Tetralogy of Fallot

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CHAPTER 47 Tetralogy of Fallot

Tetralogy of Fallot, the most common cyanotic congenital heart abnormality, is caused by a single anomaly—anterior conal septal malalignment. Despite the single defect causing the anomaly, a spectrum of clinical and imaging findings can occur depending on the extent of the conal septum deviation. A mild deviation results in minimal right ventricular outflow obstruction and severe deviation in outflow tract atresia. Patients range from “pink” to severely cyanotic. Imaging plays a major role in characterizing the extent of the abnormalities before palliative or corrective surgery. Improved surgical technique has led to improved survival with most patients surviving into adulthood. MRI and CT have an increasingly important role in the long-term follow-up of patients after initial repair.

DEFINITION

Anterior conal septum malalignment prevents fusion of the conal and ventricular septum resulting in the tetrad that characterizes tetralogy of Fallot: right ventricular tract obstruction, ventricular septal defect (VSD), overriding aorta, and right ventricular hypertrophy.

PREVALENCE AND EPIDEMIOLOGY

Tetralogy of Fallot is the most common congenital heart disease manifesting with cyanosis. The incidence is 0.06% of live births, and it constitutes 5% to 7% of all congenital heart disease.¹^,² Genetic syndromes are associated with tetralogy of Fallot in 20% of cases with 22q11 deletion and trisomy 21 being the most common.³ Complete atrioventricular septal defects are present in 2% of patients with tetralogy of Fallot, especially patients with trisomy 21.⁴ Alagille syndrome, VACTERL syndrome (vertebral anomalies, anal atresia, cardiovascular anomalies, tracheoesophageal fistula, esophageal atresia, renal anomalies, limb anomalies), and CHARGE syndrome (coloboma of the eye, heart defects, atresia of the choanae, retardation of growth and development, genitourinary abnormalities; ear abnormalities and deafness) are also associated with tetralogy of Fallot. Atrial septal defect is present in one third of patients with tetralogy of Fallot. Currently, 85% of children with tetralogy of Fallot survive to adulthood, increasing the prevalence of tetralogy of Fallot in adults.

In greater than 90% of children, right aortic arch and mirror-image branching are associated with congenital heart disease. Tetralogy of Fallot is the most common congenital heart disease associated with a right aortic arch. Approximately 90% of patients with a right aortic arch and mirror-image branching have tetralogy of Fallot. Of patients with tetralogy of Fallot, 25% have a right aortic arch, and most have a mirror-image branching pattern.

Aberrant subclavian arteries are associated in 5% to 8% of patients with tetralogy of Fallot. The prevalence of tetralogy of Fallot and anomalous subclavian artery is greater if associated with a right aortic arch or pulmonary atresia. Anomalous subclavian artery occurs in 24% of patients with tetralogy of Fallot, right aortic arch, and pulmonary atresia.⁵

Coronary artery anomalies are common, occurring in 5% to 12% of children with tetralogy of Fallot.⁶^,⁷ Coronary artery anomalies that result in major branches coursing anterior to the right ventricular outflow tract (RVOT) interfere with transannular patch angioplasty of the outflow tract.

ETIOLOGY AND PATHOPHYSIOLOGY

Tetralogy is the consequence of abnormal conotruncal development. Anterior conal septum deviation narrows the developing RVOT. The deviation may be minimal, causing little initial RVOT obstruction, or severe, resulting in pulmonary atresia. The abnormal position of the anterior conal septum prevents ventricular septal closure and results in a large VSD that does not spontaneously close. The aorta assumes a position overriding the VSD. The pressure within the ventricles equalizes, resulting in right ventricular hypertrophy. Right-sided obstruction classically involves the right ventricular infundibulum, but frequently also includes the pulmonary valve, pulmonary artery, and branch pulmonary arteries.

The pathophysiology varies with the degree of RVOT obstruction. A left-to-right shunt across the VSD might initially predominate if the RVOT obstruction is mild. Outflow tract obstruction is progressive leading to eventual right-to-left shunting and cyanosis. Pulmonary atresia or more severe RVOT obstruction occurs with immediate or early cyanosis.

MANIFESTATIONS OF DISEASE

Clinical Presentation

The initial presentation depends on the degree of right ventricular outflow obstruction. Infants may initially be acyanotic—so-called pink tetralogy. The VSD may act as a left-to-right shunt with pulmonary overcirculation. A murmur of VSD may be present. Cyanosis progresses during the first months of life owing to progressive RVOT obstruction. Infants typically present with cyanosis between 6 weeks and 6 months of age. Infants with severe RVOT obstruction can be ductal dependent and present earlier. Clubbing may be present. Outflow tract obstruction is associated with systolic murmurs.

The pulmonary infundibulum is prone to spasm, causing acute episodes of cyanosis referred to as “tet spells.” Activity and acidosis stimulate these episodes, and squatting may relieve them. In children with right aortic arch, the large ascending aorta causes tracheal compression that can result in feeding or respiratory difficulty. Patients with tetralogy of Fallot and absent pulmonary valve present with severe dilation of the pulmonary artery and central pulmonary arteries that compress the trachea and bronchi. These patients can have severe respiratory distress. Polycythemia associated with cyanosis may lead to pulmonary or cerebral thrombosis.

A child with a palliative shunt has a continuous murmur similar to that heard in a child with a patent ductus arteriosus. After corrective surgery, symptoms may reflect continued right-sided obstruction or pulmonary regurgitation. Corrective surgery for tetralogy of Fallot requires enlargement of the RVOT and frequently a transannular patch, which results in pulmonary regurgitation.⁸^–¹⁰ Pulmonary regurgitation is initially well tolerated, but later is associated with fatigue. The right ventricle and its outflow tract progressively dilate. Right ventricular enlargement leads to distortion of the tricuspid valve annulus and tricuspid regurgitation, compromising right heart function further. ECG abnormalities with lengthening of the QRS complex can lead to arrhythmia that may result in sudden death. Diastolic dysfunction can progress to irreversible systolic dysfunction.⁸ Revision of the RVOT with a valved conduit causes right ventricular remodeling and improved cardiac function. Pseudoaneurysms of the RVOT can follow corrective surgery with a transannular patch or pulmonary conduit.

Aortic root dilation and regurgitation are common in adults with tetralogy of Fallot and can lead to arterial medial abnormalities. Risk factors for aortopathy include patients with pulmonary atresia, right aortic arch, history of aorticopulmonary shunts, male sex, and chromosome 22q11 deletions.¹¹

Imaging Indications and Algorithm

Echocardiography is the first and frequently the only imaging modality necessary before surgery for evaluation of tetralogy of Fallot. The intracardiac anomalies, potential critical coronary artery anomalies, and RVOT can be characterized by echocardiography. The branch pulmonary arteries and aortic arch anomalies are usually adequately evaluated on preoperative echocardiograms, but may require additional imaging—particularly if the branch pulmonary arteries are nonconfluent or severely hypoplastic, or major aorticopulmonary collateral arteries are present. The positions of the coronary arteries need to be established before surgery. MRI and CT are the second-line imaging modalities to complete preoperative evaluation if echocardiography is incomplete. Cardiac catheterization is no longer necessary before primary surgery.

After palliative or corrective surgery, echocardiography is more limited. The windows for echocardiography evaluation diminish with age. The importance of quantifying right ventricular size and function increases in importance. MRI has a more dominant role in the postoperative patient, particularly for the evaluation of pulmonary regurgitation. CT is useful when MRI is unavailable or contraindicated.

Imaging Technique and Findings

Radiography

Radiography plays a secondary role to echocardiography. The cardiac silhouette is frequently normal in size and configuration (Fig. 47-1). The classic radiographic appearance is a boot-shaped heart caused by elevation of the cardiac apex from right ventricular hypertrophy with a diminished or unapparent pulmonary artery segment (Fig. 47-2). The appearance of a boot-shaped heart can be mimicked if the central x-ray beam is caudal to the heart causing cephalic projection of the more anterior portion of the heart (Fig. 47-3). The pulmonary vascularity is diminished, but may be undetectable on a conventional radiograph if right ventricular obstruction is mild. A right aortic arch is visible in 25% of patients with tetralogy of Fallot. Direct visualization of the aortic arch and tracheal shift can be difficult to detect in infants and young children. The side of the aortic arch is reliably determined, however, by noting the increased density of the pedicles on the side of the aortic arch. Patients with tetralogy of Fallot and absent pulmonary artery have enlargement of the central pulmonary arteries (Fig. 47-4).

FIGURE 47-1 The heart is normal in appearance on this chest radiograph in a 5-month-old infant with tetralogy of Fallot. A right aortic arch is present.

FIGURE 47-2 Tetralogy of Fallot in a 6-week-old infant shows elevation of the cardiac apex creating the classic boot-shaped heart. A right aortic arch is present.

FIGURE 47-3 A 3-month-old infant with a history of necrotizing enterocolitis and no heart disease has a seemingly boot-shaped heart on the chest-abdomen examination owing to the x-ray beam being centered over the mid-abdomen.

FIGURE 47-4 Chest radiograph in a 1-day-old infant with tetralogy of Fallot and absent pulmonary valve shows massive enlargement of the central pulmonary arteries.

Ultrasonography

Echocardiography is definitive for the prenatal and postnatal diagnosis of tetralogy of Fallot.¹² A large VSD is identified in utero on the four-chamber view, but evaluation of the outflow tracts is necessary for the diagnosis because other conotruncal abnormalities can have a VSD and an overriding aorta. A large aorta overriding the VSD is present. The aorta is larger than the pulmonary artery. Doppler interrogation of the fetal RVOT shows elevated velocity. A left-to-right shunt through the ductus arteriosus suggests severe RVOT obstruction.¹³

Postnatal echocardiography characterizes all intracardiac features of tetralogy of Fallot. The VSD and aorta override are identified on the parasternal long-axis view (Fig. 47-5). The outflow tract must also be evaluated (Fig. 47-6) because truncus arteriosus and VSD with pulmonary atresia can have the same appearance. In contrast to double outlet right ventricle, the aortic override to the right ventricle is less than 50%, and aortic valve fibrous continuity with the mitral valve is preserved. The RVOT including the infundibulum, pulmonary valve, and pulmonary artery is measured for stenosis. The measured infundibular volume is small, and owing to reduced growth compared with somatic growth becomes progressively more stenotic.¹⁴ Infants who require earlier surgical intervention have a greater reduction in infundibular volume. The pulmonary annulus can be hypoplastic and bicuspid. Hypoplasia with a z score less than 2 is an indication for a transannular patch. Branch pulmonary artery continuity and size is also determined. The position of the coronary arteries must be established. A coronary anomaly with a major coronary branch coursing anterior to the RVOT must be excluded before repair. CT or MRI may be necessary if the coronary arteries or pulmonary arteries are inadequately visualized.

FIGURE 47-5 A, Parasternal long-axis echocardiogram shows the aorta overriding (asterisk) a large VSD. B, Parasternal short-axis echocardiogram shows anterior deviation of the anterior conal septum (arrow).

(Courtesy of Dr. Himesh Vyas, Arkansas Children’s Hospital.)

FIGURE 47-6 A, Parasternal long-axis echocardiogram shows the aorta overriding a large VSD. B, Parasternal short-axis echocardiogram shows pulmonary atresia (arrow). C, Color flow Doppler image shows ductal dependent flow to the confluent branch pulmonary arteries. The ductus arteriosus is indicated by the arrowhead.

(Courtesy of Dr. Sadia Malik, Arkansas Children’s Hospital.)

Echocardiography is more limited after surgery because of mediastinal fibrosis. Palliative shunt stenosis or occlusion is difficult to image because of their positions. Branch pulmonary artery abnormalities are also more difficult to evaluate after surgery. Available windows diminish with patient age. Residual VSD, RVOT obstruction, and branch pulmonary artery stenosis can be detected. Pulmonary regurgitation is suggested by the presence of right ventricular enlargement. Color flow and spectral Doppler are helpful in detecting pulmonary regurgitation and residual VSD.

Computed Tomography

CT is a complementary imaging modality. Echocardiography is the primary imaging modality, and MRI is the preferred complementary imaging modality. The high diagnostic dose of ionizing radiation and the inability to detect flow patterns with CT make MRI the preferred complementary modality. Multidetector CT has an increasing role in the evaluation of tetralogy of Fallot in patients with contraindications to MRI, such as pacemakers or implantable cardioverter defibrillators. Multidetector CT is better than echocardiography and MRI for defining coronary artery abnormalities (Fig. 47-7) and can be used if echocardiography is inconclusive.¹⁵^,¹⁶ High spatial resolution allows for excellent demonstration of the RVOT and pulmonary arteries, allowing for noninvasive measurements of hypoplasia, focal stenosis,¹⁷ or aneurysms (Fig. 47-8). CT is preferred to MRI to evaluate vascular stents (Figs. 47-9 and 47-10). Gated studies to measure cardiac function in patients with contraindications for MRI can be performed, but they require higher radiation dose gated techniques.¹⁸

FIGURE 47-7 A and B, CT shows right (A) and left (B) coronary artery calcifications in a 61-year-old patient with a history of tetralogy of Fallot. Pulmonary artery stenosis and poststenotic dilation are also present (B).

FIGURE 47-8 A and B, CT in a 6-year-old patient after tetralogy of Fallot repair shows aneurysmal dilation of the RVOT caused by a large outflow tract patch (arrows) on multiplanar reformation (A) and three-dimensional volume rendering (B). C, A metal ring in the Hancock pulmonary conduit is present. Hypoplasia of the right pulmonary artery is present.

FIGURE 47-9 Branch pulmonary artery stents are well visualized using multidetector CT in children after tetralogy of Fallot repair. Three-dimensional volume rendering shows left pulmonary artery stenosis distal to the stent in a 14-year-old patient.

FIGURE 47-10 Multiplanar reformation shows a widely patent right pulmonary artery stent in an 8-year-old child with stenosis of right pulmonary artery origin.

Magnetic Resonance Imaging

MRI is the primary complementary imaging modality after echocardiography. The primary diagnostic findings of tetralogy of Fallot can be detected by MRI, but are adequately imaged by echocardiography. Although echocardiography is adequate for evaluation of intracardiac abnormalities, the branch pulmonary arteries and collateral arteries are better characterized by MRI (Fig. 47-11). MRI, notably using MR angiography, can characterize branch pulmonary confluence, absence, or stenosis, and the number and location of major aorticopulmonary collateral arteries. Palliative shunts are better visualized by MR angiography than echocardiography (Fig. 47-12). Associated aortic arch anomalies that should be identified by echocardiography are more completely visualized by MR angiography (Fig. 47-13).¹⁶

FIGURE 47-11 A and B, Axial T1-weighted MRI (caudal [A] and cranial [B] images) in a 3-month-old infant with tetralogy of Fallot shows pulmonary atresia (yellow arrow in A). A large overriding aorta is present, and the aortic arch is on the right. The origin of a major aorticopulmonary collateral artery from the descending aorta (red arrow in A) is identified. Branch pulmonary artery confluence (arrowhead in B) is present.

FIGURE 47-12 MR angiography volume rendering posterior view shows wide patency of a modified Blalock-Taussig shunt (arrow).

FIGURE 47-13 MR angiography volume rendering cranial view shows a double aortic arch with a dominant right arch (arrow) in a 7-year-old patient with tetralogy of Fallot.

MRI has a more dominant role in postoperative patients. Echocardiography windows become progressively more restricted and are particularly limited for evaluation of the right ventricle and branch pulmonary arteries. Complications of palliations, such as branch pulmonary artery stenosis (Fig. 47-14), distortion, or occlusion resulting from prior systemic pulmonary artery shunts, are well visualized by MR angiography. Complications of corrective surgery, including residual RVOT stenosis, pseudoaneurysm (Fig. 47-15), or dilation from pulmonary regurgitation, are well imaged by a combination of black and white blood imaging techniques or MR angiography.

FIGURE 47-14 MR angiography volume rendering in a 5-year-old patient shows left pulmonary artery stenosis after tetralogy of Fallot repair.

FIGURE 47-15 MR angiography volume rendering shows a right ventricular outflow pseudoaneurysm (arrow) projecting to the right of the RVOT in a 14-year-old patient.

Accurate right ventricular size quantification cannot be obtained by two-dimensional echocardiography because of the right ventricular shape and the poor available windows. MRI biventricular volume measurements using short-axis cine steady-state free precession white blood images to quantify right and left ventricular systolic function and the right ventricular regurgitant fraction are the gold standard (Fig. 47-16). Right ventricular enlargement and an elevated right ventricular regurgitant fraction are among the criteria used to determine the timing for further reoperation to place a valved pulmonary conduit. Criteria for determining the appropriate time for valved conduit surgery are evolving.⁹

FIGURE 47-16 A, MR angiography volume rendering in a 40-year-old patient with massive pulmonary regurgitation after tetralogy of Fallot repair shows pulmonary artery and branch pulmonary aneurysmal enlargement. B, Short-axis cine imaging shows right ventricular enlargement and is used to quantify ventricular sizes. The right ventricular regurgitant fraction was 70%, and right ventricular end-diastolic volume index was 259 mL/m². Biventricular systolic dysfunction was also present.

Pulmonary regurgitation and late diastolic forward flow consistent with right ventricular diastolic dysfunction can be quantified using phase contrast MRI flow quantification (also known as velocity-encoded MRI) (Fig. 47-17).¹⁹ A reduction in the normal E:A maximum flow amplitude ratio across the tricuspid valve detected by phase contrast MRI flow quantification across the tricuspid valve is also consistent with diastolic dysfunction and correlates with right ventriculomegaly (Fig. 47-18).²⁰^,²¹ Dobutamine stress MRI may unmask diastolic dysfunction not appreciated on rest imaging.²² Delayed hyperenhancement imaging can show fibrosis in the RVOT and is associated with outflow tract dilation.²³

FIGURE 47-17 Phase contrast MRI flow quantification of the aorta (red) and pulmonary artery (blue) in a 26-year-old patient showed initial increased systolic pulmonary artery flow (blue arrow) compared with the ascending aorta. Pulmonary regurgitation (red arrow) is severe (pulmonary regurgitant fraction = 30%). Late diastolic forward flow indicates right ventricular diastolic dysfunction (dark red arrow).

FIGURE 47-18 A 10-year-old patient with a tetralogy of Fallot repair had a pulmonary regurgitant fraction of 42%. Right and left ventricular systolic function was normal. The right ventricular end-diastolic volume was twice that of the left ventricular end-diastolic volume. Phase contrast MRI flow quantification across the atrioventricular valves was performed. Mitral flow is shown in red, and tricuspid flow is shown in blue. The tricuspid E-wave amplitude (red arrow) is decreased compared with the A-wave (blue arrow) consistent with diastolic dysfunction.

Late aortopathy associated with tetralogy of Fallot can be evaluated by MR angiography (Fig. 47-19). Associated aortic valve regurgitation is quantified by phase contrast MRI flow quantification.

FIGURE 47-19 A 37-year-old man with repaired tetralogy of Fallot had ascending aorta aneurysmal dilation that measured 5 cm. Aortic valve regurgitation was present (aortic valve regurgitant fraction = 18%).

Angiography

Angiography was the historical gold standard for evaluation of pulmonary artery size and collateral pulmonary blood flow. These functions are now performed using noninvasive MR angiography or CT angiography. Catheterization still plays a role for interventions such as balloon angioplasty and stent placement within stenotic branch pulmonary arteries or coiling collateral arteries.

Classic Signs

The classic imaging sign of tetralogy of Fallot is the wooden shoe (coeur en sabot) or boot-shaped heart characterized on radiographs by elevation of the cardiac apex and pulmonary artery segment concavity. Right ventricular hypertrophy causes elevation of the cardiac apex. The greater the right ventricular hypertrophy, the more elevated the cardiac apex.²⁴

DIFFERENTIAL DIAGNOSIS

Clinical Presentation

Tetralogy of Fallot is the most common cyanotic heart disease, but other anomalies manifest with cyanosis. D-transposition of the great arteries, truncus arteriosus, total anomalous venous return, and double outlet right ventricle all manifest with cyanosis, but are classified as admixture lesions that tend to manifest with large hearts and overcirculation. At birth, pulmonary overcirculation and cardiomegaly may be absent in patients with D-transposition because of elevated pulmonary pressures. Patients with tricuspid atresia also present with cyanosis, and similar to patients with tetralogy of Fallot are likely to have a normal-sized heart and decreased pulmonary circulation. Right ventricular hypertrophy is not present on an ECG because tricuspid atresia is characterized by absence of the right ventricle.

Imaging Findings

Echocardiography is diagnostic. The parasternal long-axis view shows the VSD and overriding aorta (see Fig. 47-5), which is common to other conotruncal anomalies such as truncus arteriosus. Imaging of the RVOT detects stenosis, determining the diagnosis. The chest radiograph may show a boot-shaped heart that is normal in size with decreased pulmonary vascularity. Admixture lesions typically have large hearts and overcirculation. Patients with D-transposition of the great arteries clinically present early, and classic manifestations of cardiomegaly and increased pulmonary vascularity are absent. The chest radiograph in infants with D-transposition is initially normal with progressive enlargement and increased vascularity manifesting during the first week of life. Patients with truncus arteriosus frequently have a right aortic arch, but the cardiac silhouette is large and the pulmonary vascularity is increased compared with infants with tetralogy of Fallot. Patients with tricuspid atresia usually have normal-sized hearts and decreased pulmonary vascularity, but no elevation of the cardiac apex is present because the right ventricle is absent.

TREATMENT OPTIONS

Medical

Surgery is the primary treatment for tetralogy of Fallot. Ductal dependent infants require prostaglandin therapy to maintain ductal patency. Acute hypercyanotic spells (“tet spells”) may be treated by calming the child and placing the child in a knee-chest or squatting position. This position increases peripheral resistance and increases pulmonary blood flow, which reduces cyanosis. Oxygen and sedatives may be necessary. β blockers help prevent tachyarrhythmias.

Surgical

Surgical palliation by systemic pulmonary shunts to increase pulmonary blood flow was initiated by Blalock in 1945. Blalock-Taussig, central, Potts, and Waterston shunts all have been used, but the most popular current palliative shunts are the modified Blalock-Taussig shunt and the central shunt. The modified Blalock-Taussig shunt uses a polytetrafluoroethylene tube graft to connect the brachiocephalic artery or subclavian artery to the ipsilateral branch pulmonary artery. A central shunt also uses a polytetrafluoroethylene tube graft, but connects the ascending aorta to the branch pulmonary artery confluence. Palliative shunts are outgrown as the child grows, but can be a bridge in time to allow sufficient growth before corrective surgery. Shunt complications include shunt occlusion, branch pulmonary artery stenosis, and branch pulmonary artery distortion.

Primary repair bypassing initial palliation is growing in popularity. Repair includes VSD closure and RVOT enlargement. RVOT reconstruction frequently requires a transannular pulmonary patch that creates pulmonary valve incompetence. Symptomatic infants with pulmonary hypoplasia can still be treated with initial palliation. Increased morbidity is associated with primary repair in the first 3 months of life, but survival is not adversely affected.²⁵ The ideal age for corrective surgery is 4 to 11 months in asymptomatic infants, but earlier repair is advocated in symptomatic infants.²^,²⁶

Long-term pulmonary regurgitation is associated with right ventricle, pulmonary artery, and branch pulmonary artery progressive dilation. Right heart dilation with associated complications can be prevented or reversed by placement of a valved conduit in the RVOT.²⁷ No criteria are universally accepted to determine the timing for valved conduit placement. A combination of clinical, ECG, and imaging criteria is used to determine the timing of valve replacement. Clinical criteria include exercise intolerance and signs of heart failure. ECG abnormalities include prolonged tachycardia. A QRS length greater than 180 ms is associated with sudden death.²⁸ MRI criteria noted by Geva⁸ include pulmonary regurgitant fraction of 25% or greater combined with increased right ventricular size or decreased right ventricular systolic function.

Right ventricular remodeling occurs in children after placement of a valved conduit with a reduction in right ventricular size when the right ventricular end-diastolic volume index was ≥150 mL/m².²⁹ Right ventricular remodeling also occurs in adults, but Oosterhof and colleagues³⁰ found that right ventricular normalization did not occur if the right ventricular end-diastolic volume index was ≥160 mL/m² or the right ventricular end-systolic volume index was ≥82 mL/m². Studies in children and adults suggest that an optimal time exists for reoperation beyond which right ventricular remodeling is incomplete.

The aneurysmal size of the pulmonary artery and central branch pulmonary arteries in infants with tetralogy of Fallot and absent pulmonary valve requires additional surgical considerations. Surgical angioplasty to reduce the size of pulmonary arteries is essential to prevent airway compression.³¹ A Lecompte maneuver can be helpful (Fig. 47-20).