung. It is important to recognize the absence of a pulmonary artery, as occlusion of the remaining pulmonary artery during operation seriously compromises the already reduced pulmonary blood flow.

TREATMENT

The treatment of tetralogy of Fallot depends on the severity of the right ventricular outflow tract obstruction. Those infants with severe tetralogy require medical treatment and surgical intervention in the neonatal period. Therapy is aimed at providing an immediate increase in pulmonary blood flow to prevent the sequelae of severe hypoxia. The infant should be transported to a medical center adequately equipped to evaluate and treat neonates with congenital heart disease under optimal conditions. It is critical that oxygenation and normal body temperature be maintained during the transfer. Prolonged, severe hypoxia may lead to shock, respiratory failure, and intractable acidosis and will significantly reduce the chances of survival after cardiac catheterization and surgery, even when surgically amenable lesions are present. Cold increases oxygen consumption, which places a further stress on the cyanotic infant, whose oxygen delivery is already limited. Finally, blood glucose levels should be monitored, as infants with cyanotic heart disease are more likely to develop hypoglycemia.

Infants with marked right ventricular outflow tract obstruction may deteriorate rapidly because as the ductus arteriosus begins to close pulmonary blood flow is further compromised. The administration of prostaglandin E1 (0.05-0.20 g/kg/min), a potent and specific relaxant of ductal smooth muscle, causes dilatation of the ductus arteriosus and provides adequate pulmonary blood flow until a surgical procedure can be performed. This agent should be administered intravenously as soon as the clinical suspicion of cyanotic congenital heart disease is made and continued through cardiac catheterization and the preoperative period. Postoperatively, the infusion may be continued briefly as a pulmonary vasodilator to augment flow through a palliative shunt or through a surgical valvulotomy.

Infants with less severe right ventricular outflow tract obstruction who are stable and awaiting surgical intervention require careful observation. The prevention or prompt treatment of dehydration is important to avoid hemoconcentration and possible thrombotic episodes. Paroxysmal dyspneic attacks in infancy may be precipitated by a relative iron deficiency; iron therapy may decrease their frequency and also improve exercise tolerance and general well-being. Red blood cell indices should be maintained in the normocytic range. In the past, oral propranolol (1 mg/kg every 6 hr) had been used to decrease the frequency and severity of hypercyanotic spells, but it is preferable to refer the patient for surgical treatment as soon as spells begin.

In general, infants presenting with symptoms and severe cyanosis in the 1st mo of life have marked obstruction of the right ventricular outflow tract or pulmonary atresia. In these infants the most common procedure is a systemic-to-pulmonary artery shunt, performed to augment pulmonary artery blood flow. The rationale of this palliative surgery is to decrease the amount of hypoxia and to improve linear growth as well as to augment the growth of the branch pulmonary arteries. In several centers, corrective open heart surgery in early infancy is being performed in critically ill patients as long as they have normal coronary artery anatomy. The advantages of corrective surgery in early infancy versus a palliative shunt and correction in later infancy are still controversial. For infants who can be maintained until 6-12 mo of age, full correction is a reasonable primary alternative when the pulmonary arteries are of sufficient size and no other complicating great vessel abnormalities are present.

The modified Blalock-Taussig shunt is currently the most common aorto-pulmonary shunt procedure and consists of a Gore-Tex conduit anastomosed side to side from the subclavian artery to the homolateral branch of the pulmonary artery. Sometimes the conduit is brought directly from the ascending aorta to the main pulmonary artery and is called a central shunt. The Blalock-Taussig operation can be successfully performed in the newborn period using 4-5 mm diameter shunts and has been utilized successfully in premature infants. The original Blalock-Taussig shunt consisted of a direct anastamosis of the subclavian artery to a branch pulmonary artery. Other shunt procedures include a side-to-side anastomosis of the ascending aorta and right pulmonary artery (Waterson) and anastomosis of the descending aorta and left pulmonary artery (Potts). These procedures are rarely done because of a higher frequency of complicating congestive heart failure and a higher risk for the development of pulmonary hypertension as well as greater technical difficulties in closing these shunts during subsequent corrective surgery.

Usually, the postoperative course of patients with a successful shunt procedure is relatively uneventful. However, postoperative complications following a lateral thoracotomy, such as chylothorax, diaphragmatic paralysis, and Horner syndrome, may occur. Chylothorax may require repeated thoracocentesis and, on occasion, reoperation in order to ligate the thoracic duct. Diaphragmatic paralysis due to injury to the phrenic nerve may result in a more difficult postoperative course. Prolonged ventilator support and vigorous physical therapy may be required, but diaphragmatic function will usually return in 1-2 mo unless the nerve was completely divided. Surgical plication of the diaphragm may be indicated. Horner syndrome is usually temporary and does not require treatment. Postoperative cardiac failure may be caused by a large sized shunt. Vascular problems, other than a diminished radial pulse and occasional long-term arm length discrepancy, are rarely seen in the upper extremity supplied by the subclavian artery used for the anastomosis.

After a successful shunt procedure, cyanosis diminishes. The development of a continuous murmur over the lung fields after the operation indicates a functioning anastomosis. However, a good shunt murmur may not be heard until several days after surgery. The duration of symptomatic relief is variable. As the child grows, more pulmonary blood flow is needed and the shunt may eventually become inadequate. When increasing cyanosis develops, a corrective operation should be performed if the anatomy is favorable. However, if this is not possible (e.g., because of hypoplastic branch pulmonary arteries) or if the 1st shunt lasts only a brief period in a small infant, a second aorto-pulmonary anastomosis may be required on the opposite side.

Corrective surgical therapy consists of relief of the obstruction of the right ventricular outflow tract by removing obstructive muscle bundles and patch closure of the VSD. If the pulmonary valve is stenotic, a valvotomy is performed. If the pulmonary valve annulus is very small or the valve is extremely thickened, a valvectomy may be performed and a transannular patch placed across the pulmonary valve ring. When there is a previously established systemic to pulmonary shunt, it must be obliterated prior to cardiotomy. The surgical risk of total correction is currently under 5%. A right ventriculotomy is performed in most patients, although in some centers a transatrial-transpulmonary approach reduces the long-term risks of a ventriculotomy. The presence of a previous Blalock-Taussig shunt does not increase the operative risk. Increased bleeding in the immediate postoperative period is common in polycythemic patients but should not seriously affect the outcome. The operative risks may be somewhat higher in small infants because these are usually the patients with the more severe forms of right ventricular outflow tract obstruction.

After successful total correction, patients are generally asymptomatic and are able to lead unrestricted lives. Immediate postoperative problems include right ventricular failure, transient heart block, residual VSD with left-to-right shunting, myocardial infarction from interruption of an aberrant coronary artery, and disproportionately increased left atrial pressure due to residual collaterals. Postoperative heart failure (particularly in patients with a transannular outflow patch) requires a positive inotropic agent such as digoxin. The long-term effects of isolated, surgically induced pulmonary valvular insufficiency are unknown, but insufficiency is generally well tolerated. Patients with marked pulmonary valve insufficiency will have moderate to marked cardiac enlargement. Patients having a severe residual gradient across the right ventricular outflow tract may require reoperation, but mild to moderate obstruction is virtually always present and does not require reintervention.

Follow-up of patients 5-20 yr after operation indicates that the marked improvement in symptomatology is generally maintained. However, even asymptomatic patients have working capacities, maximal heart rates, and cardiac outputs that are lower than those of controls. These abnormal findings are more common in patients who had placement of a transannular outflow tract patch and may be less frequent when surgery is undertaken at an early age.

Conduction disturbances are also frequent after operation. The atrioventricular node and the bundle of His and its divisions are in close proximity to the VSD and may be injured during surgery. Permanent complete heart block following surgery is rare. When present, it should be treated by placement of a permanently implanted pacemaker. Bifascicular block occurs in about 10% of patients; the long-term significance is uncertain, but in most instances there are no clinical manifestations. The additional finding of transient complete heart block in the immediate postoperative period, however, appears to be associated with an increased incidence of late-onset complete heart block and sudden death. However, unexpected cardiac arrest rarely occurs many years after surgery in patients without postoperative bifascicular block or transient complete heart block. A number of children will display premature ventricular beats following repair of tetralogy of Fallot. These are of concern in patients with residual hemodynamic abnormalities; 24-hr ECG (Holter) monitoring studies should be performed to be certain that occult short episodes of ventricular tachycardia are not occurring. In addition, exercise studies may be useful in provoking cardiac arrhythmias that are not apparent at rest. In the presence of complex ventricular arrhythmias or severe residual hemodynamic abnormalities, prophylactic antiarrhythmia therapy is warranted. Dilantin, propranolol, or combinations of these agents are most often used.