Tetralogy of Fallot is a cardiac malformation comprising ventricular septal defect (VSD), right ventricular outflow tract obstruction, aorta overriding the interventricular septum, and right ventricular hypertrophy. This malformation presumably occurs because of unequal division of the conotruncus or incorrect alignment of the ascending aorta during embryogenesis (Romero et al., 1988). A wide spectrum of severity exists for this condition, ranging from mild right ventricular outflow tract obstruction to complete pulmonary atresia, and aortic override ranging from minimal to 75% (Pinsky and Arciniegas, 1990). In addition, the hypoplasia may involve simply the infundibulum of the right ventricle or it may also involve the pulmonary valve or pulmonary arteries. Tetralogy may therefore also include pulmonary stenosis, pulmonary atresia or absent pulmonary valve, with this degree of right ventricular tract obstruction determining the amount of right-to-left shunting and cyanosis in infancy (Bashore, 2007). Approximately 3% to 6% of all cases of tetralogy will present with absent pulmonary valve (Shinebourne et al., 2006). The VSD is most often in a superior or perimembranous location, and it is usually large and nonrestrictive. The diameter of the aortic root is generally inversely proportional to that of the pulmonary artery, so that in cases of pulmonary atresia the aorta is very wide, with more than 50% committed to the right ventricle (Graham and Gutgesell, 1990).

In general, tetralogy of Fallot does not cause significant intrauterine hemodynamic compromise for the fetus because of similarities in the pressure between the systemic and pulmonary circulations. Right ventricular hypertrophy is therefore usually not seen until after birth. In cases of tetralogy with complete absence of the pulmonary valve, significant regurgitation may cause congestive cardiac failure in utero and ultimately, hydrops (Romero et al., 1988). Following birth, the closure of the ductus arteriosus, together with the narrowed right ventricular outflow tract, results in development of a significant right-to-left shunt. This leads to blood flow bypassing the pulmonary bed, resulting in hypoxemia in the systemic circulation. Because the right ventricular outflowtract obstruction is fixed, the degree of right-to-left shunt is almost entirely dependent on the systemic vascular resistance (Graham and Gutgesell, 1990). In times of low systemic resistance, such as during exercise, fever, or following feeding, the right-to-left shunt becomes more pronounced, which leads to cyanotic spells in the child. These can be treated by knee–chest positioning and by the use of peripheral vasoconstrictors.

Additional abnormalities that may coexist with tetralogy of Fallot include chromosomal abnormalities (such as trisomies 13, 18, and 21), syndromes (such as velocardiofacial), and associations (such as CHARGE and VATER) (Sanders et al., 1996). In one series, 3 of 6 cases (50%) of prenatally diagnosed tetralogy were associated with trisomy 18 (Crawford et al., 1988). In another series of prenatally diagnosed cardiac malformations, 3 of the 18 fetuses (17%) with tetralogy and known karyotype had trisomy 13 (Paladini et al., 1996). In a series of 138 liveborn infants with tetralogy, 17 (12%) had chromosomal abnormalities (Ferencz et al., 1987).

Tetralogy of Fallot is one of the most common congenital cardiac malformations, occurring in approximately 2 to 3 per 10,000 livebirths (Hoffman and Kaplan, 2002). It accounts for 5% to 10% of congenital cardiac defects diagnosed in liveborn infants (Fyler et al., 1980; Ferencz et al., 1987).

Because right ventricular hypertrophy does not generally develop in utero and visualization of fetal right ventricular outflow tract obstruction is difficult, the prenatal diagnosis of tetralogy of Fallot relies on the demonstration of the aortic outflow tract overriding a VSD at the interventricular septum (Figures 52-1 and 52-2) (Romero et al., 1988). The demonstration of a VSD with overriding aorta may be performed as early as 14 weeks of gestation with the aid of transvaginal sonography (Bronshtein et al., 1990). Because of the difficulties in visualizing small perimembranous VSDs with prenatal sonography, the diagnosis of tetralogy may be missed on a standard four-chamber view of the heart. This emphasizes the importance of careful visualization of the cardiac outflow tracts during prenatal sonography (Shinebourne et al., 2006). In one series of 22 fetuses with tetralogy, the four-chamber view was abnormal in only 1 case (Paladini et al., 1996). The VSD is best imaged by a demonstration of discontinuity between the interventricular septum and the aortic outflow tract in the left parasternal long-axis view rather than by a standard four-chamber cardiac view (DeVore et al., 1988). Color Doppler sonography may also be helpful in demonstrating flow from the right ventricle, across the VSD, and into a dilated aortic root (Anderson et al., 1994).

Additional features that may assist in the prenatal diagnosis of tetralogy include the right ventricle being slightly larger than the left, a relatively small pulmonary artery (Figure 52-3), a dilated aortic root, and axis deviation (Sanders et al., 1996). In fetuses with tetralogy, the aortic-root diameter is increased as compared with other biometric parameters, such as the biventricular diameter (DeVore et al., 1988). However, the presence of a normal pulmonary artery:aorta ratio does not exclude tetralogy; in many cases the pulmonary artery narrowing may not become apparent until late in gestation (Lee et al., 1995). In some cases of tetralogy with absence of the pulmonary valve, the pulmonary outflow tract may even appear massively dilated (Liang et al., 1997). Polyhydramnios may also be present in cases of tetralogy with absent pulmonary valve, as the massively dilated pulmonary artery may cause tracheobronchial and esophageal compression that lead to impaired fetal swallowing and an increase in amniotic fluid volume (Callan and Kan, 1991).

The usefulness of screening ultrasonography for prenatal detection of tetralogy is unclear. In one study, 13 of the 20 cases (65%) of tetralogy were detected prenatally when careful screening obstetric sonography was performed (Kirk et al., 1997). By contrast, in another study only 9 of 66 cases (14%) of tetralogy were detected using fetal echocardiography (Montana et al., 1996). In a population-based screening study from Australia, the prenatal detection rate for tetralogy was 43% (Chew et al., 2007).